Case Reports in Anesthesia

Blog with interesting cases and/or problems related to anesthesia with discussion based on best evidence in the literature.

July 23, 2018

super obese patient for shoulder surgery

Recently my partner placed an interscalene brachial plexus block with catheter in a patient who had a BMI of greater than 60.  The patient was at risk for post operative respiratory insufficiency due to the known complication of phrenic nerve palsy on the ipsilateral side of the block, which results in an elevated diaphragm on chest x ray. (see figure 1).
fig 1
Patients having arthroscopic shoulder surgery are most commonly scheduled for day surgery, and admission to the hospital for hypoxia associated phrenic nerve paralysis is suboptimal.  Patients with intrinsic lung disease are at risk for this complication.  Patients who are morbidly obese are also at risk for this complication [1].   Unfortunately, it is not always easy to tell ahead of the block who will develop clinically relevant complications as a result of phrenic nerve palsy after interscalene block.  A large retrospective review was published on cases of outpatient interscalene catheter usage (n=509) [12].  Adverse events were recorded in 6.7% of patients.  Of these only 0.6% (3 patients) had problems or complaints of dyspnea all of which were after discharge to home. However, the mean BMI was 24, with no patient having a BMI greater than 29.  In addition, all patients with any lung disease were not included in the study.  

At the level of the interscalene block, the phrenic nerve lies in close association with the brachial plexus cords/trunks. (see fig 2). Its course is most proximate to the brachial plexus at the level of the interscalene block where it typically is 18 to 20 mm away from the C5 nerve root at the level of cricoid cartilage.  
fig 2
However, as one moves caudad, the phrenic nerve moves an additional 3 mm further away from the plexus for every cm that it descends over the anterior scalene muscle.  
A review article on the options for avoidance of significant phrenic nerve block was published in the journal Anesthesiology in 2017 [2].  This article is highly recommended for the practioner who desires a more in depth understanding of brachial plexus and phrenic nerve anatomy.

Patients who are obese are more likely to experience dyspnea in association with phrenic nerve palsy. Furthermore, it is likely that morbid obesity will also increase the risk of hypoxemia in association with dyspnea. There are several strategies to reduce the chances of this outcome and will be reviewed briefly. Traditional training of the ISB has recommeneded high volumes (30 to 40 mLs) to ensure complete blockaded of the brachial plexus. Reducing this amount could reduce the incidence of phrenic nerve palsy. However, most studies indicated that volumes of 20 mL or greater will inevitably result in phrenic nerve palsy if the injection occurs around the C5-C6 nerve roots. US guided techniques have allowed practitioners to be more precise in the placement of the local anesthetic permitting a lower volume to achieve shoulder analgesia after interscalene block.  In particular, one study found that after 10 mL was injected, the chance of phrenic nerve palsy could be reduced from 100% to 60% [3]. Reducing the injected volume to 5 mL can lower the chance of phrenic nerve palsy to as low as 27% [4] without compromising the block effectiveness for up to 24H.  However, it is well known that introducing a greater volume tends to increase the chances for success, and therefore, using 5 mL as a routine is likely to lead to suboptimal outcomes in some cases. Furthermore, given the problems with admitting a patient from an outpatient facility to a hospital secondary to respiratory compromise, an incidence of phrenic nerve palsy of 27% may seem high.  Nevertheless, further improvements seem possible if the concentration is reduced. To provide dense surgical anesthesia and prolonged dense post operative analgesia, I typically opt for 0.5% bupivacaine or Ropivacaine.  By halving the concentration to 0.25%, AL-Kaisy et al was able to reduce the incidence of phrenic nerve palsy from 100% to 17% [4].  However, this study only had 5 volunteers in each group. In a slightly larger group of patients (30), Thackeray et al.[5] were able to reduce the incidence of phrenic nerve palsy from 78% to 21% by halving the bupivacaine concentration with a 20 mL injection.  This reduciton in  concentration seemed to come at the expense of more opioid requirements over a 72 hours time frame and also a shorter block duration (18 hr vs. 11.9 hr) [6]. To add confusion to the previous studies, Zhai et al. couldn't find a significant differenence in phrenic nerve palsy when using a fixed dose of 50 mg of Ropivacaine for US guided interscalene block using concentrations of 0.25, 0.5 or 0.75% [7].
Perhaps more effective, is an injection of local anesthetic around the C7 nerve root.  When using 5 mL of a 0.75% concentration of Ropivacaine, no patients (n=20) had any diaphragmatic paresis after 2 hours post injection [8].  The calculated ED 95 was 3.6 mL of 0.75% ropivacaine for this study when injected around the C7 nerve root.
Performing a supraclavicular block as been studied as a method of reducing phrenic nerve palsy.  More specifically, targeting the superior trunk of the brachial plexus (formed by the union of the C5/C6 nerve roots), has been reported in two case reports to provide analgesia after shoulder surgery without blocking the phrenic nerve. At this level the phrenic nerve has migrated away from the brachial plexus.  An approach to target the superior trunk is to do a supraclavicular brachial plexus block. Mulltiple studies have looked at the supraclavicular block and the results are somewhat equivocal. For example, with injection volumes of 20 to 30 mL, phrenic nerve palsy occured in 25 to 51% of patients.  Furthermore, in some of the studies, patients receiving a supraclavicular block had inferior post op analgesia. On the other hand, Kim et al. were able to show that a supraclavicular block  performed equally to ISB for patients having shoulder surgery without GA in terms of conversion to GA (0 patients) or fentanyl requirements [14]. Given the above data, I have begun to modify my block for shoulder surgery, seeking to do what I consider to be a high supraclavicular block or perhaps a low ISB. I further modify my block in patients who I consider to be at higher risk for phrenic nerve palsy by reducing the volume of local anesthetic and perhaps also decreasing the concentration. 
    The above techniques may reduce the incidence of phrenic nerve palsy, but don't seem to eliminate the risk altogether. The risk of phrenic nerve palsy may be completely eliminated by avoiding any injection around the brachial plexus.   In 2012, Siegenthaler et al. desribed a novel approach to the suprascapular nerve [9].  In their study, a comparison study was done between locating the suprascapular nerve in the supraclavicular space vs. in the suprasspinous fossa.  They determined that  suprascapular nerve identification was much better in the supraclavicular space (81% identified vs. 36%). In my own practice, attempts at identification of the correct space using the supraspinous fossa (also known as the suprascapular notch) proved very difficult due to the greater amount of thick tissue overlying the area.  Furthermore, This approach requires a cooperative patient who can sit upright in order for the approach.  Lastly, I found that due to the depth of the area to be blocked, an acute angle was required making visualization of the needle problematic for accurate injection.  
The suprascapular nerve provides aproximately 70% of the innervation to the glenohumeral joint.  The majority of the remaining 30% derives from the axillary nerve.  Two recent studies published in Anesthesiology have looked at the effectiveness of suprascapular nerve block vs. interscalene block to determine non inferiority.  One was a meta analysis comparing the two approaches [10].  The meta analysis determined that a suprascapular nerve block alone was not inferior to an ISB.  In this meta analysis, only one study utilized a supraclavicular block in the supraclavicular fossa (all other studies approached the nerve from supraspinous fossa). The primary outcome for which the blocks were compared and found to be similar were for post operative morphine consumption (24H) and the cumulative difference between ISB and SSNB in the area under the curve for rest pain during the first 24H interval. This meta analysis did note, however, that during a 1 hour interval in the PACU, ISB provided superior pain control.  At 6,12,24, and 48hr there was no statistical difference. In this same meta analysis, it was found that ISB was associated with more respiratory complications, undesirable blockades, and block-related complications. 
    This month (july 2018), a head to head to head trial was published comparing analgesic efficacy between the anterior suprascapular, supraclavicular and interscalene nerve blocks [11]. The primary outcome was pain scores in the PACU.  The pain scores were 1.9,2.0 and 2.3 for the ISB, anterior suprascapular, and supraclavicular blocks respectively. The authors concluded that the anterior suprascapular nerve block was non inferior to the ISB. They also concluded that the supraclavicular block did not meet their prespecified criteria for non inferiority. They also found a significant decrease in respiratory function (measuring VC) for the ISB. (see fig 3).

In general, ISB is the gold standard for providing effective and consistent post op analgesia for shoulder surgery.  However, in day surgery patients who are likely to become hypoxemic (sat less than 90%) on room air after phrenic nerve block, the ISB is a relative contraindication.  Current research indicates that the only method found to virtually provide a 0% incidence of phrenic nerve palsy is a suprascapular nerve block.  Fortunately, several studies indicate that this block is likely non inferior to ISB in providing ample pain control.

Technique-Anterior Surpascapular nerve block:

place US probe in standard location as you would for ISB. Move the probe in a caudad direction as you follow the brachial plexus. (see figures below).

After determining the location of isolated SCN, only 3 to 4 mL are required to block this nerve at this level. Furthermore, due to the location of the nerve, a catheter could easily be inserted and secured in this location.
1. Hartrick CT et al. BMC Anesthesiol 2012;12:6.
2.  Deborah Culley Anesthesiology 2017;127:173-91.
3. Lee, JH, Cho, SH, Kim, SH, Chae, WS, Jin, HC, Lee, JS, Kim, YI R Can J Anaesth 2011; 58:1001–6
Stundner, O, Meissnitzer, M, Brummett, CM, Moser, S, Forstner, R, Koköfer, A, Danninger, T, Gerner, P, Kirchmair, L, Fritsch, G  Br J Anaesth 2016; 116:405–12
4. Al-Kaisy, AA, Chan, VW, Perlas, Br J Anaesth 1999; 82:217–20
5. Thackeray, EM, Swenson, JD, Gertsch, MC, Phillips, KM, Steele, JW, Burks, RT, Tashjian, RZ, Greis, PE  J Shoulder Elbow Surg2013; 22:381–6
6.Wong, AK, Keeney, LG, Chen, L, Williams, R, Liu, J, Elkassabany, NM Pain Med 2016; 17:2397–403
7.Zhai, W, Wang, X, Rong, Y, Li, M, Wang, H BMC Anesthesiol 2016; 16: 1–8
8. Renes, SH, van Geffen, GJ, Rettig, HC, Gielen, MJ, Scheffer, GJ  Reg Anesth Pain Med 2010; 35:529–34 
9.  Siegenthaler, A, Moriggl, B, Mlekusch, S, Schliessbach, J, Haug, M, Curatolo, M, Eichenberger, U Reg Anesth Pain Med 2012; 37:325–8 
10. Hussain N, Goldar G, Ragina N, Banfield L, Laffey J and Abdallah F. Anesthesiology 2017;127:998-1013.
11. Auyong DB, Hanson NA, Joseph RS, Schmidt BE, Slee AE, and Yuan SC. Anesthesiology 2018;129:47-57.
12. Marhofer P, Anderl W, Heuberer P, Fritz M, Kimberger O, Marhofer D, Klug W, and Blast J. Anaesthesia 2015, 70, 41-46.
13.Urmey WF, Talts KH, Sharrock NE Anesth Analg. 1991 Apr; 72(4):498-503.
14.Ryu, T, Kil, BT, Kim, JH  Medicine (Baltimore) 2015; 94:e1726

March 4, 2018

63 year old male for shoulder arthroscopy and polycythemia vera

Today I was informed by the orthopedic surgeon that the next patient scheduled for shoulder surgery had a Hgb of 19.4 g/dL.  He was 63 and appeared to be in poor health.  He was on several medications including:

  • ASA 81 mg
  • Buprion 300 mg qd
  • Citalopram 20 mg 
  • Fluoxetine 40 mg
  • Gaba 300 mg
  • Glipizide 5 mg
  • Lantus 50 units qd
  • NPH 30 units qd
  • Metformin 1000 mg
  • pravastatin 40 mg
  • Na+ 141
  • K+ 4.8
  • BUN 10
  • Cr 0.83
  • Glucose (finger stick) 63 mg/dL patient without symptoms of hypoglycemia.
  • Hct 57.8
  • Platelets 245,000
  • total CO2 33
In addition to untreated HTN, depression and diabetes, the patient had significant OSA.
The surgeon was eager to proceed with surgery.  Therefore, I had a conversation with the patient in an attempt to determine the etiology of his polycythemia.  He admitted to a chronic smoking history, but denied any trouble with SOB, orthopnea or lung disease.  His room air sat was 94%.  He had previously been made aware of his elevated Hgb levels, and it was recommended to him to donate blood.  However, he stated that he had never donated blood because they would not accept his blood due to his diabetes.  I explained to him the importance of immediate follow up with his doctor to treat his condition.  I made a presumptive diagnosis of polycythemia vera since I could not find any obvious source of hypoxemia in this patient. A formal diagnosis requires that several criteria be met as listed below.

I placed an interscalene catheter under US guidance.  In the meantime, as we waited for surgery, I gave the patient 2 L of LR in order to mitigate his PV to a small degree.  We also planned to extract a small amount of blood once we arrived to the OR (between 60 and 100 mL's).  The patient underwent an uneventful shoulder scope with a surgical time of 63 min with un uneventful PACU stay.

Polycythemia Vera (PV) is a myeloproliferative disease which results in excessive production of RBCs. In addition, it can also lead to luekocytosis and thrombocytosis.  Other myeloproliferative disorders include essential thrombysthenia and primary myelofibrosis. PV is considered when the Hct is greater than 48% in women and 52% in men. This is an important disease to recognize in the perioperative period because patients with this condition are at greater risk for venous and arterial thrombosis, and hemorrhage [1,2], due to hyperviscosity of the blood and dysfunctional platelets.  Diagnosis can be challenging and is not easily made at the bedside. In order to be positively diagnosed with polycythemia vera, two major criteria and one minor criterion must be present

Major criteria include:

  1. hemoglobin greater than 18.5 g/dl in men, greater than 16.5 g/dl in women
  2. presence of the JAK2 V617F or similar mutation.
Minor criteria include: 
  1. bone marrow biopsy showing myeloproliferation. 
  2. serum erythropoietin level below the normal range.
  3. endogenous erythroid colony formation in vitro [3].  
In additioon, in patients who NOT have the JAK2 mutation, but do have elevated Hgb, AND two minor criteria, a positive diagnosis of PV can be made.

The only available information from the above list in my patient was a Hbg of 19.4 md/dL.  My patient also had significant OSA.  I questioned whether this might account for his polycythemia rather than polycythemia vera.  A recent retrospective analysis of polycythemia in patients with OSA considered over 527 patients who had polysomnography indicating OSA.  The authors concluded that the incidence of polycythmia in OSA patients was indeed very rare.  They also reported that severity of OSA did not correlate with greater increases in Hgb levels.  The authors did concede that, "
In those rare polycythemic OSA patients, polycythemia is corrected by CPAP therapy in the majority."  In addition, my patient had diabetes and a finger strip glucose reading prior to surgery read 63 mg/dL.  I did not treat this as the patient stated that he had no symptoms and usually did not feel bad until his sugar reading fell below 50 mg/dL.  It should be noted, that glucose bedside monitors can read artificially low levels of glucose in patients with elevated hematocrits.

Polycythemia results in hyper viscous blood.  It should be recognized that Poiseuille's equation indicates that flow through a tube is proportional to the fourth power of the radius, the pressure differential, and inversely proportional to the length and viscosity of the fluid. Therefore, as viscosity increases with increasing hematocrit, the pressure differential must increase in the exact amount to maintain an equal blood flow.  Unfortunately, as the hematocrit increases it results in a semilogarithmic increase in the blood's viscosity.  In fact, as the hematocrit hits 45%, it can be said that the increase in viscosity "takes off". (see figure)

Therefore, cardiac work must increase to maintain forward flow, and indeed, as polycythemia progresses much beyond a hematocrit of 45% cardiac output decreases.  Clinically, this can manifest as SOB, lack of energy, tiredness, and even angina.  Therefore, a careful history is critical in these patients. It should be noted that hyperviscous blood is particularly problematic for the small capillaries which can lead to ischemia especially in patients with  poor heart function.

Polycythemia Vera can also result in paradoxical hemorrhage. This is believed to be a result of platelet dysfunction. Acquired  Von Willebrand's diesease could be another cause of increased bleeding tendency in these patients. Most authors recommend the avoidance of neuraxial anesthesia in patients with PV unless a coagulation profile is normal (i.e.thromboeslastography) due to known platelet dysfunction.

These patients may be at high risk for complication and death after surgery. In a review from 1963, patients with uncontrolled polycythemia had a 79% rate of complications and a mortality rate of 36% [4]. However, patients who were treated had their rate of complications decreased to 28% and mortality rate to 5%. Another study found that the incidence of thrombosis was 7.7% and that of major hemorrhage was 7.3% [5].

A lower hematocrit can be favorable in the perioperative period.  The increased viscosity of blood is particularly problematic during this period.  Some evidence suggests that the brain oxygenation and brain oxygen delivery is optimized when the hematocrit is decreased to below 45% [5].
In several case reports dealing with patients who have presumptive PV, the authors desribe performing acute normovolemic hemodulition.  The formula that should be used in order to arrive at the proper Hgb level preoperatively is V=EBV X (Ho - Hf)/Have.    

V=volume of blood to extract
EBV = Estimated Blood Volume (in adult males 70 mL/kg is often used)
Ho= starting Hematocrit level of patient 
Hf= final Hematocrit or minimal allowable (around 40 to 45%).
Have= the average between the start and final hematocrit.

An equivalent volume to replace the blood withdrawn should be infused during the extraction of blood.  I.e. if 500 mL of blood is extracted, then 500 mL of colloid might be infused.   In general, the goal is to reduce the viscosity of blood, and generous intravenous fluids (LR or NaCl) should be infused.

Even in patients that have normalized their hematocrit prior to surgery there is elevated risk of thrombosis.  A retrospective review of patients with either PV or ET and receiving anticoagulation therapy during their surgery estimated that the incidence of DVT after major surgery was increased 5-fold. Unfortunately, the risk of major bleeding was also increased in this retrospective review demonstrating the paradoxical risk of hemorrhage in PV. The authors recommended using LMWH for prevention of DVT during the perioperative period.

One final note on this particular patient was his list of medications.  He was taking bupropion, a norepinephrine/dopamine reuptake inhibitor for depression, citalopram a norepinephrine/serotonin reuptake inhibitor and fluoxetine a selective serotonin reuptake inhibitor.  This patient was therefore, at some risk for serotonin syndrome should. This syndrome is characterized by changes in autonomic, neurological, and cognitive behavioral functions and appears to result from over-stimulation of 5-HT1a and 5-HT2 receptors in the central grey nuclei and medulla.  Therefore, Demerol, tramadol and dextromethorphan would be relatively contraindicated in this patient. Furthermore, as mentioned, this patient had a glucose reading of 63 mg/dL preop.  There are a number of articles and case reports indicating the risk of hypoglycemia in patients taking SSRIs.  It has been shown that SSRIs can enhance insulin sensitivity in man [7-10].  Therefore, this patient had three possible causes of low blood sugar: 1) too much diabetic medication plus a missed meal, 2) polycythemia induced misreading of glucometer, and 3) SSRI induced insulin enhanced sensitivity.

In general, the approach to patients with PV with scheduled surgery needs to consider any clinical symptoms resulting from the PV, current medical condition of the patient, age, degree of polycythemia, and stress and duration of surgery.  Therefore, a patient having a relatively short procedure, with no evidence of any symptoms from the polycythemia and in otherwise decent health may tolerate surgery with acute preoperative normovolemic hemodilution.  A patient with uncontrolled PV undergoing major surgery on the other hand may need to be delayed to allow for a hematologist consult for preoperative treatment.

1. Berk PD, Goldberg JD, Donovan PB, Fruchtman SM, Berlin NI, Wasserman LR. Therapeutic recommendations in polycythemia vera based on Polycythemia Vera Study Group protocols. Semin Hematol. 1986;23:132–143.
2. Ruggeri M, Rodeghiero F, Tosetto A, Castaman G, Scognamiglio F, Finazzi G, et al. Postsurgery outcomes in patients with polycythemia vera and essential thrombocythemia: a retrospective survey. Blood. 2008;111:666–671.
3. Wadleigh M, Tefferi A. Classification and diagnosis of myeloproliferative neoplasms according to the 2008 World Health Organization criteria. Int J Hematol. 2010;91:174–179. 

4. Wasserman LR, Gilbert HS. Ann New York Acad Sci 1964;115:122-38.

5. Thomas DJ, Marshall J, Russell RW, Wetherley-Mein G, du Boulay GH et al.  Lancet. 1977;2(8045): 941-943

6. Ruggeri M, Rodeghiero F, Tosetto A, Castaman G, Scognamiglio F, et al.  Blood 2008; 111(2): 666-671

7. Araya V, Contreras P, Aguirre C, Depix MS, Zura ML. The effect of fluoxetine on insulin resistance in nondiabetic obese patients [in Spanish]. Rev Med Chil. 1995;123(8):943-947.

8. Potter van Loon BJ, Radder JK, Frölich M, Krans HM, Zwinderman AH, Meinders AE. Fluoxetine increases insulin action in obese nondiabetic and in obese non-insulin-dependent diabetic individuals. Int J Obes Relat Metab Disord. 1992;16(2):79-85.
9.  Lustman PJ, Clouse RE, Nix BD, et al. Sertraline for prevention of depression recurrence in diabetes mellitus: a randomized, double-blind, placebo-controlled trial. Arch Gen Psych. 2006;63(5):521-529.
10.  Paile-Hyvärinen M, Wahlbeck K, Eriksson JG. Quality of life and metabolic status in mildly depressed women with type 2 diabetes treated with paroxetine: a single-blind randomised placebo controlled trial. BMC Fam Pract. 2003;4:7.

February 1, 2018

WPW syndrome in patient with significant intraoperative hyperthermia

Recently I relieved one of our CRNAs. The patient was undergoing a marathon thyroidectomy which had started at 9:30 that morning. It was now 3pm.  The report I received indicated that the patient had developed hyperthermia with a  temperature (esophageal) of 38.8 C. The patient was receiving sevoflurane in oxygen and the HR was 110's.  BP was stable, and etCO2 was 36 mmHg with a minute ventilation of close to 5 to 6 L/min.  

I immediately verified that the room temperature was turned down and that the bear hugger was blowing cool air.

The main considerations when confronted with sudden intraoperative hyperthermia are:
  • malignant hyperthermia
    • in this case ruled out by normal minute ventilation and etPCO2, and lack of muscle rigidity.
  • Thyroid Storm
    • A possibility in this case, considering that the patient was having a thyroidectomy that required nearly seven hours of surgery. 
    • often causes tachycardia, hyperthermia, hyper or hypotension, hypokalemia and significant mental status changes that would not be evident until emergence.
  • cocaine abuse
    • excessive preoperative use can result in hyperpyrexia along with other symptoms like tachycardia, seizures, tachypnea, and dysrhythmias can often mimic MH.
  • sepsis
    • usually clinical history can clue in to this cause as well as tachycardia and hypotension.
  • pheochromocytoma
    • clinical history of headaches, sudden onset hypertension and tachycardia will lead the clinician to suspect this diagnosis
    • Catecholamine excess usually results in significant tachycardia and hypertension intraoperatively, but hyperthermia may also be present.
  • excessive medication administration
    • ketamine, atropine, dopamine, droperidol, or tricyclics 
  • Neuroleptic malignant syndrome (NMS)
    • from central dopaminergic blockade
    • butyrophenones, phenothiazines, metoclopramide, lithium,  tricyclics, monoamine oxidase inhibitors, selective serotonin  reuptake inhibitors, and haloperidol.
    • clinical symptoms similar to MH; tachycardia, hyperthermia, metabolic acidosis, and increased muscle tone.
    • The only possible way to distinguish NMS  from MH is that severe hypercapnia is not seen with NMS.
This patient also was diagnosed with Wolf Parkinson White (WPW) syndrome and was currently tachycardic.  WPW is a pre excitation syndrome where atrial cardiac impulses may bypass the AV node via Kent's bundle which can lead to pre excitation of the ventricle leading two main arrythmias: parosxymal supraventricular tachycardia (PSVT) or atrial fibrillation (AF). The accessory pathway (Kent's bundle) utilizes a sodium-dependent fast inward current for electrical impulse transmission, thus conducting signals more quickly from the atrium to the ventricle than the AV node, where a calcium-dependent slow inward current slows conduction.  Therefore, the general goal is to increase the refractory period of the bundle of kent relative to the AV node. However, conduction of cardiac impulses may travel in either a retrograde or anterograde direction through the bundle of Kent creating a clinical challenge in both diagnosis and proper treatment of pathologic tachycardias in patients with WPW syndrome. In some cases The bundle of kent  conducts impulses in only a retrograde direction as seen below in the graphic. In this case, no delta wave is visible on a normal EKG.
However, the vast majority of WPW patients can experience both anterograde and retrograde conduction via the accessory pathway. This is important because the EKG appearance is altered by the direction of conduction of cardiac impulses through the accessory pathway.  Patients with WPW syndrome who develop PSVT will have a regular R to R interval and may have a narrow complex tachycardia.  However, Antidromic AVRT can lead to wide complex tachycardia that is very difficult to differentiate from ventricular tachycarida (VT).  Antidromic AVRT is rare (~5% of PSVT in WPW).  Orthodromic AVRT is more common and results in a narrow complex tachycardia (200 to 300 bpm).  Patients with WPW syndrome and AF have an irregularly irregular wide complex tachycardia. (see graphic of AF in WPW below).
Therefore, the approach to a patient who develops pathologic tachycardia and has WPW syndrome in the preoperative period requires determining most importantly whether there is a regular R to R interval (see discussion below). In many cases, amiodarone is the preferred for treatment of pathologic tachycardia in WPW only because amiodarone can be safely used regardless of the etiology, i.e. AF, VT, PSVT.  If a clear diagnosis is possible from the cardiac tracing or 12 lead EKG, then a more selective medication may be chosen. For example, it may be best to begin treatment of PSVT  with vagal maneuvers if the patient is otherwise stable. These include valsalva, gag reflex (fingers in throat), or ice on face (diving reflex).  If prompt attempts at vagal maneuvers fail, then medications that can abruptly prolong the refractory period of the AV node (PSVT) such as adenosine 3 to 12 mg IV, verapamil 2.5 to 10 mg IV,  or esmolol 50 to 100 mg IV can be tried. It should be noted that several case reports of WPW syndrome patients receiving general anesthesia have commented on the use of lidocaine to prevent re entrant tachycardia.  It is unlikely that lidocaine will play a significant role in preventing re entrant tachycaridias in this patient population as noted in a paper published by Barrett et al. [2] In another study, the authors showed that in patients with WPW syndrome in A Fib with RVR, lidocaine was likely to increase (make worse) the ventricular rate, or have no beneficial effect [3]. Lidocaine is a class IB anti arrhythmic (blocks Na+ channels), but unlike procainamide (used in WPW syndrome with A fib), lidocaine decreases the effective refractory period (procainamide class IA increases the effective refractory period). Therefore, lidocaine based on its pharmacology alone would be predicted to be less useful in treating supra ventricular tachycardias as compared to procainamide. In the case of WPW with AF, the goal is exactly the opposite of that in a non WPW syndrome patient with AF. i.e. the goal in non WPW A fib is to slow conduction through the AV node, via medications such as verapamil.  In WPW AF, verapamil (and digitalis), which slows AV node conduction, is strictly contraindicated. Procainamide, on the other hand,  can be used since it prolongs the refractory period of the accessory pathway. In daily clinical practice, procainamide may not be readily available.   Amiodarone is a class III anti arryhmic and indeed can treat AF in WPW.  However, a  2010 review [4] found several studies that were able to identify a small risk of ventricular fibrillation (similar to the concern when using lidocaine for AF in WPW syndrome).  The study, therefore, concluded that amiodarone was not superior to procainamide in rate control for WPW with AF and could be dangerous.
  In any patient who becomes unstable due to the arrhythmia, synchronized cardioversion is the treatment of choice.   Opioids like fentanyl, benzodiazepines including midazolam have been found to have no effect on the EP effects of the accessory pathway. There is a case report of disappearance of delta wave after propofol administration making it the induction drug of choice if GETA is required [1]. Both isoflorane and sevoflorane do not have any effect on AV node conduction, however, desflurane if given at greater than a 6% concentration initially can lead to increased sympathetic output.  Therefore, if desflurance is chosen, slowly increasing the concentration would be required. 

Typically, the anesthetic management of a patient with WPW calls for the avoidance of increased sympathetic activity (pain, anxiety, fear, stress response from any cause, lighter planes of anesthesia, hypovolemia and the avoidance of anticholinergic medications.  Since the avoidance of lighter planes of anesthesia is critical in these cases, monitoring anesthesia depth with a BIS monitor would not be unreasonable in any patient with WPW syndrome who has not had the accessory pathway ablated.  

In this patient who was at risk for thyroid storm having a prolonged thyroidectomy surgery, significant hyperthermia with sinus tachycardia and WPW syndrome, presumptive treatment for thyroid storm was reasonable.  It turns out that many of the indicated treatments for a patient with possible thyrotoxicosis are also indicated to modify the risk in a patient having surgery with WPW syndrome.  An esmolol infusion is recommended to control HR in thyroid storm [5], and is not a bad choice to control HR and attenuate the sympathetic nervous syndrome in a WPW syndrome patient.  Also recommended for patients suspected of thyroid storm and persistent hypotension is cortisol IV (100 to 200 mg).  Decadron may inhibit the conversion of T4 to T3 (T3 is the main culprit in promoting symptomatology in thyrotoxicosis) [5]. 

In the end, the patient was extubated and taken to PACU.  Despite efforts to cool the patient, the temperature was still elevated in the PACU.  Fortunately, other than hyperthermia, the patient suffered no apparent ill effect from the surgery and recovered without sequelae.  However, vigilance and the ability to prioritize treatments in patients who present with two or more conditions simultaneously in the OR requires a more intimate understanding of the underlying goals at a mechanistic level.  In this case, the treatments recommended were beneficial for both conditions.

1. 8. Seki S, Ichimiya T, Tsuchida H, Namiki A. A case of normalization of Wolff-Parkinson-White syndrome conduction during propofol anaesthesia. Anesthesiology. 1999;90:1779–81.
2. Barrett PA, Laks MM, Mandel WJ, Yamaguchi I. Am Heart J. 1980. Jul;100(1):23-33.
3. Akhtar M, Gilbert CJ, and Shenasa M. Circulation 1981. Vol. 63(2):435-441.
4. Simonian SM, Lotfipour S, wall C, and Langdorf MI.  Intern Emerg Med. 2010, 5(5): 421-6.
5. Stoelting RK. Anesthesia and Co Existing Disease. p. 349

September 21, 2017

Three cases of spinal for c-section; an eval of each one.

Last weekend on call I had three sections during the night.  The first case was a young nurse, healthy with excellent anatomy for placing a spinal anesthetic.  She was admitted around 4pm and went to c-section at 10:00pm for lates and variables.

I performed a spinal in my usual fashion.  The hospital supplies us with Spinocan spinal trays.  These contain a 25 G spinocan needle, 2 mL of 0.75% hyperbaric bupivacaine, Lidocaine 1% 5 cc and a 3 cc syringe plus a 5 cc syringe for drawing the intrathecal dose.  I also administer 12 mg  bupivacaine mixed with 20 mcg of fentanyl + 150 mcg of preservative free morphine (duramorph).  I also, always grab a 25 G whitacre needle from the anesthesia cart to decrease the risk of PDPH.

I finished this case at 11:15pm and went to bed.  At 12:30am I was called by the obstetrician who told me she had a drop in patient with severe PIH.  She planned on delivering her, but she her platelets went from 88K to 84K.  She wanted to know  what I thought.  I told her I would place a spinal in this patient as long as the platelets were greater than 75K.  We rolled back to the OR at 2:10am and finished at 3:30am.  In this patient, due to challenging anatomy (obese patient with lots of adiposity in the lumbar back), I was unable to get the spinal needle (once again a spinocan) into the space with just 2 to 3 attempts.  Due to the relative urgency of the case, I opted to use a touhy needle (18G) to locate the midline and use a gertie marx 26G spinal needle through the touhy to inject my usual mix.  With the touhy, I rapidly (with one pass) identified the midline and epidural space, inserted the spinal needle, and got a great spinal.  The case went smoothly, and blood pressures after spinal normalized almost immediately with no further issues.

At 5:30am, I was called for an urgent c-section.  A young healthy patient who was schedule for c-section that very morning but at 10:00am had arrived to the hospital early contracting. She had been scheduled because she was carrying twins who were breech.  Because of the urgency, the obstetrician was standing at my side during the spinal placement.  I noticed that her anatomy was also very poor for easy identification of the midline.  She was obese, with the vast majority of her adipose tissue located in the lumbar region.  Palpation of a midline area was very challenging.  The patient was also in terrible pain due to near constant uterine contractions and had trouble sitting and holding her position.  I elected to immediately start with a tuohy needle as I sensed that I might again have a difficult time identifying the midline by feel with a thin 25G spinal needle.  I inserted the touhy needle to a moderate depth without feeling any resistance that I could identify as ligament or bone.  No sooner had I got the touhy to a moderate depth (approximately 4.5 cm) did I notice a flash of clear liquid at the hub.  I feared a wet tap, so I removed the stylet from the touhy, without any CSF flow.  It was clear that there was however some residual clear liquid in the touhy.  I removed the touhy and retried with no luck.  After searching a minute or two for the midline, I again noticed a small flash of clear liquid in the hub and withdrew the stylet.  This time clear fluid drained from the tuohy.  While disappointed in this result, my only real option at this time was to inject the spinal cocktail I had prepared.  I attached my syringe, aspirated to verify continued CSF drainage, and once this was verified, I injected the anesthetic (12 mg bupivacaine, 20 mpg fentanyl, 150 mpg duramorph).  The patient laid down, and very slowly the contractions eased, but did not go away completely.  Very quickly it was evident, that the patient did not develop a proper spinal block. GETA via rapid sequence was rapidly induced and the twins were delivered without incident.  We finished at 7:05am, the end of my shift.

Case 1 breakdown- discussion of spinocan vs other needle types.

This case went as uneventful as you might expect in a healthy 28 year old female having her 2nd child for NRFHT.  The only difference in this cases was that I was forced to use a spinocan needle (cutting type tip) rather than my usual choice of a whitacre.  This case was performed at 10PM. I visited the patient on POD 2 at about 4:30pm. She was complaining of a headache which started mid morning on POD 1.  It was gone in the morning of POD 2 after a good nights sleep, but returned with a vengeance by early afternoon.  It was totally relieved by lying down, and clearly aggravated when the head was elevated.

Modifiable risk factors of PDPH include the needle size, needle shape, bevel orientation and inserting angle, stylet replacement, and operator experience. Needle size might be the most significant factor in the development of PDPH.  In our hospital, we only have access to 25G needles. A previous meta-analysis published in 2000 has compared the frequency of PDPH between Quincke (a cutting-point spinal needle) and pencil-point spinal needles which suggested that pencil-point spinal needle will significantly reduce PDPH rate compared with Quincke spinal needles. They found a RR of PDPH of 0.38 if a pencil point needle was used.  Criticism of this meta analysis related to its small size (only 15 PDPH occurred in the study), and only two needle types compared (Quincke vs whitacre).  One study estimated that the incidence of PDPH when a 25 G cutting needle is used (Quincke) is about 23% [1] but may be less than 2% if a 27G needle is used. However, a more recent study [3] a prospective comparison of 5 needle types showed that PDPH went from an incidence of 8% in the quincke style needle to 3% with a whitacre. The requirement for blood patch in this study went from 12.5% with a 25G quincke to 0% with a whitacre.  Studies also find that there is less chance of successful spinal anesthesia when smaller gauge needles are used, therefore, the law of diminishing returns becomes significantly apparent with needles smaller than 27G.  In a recent meta analysis looking at over six thousands patients, an overall incidence of 4.6% was found for PDPH [2]. The incidence of PDPH was 6.6% for cutting type needles, and 2.6% for pencil point styled needles. This resulted in a RR of PDPH of 2.5 when a cutting style needle was used based on the studies included in this meta analysis.

Other risk factors for PDPH include female sex, younger age, and pre existing headache.  Pregnancy may be yet another risk factor for developing PDPH.  Ironically, it is this patient population that is most exposed to dural puncture.    In this case, the patient suffered a PDPH likely avoided if a pencil point styled needle was used.  She was treated with a blood patch on POD 2, and her headache was relieved.

Case 2 breakdown-  low plateles requiring urgent delivery due to sever PIH

For a general review on PIH and anesthesia click here.   There is no evidence to support any arbitrary cut off when performing neuraxial anesthesia in a patient whose platelets are low (i.e. less than 150k).  However, we have indirect evidence that there is some risk associated with neuraxial anesthsia in thrombocytopenic patients.  In PIH about 30% of patients will develop thrombocytopenia.  The cause is unknown, but damage to the endothelium has been implicated. This is known to cause the release of thromboxane and serotonin from activated platelets and a platelet consumption cascade ensues.   As an anesthesia resident, I was taught that below 100k platelets, neuraxial anesthesia is contraindicated in PIH.  However, practice patterns are changing.  In a small retrospective study of 30 parturients, epidural anesthesia was conducted when platelets were between 69K and 98K [4]. in 1989, Rasmus et al. [5] found 14 parturients who received neuraxial anesthesia with platelet counts ranging from 15K to 99K.  No reports of epidural hematoma were found in this review.  In a more recent review Goodier et al. [6] looked at 174 parturients with low platelet count (less than 100K) and neuraxial anesthesia and found no cases of hematoma formation.   Nevertheless, at this time, the numbers of patients documented to have neuraxial anesthesia with platelets less than 100k  (~499) is small.  In a review by Vandermuelen et al revealed that in cases of epidural hematoma, 75% were in patients who had EA instead of SAB. This rare condition may still be a relevant concern given that it may lead to permanent paraplegia. In the past, many have advocated for epidural anesthesia in PIH or severe PIH for Cesarean delivery to avoid severe hypotension by using a gradual block onset.  This is related to the fact that in PIH, intervillous blood flow is decreased, therefore, making these patients particularly vulnerable to hypotension.  However, in clinical practice this has not born out. Dyer at al stated that current evidence suggests that parturients with PIH have less susceptibility to hypotension and perhaps less impairment of cardiac output vs their healthy counterparts [7]. In a recent prospective study, hypotension was more frequent in SAB vs. EA for C-section, however, the duration of hypotension was short (less than 1 min) in both groups.  Also, hypotension was easily treated in both groups, and the study concluded that  SAB was safe for C-section in patients with severe pre eclampsia [9]. In severe PIH, a major concern for maternal health is the avoiding severe hypertension that can result in cerebral bleeding.  Ramanathan et al. showed that neuraxial anesthesia is superior to GA in avoiding hypertension during cesarean delivery [8]. Therefore, in patients with severe PIH and low platelets requiring urgent cesarean delivery, SAB is better than EA is probably better than GA.  

Case 3 breakdown:
This case was FUBAR.  Some of the issues making this case difficult, included:  a patient in near hysterics due to near constant contractions, significant obesity located to the lower back area, and the fact that I was exhausted by this point having been working on and off for the last 24 hours.  Nevertheless, my decision to use a touhy needle (a routine for me) as my first attempt in a difficult spinal was a problem.  It lead to an immediate dural puncture.  Unfortunately, when I removed the stylet to verify my suspicion, there was no flow of CSF.  The next "dural puncture" was likely not a dural puncture after all.  Although there was flow of CSF from the tuohy, no level was achieved.  My explanation for this is that a pool of CSF must have built itself up in the epidural space from the initial dural puncture. I therefore, had plenty of CSF flow and could withdraw CSF prior to injection, but the injection was placed into the epidural space.  The very slight relief of pain that occurred just a few minutes after my "spinal injection" is likely attributed to some medication finding its way through the dural tear into the IT space.   In hind site, and going forward, I will always make my first attempt at spinal anesthesia with the standard 25 g spinal needle (pencil point type).  In this case, I did have some redemption in choosing the tuohy first for two reasons: 1) I had had to resort to a touhy in the previous spinal after multiple failed attempts with the spinal needle and this had solved immediately my difficulty with gaining access to the IT space, and 2) we had 5 inch gertie marx needles (pencil point styled) which are meant to be inserted through the touhy after gaining access to the epidural space, but I only 3.5 inch 25 G spinocan needles for a straight IT approach.  (BTW, i have since talked to the anesthesia tech to ensure that we have a large supply of 25G whitacre needles in each OB OR).  

In summary, in one night on OB call, I performed 3 anesthetics. Case one was a straight forward very forgettable spinal anesthetic that resulted in an early and severe PDPH requiring blood patch.  Case two was a severe PIH patient with very low platelets that underwent uneventful spinal anesthesia despite unfavorable anatomy using a pencil point spinal needle with no sequelae.  Case three was a healthy breech twin parturient in extreme pain who suffered a dural puncture with 18G touhy needle with accompanied failed SAB requiring conversion to GETA via RSI. She developed a PDPH 36 hours after puncture and eventually requested a blood patch which resolved her headache.  

1. Castrillo A, Tabernero C, Garcia-Olmos LM, et alSpine J 2015;15:1571–6
2. Hong Xu, MD, Yang Liu, MD, WenYe Song, MD, ShunLi Kan, MD, FeiFei Liu, MD, Di Zhang, MD, GuangZhi Ning, PhD, and  ShiQing Feng, PhD  . 2017 Apr; 96(14): 
3. Vallejo MC1Mandell GLSabo DPRamanathan S 2000 Oct;91(4):916-20.
4.   Beilin Y, Zahn J, Comerford M. Safe epidural analgesia in 30 parturients with platelet count between 69000 and 98000 cumm-1. Anesth Analg. 1997;85:385–8.  [PubMed]
5.  Rasmus KT, Rottman RL, Kotelko DM, Wright WC, Stone JJ, Rosenblatt RM. Unrecognised thrombocytopenia and regional anaesthesia in parturients: A retrospective review. Obstet Gynecol. 1989;73:943–6.  [PubMed]
6. Goodier CG, Lu JT, Hebbar L, Segal BS, Goetz L.  Anesth Analg 2015 Oct;121(4):988-91.
7.  Dyer RA, Piercy JL, Reed AR. Currently Open Anaesthesiol. 2007 Jun;20(3): 168-74.
8. Ramanathan J, Coleman P, Sibai B. Anesthetic modification of hemodynamic and neuroendocrine stress responses to caesarean delivery in women with severe preeclampsia. Anesth Analg. 1991;73:772–9.
9.  15. Visalyaputra S, Rodanant O, Somboonviboon W, Tantivitayatan K, Thienthong S, Saengchote W. Spinal versus Epidural anaesthesia for caesarean delivery in severe pre-eclampsia: A prospective randomised multicenter study. Anesth Analg. 2005;101:862–8.

September 3, 2017

preoperative hypokalemia, replace and proceed or cancel?

The other morning, while I was in the middle of my first of three scheduled cases I received a call from a nurse to report that the K+ level was 2.8 mEq/L on my next scheduled patient.   The patient was a 76 year old female who had a history of Afib, CHF, CAD. Previously, an AICD was placed for unclear reasons.  The patient stated that she believed the reason for the AICD was due to her poor cardiac function.  She reported no history of previous ventricular fibrillation/tachycardia.  She was on diuretics, with oral potassium supplements.

Basic Science review: Potassium homeostasis
Total body stores of K+ are approximately 3000 to 4000 meq with 98% of this located inside the cells.   This concentration differential is maintained by the Na+-K+ ATPase pump, located in the cell membrane which pumps out 3 Na+ ions for every 2 K+ions pumped in.  K+ plays an important role in cell metabolism, and therefore, a myriad of cellular functions deteriorate with an imbalance of K+ ion concentration. For example, significant hypokalemia can result in polyuria due to a reduced sensitivity to ADH.  Also critical is the intracellular to extracellular concentration gradient which largely determines resting membrane potential.

Em=-61 log r[K+]cell+0.01 [Na+]cell / r[K+]ecf + 0.01[Na+]ecf

Alterations in relative concentrations of K+ inside versus outside cells can significantly alter the resting membrane potential leading to cardiac arrhythmia as well as other skeletal muscular symptoms.

The maintenance of whole body potassium  stores is handled by the kidney, in particular by the principal cells of the cortical collecting tubule.  Here aldosterone (secreted in response to a minimal rise in serum potassium) stimulates increased activity of the Na+-K+ ATPase pump located in the basolateral membrane of the principal cell.   This pump pulls K+ from the extracellular space and pumps it into the principal cell while exchanging this for Na+ (3 Na+ : 2 K+ ratio).  As the intracellular K+ ion concentration increases inside the principal cells,  K+ passively leaves the principal cell into the collecting tubule lumen as it follows its concentration gradient. Also in the collecting tubules are located the intercalated cells.  These cells work to reabsorb K+ (exactly opposite the Principal cells).  These cells are particularly important in hypokalemic patients where an H+-K+ ion pump located in the luminal membrane actively exchanges intracellular H+ ions for K+ ions, allowing K+ to be reabsorbed back into the plasma.  As mentioned above, ADH, plays a role in K+ ion secretion by increasing the number of luminal K+ ion channels in the collecting tubules. The K+ ion itself can mimic all of the changes in the kidney that aldosterone initiates.  In other words, elevated K+ levels produce an aldosterone like effect where K+ excretion and Na+ reabsorption are enhanced in the principal cells.  This is due to increased luminal membrane permeability to Na+ and K+ (by increasing the number of open channels for passive diffusion) and increased activity of the Na+-K+ ATPase pump located on the basolateral membrane.

Also critical to K+ homeostasis is the urine flow rate in the cortical collecting tubule.  There are two mechanisms that allow a greater urinary flow rate in the cortical collecting tubule to increase net K+ secretion leading to hypokalemia.  1) increased flow lowers the intraluminal (inside the collecting tubule) K+ concentration, favoring the passive movement of K+ out of the principal down its concentration gradient via the K+ channels in the luminal membrane of the principal cells. 2) increased flow through the kidney brings more Na+ to the collecting tubule leading to increased Na+ reabsorption in the collecting tubule causing the lumen to become more electronegative favoring passive K+ diffusion to maintain electroneutrality.  Furthermore, as more Na+ is reabsorbed into the principal cell, the Na+-K+ ATPase pump removes the Na+ from the cell back into the body, leading to increased K+ entry into the principal cell which increases the intracellular K+ concentration of the principal cells of the collecting tubule. Increased distal flow of urine in the collecting tubule seems to be the mechanism by which the loop and thiazide diuretics induce hypokalemia. These agents increase distal flow by diminishing Na+ and water reabsorption in the loop of Henle and distal tubule respectively.

Clinically, chronic K+ alterations from so called normal levels are not as likely to cause outward clinical symptoms.  This is because, the intracellular to extracellular gradient is the more important than the overall measured serum concentration.   Nevertheless, hypokalemia does result in symptoms even if chronic in nature.  When considering the hypokalemic patient in the immediate preoperative period, it is important to consider three main things:  1)  degree of hypokalemia, 2) Invasiveness of surgery and 3) patient's co morbid conditions (i.e. concomitant coronary artery disease or congestive heart failure).   Per Miller’s Anesthesia, p. 1107, “As a rule, all patients undergoing elective surgery should have normal serum potassium levels.  However, we do not recommend delaying surgery if the serum potassium level is above 2.8 mEq/L or below 5.9 mEq/L, if the cause of the potassium imbalance is known, and if the patient is in otherwise optimal condition.”

Patients without underlying cardiac disease are unlikely to suffer myocardial effects, even at levels below 3.0 mEq/L. However, those with ischaemic heart disease, heart failure or left ventricular dysfunction are at risk of arrhythmias with only mild or moderate hypokalaemia. This fact was highlighted in 1981 when Hulting followed patients admitted for treatment of an MI.  This paper showed that patients who had a baseline risk of arrhythmia of 3.5% increased to 8% if their serum K+ was less than 3.5 mEq/L. They also found that no patients suffered arrhythmias if their serum potassium was greater than 4.6 mEq/L.

In patients undergoing very quick surgeries such as cataract, egd, etc, it is routine to not check labs in the first place so that hypokalemia if it existed would be unknown to the provider.  In patients who require diuretics undergoing intermediate or high risk surgery, checking potassium levels is common and hypokalemia should prompt a decision tree based on the degree of hypokalemia, patient co morbidities, and type of surgery.

By far the most common cause of hypokalemia in the preoperative period is diuretic use. Diuretic induced hypokalemia has been associated with an increased rate of arrhythmias [4]. Furthermore, diuretic therapy in hypertension and heart failure has been associated with an increased rate of arrhythmic death that can be prevented by a K+ sparing diuretic and therefore, may be related to K+ depletion [5,6].  In the Framingham Heart Study [10], they reported increased frequency of PVCs to be associated with hypokalemia.   They estimated that the arrhythmia increased by 27% wit each 0.5 mEq/L decrease in K+ level. Understanding diuretic physiology makes sense given its importance in both inducing and treating hypokalemia.  A brief primer on diuretics follows:

Loop Diuretics (furosemide, bumetanide, torsemide, and ethacrynic acid)

  • may lead to the excretion of 20 to 25% of filtered Na+ at max doses.
  • Act in thick ascending limb of loop of henle. 
  • loop diuretics inhibit Na+ reabsorption by binding to Cl- site of the Na+-K+-2Cl- carrier on luminal membrane.  The carrier only works when all four sites are occupied.
  • Secondarily inhibit Ca2+ reabsorption which is passive down an electronegative gradient by the absorption of Na+/K+, which leads to a caliuresis.
  • Ca2+ effects make loop diuretics excellent method of treating hypercalcemia when combined with saline loading.
Thiazide type diuretics

  • primarily inhibit NaCl transport in the DCT
  • at max doses only inhibit up to 3 to 5% of filtered Na+ (far less potent than loop diuretics).
  • Diuresis offset by increased reabsorption in the cortical collecting tubule.
  • Thiazide type diuretics also compete with Cl- at the Na+-Cl- cotransporter
  • In contrast to loop diuretics, thiazide type diuretics can increase the reabsorption of Ca2+ in the DCT and early collecting tubule which may be useful in the treatment of recurrent kidney stones due to hypercalciuria.
Potassium Sparing Diuretics (Amiloride, Spironolactone, Triamtere)

  • Act in principal cell in the cortical collecting tubule where Na+ reabsorption occurs passively through Na+ channels (which are increased via aldosterone)
  • amiloride and triamterene directly decrease the number of open Na+ channels decreasing Na+ reabsorption leading to decreased K+ (and H+) secretion.  
  • Spironolactone competitively inhibits aldosterone resulting in same result.
  • Weak natriuretic effect (1 to 2% of filtered Na+).
  • Amiloride is very effective in the treatment of polyuria/polydipsia from lithium-induced nephrogenic diabetes insipidus where the tubular cells of the collecting ducts become insensitive to ADH from the accumulation of lithium.
  • Triamterene is a potential nephrotoxin, possibly leading to crystalluria and cast formation which in severe cases has lead to renal failure particularly if given in conjunction with NSAIDs.

 In a patient with hypokalemia not taking any loop or thiazide diuretics, other causes should be considered.  The next most common cause would be increased GI losses, either from diarrhea, vomiting or NG suction.  Magnesium levels should also be checked.  The loop and thiazide diuretics also result in Mg2+ wasting, and hypomagnesemia promotes K+ wasting. The mechanism whereby Mg2+ effects K+ homeostasis is unclear.  However, replacement with MgSO4 (as is common in the perioperitve period) could be problematic.  The sulfate acts as a non resorbable anion in the collecting tubule (leading to a greater negative intraluminal charge).  The negative charge in the lumen prevents the passive diffusion of K+ out of the lumen into luminal cells of the collecting tubules leading to increased K+ losses. Repletion with magnesium chloride or magnesium lactate would avoid this problem. 

In the patient who is hypokalemic prior to anesthesia, it is important for the anesthesiologist to consider all of the ways in which acute shifts of K+ into the intracellular space may occur exacerbating the pre existing hypokalemia.  Avoidance of respiratory or metabolic alkalemia. is very important since alkalemia results in a transfer of H+ ions from the intracellular space into the plasma to counter act a rising pH.  To preserve electroneutrality, Na+ and K+ ions  enter cells.  In general the plasma concentration falls less than 0.4 meq/L for 0.1 unit increase in pH.  Therefore, a patient with a serum potassium of 3.1 meq/L who develops a respiratory alkalosis to a pH of 7.5 due to inadvertent hyperventilation, will potentially see an acute intracellular shift of K+ ions leading to a serum K+ concentration of 2.7 meq/L. 

Insulin directly stimulates the entry of K+ ions into skeletal muscle and hepatic cells via a Na+-K+ ATPase pump. Thusly, it would be important to avoid using a dextrose solution to replace potassium, as the dextrose can stimulate insulin release causing a paradoxical further decrease in the serum potassium concentration.

The Na+-K+ -ATPase pump is also stimulated by beta 2 adnergic receptors.  This is a particular concern in the preoperative period when stress related events causes a surge in catecholamines. In fact, a catecholamine surge can acutely lower the plasma K+ concentration by approximately 0.5 to 0.6 meq/L.  This large change in serum K+ ion concentration may be partially due to the effects of insulin which is secreted in response to increased B2 adrenergic activity. Prompt recognition of this may be treated by adequate opioids +/- non selective beta blockers (i.e. propranolol). Administration of beta agonists such as albuterol for a breathing treatment, may induce a 0.5 to 1 meq/L drop in serum K+ concentration.

Clinical studies looking at preoperative K+ levels and patient outcomes

One study done on patients with coronary artery disease who underwent non-cardiac surgery suggested that a pre-operative serum potassium level of less than 3.5meq/ L was independently associated with peri-operative mortality. Other studies have failed to find any increased incidence of arrhythmia in patients at high risk (major vascular or cardiac surgery with cardiac disease) who also had significantly decreased K+ levels [2].  However, this study was probably under powered.  They examined only 447 patients, and of these only 9% had significant hypokalemia (less than or equal to 3.0 mEq/L). In contrast to this, another study of 2402 patients undergoing CABG, were followed for a variety of outcomes.   In this study, a serum potassium level less than 3.5 mEq/L was a predictor of serious perioperative arrhythmia (OR 2.2), intraoperative arrhythmia (OR 2.0) and post operative atrial fibrillation/flutter (OR 1.7) [8].  In another study looking at patients undergoing non cardiac surgery were analyzed for predictors of preoperative myocardial infarction (PMI) or cardiac death. They found that  among several risk factors hypokalemia (serum level less than 3.5 mEq/L) was identified as a predictor of these outcomes [9]. Myocardial ischemia seems to be a significant risk factor  leading to arrhythmias in the setting of hypokalemia [3,7]. Therefore, it becomes particularly important to monitor potassium levels in any patient at significant risk for preoperative ischemia.

Repletion of K+ prior to surgery is fraught with problems due to the logistics of K+ administration.  KCl is painful in peripheral IVs and can cause severe phlebitis.  In my patient, she could not tolerate KCl being infused in her peripheral IV at a concentration of 20 mEq per 100 mL any faster than 50 mL per hour (or 20 mEq per 2 hour time period).  Therefore, it was decided to delay surgery until later that afternoon to allow adequate time for her to receive 40 mEq (requiring 4 hours) and to recheck her potassium level.  After 5 hours, we returned to perform her surgery.  Her repeat potassium was 3.3 mEq/L.  She underwent GETA with Sevoflurane.  Because she had a medtronic AICD, defibrillator pads were placed prior to surgery and a magnet was placed over the device to disable anti tachycardia therapy.  The anesthetic was uneventful and she was observed overnight in the hospital.

1.  Shah, K.B., Klienman, B.S., Rao, T.L., Jacobs, H.K., Mestan, K. and Schaafsma, K. (1990) Angina and other risk factors in patients with cardiac diseases undergoing non-cardiac operations. Anesth. Analg., 70, 240-247. 

2.Hirsch IA1Tomlinson DLSlogoff SKeats AS.   1988 Feb;67(2):131-6.
3.  Hulting, J. (1981) In hospital ventricular fibrillation and its relation to serum potassium. Acta Med. Scand. Suppl., 647, 109-116.
4. Kuller LH, Hülle SB, Cohen JD, Neaton J. Circulation 73:114, 1986
5. Siscovick DS, Raghunathan TE, Psaty BM, et al. N Engl J Med 330:1852, 1994
6. Pitt B, Zannad F, Remme WJ, et al. N Engl J Med 341:709, 1999
7. Nordrehaug JE, von der Lippe G. Hypokalaemia and ventricular fibrillation in acute myocardial infarction.  Br Heart J.1983;50:525-529.
8. Wahr JA, Parks R, Boisvert D et al. JAMA 1999; 28(23):2203-10.
9. Shah KB, Kleinman BS, Rao TL, Jacobs HK, Mestan K, Schaafsma K.  Anesth Analg.1990;70:240-247.
10. Tsuji H, Venditti FJ Jr, Evans JC, et al. Am J Cardiol 74:237, 1994