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

April 24, 2009

pacemakers and defibrillators-what does the anesthesiologist need to know

Case report

71 year old male is scheduled to undergo AAA repair via endovascular approach. The pt experienced an MI in 1999 and underwent 4 vessel CABG with good results. An echocardiogram from this time period demonstrated an EF of about 28%. The pt also has a history of HTN and Tobacco use for many years. He was implanted with an ICD about 5 or 6 years ago. The pt comes to surgery with no information on the manufacturer or type of ICD, why it was indicated or how often if at all he is dependent on it for cardiac pacing. His EKG shows an old anterior MI, but is otherwise unremarkable with no pacer spikes noted. The pt reports reasonable exercise tolerance, stating that he is able to climb one flight of stairs without trouble and without getting short of breath. He denies any orthopnea or paroxysmal nocturnal dyspnea.
The case is begun, the surgeon is notified that the pt has an ICD which is still activated and that if any electrocautery is required he should notify me first and a magnet will be placed over the ICD for deactivation. A magnet is placed at bedside for use only if necessary.
The case begins and pacing spikes are noted when the HR drops below 65 bpm. The case procedes uneventfully with successful deployment of a large infrarenal endovascular stent.
Pts are presenting more and more frequently with ICD/pacemakers. Anesthesiologists are expected to prescribe a proper anesthetic and are expected to be knowledgeable about the implications of both the surgery and medications on the function of the cardiac devices. Unfortunately, the number of devices has proliferated dramatically in the last decade in both types but also features making it more and more difficult for anesthesiologists who do not routinely care for these types of patients to stay abreast of the various subtle differences between manufacturers. There are several manufacturers each with their own idiosyncracies.
The most relevant are listed: Medtronic (, St. Jude, (, and Guidant (merged with Boston Scientific
The pacemaker code is an important first step in understanding what a patient's pacemaker is doing. The first letter in the code indicates the heart chamber that is paced when the pacer is active: A-atrial, V-ventricular, D-dual (both atrial and ventricular pacing). The second letter indicates the chamber in which the electrical activity of the heart is being sensed with the same letters as before. An 'O' when pacemaker discharge is not dependent on sensed electrical activity. The third letter refers to the pacemaker's response to a sensed electrical signal. The letter 'O' means no response to an electrical signal and is usually related to the absence of an associated sensing function. The letter I-inhibition, means that when the pacemaker senses electrical activity it will not discharge, but remain quiet and allow the intrinsic heart to function as if there were no pacemaker present. The letter T-trigger, means that if the pacemaker senses electrical activity, it will trigger output from the pacemaker. The letter D-dual, means that the pacemaker will be inhibited if it senses normal atrial and ventricular activity, but it will be triggered if it senses atrial activity but does not detect the normal ventricular activity in which case it can pace the ventricle in time with atrial activity. The 4th position indicates whether the pacemaker has rate modulation programmed into it's algorithms and the 5th position indicates whether there is anti-tachycardia functions.

Currently, the only guidelines available are provided by ACC/AHA (ACC/AHA 2007 Guidelines on perioperative cardiovascular evalutaion and care for noncardiac surgery, Circulation< The guidelines indicate that it is imperative to determine firstly if the patient is pacemaker dependent. This can be done by a chart review and examination of the ECG. The guidelines specify that if the patient is not pacemaker dependent and/or the cautery is remote (to the pacemaker leads/generator) and the operative team can monitor closely the ECG and pulse oximeter, it may be unecessary to interrogate the pacer at all. If however, a patient is pacemaker dependent, it is recommended in these guidelines that the patient have an evaluation of the device within 3 to 6 months of significant surgical procedures, and also after surgery. Significant surgical procedures include major abdominal or thoracic surgery, especially if the surgery will require large amounts of electrocautery. An evaluation will need to determine the type of device, whether the patient is dependent on the device for antibradycardic pacing and finally, determining device programmed settings and battery status. For significant surgical procedures in patients found to be pacemaker dependent, the device should be programmed to an asynchronous mode during surgery (VOO or DOO), or a magnet can be placed over the device. Unfortunately, placing a magnet over a device with no prior knowledge of the patient's condition or the possible reaction to the magnet could be dangerous. This is because applying a magnet over a device that results in a VOO mode may result in competitive pacing or the magnet induced asynchronous pacing competes with the patient's own heart rhythm. If this occurs risk exists that a stimulus from the pacemaker could hit the myocardium during the vulnerable period and result in ventricular fibrillation. Fortunately, if a patient has a more modern pacemaker this risk is avoided because the switch to asynchronous pacing is coupled to the next cardiac event which avoids competition. Patients who depend upon the pacemaker and are exposed to EMI will potentially devlop asystole as the pacemaker generator is inhibited. ICDs, on the other hand, should have their antitachycardia pacing turned off before surgery and turned on after surgery. Patients should be continuously monitored during this period of time (both ECG and pulse oximetry), and cardioversion pads should be immediately available should the need arise.

It is noted that placement of a magnet over a pacemaker used to treat bradycardia may not elicit a response in certain pacemakers. Other responses may include: brief asynchronous pacing, continuous or transient loss of pacing, asynchronous pacing without rate response. Guidant pacemakers (or Boston Scientific) all respond to a magnet with asynchronous pacing at 100 bpm. If a magnet is placed over a Guidant pacemaker and it paces at 80 bpm or less, this indicates that the battery is low and the pacemaker needs to be serviced if the patient is pacemaker dependent. Nevertheless,The ACC/AHA guidelines suggest that a magnet be placed over the generator prior to surgery to determine what response will occur if the patient is pacemaker dependent. A magnet will not change the pacing function of an ICD, but will affect the antitachycardia function of an ICD. There are various responses depending on the manufacturer, therefore, this knowledge is critical in caring for these patients. According to Dr. Rozner, Medtronic ICDs always respond to appropirate magnet placement by turning off the anti-tachy-therapy, but there is no feedback on this from the generator, and consequently, the physician can't be sure that he/she has successfully disabled the anti-tachycardia therapy. St. Jude ICDs can be programmed to ignore a magnet and there is no way to tell if this is the case when a magnet is placed. Therefore, with St. Jude ICD's it is imperative to ensure that they are programmed to off prior to surgery requiring monopolar electrocautery since there is no way to be certain whether a magnet will turn off anti-tachy therapy or not. Guidant ICD's can also be programmed to ignore a magnet. Fortunately, when a guidant ICD has magnet mode enabled it will be beep with each R wave (just listen with a stethoscope bell over the hole in the magnet placed over the generator) when the magnet is placed which indicates that the tachytherapy is off. However, some guidant ICDs will respond to a magnet in a very different fashion than described above. In this case, after having a magnet in place over the generator for 30s, the guidant ICD will be programmed off. When this occurrs the ICD will emit a constant tone indicating that its anti-tachy therapy is now programmed off. The magnet can be removed at this point and this particular guidant ICD will remain programmed off. Following surgery it will be important to remember to program this ICD back on by replacing the magnet for 30s and listening for restoration of the beep with each R wave. Now removing the magnet will restore anti-tachy therapy.

Clearly, routine application of a magnet to a patient with an ICD may be problematic unless the clinician is well versed in the function and type of the ICD and its expected reaction to a magnet.

All sources of electromagnetic interference (EMI) should be avoided in these patients if possible. Common sources include unipolar electrocautery, radiofrequency ablation, lithotripsy, MRI, and radiation therapy. EMI is far morel likely to cause problems to patients who have a pacemaker and /or ICD with unipolar circuitry, where sensing or pacing occurs between an electrical pole at the lead tip and another within the pacemaker generator creating a larger antenna loop. Bipolar pacemakers have both electrical poles located at the lead tip and are far less susceptible to EMI.
In conclusion, it is important to recognize as perioperative physicians the implications of patients who come to the operating room with cardiac devices (either pacemakers or pacemaker/ICD combinations). We must also encourage our surgical colleagues to recognize the importance of having their patient's have their devices evaluated prior to arriving for elective or semi-elective surgery. All clinical sites where the potential for patients to come into contact with EMI need to have a comprehensive policy in place that is strictly followed. Priority should be given to determining if patients are pacemaker dependent, and if so, what is the back up mode of the pacemaker, what is the battery status, and who is the manufacturer. Bipolar cautery should be used if possible in this population of patients. If not feasible, the return pad should be placed as far away as possible from the pulse generator, and the surgeon should avoid long, continuous bursts from the electrocautery.

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