I work for a large anesthesia group that has communicated recommendations on how we should protect ourselves as we ramp up. Some of the recommendations have caused me to think about the rationale behind them a little more in depth to see whether they may apply to our community hospital.
The current recommendation in general is that all patients should be tested using RT PCR as well as tested for IgM/IgG antibodies. The idea behind this is to reduce the chance of transmission of infection from an asymptomatic patient who has COVID-19 and also reduce the burden of PPE burden. The general recommendation however, continues on that it is recommended in asymptomatic patients who test negative via both serology and PCR (also known as molecular testing) that providers who intubate/place an LMA for surgery should don an N95 mask and gloves per the usual method as we now do for untested patients. Furthermore, it is still recommended to don maximal barrier protection (i.e. face shield, double gloves, gown, n95 or PAPR) in cases on the respiratory tract such as bronchoscopies, ent etc even if the patient has tested negative using both PCR and serology.
The argument put forward for this recommendation relates to the sensitivity of the two tests. Our gurus have quoted a sensitivity of 98.6 % when the two tests are done together. However, I do not believe this gives us a full picture of the actual risk to any given patient under our care. First of all, as clinicians in practice, we really don't care about the sensitivity of any test except to use it in the calculation of the negative predictive value of our test. The negative predictive value relates to Baye's rule where the pretest probability of the disease is considered. In this case, we are concerned with asymptomatic patients, no known contact with a COVID-19 positive patient living in our local community who has not traveled or taken excessive risk related to work environment etc. The pretest probability in this case is simply the current prevalence of disease in the community. Unfortunately, we don't really know the current prevalence in real time because it is changing everyday. Fortunately, my county publishes daily the prevalence in all of the individual surrounding communities. Currently, the prevalence is 1%. This can be doubled (to account for positive patients who have not been tested). After considering the prevalence, the negative predictive value can change dramatically given the same sensitivity.
Understanding why we do what we do in the OR to prevent transmission of infection is important and should be based on rational science underlying infectious disease. The first principal of infectivity of any pathogen has to do with inoculum: A larger inoculum of pathogen is more likely to cause an infection than a lower inoculum. The second principal considers how virulent the pathogen is.
The reason that Sars CoV-2 has been so easily transmitted is tied to the ability of the host's viral load to reach near peak levels prior to the symptom onset. (See graph)
From ASA townhall
Therefore, SARS-CoV-2 is very contagious largely because asymptotic individuals can have very high viral loads allowing them to deliver a large inoculum without realizing it. Therefore, the ability to detect viral burden in a patient without symptoms can aid us in determining how to handle the person in question. Unfortunately, as per the slide above, PCR will only detect virus (turn positive) if the viral load is relatively high AND the test is done correctly. When the specimen is collected in appropriately, or the viral load is low ( i.e. it's day one of the infection), then the test will indicate negative while the patient is really infected. This results in published sensitivities of anywhere from 60% to 90%. The important point to keep in mind about sensitivity and specificity is that they are characteristics of the test, and have nothing to say about the population the test is used in. (Sensitivity = Tp/Tp+Fn) We really need to calculate the negative predictive value of our negative result. For this we need to know the prevalence in our community (or the community in which the patient resides and spends most of their time). In my community, the current prevalence is less than 1%, but I'm going to assume 2% for a margin of safety and to presume a higher real prevalence due to lack of testing of all individuals in the community. So lets go through what we are looking at with negative predictive value:
(prevalence 20% is due to known positive contact: best guess)
However, at this point perhaps it is reasonable to move forward with the more conservative approach. What would you do? Would you don full PPE (i.e. double glove, PAPR throughout entire case, gown entire case, videolargyngoscope for intubation)? I personally would be comfortable using an N95 for intubation and extubation with eye shield and gloves. But during the remainder of the case I would not don an N95 if the case was not considered to be aerosolize generating.
Fortunately, I live in an area with a known prevalence of less than 1%. Obviously, there are probably a fair number of asymptomatic individuals who have the virus and could show up for surgery. The other side of the coin is to consider that there is a cost to using full scale PPE, the resource is limited, there are few surgeries that are elective to the point that they are never needed (i.e. eventually the patient needs their surgery), and the longer we go without providing services for elective surgeries, the larger the back log of cases will be. Therefore, ramping up will require an intelligent approach utilizing a full understanding of the true risk associated with caring for asymptomatic patients.
At my small private hospital the intention is to provide both molecular (PCR) testing and serology (antibody) testing for all patients. PCR testing is recommended to be done about 48 to 72 hours prior to surgery. This is because the test will be negative in an infected patient very early on in infection due to low viral load. Please see the above graph showing how the viral load ramps up over 5 days or so. Serology testing is recommended to be done just prior to surgery with enough time to get results. This is because it can take several days to weeks for IgM antibodies to appear and up to four weeks for IgG antibodies to appear. A recent study was able to show that when PCR alone was used the sensitivity of test was 51.9%, but when adding serology (IgM) testing into the mix, the sensitivity improved dramatically (98.6%). So lets look at the graph below with this new sensitivity and we will assume a very conservative prevalence of 5%.
In raw numbers, I would have to care for 1,000 patients before I ran into one patient who was a false negative. Some practices are busy enough that this isn't good enough. Therefore, the cost benefit of depleting PPE for every case becomes questionable. I personally do not think it unreasonable to approach a healthy patient coming from a low prevalence area to this particular facility with routine universal precautions if they are negative for SARS-CoV-2 after both PCR test (done 72 hrs prior to surgery) and a serology test done no more than 24 hours prior to surgery. The critical piece to this is to have access to updated prevalence data from the community from which the patient resides and to verify that the patient has not traveled in the last 14 days nor had contact with others who might have traveled or had symptoms similar to COVID-19. This could be accomplished prior to surgery by simply asking the patient a few questions.
In the real world different facilities are currently planning different testing patters. In my area, a large nationwide hospital chain is not requiring ANY test prior to elective surgery in asymptomatic patients with no known risk factors. They are asking the surgeons to request the test (PCR) and then it will be done. As an example, the Mayo clinic is performing dual testing (PCR/serology) on all asymptomatic patients 48 hours prior to surgery. However, vanderbilt is using just the PCR test. This is also true of other major institutions. Cost is likely to be a major factor in making this decision. Each test can be around $100.00. In addition, different facilities are requiring different levels of PPE depending on the test results. Again, Vanderbilt who is performing the PCR nasal swab, will not require N95 if the patient tests negative. UCSF (also using single molecular testing) will allow the provider to opt out of N95, but allows the provider to choose if the patient tests negative. Mayo, (dual testing), still requires N95 regardless of test results. The takeaway, we still really don't know what the right answer is. However, I believe the pre test probability (baye's rule) should be a large factor in determining whether you believe the test. In a patient who has been living with a COVID-19 patient in close quarters, and comes for elective surgery, testing would allow you to decide to proceed with surgery should it be negative, but the NPV of the test would likely be low. In that case, the testing would not impact my decision to use enhanced PPE.
There are also questions related to what actually results in aerosolization of SARS-CoV-2. The current recommendations for PPE during intubation and extubation are largely based on the experience of HCW during the SARS-CoV-1 outbreak. During that outbreak, HCW doing intubations had a higher rate of infection than others. Other data comes form the recent experience in Wuhan. For example, there is anecdotal evidence of cases of contracting SARS-CoV-2 after an arduous intubation where nasal intubation was ultimately required. In this case, several HCWs contracted SARS-CoV-2 despite the use of N95 masks. However, the anesthesiologist, who was utilizing a PAPR, did not contract the disease. Other anecdotes are similar, for example, all HCWs contracted SARS-CoV-2 after transphenoidal puitary surgery except the anesthesiologist who again was using a PAPR. The takeaway for me is that first, there is a lot we don't know right now about how different procedures may lead to different aerosolized viral particles. Second, a one size fits all approach is not the best. Thirdly, as far as I can tell, at least I could not find any published papers, where someone measure the amount of aerosolized particles after an intubation. It is clear that difficult and prolonged airway manipulation will lead to higher risk (i.e. prolonged ongoing surgery using a high speed drill (Transphenoidal pituitary surgery). However, gently placing a blade into the mouth, with careful ETT placement may cause very little in the way of a viral plume. Extubation can be much more dangerous. The main documentary evidence leading to the recommendation of N95's indicates that HCW doing intubations were more likely to contract SARS CoV-1, but did not specify whether the exposure might have occurred during extubation. There is some data that the virus can be aerosolized but the study was unable to determine the amount of viable virus in the aerosolized form only that in some cases viral particles may remain viable for several hours in aerosol form. This same study also found that the floor is the most contaminated surface due to droplets migrating to the ground via gravity. The study seemed to indicate that the quantity of viable virus decreases significantly with distance from the source.
Many facilities want to preserve PPE. In a drastic maneuver to do this, they are demanding that all staff not required for intubation/extubation remain out of the OR for varying amounts of time. The rational is by allowing the viral particles to clear via air exchange, the OR crew may enter and perform the surgery without enhanced PPE. It is not clear to me whether the decrease in throughput, expense of staff and OR for additional time, risk to patient who must remain under anesthesia for a longer period and other factors may outweigh the benefit. Or would dual testing be cheaper and do away with this requirement for negative patients? Furthermore, the wait times vary from institution to institution and the prescribed wait time seems arbitrary. It is not clear if the times are based on good data, or whether a best guess. The CDC published the following related to this topic:
| ACH | Time (mins.) required for removal 99% efficiency |
|---|
| 10+ | 28 |
| 12+ | 23 |
ACH-air changes per hour (dependent upon each facilities air conditioning units)
t2 – t1 = – [ln (C2 / C1) / (Q / V)] X 60
The above equation is used to calculate the time listed above and is highly dependent on the volume of the room. The C relates to the viral concentration. This is totally unknown and will be much higher for certain intubations than others. I would also argue, that the initial viral load will be much higher after extubation than intubation.
In summary, SARS-CoV-2 is a novel virus that has an Ro of around 3 or higher. This indicates that it is highly contagious. Current tests are not uniformly highly sensitive (i.e. PCR), but dual testing patients provides sensitivity of 98.6%. In low prevalence areas, the NPV can be as as high as 99.9% after dual testing. For providers in low prevalence areas providing care to small to moderate numbers of patients, forgoing enhanced PPE during cases where the patient was negative on two different tests (rt PCR and serology) may be reasonable. The decision to preserve PPE may also depend on the type of case (i.e. an airway case with prolonged manipulation of the airway with high impact equipment). The requirement for requiring staff to wait outside the OR during intubation and extubation likewise could depend on other factors as well? Local prevalence, what kind of testing was done, was the intubation difficult or very smooth and non traumatic, did the patient have any known risk factors to raise your suspicion etc. Furthermore, N95's are susceptible to failure in certain high viral load environments. However, PAPRs are total overkill for routine non airway surgeries in COVID-19 negative patients or unknown but low risk patients.
Please feel free to use the below link to a web based calculator to help you individualize your care. Please note that patients who have SARS-CoV-2 contact, or symptoms (such as fever, cough, SOB, dyguesia, anosmia, headache) should be assumed to be positive no matter what the test states when determining whether to use enhanced PPE. This tool is related to asymptomatic patients with no known contact with a SARS-CoV-2 positive patient.
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