Neuromuscular Monitoring

Neuromuscular blocking drugs have profound effects on the respiratory system, and this has been recognized for a long time. After all, these drugs block transmission at the neuromuscular junction of skeletal muscle and breathing involves skeletal muscle. It follows logically that if the effects of neuromuscular blocking drugs are allowed to persist after surgery, adverse respiratory events are likely to occur. To confirm this physiologically based prediction, there are abundant epidemiological data demonstrating that residual paralysis is common and carries morbidity and mortality. Despite all this compelling evidence, this issue of Anesthesia & Analgesia contains a survey conducted in the United States (US) and several European countries indicating that the use of neuromuscular monitoring is far from universal, and that anesthesiologists on both sides of the Atlantic underestimate the consequences of residual paralysis.1 The case for avoiding residual neuromuscular blocking in the postanesthetic care unit (PACU) has built up over the last 4 decades. The evidence that emerges is remarkably consistent, with new data confirming and refining previous findings. In short, the story can be summarized as follows: with the introduction of train-of-four (TOF) monitoring in the early 1970s, it became essential to determine the level of TOF recovery that was compatible with normal respiratory function. Early studies suggested a TOF ratio of 0.7,2 and this level was considered as the target to be achieved before patients could be tracheally extubated. In the late 1970s and 1980s, it became apparent that clinicians left their patients in the PACU with considerable degrees of paralysis. Viby-Mogensen et al.3 reported that 30% of patients had a TOF ratio <0.7 in the PACU, despite routine reversal. Monitoring was proposed as a solution, but it was recognized later that human senses could not appreciate TOF fade if the actual, measured value was >0.4.4 Thus, what was later called “subjective” monitoring could not identify residual blockade reliably. The term “blind paralysis” was coined for TOF ratio values in the range 0.4 to 0.7. In the 1990s, the target for neuromuscular recovery was reassessed because it was appreciated that subjects could have signs and symptoms of residual blockade even with a TOF ratio >0.7. This was largely due to the previously poorly recognized effects of neuromuscular blockade on upper airway patency and swallowing, leading to the recommendation to increase the threshold for adequate recovery to 0.9.5 There was also mounting evidence that residual blockade mattered and had deleterious effects such as hypoxia,6,7 hypercarbia,6 atelectasis,8 and airway obstruction.7 Thus, the range of TOF ratio values associated with “blind paralysis” when using “subjective” or “qualitative” monitoring increased to include TOF ratio values ranging from 0.4 to 0.9. In response to the shortcomings of such a monitoring method, there was a call to adopt “objective” or “quantitative” monitoring, which involves the use of a stimulator coupled with a measuring device that displays a numerical value of the response.5 Accelerometers and displacement transducers can provide this information. Management of neuromuscular blockade then is simple: monitor neuromuscular blockade quantitatively and make sure that the TOF ratio returns to at least 0.9 before tracheal extubation. The results of the survey conducted by Naguib et al.1 show that many anesthesiologists in the US and Europe just do not follow this recommendation. Quantitative monitors are available to only a minority of American practitioners (22.7%), and even if 70.2% of European practitioners have access to such devices, they are not available in all operating rooms. Even if both conventional and quantitative nerve stimulators are available, the latter are used by only half the Europeans and fewer than 20% of Americans, and many on both sides of the Atlantic do not use nerve stimulators at all. One might suspect that the situation might actually be worse, because people who answered the survey probably had some interest in neuromuscular blockade, perhaps more than those who did not participate in the survey. There are many reasons why evidence-based medicine is not applied in practice, one of which is lack of dissemination of knowledge. However, the survey suggests that in this case, most American and European respondents were aware of the most relevant facts. The majority of respondents (73% overall) correctly identified a TOF ratio of 0.91 to 1.0 as required for tracheal extubation, and most of those who were wrong put this value at 0.81 to 0.90, not far from the optimal value. Thus, lack of knowledge is certainly not a major explanation for failure to apply the recommendations. Perhaps respondents thought that another method was better than measuring the TOF ratio. The survey is not clear about how often “clinical” methods are used by respondents, but when asked about the usefulness of head lift, more than half the respondents thought that head lift for 5 seconds was a reliable indicator of recovery. Evidence indicates that this is a misconception.9 Sometimes, evidence is not followed because the problem to be handled is not considered important and ranks very low in the list of priorities. Again, this reason must be rejected in this case because most respondents said they considered residual paralysis a “significant health problem.” However, more than 80% claimed that they never saw clinically significant residual paralysis in their practice and approximately 70% estimated that the incidence of “significant” residual paralysis is <1%, and virtually all the rest estimate this incidence to be 1% to 5%. In other words, the message seems to be, “I must be doing things right, because it never happened to me.” How do we explain this contradiction? In a meta-analysis, Naguib et al.10 calculated that under the best conditions, that is, if intermediate acting neuromuscular blocking drugs were given and if monitoring was used, the estimate of the rate of residual paralysis based on a TOF ratio of 0.9 was 34.8%, a much higher number than the <1% figure mentioned by the respondents. In real life, outside the rigorous conditions imposed by studies, chances are that the actual rate might be higher. Why then would anesthesiologists state that they never saw a case of significant residual paralysis and why would they estimate the incidence at <1%? It may be that busy practitioners are more concerned with starting their next case than watching patients in the PACU, but this may be only part of the answer. One other explanation is that patients get by despite a TOF ratio <0.9 or worse, and their problems might not be noticed. The question might be investigated by looking at respiratory difficulties in the PACU and working backward to determine the role of residual blockade. Studies report that respiratory difficulties in the PACU are relatively common, with rates as high as 10%.11,12 Many of the events reported, such as bronchospasm and abundant secretions, probably had little to do with residual neuromuscular blockade, but inadequate neuromuscular recovery was recognized as a factor in only 0.3% of patients.11,12 However, blockade was not monitored, and it seems that if neuromuscular function is assessed in a more rigorous fashion, residual blockade becomes a major risk factor for adverse respiratory events. Murphy et al.7 measured TOF ratio in patients with respiratory problems such as hypoxemia and airway obstruction in the PACU and matched these cases with patients who did not have the complications. The incidence of such complications was 0.8%. The mean TOF ratio was 0.61 in patients who had respiratory events, compared with 0.98 in patients who did not.7 This suggests that residual blockade is a common cause of respiratory difficulties in the PACU. If monitoring is not used, such adverse events are unlikely to be attributed to residual blockade. This might explain why most clinicians claim that they never saw a case of residual blockade in the PACU. One can also imagine that some patients did not show detectable respiratory problems despite inadequate restoration of neuromuscular function, but that they could have had a smoother recovery if they did not have to struggle with residual blockade. Objective monitoring should then be one of the key strategies used by anesthesiologists to avoid the consequences of neuromuscular blockade. When not available, such monitoring should be replaced by other methods. For example, the survey indicates that when objective monitoring is not available (in the US), anesthesiologists rely more often on reversal than they do in Europe.1 If neostigmine is given when 4 twitches are visible, one can be reasonably confident that adequate neuromuscular function will be restored within 10 to 15 minutes.13 If reversal is omitted, then objective monitoring is virtually necessary because significant paralysis can be missed in the TOF ratio range 0.4 to 0.9.4 Whatever the strategy, the key is to use it in a consistent manner and to check the results. One suspects that the complication rate of 0.8% mentioned in the study by Murphy et al.7 is just the tip of the iceberg. Chances are that many patients had some difficulty or discomfort, managed to compensate, and did not need any specific intervention. It is possible that rigorous management of residual blockade might have led to more comfort, a more rapid discharge, and more satisfaction. Faced with a patient who is hypoxemic, has airway obstruction, or is not comfortable in the PACU, most anesthesiologists do not spontaneously think of residual neuromuscular blockade as a likely cause. In fact, there is strong evidence of a link between respiratory complications and residual blockade. If such adverse events do occur, the possibility of blockade should be excluded, and better, all the necessary steps should be taken to avoid this situation in the first place.