Managing Neuromuscular Block

Curare was introduced into the practice of anesthesiology in 1942 and the subsequent decade saw its widespread acceptance as a valuable addition to the anesthesiologist's armamentarium. Nevertheless, at that time, suggestions as to how to monitor the clinical effects of neuromuscular blocking drugs were scant and anecdotal at best. Perhaps the best counsel of that era was expressed by Morris et al.1 who in 1953 advised that at the end of surgery small doses of an anticholinesterase should be administered until ventilatory exchange seems improved. Additional doses should be administered at 5-minute intervals until no detectable change for the better was apparent. Several years later as a young postgraduate year 2 resident (1964), I received essentially identical advice. When I asked a well-known and distinguished attending "How do you determine the dose of neostigmine to administer?" the answer I received was "Give one ampoule (0.25 mg) every few minutes until the tidal volume seems adequate." The ability to perform a "head lift" was considered evidence that the patient would continue to be able to care for oneself satisfactorily without danger of hypoventilation or airway obstruction. However, by the mid-1960s, more rigorous criteria began to emerge. An editorial in Anesthesiology opined that "The only satisfactory method of determining the degree of neuromuscular block is to stimulate a motor nerve with an electric current and observe the contraction of the muscles innervated by that nerve,"2 and by 1965, the first commercially available peripheral nerve stimulators (PNSs) became available. By the mid-1970s, the train-of-four (TOF) fade ratio was well on the way to becoming the benchmark by which adequate recovery from nondepolarizing block was measured.3 Nevertheless, the survey by Naguib et al.4 in this issue of Anesthesia & Analgesia makes it clear that monitoring the indirectly evoked muscle response to nerve stimulation is still not universally practiced when neuromuscular blocking drugs are administered, reversal of residual block is far from routinely implemented, and that misinformation regarding optimal muscle relaxant management is still widespread. Why, 40 plus years after PNS devices became readily available, is there often such a vast "disconnect" between editorial opinion regarding optimal management of neuromuscular blocking drugs ("… objective neuromuscular monitoring [real time measurement of the train-of-four ratio] … should … be used whenever a nondepolarizing neuromuscular blocking agent is administered")5 and actual day-to-day clinical practice? Although the answer to this question is no doubt multifaceted, I suspect that at least 2 factors have played a major role in producing the diverse clinical practices that now exist. Anesthesiologists have historically been early adopters of new technologies. Capnography, end-tidal gas analysis, pulse oximetry, and even neurophysiologic monitors are now found in virtually every modern operating room. However, each of these monitors provided the user with an objective real-time value that was reasonably easy to interpret when the instruments first became commercially available, but not so with neuromuscular "monitors." Although the literature is replete with investigations on the clinical correlates of variations in the TOF fade ratio, most clinicians have had little incentive to give much heed to these reports. The TOF ratio until fairly recently was simply a parameter the average practitioner could not measure. User-friendly, portable, battery-operated devices capable of measuring the TOF ratio in real time did not begin to appear until the late 1990s. Subjective evaluation of the evoked muscle response with its many limitations was the best the anesthesiologist could do. Clinicians who did wish to use PNSs faced additional problems. Access to a patient's arm was not always available, and the response of the facial muscles did not necessarily mirror what was happening at the adductor pollicis. To make matters worse, many (if not a majority) of the nerve stimulators available even today fail to display the delivered current (milliampere) and it is still possible to purchase units incapable of providing supramaximal stimuli to the average adult patient. Thus, most PNS devices have been viewed as less than scientific instruments. The reasons for the failure of the majority of the anesthesia community to adopt objective or quantitative monitors into their practice are less easy to understand. One such device, the TOF-Watch®, costs approximately $800 in the United States. The price of top of the line conventional PNS units may exceed $500, so cost is probably not a major determinant in explaining the resistance to acquiring these units. In addition, I am personally aware of 2 institutions where objective monitors reside unused in the bottom drawers of their anesthesia machines. Comments from staff such as "too complicated," "takes too long to set up," and "displayed results do not match clinical signs" are common. I suspect that Murphy and Brull6 have it right when they state that "few clinicians perceive residual block as an important safety issue." As they point out, the majority of patients who arrive in the postanesthesia care unit with TOF ratios <0.90 do not experience complications. Nevertheless, these authors also present convincing evidence that not all individuals are so fortunate. Undetected postoperative residual neuromuscular block is clearly associated with potentially adverse respiratory events and increased morbidity. However, I am not optimistic at this late date that additional review articles and strongly worded editorials will have much impact on the day-to-day management of neuromuscular blockers in the "real world." A key element necessary to modify traditional practices seems to be missing: any recommendations or direction from influential professional organizations such as the American Society of Anesthesiologists (ASA) on how neuromuscular blocking drugs should be administered, monitored, or antagonized. In its published Standards for Basic Anesthetic Monitoring (last amended by the House of Delegates in October 2005),* the ASA remained silent on the need for neuromuscular monitoring. The recent Report of the ASATask Force on Postanesthetic Care7 stated that "Assessment of neuromuscular function primarily includes physical examination and on occasion may include neuromuscular monitoring." In the absence of any clear directives, almost any practice protocol can claim to represent "standard of care." Thus, the recommendations of Brull and Murphy8 in this issue of Anesthesia & Analgesia have no "official" standing. It is for this reason that I firmly believe that some policy statement from the ASA regarding the management of neuromuscular block is long overdue. In defense of the ASA, it should be noted that the issuance of a policy statement on a practice parameter is a costly, complex, and time-consuming undertaking. A detailed review of the functions of the ASA Committee on Standards and Practice Parameters may be found on their Web site. The following outline is excerpted from that committee's report last amended in 2008. Two major categories of practice parameters are recognized: evidence-based and consensus-based. Evidence-based practice parameters may take the form of standards, guidelines, or advisories. Standards contain recommendations that are supported by meta-analyses of findings from multiple clinical trials. They provide rules or minimal requirements for clinical practice and are regarded as generally accepted principles of patient management. Guidelines provide recommendations for patient care that describe a basic management strategy or a range of basic management strategies. Practice advisories provide statements to assist decision making in areas of patient care where there is not a sufficient number of adequately controlled studies to permit meta-analysis. Consensus-based practice parameters are developed by ASA-appointed experts. When available, scientific evidence may be considered. Unlike evidence-based practice parameters, consensus-based practice parameters do not use a systematic and standardized approach to data collection, assessment, analysis, and reporting. In this issue, Brull and Murphy8 opine that "perioperative monitoring of evoked neuromuscular responses … guides the administration of anticholinesterases and documents return of neuromuscular function, [and] should be a standard of care." It is hard to fault this statement. It should be noted, however, that these authors did not suggest mandating objective monitoring in all situations. When the tactile or visual TOF count is 4 at the adductor pollicis (with minimal fade), neostigmine or edrophonium are reliable antagonists of residual nondepolarizing block. Nevertheless, circumstances frequently arise where quantitative monitoring markedly simplifies patient management and decision making,9, 10 and these monitors should be readily available to every anesthesia provider. The ASA makes clear that even "standards" may be modified or rejected according to clinical needs and constraints, are not intended to replace local institutional policies, and variance from practice parameters may be acceptable, based on the judgment of the responsible anesthesiologist. Nevertheless, practice standards and guidelines send a strong message to the anesthesia community. The clinician who consistently ignores them at some point must ask, "Why am I off the bell-shaped curve." It is time for anesthesia's professional organizations to finally draft evidence-based guidelines detailing how best to monitor and manage the perioperative administration of neuromuscular blocking drugs.