The use of paralytics has become common in modern surgical care; yet these drugs pose risk, particularly during the recovery process. To minimize the duration of acute adverse effects, it is important to consider certain factors that may amplify or prolong the effects of paralytic agent postsurgery. These factors include: (1) residual blockade; (2) preexisting neuromuscular diseases; and (3) conditions that may mimic residual blockade.
Residual blockade is the most common neuromuscular complication encountered during a patient’s postanesthetic care unit (PACU) course. Each case varies in severity and has a multitude of factors influencing the outcome. Some stem from the types of paralytic used (mechanism of action), others from inadequate reversal administration and/or suboptimal monitoring throughout the procedure. In general, residual blockade can cause serious complications, which include, but are not limited to: hypoxemia, upper airway obstruction, prolonged PACU visit, prolonged ventilator time, and postoperative pulmonary complications.
DEPOLARIZING VERSUS NONDEPOLARIZING AGENTS
There are two main types of paralytics used in anesthesia. Depolarizing agents (ie, succinylcholine) are direct acetylcholine receptor agonists that bind to the acetylcholine receptor and propagate action potentials. Since they are not metabolized by acetylcholinesterase, prolonged depolarization occurs, leaving the end plate unable to repolarize, which in turn causes Phase I blockade. Eventually, the depolarizing agent leaves the neuromuscular junction and becomes metabolized by pseudocholinesterase in the plasma. The nondepolarizing agents (ie, rocuronium, veruronium) act as competitive antagonists at the acetylcholine receptor site. They block the binding of acetylcholine to its receptor, preventing an action potential from occurring. The nondepolarizing agent’s reversal is dictated by the rate of redistribution and metabolism, making its half-life longer than that of a depolarizing agent.
Due to the mechanism of action, nondepolarizing agent more commonly causes residual neuromuscular blockade in the PACU than depolarizing agent. This complication can be avoided by administering the appropriate amount of reversal prior to emergence. The most common reversal agents used are cholinesterase inhibitors (ie, neostigmine, pyridostigmine). These are routinely administered with anticholinergic agents (eg, glycopyrrolate, atropine) to reduce the cholinergic effects.
The most common method of monitoring neuromuscular blockade in the operating room is train-of-four (ToF). ToF nerve stimulation consists of four supramaximal stimuli delivered in 0.5 seconds intervals. The degree of muscle response to the stimulation determines the level of blockade. The level of fade is directly proportional to the level of neuromuscular blockade, making ToF the gold standard of monitoring. The addition of ToF monitoring has significantly reduced the amount of residual blockade seen in the PACU. There are also secondary measures of neuromuscular blockade used such as: five-second head lift, grip testing, or eye opening. These are less reliable, but still used in addition to ToF monitoring.
EXISTING NEUROMUSCULAR DISEASE