Monitoring Neuromuscular Blockade

Neuromuscular-blocking agents (NMBA) have been used in clinical anesthesia for nearly 60 years. In the perioperative period, a variety of factors can influence a patient’s response to these drugs. To precisely titrate NMBAs to clinical effects in the anesthetized patient, the application of neuromuscular monitoring is essential. The use of a peripheral nerve stimulator allows the clinician to assess the intensity of the neuromuscular block. Profound muscle relaxation (to permit tracheal intubation and ensure immobility), moderate muscle relaxation (to facilitate surgical procedures), and recovery of neuromuscular function can be evaluated by monitoring evoked responses to nerve stimulation. This chapter reviews the physiology of neuromuscular transmission, the fundamental principles of neurostimulation, the patterns of nerve stimulation used in the operating room, and the assessment of evoked responses to peripheral nerve stimulation. In addition, the role of neuromuscular monitoring in detecting, and possibly avoiding, residual paralysis in postoperative patients is examined. Physiology Of Neuromuscular Transmission The neuromuscular junction consists of the motor nerve terminus and the end plate of the muscle fiber (Fig. 1). These 2 structures are separated by a narrow gap called the synaptic cleft. Acetylcholine (ACh) released from the motor nerve terminus is able to rapidly transverse the 20- to 50-nm synaptic cleft and bind with specialized receptors on the postsynaptic end plate.