Explain the different patterns of nerve stimulation and the relevance of pattern choice.
The simplest pattern is single-twitch stimulation (Figure 26.1). The stimulus should be a rectangular pulse of 0.1–0.3 millisecond duration. Stimulus frequencies of greater than 0.15 Hz may cause fade in single-twitch meas-urements; therefore, measurements are generally made at 0.1 Hz intervals. A supramaximal stimulus is required. A control (pre-neuromuscular blockade) single twitch (Tc) is measured and recorded. Subsequent single-twitch meas-urements are compared with the control value (T1/Tc). This mode is primarily useful in research. Relative poten-cies of neuromuscular blocking agents are measured, and recovery rates are quantified. Single-twitch stimulation has its limitations. It requires recording of control values before administration of neuromuscular blockers. In addi-tion, full return of single-twitch height to control levels does not guarantee normal function of the neuromuscular junction.
A clinically useful stimulation pattern (Figure 26.2) is the train-of-four (TOF). When a peripheral nerve is stimulated at a rate greater than 0.15 Hz, the muscular response will show fade.
The ratio of the fourth to the first twitch is measured (T4/T1), and called the T4 ratio. Four stimuli applied at a frequency of 2 Hz are employed clinically. Multiple TOF stimuli are separated by 10- to 12-second intervals to ensure maximal twitch height. Fade on the TOF is a property of nondepolarizing muscle relaxants. It is a presynaptic phenomenon. Depolarizing muscle relaxants do not show fade on TOF unless a phase II block has occurred. An advantage of TOF monitoring compared with single-twitch monitoring is that a recorded control value is not required.
Furthermore, fade of the fourth twitch is still seen when the first twitch has fully recovered, making TOF measure-ments a more sensitive indicator of neuromuscular block-ade than single-twitch measurements. A T4 ratio of greater than 0.7 on MMG or 0.9 on EMG correlates well with clinical signs of recovery from nondepolarizing neuromus-cular blockade. When TOF is measured by manual palpation or observation, as in most clinical situations, the degree of fade is generally underestimated. Therefore, the trachea should not be extubated when fade is visible or palpable.
The fourth twitch disappears completely when the first twitch height is 25% of control. This usually correlates well with adequate surgical relaxation. Disappearance of the third twitch occurs when the first twitch is 80% depressed, and at 90% first twitch height depression, the second twitch disappears. This is the degree of relaxation required to facilitate endotracheal intubation.
High-frequency (30–200 Hz) nerve stimulation is called tetanic stimulation and evokes a tetanic response without fade in the unrelaxed muscle (Figure 26.3). Physiologic frequencies of 50 Hz are most commonly employed in the operating room. Higher frequencies may produce fade even in the absence of neuromuscular blockade and are, therefore, overly sensitive for clinical situations. At a stimulus frequency of 50 Hz, tetanic fade is seen only in the presence of nondepolarizing or phase II neuromuscular blockade. As previously alluded to, fade is felt to be a presynaptic phenomenon resulting from a decrease in presynaptic acetylcholine stores in the presence of a postsynaptic com-petitive blockade of acetylcholine receptors. Blockade of presynaptic acetylcholine receptors may play a role as well. A sustained response to a 50 Hz tetanic stimulus correlates very well with the ability to protect the airway.
Application of a tetanic stimulus elicits a phenomenon called post-tetanic facilitation. Following tetanic stimula-tion, responses to single-twitch or TOF stimulation are aug-mented. Tetanic stimulation at 50 Hz applied for 5 seconds requires a 2-minute recovery period to avoid post-tetanic facilitation. Care must therefore be taken not to apply a tetanic stimulus shortly before a TOF stimulus. When profound neuromuscular blockade is present and the TOF completely obliterated, the presence of TOF response following tetanic stimulation (post-tetanic facilitation) portends the return of TOF and eventual reversibility.
Another modality is double-burst stimulation. Two brief (0.2 milliseconds) tetanic stimuli are separated by 750 milliseconds. Consecutive double-burst stimulations must be separated by at least 15 seconds. Two palpable twitches are elicited. These twitches may exhibit fade, which correlates well with fade on the TOF. As previously mentioned, absence of palpable or visible fade on standard TOF monitoring may be associated with residual, measur-able fade on TOF by MMG or EMG. The presence of two equivalent twitches following double-burst stimulation accurately indicates the absence of neuromuscular blockade and more sensitively indicates return of neuromuscular function than tetanus or TOF.