Electrophysiologic techniques for the assessment of respiratory muscle function

This Section of the Statement has described available electrophysiologic tests, the functions of which are to assess the integrity ofthe respiratory neuromotor apparatus. These electrophysiologic tests are technically complex and require considerable expertise. There are two main types of test: electromyography (EMG) and stimulation tests. Type 1: EMG. For the respiratory muscles the EMG can be used to assess the level and pattern of activation, to detect and diagnose neuromuscular pathology, and when combined with tests of mechanical function to assess the efficacy of Contraction. The EMG can be recorded with surface electrodes (for diaphragm, intercostal, scalene, abdominal, and accessory muscles) or an esophageal electrode (for the crural diaphragm). Surface electrodes are noninvasive and sample a large number of motor units, but contamination (cross-talk) from other muscles is a substantial problem, as is the effect of body size and shape on signal amplitude. Esophageal electrodes provide more specific information, but the technique is invasive and complex. Surface and esophageal electrodes can record the interference pattern EMG (raw EMG) of the respiratory muscles and are useful to determine the timing and level of respiratory muscle activation during breathing, at rest, and under load. Frequency domain analysis of the EMG is used, as a research tool, to investigate respiratory muscle fatigue (discussed in Section 5 of this Statement). Intramuscular electrodes can be used to record, relatively selectively, from the diaphragm and intercostal muscles. Motor neuron firing frequency can be measured and neuromuscular disorders diagnosed. However, the techniques are invasive and technically difficult. Type2: Stimulation Tests. Stimulation tests measure the efficiency of neural and neuromuscular transmission. Nerve stimulation can be achieved with electrical or magnetic stimulators. Electrical stimulation is inexpensive and relatively selective, but is uncomfortable and can be technically difficult. Magnetic stimulation is easier to achieve and less uncomfortable, but can be less selective and is expensive. Most commonly the phrenic nerves are stimulated and the diaphragm EMG elicited, for the measurement of phrenic nerve/diaphragm latencies and CMAP amplitudes. Latencies are prolonged in some neuromuscular disorders (e.g., demyelination) and CMAP is reduced in amplitude (e.g., traumatic damage to the phrenic nerves). Cortical stimulation is most commonly performed with a magnetic stimulator, and permits the measurement of central conduction times (CCT) for limb muscles and diaphragm. CCT can be prolonged as, for example, in multiple sclerosis. Cortical stimulation is not selective, and the application of the technique to the respiratory system is a highly specialized skill.