Repetitive phrenic nerve stimulation study in normal subjects.

In patients with myasthenia gravis (MG), it may be difficult to determine by clinical methods if respiratory insufficiency is due to a defect in neuromuscular transmission. We therefore studied the technique of repetitive electrical stimulation of the phrenic nerve in 6 healthy subjects. It was easily performed and quite reproducible. Responses at 3-Hz stimulation were recorded from surface electrodes of the ipsilateral hemidiaphragm, before and after exercise. We analyzed the percent decrement of the negative peak (NP) amplitude, area, and duration of the diaphragmatic compound muscle action potential (CMAP) between the first and the fifth or sixth potentials. The mean percentage change of the area was -2.2% (+/-4.3), and in all tests the change was <10.6%, yielding a normal range of <11%. The change in the NP amplitude was 12.1% (+/-8.3); in duration, the change was -8.7% (+/-9.6). Producing diaphragm fatigue did not change these results. The increase in amplitude and decrease in duration with little change in area, termed pseudofacilitation, may be due to shifts in the position of the diaphragm affecting volume conduction. The technique is a promising tool in the diagnosis of respiratory involvement from neuromuscular transmission disorders.