Function of respiratory muscles during partial curarization in humans

The effects of submaximal neuromuscular blockade (SMNB) on the recruitment (or derecruitment) of the respiratory muscles during different types of respiratory maneuvers were studied in four healthy males infused slowly with pancuronium. The effects on lung mechanics were similar to those observed previously in that lung recoil pressure during inspiration did not change while the chest wall pressure-volume (PV) curve was shifted to the right (Rahn diagram). In each subject, SMNB produced a large increase in abdominal (gastric) and transdiaphragmatic pressures at any given lung volume during inspiration, reflecting greater diaphragmatic contribution to respiratory pressure swings. In addition, using concentric needle electrodes, we observed a marked fall in electrical (tonic and phasic) activity in the abdominal and in the intercostal/accessory muscles during SMNB but a slight increase in diaphragmatic activity. This pattern of changes was accentuated as ventilation increased. These findings indicate that the diaphragm is more resistant to curare than the other respiratory muscles in humans and that the transposition of the chest wall PV curve during SMNB is related to a loss of tonic activity in the intercostal musculature. The difference in sensitivity toward curare between the diaphragm and the other respiratory muscles is probably related to a difference in the safety margin at the neuromuscular synapses.