[Effect of nasal-CPAP and hypercapnia on human abdominal muscles]

The effect of nasal continuous positive airway pressure (nasal-CPAP) and hypercapnia on abdominal muscles was investigated in 10 healthy volunteers in the supine position. Pairs of fine wire electrodes were inserted into the rectus abdominis, external oblique, internal oblique and transversus abdominis under direct vision provided by high resolution ultrasound echography. Ultrasound provided clear visualization of the individual muscle layers and the guide needle for electrode insertion. Monitoring end tidal CO2 (ETCO2), 1) nasal-CPAP of 15 cmH2O was applied, 2) CO2 was then added to the inspiratory limb of the nasal-CPAP system, and 3) CO2 rebreathing was also performed without nasal-CPAP. Electromyograms (EMG) were sampled and integrated. Peak values of integrated EMG were measured under the following three conditions, normocapnia, 7% ETCO2, and 9% ETCO2, with and without nasal-CPAP. In each abdominal muscle, the percentage of patients with expiratory EMG activity increased with increasing ETCO2, regardless of nasal-CPAP. Among the four muscles, the transversus abdominis was recruited most frequently, the rectus abdominis least frequently. The EMG from each muscle was activated by nasal-CPAP of 15 cmH2O under each of the three conditions. We conclude that in humans 1) nasal-CPAP and hypercapnia synergistically activate abdominal muscles, 2) the transversus abdominis is the primary expiratory muscle, and 3) the rectus abdominis is an accessory expiratory muscle.