CHANGES IN RESPIRATORY MUSCLE-ACTIVITY IN CONSCIOUS CATS DURING EXPERIMENTAL DIVES AT 101-ATA

The rationale for the present study was to test the hypothesis that increased work of breathing during experimental deep diving may lead to respiratory muscle fatigue. For this purpose, electromyograms (EMGs) of respiratory and skeletal muscles, plus electrocardiogram and electroencephalogram (EEG) derivatives, were continuously recorded in conscious cats. In each muscle group, the ratio of power in a high (H) to that in a low (L) band of EMG frequencies was computed. Direct diaphragmatic stimulation in selected animals produced a mass action potential to obtain the muscle fiber conduction velocity (MFCV). The maximal pressure was 101 ATA (1,000 msw) with a maximal duration of 72 h. Four cats breathed an He-O2 mixture and five others a ternary mixture (10% N2 in He-O2). Inspired O2 partial pressure was 350 Torr. With the He-O2 mixture, all the animals died within 2-54 h during the study at maximal depth. EEG signs of high-pressure nervous syndrome (HPNS) were present in all cats, and low-frequency (11-14 Hz) hyperbaric tremor discontinuously contaminated all EMG tracings. The H/L ratio computed from diaphragmatic and intercostal muscle EMGs increased after 12 h at 101 ATA. With the He-N2-O2 mixture, the cats survived until the end of the sojourn at 101 ATA, during which no hyperbaric tremor was detected from EMG tracings, and EEG signs of HPNS were weak or absent. From 31 ATA, the H/L ratio decreased significantly in respiratory but not in skeletal muscles; this was associated with decreased MFCV in the diaphragm after several hours at maximal depth. In both circumstances (binary or ternary mixture), a marked increase in inspiratory activity was measured at maximal depth, and bradypnea progressed until the end of the dive. These observations suggest that respiratory muscle fatigue is present under hyperbaric conditions and that EMG fatigue signs may be masked by the neurological disorders of HPNS.