Diaphragm metabolism during supramaximal phrenic nerve stimulation

The metabolic changes accompanying diaphragm fatigue caused by supramaximal stimulation of the phrenic nerves are incompletely described. In particular, we wished to determine whether the occurrence of anaerobic metabolism correlated with fatigue as defined by decline in force generation. In 10 anesthetized mechanically ventilated mongrel dogs we measured arterial pressure, transdiaphragmatic pressure (Pdi), phrenic arterial flow (Qdi-Doppler flow probe), arterial and phrenic venous blood gases, and lactate levels. From these we derived indexes of diaphragm O2 consumption (VO2) and lactate production. Bilateral phrenic nerve pacing was carried out (50 Hz, duty cycle 0.4, 24 contractions/min) for two 15-min pacing periods separated by a 45-min rest period. Over each pacing period Pdi decreased from approximately 16 to approximately 10 cmH2O (P less than 0.01, no significant difference between periods). Initially, during pacing, Qdi and VO2 each increased fivefold over prepacing base line. Qdi remained elevated at this level whereas VO2 decreased over the pacing period by approximately 25%. Hence, the change in VO2 over the pacing period was due primarily to changes in O2 extraction. During the first pacing period lactate production was observed early and declined throughout the pacing period. No lactate production was observed during the second pacing period, although Pdi, VO2, and Qdi responses were the same for both pacing periods. Phrenic venous PO2 remained greater than 30 Torr throughout both pacing periods.(ABSTRACT TRUNCATED AT 250 WORDS)