CONTRACTION-DEPENDENT MODULATIONS IN REGIONAL DIAPHRAGMATIC BLOOD-FLOW

Blood flow (Q̇) of the diaphragm was measured simultaneously with Doppler probes placed on diaphragmatic veins and an artery and by direct volumetric measurements obtained from cannulation of diaphragmatic blood vessels. The Doppler converting coefficients obtained were 6.27, 7.25, 4.21, and 41.07 ml · min-1 · kHz-1 for left phrenic artery flow (Q̇pha), phrenic vein flow (Q̇phv), internal mammary vein flow (Q̇imv), and azygos vein flow (Q̇azv), respectively. The time course of Q̇pha, Q̇phv, Q̇imv, and Q̇azv after imposed patterns of diaphragmatic contraction was measured in nine anesthetized dogs. Each pattern consisted of various combinations of transdiaphragmatic pressure (Pdi), frequency of pacing (f), and duty cycle obtained by bilateral phrenic nerve stimulation. The dogs were prepared with chests open and loosely casted abdomens. Q̇pha, Q̇phv, Q̇imv, and Q̇azv were measured at rest (control, passive diaphragm, mechanical ventilation) and at two submaximal levels of stimulation (30 and 60% of Pdi(max)). The f was 10 or 30 cycles/min and the duty cycle was 0.25, 0.50, and 0.75. The results show 1) Q̇pha, Q̇phv, Q̇imv, and Q̇azv reached stable values (equilibration) after 30-36 s of pacing 2) the steady Q̇pha, Q̇phv, and Q̇imv were linearly related to Pdi, and they were related by a parabolic function to duty cycle, whereas Q̇azv was not significantly affected by Pdi and increased linearly as a function of the duty cycle; 3) the diaphragmatic blood drainage was ~ 60% through the intercostal veins leading into the azygous trunk, 25% through the phrenic vein, and 15% through the internal mammary vein during pacing of the diaphragm at a duty cycle of 0.50 and 60% Pdi(max); and 4) for a given pacing pattern, Q̇pha and Q̇phv increased with f, but Q̇imv and Q̇azv did not.