Contraction-dependent modulations in regional diaphragmatic blood flow.

Blood flow (Q) 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 (Qpha), phrenic vein flow (Qphv), internal mammary vein flow (Qimv), and azygos vein flow (Qazv), respectively. The time course of Qpha, Qphv, Qimv, and Qazv 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. Qpha, Qphv, Qimv, and Qazv were measured at rest (control, passive diaphragm, mechanical ventilation) and at two submaximal levels of stimulation (30 and 60% of Pdimax). The f was 10 or 30 cycles/min and the duty cycle was 0.25, 0.50, and 0.75. The results show 1) Qpha, Qphv, Qimv, and Qazv reached stable values (equilibration) after 30-36 s of pacing; 2) the steady Qpha, Qphv, and Qimv were linearly related to Pdi, and they were related by a parabolic function to duty cycle, whereas Qazv was not significantly affected by Pdi and increased linearly as a function of the duty cycle; 3) the diaphragmatic blood drainage was approximately 60% through the intercostal veins leading into the azygos 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% Pdimax; and 4) for a given pacing pattern, Qpha and Qphv increased with f, but Qimv and Qazv did not.