Pressure-flow relations of diaphragm and vital organs with nitroprusside-induced vasodilatation

We determined maximal conductance in the diaphragm and other vital organs in 14 anesthetized dogs, weighing 22.8 +/- 4.2 kg, which were given maximal vasodilating doses of nitroprusside (mean dose 13.9 +/- 4.3 micrograms X kg-1 X min-1) and the blood pressure was lowered in stages by hemorrhage. Blood flow in the diaphragm, brain, heart, kidney, gut, and quadriceps was measured with radiolabeled microspheres. To ensure maximal vasodilatation of diaphragmatic vessels, we stimulated the phrenic nerves to produce diaphragmatic contractions at 0.3 Hz. The mean cardiac output was 2.13 +/- 0.42 l/min (thermodilution) before nitroprusside and 4.68 +/- 1.45 after (P less than 0.001). Nitroprusside failed to break the autoregulation of the brain. Pressure-flow relations (P-F) in other regions were linear (r = 0.70 +/- 0.03, P less than 0.001) and blood pressure at zero flow (X-intercept) was always greater than venous pressure (diaphragm = 11, kidney = 19, heart = 8, gut = 8, quadriceps = 32 mmHg). The flow to the diaphragm (Qdi) could be predicted by Qdi (ml X min-1 X g-1) = [(3.13 +/- 0.56) X Pa X 10(-2)] -0.52 (r = 0.71), where Pa is mean arterial pressure. The maximal vascular conductance (i.e., slope of the P-F relation) of the diaphragm was 27% of the conductance in the kidney, 87% of the value in the gut, and 42% of that in the heart. In conclusion the maximal diaphragmatic blood flow at a given blood pressure is much larger when the muscle is stimulated than is observed in spontaneously breathing animals.