RESPIRATORY FREQUENCY AND DISTRIBUTION OF CARDIAC-OUTPUT IN RATS BREATHING GAS WITH DIFFERENT DENSITIES

The effect of increased and reduced breathing-gas density on cardiac output (CO) and organ blood flow was studied with radiolabelled microspheres on pentobarbital anaesthetized rats using normoxic sulphurhexafluoride (SF6) and normoxic helium (He). The mean arterial pressure (MAP), heart rate (HR), and CO remained unchanged during exposure to any gas composition. The respiratory frequency (RF) was reduced from 98 +/- 6 (mean +/- SE) to 85 +/- 3 min-1 (p < 0.01) in the dense breathing-gas, while the RF increased from 96 +/- 6 to 108 +/- 4 (p < 0.01) in the He-O2 atmosphere. The arterial acid-base chemistry was mainly unaltered in any situation, indicating almost unchanged alveolar ventilation. Since the cerebral blood flow increased (30%, p < 0.01), we suggest that SF6 gas is not a truly inert gas; blood flow to the eyes was also increased (30%, p < 0.01). Though the pumping action of the heart (MAPxHR) remained constant, the blood flow to the left ventricular myocardium was increased (28%, p < 0.01) by the elevated gas density. Reduced breathing-gas density increased the myocardial blood flow in the right ventricle (20%, p < 0.02) suggesting a constrictive effect of He on the pulmonary arteries. Except for reduced renal blood flow, no blood flow changes were observed in any organ in this situation.