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 the 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 (MAP x HR) 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.