Respiratory frequency and distribution of cardiac output in rats breathing gas with different densities

Abstract

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 (MAP×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.

Key word:

• breathing-gas density
• cardiac output distribution
• microspheres
• rat
• respiratory frequency