Increased breathing gas density enhances cardiac workload.

The effect of increased breathing gas density on the left ventricular pressure (LVP), cardiac contractility (dP/dt), heart rate (HR), intrapleural pressure (P(ip)) and respiratory frequency (RF) was evaluated in pentobarbital-anaesthetized rats (n = 8) and cats (n = 6). Catheters were placed in the left cardiac ventricle via the right carotid artery to measure the LVP, in the oesophagus for indirect measurements of P(ip) and RF, and into the aorta from the right femoral artery for arterial pressure measurements. The RF fell significantly within the first 30 s and had reached a stable value 2 min after gas shift in both rats and cats. Concomitant with the RF fall, the depth of inspiration and intrapleural pressure differences in both rats and cats increased. The acid-base balance remained at control levels in both animal groups. LVP and dP/dt started to increase during the first half-minute, and reached their maximum values 2-5 min after the introduction of normoxic sulphur hexafluoride. A linear relationship between the enhanced dP/dt and the P(ip) increase was found. The HR remained unchanged in both cats and rats. These findings indicate that the breathing gas density might influence the cardiac contractility found during hyperbaric exposure, and that a gas density of five times that of air at 1 bar does not influence the diffusion of O2 and CO2 in the lung. The O2 consumption of the heart in cats and rats was calculated to rise by 25% and 30% respectively in the dense breathing gas atmosphere.