EFFECTS OF GAS-DENSITY AND AMBIENT-PRESSURE ON MYOCARDIAL-CONTRACTILITY IN THE RAT

Background: Cardiac contractility and myocardial blood flow have been shown to increase when anesthetized and awake rats were exposed to nor moxic 0.5 MPa ambient pressure, independent of inert gas composition. Similar changes have been demonstrated in anesthetized rats breathing a dense (relative density (RD) 5) SF6-O-2 gas mixture at normobaric pr essure. Hypothesis: The purpose of the present study was to further ex plore whether cardiac contractility increases during hyperbaric exposu re as a response to the elevated atmospheric pressure per se or rather as a response to increased breathing gas density. Methods: Arterial p ressure, left ventricular pressure (LVP), central venous pressure and intra-esophageal pressure were monitored in anesthetized rats during s imulated dives. The Fats were exposed to various gas mixtures (air, SF 6-N-2-O-2, He-N-2-O-2), partial pressures of O-2 (PIO2 0.02 and 0.03 M Pa) ambient pressure (PTot 0.1-0.3 MPa) and gas density (RD 1-10.1). R esults: Cardiac contractility increased briefly by 5-10% (p < 0.05) du ring mild hyperoxia (PIO2 0.03 MPa). A concomitant stepwise increase i n RD (1-10.1) and PTot (0.1-0.3 MPa) by adding SF6 to air, increased m aximal rate of LVP rise (+dP/dt) and fall (-dP/dt) by 30% (p < 0.01). Two groups of rats exposed to either a high density SF6-N-2-O-2 (RD 5. 5) or a normal density He-N-2-O-2 (RD 1.2) breathing gas of identical PTot 0.2 MPa demonstrated similar rise in dP/dt (peak 72%, p < 0.05). Conclusion: At moderately increased ambient pressure, pressure per se increases cardiac contractility independently of the breathing gas den sity.