Repeated normoxic hyperbaric exposures induce haemodynamic and myocardial changes in rats

The effect of repeated exposure to ambient pressures of 5 bar (500 kPa), in atmospheres comprising normal partial pressures of oxygen [0.2 bar (20 kPa)] and nitrogen [0.8 bar (80 kPa)] and 4 bar (400 kPa) helium, on cardiac function and morphology was assessed in conscious rats. Ten test rats underwent chamber dives daily for 40 consecutive days, and ten control rats were exposed in the same chamber for an equal period of time, but in air at 1 bar (100 kPa). Cardiac output (Qc) and myocardial blood flow (Qmyocardial) were determined by the microsphere method. After 40 days, the body mass was 7% greater in the control than in the test rats (P<0.05), although they were given exactly the same amount of standard food. The test rats had a significantly higher (7% absolute, 12% ventricular mass to body mass, P<0.05) heart mass (left ventricular myocardium, including the ventricular septum) than the control rats. The percentage tissue dry mass of the right and left ventricles was equal in the two groups. Microscopic examination revealed a number of small focal necroses in the left ventricle of the test rats but none in the control rats. The left ventricular pressure (LVP) and the maximum velocity of LVP increase (contractility) and decrease were significantly increased (25%–96%, P<0.001) in the pre-exposed compared to the control rats at 1 bar (100 kPa). The systolic arterial pressure, heart rate and respiratory frequency were similar in the two groups at 1 bar (100 kPa). The LVP and + dP/dt increased linearly and in parallel in both groups during compression, although at 5 bar (500 kPa) the test rats had reached a significantly higher LVP and + dP/dt level. However, the heart rate was unchanged in both groups. The pre-exposed rats had a higher left Qmyocardial [1 bar (100 kPa)=33%, P<0.05; and 5 bar (500 kPa)=maximum 40%, P<0.05] than the control rats. The systolic arterial blood pressure also increased during compression to its maximum after 20 min at 5 bar (500 kPa) in both groups. The mean arterial pressure, respiratory frequency, end-diastolic pressure and Qc were unchanged throughout the experiments. A pressure drop of 42 mmHg (5.6 kPa) between the left ventricle and the arteries would suggest stenosis in the aortic valve region in the test rats. In conclusion, the cardiac function as well as myocardial mass and morphology were changed after 40 consecutive exposures to 5 bar (500 kPa) in conscious rats.