O2 reserve of left ventricle of isolated, saline-perfused rabbit heart

The heart from the pentobarbital-anesthetized rabbit was isolated, and a fluid-filled balloon was placed in the left ventricular chamber for assessment of isovolumic pressure development. The bicarbonate-buffered, physiological perfusate was aerated initially with 95% O2-5% CO2, and then progressive decreases in arterial O2 content (CaO2) were produced in a stepwise fashion by substituting N2 for O2 in the aerating gas mixture. If, for a specified set of experimental conditions, no change in ventricular function occurred after the initial decrease in CaO2, it was concluded that the heart was oxygenated adequately prior to the first CaO2 decrement. Accordingly, with perfusate flow constant at 35 ml/min, adequate oxygenation was achieved during aeration of perfusate with 95% O2 when ventricular contraction rate was 30 beats/min and temperature (T) 22 degrees C. The preparation may have been just marginally O2 sufficient when rate was 60 beats/min (T, 25 or 30 degrees C), but probably was hypoxic when contraction rate was 120 beats/min (T, 30 or 37 degrees C). Perfusion with pressure kept constant at 80 mmHg did not appear to alleviate this hypoxia when metabolism was elevated (rate, 120 beats/min; T, 37 degrees C). Thus, unless the contraction rate is very low, the addition of an O2-carrying vehicle to the perfusate appears to be necessary if O2 delivery to the isolated rabbit heart preparation is to be supramaximal.