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 (Ca(O₂) 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 Ca(O₂), it was concluded that the heart was oxygenated adequately prior to the first Ca(O₂) 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°C. The preparation may have been just marginally O₂ sufficient when rate was 60 beats/min (T, 25 or 30°C), but probably was hypoxic when contraction rate was 120 beats/min (T, 30 or 37°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°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.