Oxygen supply and oxygen demand in the isolated working rabbit heart perfused with asanguineous crystalloid solution

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Abstract
Study objective – It has been suggested that oxygenation of the isolated, crystalloid perfused, Langendorff rabbit heart is inadequate and that consequent hypoxia limits mechanical performance. The isolated working rabbit heart, which has a higher oxygen requirement than the Langendorff preparation, was used to investigate the relationships between the ability of the heart to perform external work, myocardial oxygen consumption, and tissue high energy phosphate content. Design – Hearts from adult rabbits (1.5-2.2 kg), paced at 200 beats·min−1, were perfused in the working mode with bicarbonate buffer (1.8 mmol·litre−1 Ca2+; Po2 >85 kPa, 630 mm Hg). Measurements and main results – The effects of (a) increasing preload (10,14,18, 22, 26 cm H2O) at fixed afterload (80 cm H2O) and (b) increasing afterload (60, 80,100,120 cm H2O) at fixed preload (18 cm H2O) on left ventricular power and myocardial oxygen consumption were determined. Increasing preload (from 10 to 26 cm H2O) resulted in linear increases in left ventricular power [from 1.36(SEM 0.06) to 2.81(0.19) J·min−1] and myocardial oxygen consumption [from 554(37) to 801(59) μl·g dry wt−1·min−1]; this was made possible by increased oxygen supply secondary to increased coronary flow. Increasing afterload (from 60 to 120 cm H2O) also increased . left ventricular power [from 1.76(0.05) to 2.16(0.09) J·min−1] and myocardial oxygen consumption [from 617(48) to 903(30) μl·g dry wt−1·min−1]. For a given increase in left ventricular power, the increase in myocardial oxygen consumption was greater if the increased workload was due to an increased afterload than if it was due to an increased preload, suggesting that increasing afterload resulted in a reduced external mechanical efficiency. Prolonged perfusion under conditions of low and high workload (80 and 120 cm H2O afterload for 60 min) was associated with comparable myocardial contents of adenosine triphosphate [22.5(1.4) and 21.7(0.4),μmol·g dry wt−1] and creatine phosphate [21.8(4.5) and 23.7(1.9) μmol·g dry wt−1] and comparable rates of lactate efflux [34.7(13.0) and 36.7(12.5) μmol·g dry wt−1·60 min−1]. Conclusion – Our observation of increased myocardial oxygen consumption and comparable contents of high energy phosphates with increased workload would suggest that the crystalloid perfused working rabbit heart is adequately oxygenated.

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