EFFECT OF HYPERVOLAEMIA AND HYPOVOLEMIA ON REGIONAL MYOCARDIAL OXYGEN-CONSUMPTION

Study objective - The purpose of the investigation was to study the effect of preload on coronary blood flow and myocardial oxygen consumption in subendocardial and subepicardial regions of left ventricular free wall.Design - Ventricular volume in anaesthetised open chest dogs was altered over a range of 20% to produce hypovolaemia (10-15% exsanguination) or hypervolaemia (colloid infusion), allowing measurements of regional blood flow and oxygen consumption with varying preloads. β Adrenergic blockade was used to limit changes in inotropy, and heart rate was kept constant by pacing at 150 beats·min-1.Subjects - 9 mongrel dogs of either sex weighing 24.9 (SEM 4.1) kg were studied.Measurements and results - Left ventricular volume was calculated from ultrasonic measurements of long and short axis end diastolic diameters and wall thickness. Regional myocardial blood flow was estimated using radioactive microspheres, and oxygen consumption in each region was determined from microspectro-photometric measurements of oxygen saturations in small arteries and veins. Hypervolaemia increased subepicardial blood flow from 66.8(6.9) (normovolemic) to 114.1(13.5) ml·min-1·100 g-1, and regional oxygen consumption from 4.08(0.57) to 6.44(1.08) ml·min-1·100 g-1 Values in the sub-endocardium were similar, except for oxygen consumption, which increased less than in the subepicardium. Left ventricular end diastolic volume, pressure, and output were each increased in hypervolaemia, but not dP/dt and systolic aortic pressure. Hypovolaemia reduced blood pressures without reducing end diastolic volume.Conclusions - Augmented flow work produced by increased preload (even in the absence of changes in pressure work) increases myocardial oxygen supply equally in the subepicardium and the subendocardium, while oxygen extraction and consumption are preferentially augmented in the subepicardium.