Abstract
An increase in coronary flow has been observed during acute hypercapnia (1-4). On the other hand, metabolic acidosis has been shown to produce an increase (5), or a decrease (2, 6) in coronary vascular resistance. Similarly inconsistent results have been observed when either respiratory or metabolic alkalosis was produced (2, 4-6). The apparent contradiction between different studies may be due to one or more of the following. In the first place, changes in the coronary circulation were observed in the presence of changes in the systemic circulation, notably arterial blood pressure and heart rate, making it difficult to determine to what extent coronary changes are a reflection of systemic circulatory changes (1, 5, 6). Secondly, acid-base changes are capable of altering the O2 requirements of the heart which in turn strongly affect the coronary circulation. This variable has not always been taken into account in the interpretation of the results (2, 4, 6). Finally, in some cases simultaneous changes in pH, PCO2 and HCO3 concentration have thwarted attempts to ascertain which of these parameters is responsible for the changes observed (5, 6).
The present experiments were undertaken to study the effect of metabolic and respiratory acidosis on coronary vascular resistance. A preparation where hemodynamic variables and O2 requirements could either be controlled or their influence accounted for was employed.
Methods. The experiments were performed using isolated perfused rabbit hearts. Rabbits were anesthetized with a mixture of chlora-lose and urethane. The thorax was opened under artificial ventilation and a metal cannula was introduced into the aorta, with its tip oriented towards the ventricle and close to the valve. Perfusion of the coronary circulation with Ringer's solution, equilibrated at 37° with 95% O2; 5% CO2 was started immediately with a roller pump.
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