Importance of vasomotor tone to myocardial function and regional metabolism during constant flow ischaemia in swine

Study objective - The aim was to test the hypothesis that the release of vascular tone with adenosine during constant flow ischaemia alters both transmural function and regional metabolism in a detrimental way.Design - In one group of anaesthetised swine, the effects of graded reductions of flow on segmental left ventricular function, myocardial oxygen consumption (MVo₂), and lactate production in the distribution of the left anterior descending coronary artery (LAD) were determined. In a second group, a model of constant flow ischaemia was induced to test how altering vascular tone with adenosine changed the relationship of flow, function, and metabolism.Experimental material - The experiments were performed in 20 open chest, anaesthetised swine. Protocol A consisted of 11 animals and protocol B of nine animals.Measurements and main results - In protocol A, during graded ischaemia, reductions in flow, % systolic wall thickening (WTh), normalised MVo₂ and % lactate extraction (%LE) correlated well with reductions in coronary perfusion pressure when fitted with 3rd order polynominal curves (r = 0.78, 0.87, 0.85 and 0.81 respectively; p<0.0000l). In protocol B, during constant flow ischaemia, at control, % WTh was 33 (SD 11)%, mean coronary artery pressure was 72(10) mm Hg, mean LAD transmural flow was 0.99(0.43) ml·min^-1·g^-1, and % LE was +14(9)%. With inflation of a hydraulic occluder on the LAD, perfusion pressure was lowered to 38(5) mm Hg and transmural flow dropped to 0.76(0.31) ml·min^-1·g^-1 (intact vasomotion). During an infusion of intracoronary adenosine with flow held constant (absent vasomotion), %WTh was further reduced from 27(9) to 13(10) (p<0.00l), and %LE from -18(42) to -70(61) (p<0.05). MVo₂ with and without vasomotion did not differ significantly at 3.14(0.75) and 3.18(0.86) ml·min^-1·g^-1 respectively .Conclusion - In swine coronary circulation, reductions in regional function, MVo₂ and lactate production correlate well with reductions in flow and perfusion pressure during ischaemia with vasomotor tone intact. The effect of adenosine on vascular tone during constant flow ischaemia caused dramatic reductions in function and lactate extraction without altering MVo₂. This emphasises the important role of vascular tone in protecting both transmural function and regional metabolism during moderate ischaemia.