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Coronary perfusion related changes in myocardial contractile force and systolic ventricular stiffness.

OBJECTIVE
The mechanism by which changes in coronary perfusion alter myocardial oxygen consumption (MVO2; Gregg phenomenon) is controversial. This study examined the effect of coronary perfusion on myocardial contractile force and systolic ventricular stiffness in the intact, ejecting heart.


METHODS
During selective perfusion of the left anterior descending coronary artery, coronary blood flow was changed with or without concurrent changes in coronary perfusion pressure in 19 alpha chloralose anaesthetised dogs. Regional myocardial segment length (end diastolic length; end systolic length) and developed force were measured with piezoelectric crystals and with a miniature force transducer, respectively. MVO2 was calculated from coronary flow and arteriovenous O2 difference. The slope of the force-length curve during ejection period (delta F/delta SL) was used as an index of systolic myocardial stiffness.


RESULTS
When coronary perfusion pressure was varied from 60 to 180 mm Hg (protocol 1, n = 11), maximum developed force (Fmax), delta F/delta SL, and MVO2 increased with perfusion pressure while end diastolic length, segmental shortening, and other haemodynamic variables stayed constant. When coronary blood flow was increased at constant perfusion pressure by infusion of either a low dose or a high dose adenosine (protocol 2, n = 8), Fmax, delta F/delta SL, and MVO2 increased while end diastolic length, segmental shortening, and other haemodynamic variables stayed constant. MVO2 and delta F/delta SL increased more steeply with flow in protocol 1.


CONCLUSIONS
(1) Increased coronary blood flow enhances myocardial contractile force, systolic ventricular stiffness, and MVO2 in the intact, ejecting heart. (2) Coronary blood flow induced changes in myocardial contractile force and systolic ventricular stiffness, but not end diastolic length, are probably responsible for coronary blood flow related changes in MVO2.