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Paradoxically the heart, which is solely dedicated to supplying blood to the organs, restricts its own blood supply during peak contraction (fig 1). To overcome this restriction, the heart has numerous mechanisms which delicately balance the blood flow demands of the systemic and coronary circulation.
PARADIGMS OF CORONARY FLOW
Scaramucci was the first to consider the effects that the contracting heart had on coronary arteries and on coronary blood flow in 1696.1 However, it was more than 250 years later that Gregg and Sabiston reported experimental evidence in a landmark paper which described phasic coronary flow.2 They described how the contracting heart muscle behaves as a “throttling mechanism and impedes coronary flow” by compression of the myocardial vascular bed.
While this work clearly demonstrated that coronary blood flow was phasic, as a result of contraction of the surrounding ventricle, the precise interplay between the ventricle, coronary blood flow and the aorta remained unresolved. Initially, it was proposed that myocardial contraction imposed a time-varying resistance to coronary blood flow, but, this proposal was inconsistent with the increase in coronary venous flow that occurs in systole. Currently, the influence of the contracting ventricle on coronary blood flow is understood in terms of two related models: the intramyocardial pump model3 and the time-varying elastance model.4 5 The intramyocardial pump model proposed by Spaan and colleagues emphasises that ventricular contraction actively pumps blood out of the coronary microvasculature as a result of compression of the intramural vessels. This model was based initially on observations that ventricular systole could be associated with net retrograde coronary artery flow if perfusion pressure were lowered sufficiently.3 Subsequently, Krams and coworkers,4 proposed the time-varying elastance model based on the ventricular elastance concept of Suga et al.6 This model emphasises the importance of …
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