Myocardial blood flow distribution across the left ventricular wall III. Mechanical factors

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Abstract
In an isovolumic dog heart perfused at constant coronary blood flow (CBF) beating spontaneously with a left ventricular end diastolic pressure of 10 ± 2 mm Hg the flow to the inner layer of the free wall slightly exceeds the flow to the subepi-cardium leading to an average endocardial/epicardial (endo/epi) ratio of 1.30 ± 0.05. These values are similar to the ones obtained in the working heart and suggest that in absence of shortening myocardial blood flow distribution is not altered. When the isovolumic mechanical systole was changed to ventricular fibrillation (empty heart) this pattern of myocardial blood flow distribution was not significantly altered with endo-epi ratio of 1.41 ± 0.07. When the mechanical systole was arrested potassium in empty hearts, the myocardial blood flow to the inner layers doubled the flow to the subendocardium with endo/epi ratio of 2.21 ± 0.07. Similar blood flow distribution was obtained in empty, spontaneously beating hearts in which the endo/epi ratio was 2.00 ± 0.08. This series in which developed tension was presumably minimal, and shortening maximal is suggesting again that shortening is not playing an important rôle in determining blood flow distribution. When in high potassium-arrested hearts a distending pressure similar to the left end diastolic pressure was induced the endo/epi ratio was 1.86 ± 0.08, suggesting that this pressure was not an important factor in determining the myocardial blood flow distribution. To obtain a myocardial blood flow distribution similar to the one obtained in normal working hearts or isovolumic beating hearts a distending pressure of 91 ± 5 mm Hg was necessary in high potassium-arrested hearts. The average endo/epi ratio in this serie was 1.27 ± 0.02. Our data seem to support the idea of a decreased resistance in the inner layers of the left ventricular wall which could determine a doubled flow to this area when the mechanical factors are not playing any rôle. Due to the fact that under these circumstances of minimal oxygen consumption it is unlikely that local active vasodilatation could play an important rôle, an anatomical factor with a gradient of vascularity is suggested. The passive influence of intraventricular diastolic pressure seems not to be an important factor in determining the myocardial blood flow distribution.