Changes in contractility and afterload have only slight effects on subendocardial systolic flow impediment.

To test the hypothesis that contractility did not greatly influence systolic impediment to subendocardial flow we perfused the coronary artery at 70 mmHg and altered aortic pressure in 7 dogs and contractility in another 7. We measured myocardial systolic flow impediment (SFI) by comparing regional flows while beating and during asystole. Cardiac contraction impeded 29% of subendocardial asystolic flow, which was not affected by either intervention. In subepicardium, contraction increased flow by 25%, but dobutamine impeded systolic flow. Subepicardial SFI was only 16% of subendocardial SFI. Dobutamine slightly decreased estimated percent systolic myocardial blood flow (%SMBF) in subendocardium (+/- 12%) but decreased subepicardial %SMBF (45.5 to 17.4%). Phasic coronary flow pulsatility increased more with dobutamine than increased afterload, and pulsatility and SFI correlated only in subepicardium. Systolic-to-total coronary flow ratio and %SMBF did not correlate closely in subendocardium. SFI was most prominent in the subendocardium, whereas subepicardial SFI mainly determined epicardial coronary flow pulsatility. We conclude that the effects of contractility changes differ when evaluating regional SFI vs. phasic flow pulsatility.