The cardiac output, heart rate, and coronary artery perfusion pressure can be abruptly increased or decreased from the control level while maintaining constant aortic pressure. The point where the two lines cross indicates the control level. The immediate flow changes after the change in perfusion pressure are represented by solid circles, while the steady-state flows at the new pressures are represented by open circles. Autoregulation of blood flow tends to return the flow towards the control level, especially in the intermediate pressure range of approximately 60 to 180 mmHg.

During early ventricular systole, there is significant extravascular compression, causing a brief reversal of blood flow in the large coronary arteries supplying the left ventricle. The maximum inflow in the left coronary artery occurs during early diastole when the ventricles have relaxed, and extravascular compression is virtually absent. After the initial reversal in early systole, blood flow in the left coronary artery follows the aortic pressure until early diastole when it abruptly rises and then slowly declines as the aortic pressure falls during the remainder of diastole.

In addition to the pressure provided to move blood through the coronary vessels, the contracting myocardium also affects its own blood supply through extravascular compression. The flow patterns in the left and right coronary arteries are similar. The coronary perfusion pressure in the left ventricle is determined by the difference between aortic diastolic pressure and left ventricular end-diastolic pressure.

The left ventricular myocardial pressure is highest near the endocardium and lowest near the epicardium. Despite this pressure gradient, endocardial blood flow is not impaired due to the compensatory greater blood flow to the endocardium during diastole compared to the epicardium during systole. Under normal conditions, both the epicardial and endocardial halves of the left ventricle receive approximately equal blood flow, indicating that the tone of the endocardial resistance vessels is lower than that of the epicardial vessels.

In patients with a damaged myocardium and low blood pressure, myocardial perfusion can be improved using counterpulsation. This technique involves inserting an inflatable balloon into the thoracic aorta through a femoral artery. The balloon is inflated during each ventricular diastole and deflated during each systole, enhancing coronary blood flow during diastole by raising diastolic pressure when coronary extravascular resistance is lowest. Additionally, it reduces cardiac energy requirements by lowering aortic pressure (afterload) during ventricular ejection.

The flow pattern in the right coronary artery is similar to that in the left coronary artery.
Bed. Asaorticpressurewasheldconstant,thecardiacoutput,heart rate,andcoronaryarteryperfusionpressurewereabruptlyincreased ordecreasedfromthecontrollevel,whichisindicatedbythepointatwhichthetwolinescross.Thesolid circles representtheflowsthat resultedimmediatelyafterthechangeinperfusionpressure;theopen circles representthesteady-stateflowsatthenewpressures.Thereisatendencyforflowtoreturntowardthecontrollevel(autoregulationofbloodflow),andthisismostprominentovertheintermediatepressurerange(≈60to180mmHg).(FromBerneRM,RubioR.Coronarycirculation.InPageE,ed.Handbook of Physiology: Section 2: The Cardiovascular System: The Heart. Vol1.Bethesda,MD:AmericanPhysiologicalSociety;1979.) limits by baroreceptor reflex mechanisms. Hence, changes in coronary blood flow are caused mainly by changes in the diameter of coronary resistance vessels in response to the metabolic demands of the heart.
•Fig. 17.31 Comparison of Phasic Coronary Blood Flow in the Left and Right Coronary Arteries. Extravascularcompressionissogreatduringearlyventricularsystolethatthedirectionofbloodflowinthelargecoronaryarteriessupplyingtheleftventricleisbrieflyreversed.Maximalinflowintheleftcoronaryarteryoccursinearlydiastole,whentheventricleshaverelaxedandextravascularcompressionofthecoronaryvesselsisvirtuallyabsent.Afteraninitialreversalinearlysystole,bloodflowintheleftcoronaryarteryfollowstheaorticpressureuntilearlydiastole,whenitrisesabruptlyandthendeclinesslowlyasaorticpressurefallsduringtheremainderofdiastole.
In addition to providing the pressure to move blood through the coronary vessels, the heart also affects its blood supply by the squeezing effect (extravascular compression) of the contracting myocardium on its own blood vessels. The patterns of flow in the left and right coronary arteries are shown in
Fig. 17.31 . In the left ventricle, coronary perfusion pressure is the difference between aortic diastolic pressure and left ventricular end-diastolic pressure.
Left ventricular myocardial pressure (pressure within the wall of the left ventricle) is highest near the endocardium and lowest near the epicardium. This pressure gradient does not normally impair endocardial blood flow because the greater blood flow to the endocardium during diastole compensates for the greater blood flow to the epicardium during systole. Measurements of coronary blood flow indicate that the epicardial and endocardial halves of the left ventricle receive approximately equal blood flow under normal conditions. Because extravascular compression is greatest at the endocardial surface of the ventricle, the equality of epicardial and endocardial blood flow indicates that the tone of the endocardial resistance vessels is less than that of the epicardial vessels.
Theminimalextravascularresistanceandabsenceofleftventricularworkduringdiastolecanbeusedtoimprovemyocardialperfusioninpatientswithadamagedmyocardiumandlowbloodpressure.Inamethodcalledcounterpulsation, aninflatableballoonisinsertedintothethoracicaortathroughafemoralartery.Theballoonisinflatedduringeachventriculardiastoleanddeflatedduringeachsystole.Thisprocedureenhancescoronarybloodflowduringdiastolebyraisingdiastolicpressureatatimewhencoronaryextravascularresistanceislowest.Furthermore,itreducescardiacenergyrequirementsbyloweringaorticpressure(afterload)duringventricularejection.
The flow pattern in the right coronary artery is similar to that in the left coronary artery (see