During which phase of cardiac cycle is coronary blood flow maximum?
**Core Concept**
The cardiac cycle is divided into several phases, including isovolumetric contraction, ventricular ejection, and isovolumetric relaxation. During these phases, coronary blood flow varies due to changes in intracardiac pressures and myocardial oxygen demand.
**Why the Correct Answer is Right**
Coronary blood flow is maximum during diastole, specifically during the isovolumetric relaxation phase. This is because the diastolic phase allows for a decrease in intracardiac pressures, which reduces the resistance to coronary blood flow. Additionally, the myocardial oxygen demand is lower during diastole, allowing for increased coronary blood flow. The coronary arteries are also more compliant during diastole, allowing for greater distension and increased blood flow. The increased coronary blood flow during diastole helps to meet the high oxygen demands of the myocardium during periods of increased contractility.
**Why Each Wrong Option is Incorrect**
* **Option A:** Isovolumetric contraction is not associated with increased coronary blood flow because the high intracardiac pressures increase resistance to coronary blood flow.
* **Option B:** Ventricular ejection is not the phase of maximum coronary blood flow because the high intracardiac pressures and increased myocardial oxygen demand during this phase reduce coronary blood flow.
* **Option D:** Systole, in general, is not associated with increased coronary blood flow because the high intracardiac pressures and increased myocardial oxygen demand reduce coronary blood flow.
**Clinical Pearl / High-Yield Fact**
The coronary circulation is unique in that it is a low-pressure, high-flow system, which allows for efficient delivery of oxygen and nutrients to the myocardium. This is in contrast to other vascular beds, which are characterized by high pressures and low flows.
**Correct Answer:** C.