Which phase of the cardiac cycle follows immediately after the beginning of the QRS wave?
The cardiac cycle has several phases: atrial systole, isovolumetric contraction, ventricular ejection, isovolumetric relaxation, and ventricular filling. The ECG has P wave, QRS complex, and T wave. The P wave corresponds to atrial depolarization, leading to atrial contraction. The QRS complex is ventricular depolarization, which causes ventricular contraction. The T wave is ventricular repolarization.
So, the QRS marks the start of ventricular depolarization. Immediately after the QRS, the ventricles begin to contract. The first phase after depolarization would be isovolumetric contraction. During this phase, the ventricles contract, but the volume doesn't change because the AV valves are closed (due to pressure being lower than atrial) and the semilunar valves are closed. Once pressure exceeds aortic and pulmonary pressures, the semilunar valves open, leading to ventricular ejection.
Wait, but the question is which phase follows immediately after the QRS. The QRS is the depolarization, so the next phase would be the isovolumetric contraction. Because the contraction starts right after depolarization, but before the semilunar valves open. So the answer is isovolumetric contraction. Let me check the options again. The correct answer is C, assuming the options are listed in order. But since the options aren't provided, I need to make sure. The key is that after QRS comes isovolumetric contraction. The other phases: atrial systole is after the P wave. Ventricular ejection comes after isovolumetric contraction. Isovolumetric relaxation is after the T wave. So the correct phase is isovolumetric contraction.
**Core Concept**
The QRS complex on the ECG corresponds to ventricular depolarization, which triggers **isovolumetric contraction**—the phase where ventricles contract without volume change due to closed valves. This phase precedes ventricular ejection.
**Why the Correct Answer is Right**
The QRS complex marks the start of ventricular depolarization. Immediately afterward, the ventricles begin contracting, increasing intraventricular pressure. However, since both the atrioventricular (AV) and semilunar valves remain closed during this phase (AV valves due to pressure gradients, semilunar valves due to aortic/pulmonary pressures), no blood is ejected. This is **isovolumetric contraction**, characterized by rising pressure and zero volume change.
**Why Each Wrong Option is Incorrect**
**Option A:** *Atrial systole* occurs during the P wave, not post-QRS.
**Option B:** *Ventricular ejection* follows isovolumetric contraction but begins after the semilunar valves open, not immediately after the QRS.
**Option D:** *Isovolumetric relaxation* occurs during ventricular repolarization (T wave), not after depolarization.
**Clinical Pearl / High-Yield Fact**
Remember the ECG-cardiac cycle sequence: **P wave (atrial depolarization) → QRS (