Which of the following correctly explains a phase`//`event in cardiac cycle in a standard electrocardiogram

Options

A. QRS complex indicates atrial contraction.
B. QRS complex indicates ventricular contraction.
C. Time between S and T represents atrial systole
D. P-wave indicates beginning of ventricular contraction.

Explanation

To solve the question regarding the phases/events in the cardiac cycle as represented in a standard electrocardiogram (ECG), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Cardiac Cycle**: The cardiac cycle consists of a series of events that occur during one complete heartbeat, including both contraction (systole) and relaxation (diastole) of the heart chambers.

2. **Identify the Components of an ECG**: The standard ECG has several key components:
- **P Wave**: Represents atrial depolarization (contraction of the atria).
- **QRS Complex**: Represents ventricular depolarization (contraction of the ventricles).
- **T Wave**: Represents ventricular repolarization (relaxation of the ventricles).

3. **Analyze the Options Provided**:
- **Option 1**: QRS complex indicates atrial contraction. (Incorrect)
- **Option 2**: QRS complex indicates ventricular contraction. (Correct)
- **Option 3**: Time between S and T represents atrial systole. (Incorrect)
- **Option 4**: P wave indicates the beginning of ventricular contraction. (Incorrect)

4. **Determine the Correct Explanation**: The QRS complex is specifically associated with ventricular depolarization, which leads to ventricular contraction. Therefore, the correct explanation is that the QRS complex indicates ventricular contraction.

5. **Conclusion**: The correct answer to the question is **Option 2**: "QRS complex indicates the ventricular contraction."