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

Options

A. (a) QRS complex indicates atrial contraction
B. (b) ORS complex indicates ventricular contraction
C. (c) Time between S and T represents atrial systole
D. (d) P-wave indicates beginning of ventricular contraction

Answer

To solve the question regarding the phases of the cardiac cycle as represented in a standard electrocardiogram (ECG), we will analyze the components of the ECG and their corresponding physiological events.

### Step-by-Step Solution:

1. **Understanding the ECG Components**:
- The ECG consists of several key components: P wave, QRS complex, and T wave. Each of these components corresponds to specific electrical events in the heart.

2. **Identifying the P Wave**:
- The P wave represents atrial depolarization. This is the phase when the atria contract (atrial systole). Therefore, the P wave indicates the beginning of atrial contraction.

3. **Analyzing the QRS Complex**:
- The QRS complex represents ventricular depolarization. This is the phase when the ventricles contract (ventricular systole). It is important to note that during this phase, atrial repolarization also occurs, but it is not visible in the ECG because it is overshadowed by the larger electrical activity of the ventricles.

4. **Understanding the T Wave**:
- The T wave represents ventricular repolarization. This is the phase when the ventricles relax (ventricular diastole).

5. **Evaluating the Options**:
- **Option A**: "QRS complex indicates atrial contraction." - This is incorrect because the QRS complex indicates ventricular contraction, not atrial contraction.
- **Option B**: "QRS complex indicates ventricular contraction." - This is correct as it accurately describes the QRS complex.
- **Option C**: "Time between S and T represents atrial systole." - This is incorrect because the time between S and T represents ventricular systole.
- **Option D**: "P wave indicates beginning of ventricular contraction." - This is incorrect because the P wave indicates the beginning of atrial contraction.

6. **Conclusion**:
- The only correct statement is Option B, which states that the QRS complex indicates ventricular contraction.

### Final Answer:
**Option B: QRS complex indicates ventricular contraction.**