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

Options

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

Answer

To solve the question regarding which option correctly explains a phase/event in the cardiac cycle as represented in a standard electrocardiogram (ECG), we need to analyze each option based on our understanding of the ECG waves and their corresponding events in the cardiac cycle.

### Step-by-Step Solution:

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

2. **Identify the Meaning of Each Wave**:
- **P wave**: Represents atrial depolarization, which leads to atrial contraction (atrial systole).
- **QRS complex**: Represents ventricular depolarization, which leads to ventricular contraction (ventricular systole).
- **T wave**: Represents ventricular repolarization, which is associated with ventricular relaxation (ventricular diastole).

3. **Analyze Each Option**:
- **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 because the QRS complex corresponds to the depolarization of the ventricles, leading to their contraction.

- **Option C**: "Time between S and T represents atrial systole."
- This is incorrect because the time between S and T represents the period of ventricular contraction and relaxation, not atrial systole.

- **Option D**: "P wave indicates beginning of ventricular contraction."
- This is incorrect because the P wave indicates atrial depolarization, not the beginning of ventricular contraction.

4. **Conclusion**:
- The correct answer is **Option B**: "QRS complex indicates ventricular contraction."