Excessively high hear rate `( gt 180 )` can reduce cardiac output because

Options

A. blood is moving too fast through the lungs to pick up enough oxygen
B. it tires out the heart muscles and so they pump slower
C. it reduces the time for ventricular filling which reduces storke volume
D. the PR-interval increases which leads to longer ventricular diastole and shorter ventricular systole

Explanation

To answer the question regarding why an excessively high heart rate (greater than 180 beats per minute) can reduce cardiac output, we will analyze each option provided and determine the correct reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Cardiac Output**:
- Cardiac output (CO) is defined as the volume of blood the heart pumps per minute. It is calculated as the product of heart rate (HR) and stroke volume (SV).
- CO = HR × SV

2. **Analyzing the Options**:
- **Option 1**: "Blood is moving too fast through the lungs to pick up enough oxygen."
- This statement is incorrect. While an increased heart rate may lead to faster blood flow, it does not necessarily mean that the blood cannot pick up enough oxygen. The lungs can still oxygenate blood efficiently even at higher heart rates.

- **Option 2**: "It tires out the heart muscle and so they pump slower."
- This option is also incorrect. Under normal circumstances, heart muscles do not fatigue in a way that would cause them to pump slower due to an increased heart rate. The heart can adapt to higher rates without losing its pumping ability.

- **Option 3**: "It reduces the time for ventricular filling which reduces stroke volume."
- This statement is correct. At excessively high heart rates, the time available for the ventricles to fill with blood during diastole (the relaxation phase) is reduced. This leads to a decrease in stroke volume, as there is less blood available to be pumped out with each heartbeat.

- **Option 4**: "PR interval increases which leads to longer ventricular diastole and shorter ventricular systole."
- This option is misleading. While the PR interval can change, an increase in the PR interval does not necessarily lead to a significant change in cardiac output. The relationship between diastole and systole in this context does not directly affect cardiac output in the way described.

3. **Conclusion**:
- The correct reason why an excessively high heart rate can reduce cardiac output is that it reduces the time for ventricular filling, which in turn decreases stroke volume. Therefore, the correct answer is **Option 3**.