A 28-year-old woman from Delhi presents to the outpatient clinic with dyspnea on exertion and palpitations for 3 months. On examination, her heart rate is 88 bpm, blood pressure 118/76 mmHg. Auscultation reveals a high-pitched, blowing, holosystolic murmur best heard at the apex with radiation to the axilla. The murmur increases with handgrip and decreases with amyl nitrite inhalation. Echocardiography confirms mitral regurgitation. Which of the following best explains the physiological mechanism underlying the decrease in murmur intensity with amyl nitrite?

Question

Accepted Answer

A. Decreased systemic vascular resistance reduces the pressure gradient across the mitral valve. ## Physiological Mechanism of Murmur Changes in Mitral Regurgitation

### Understanding Amyl Nitrite's Effect

**Key Point:** Amyl nitrite is a vasodilator that decreases systemic vascular resistance (SVR), thereby reducing the pressure gradient driving blood flow from the left ventricle into the left atrium during systole.

### Pressure Gradient and Murmur Intensity

The intensity of a regurgitant murmur is directly proportional to the pressure gradient across the incompetent valve:

$$	ext{Murmur intensity} \propto \Delta P = P_{LV} - P_{LA}$$

When SVR decreases (amyl nitrite effect):
- LV afterload ↓
- LV pressure during systole ↓
- Pressure gradient across mitral valve ↓
- Murmur intensity ↓

### Contrast with Handgrip (Isometric Exercise)

| Intervention | SVR | LV Afterload | ΔP across MR | Murmur | Clinical Sign |
|---|---|---|---|---|---|
| **Handgrip** | ↑ | ↑ | ↑ | **Louder** | Increases |
| **Amyl nitrite** | ↓ | ↓ | ↓ | **Softer** | Decreases |
| **Squatting** | ↑ | ↑ | ↑ | **Louder** | Increases |
| **Standing** | ↓ | ↓ | ↓ | **Softer** | Decreases |

**High-Yield:** Maneuvers that increase SVR (handgrip, squatting, leg raise) **increase** MR murmur; maneuvers that decrease SVR (amyl nitrite, standing, Valsalva) **decrease** MR murmur. This is the reverse of aortic regurgitation.

### Clinical Pearl

The response to amyl nitrite is a classic bedside test to differentiate mitral regurgitation (murmur decreases) from aortic regurgitation (murmur increases). This principle is based on the load-dependence of regurgitant flow.

[cite:Harrison 21e Ch 237]

Answer

B. Increased systemic vascular resistance reduces the pressure gradient across the mitral valve

Answer

C. Decreased heart rate prolongs diastole

Answer

D. Increased cardiac contractility enhances valve closure

Explanation

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