A 52-year-old man with a 15-year history of hypertension presents with exertional chest discomfort and dyspnea. On examination, blood pressure is 162/98 mmHg, heart rate 76 bpm. Auscultation reveals a late systolic murmur at the apex that increases with standing and decreases with squatting. Echocardiography shows severe left ventricular hypertrophy with systolic anterior motion of the mitral valve and a left ventricular outflow tract (LVOT) gradient of 85 mmHg at rest. Which physiological principle explains why the LVOT murmur increases when the patient stands up?

Question

Accepted Answer

B. Standing decreases left ventricular volume, reducing the LVOT diameter and increasing flow velocity. ## Hypertrophic Cardiomyopathy (HCM) and LVOT Obstruction Physiology

### Clinical Presentation

This patient has **obstructive HCM** (also called HOCM or IHSS — idiopathic hypertrophic subaortic stenosis). The key finding is **systolic anterior motion (SAM)** of the mitral valve, which narrows the LVOT during systole, creating a dynamic obstruction and a late systolic murmur.

### Pathophysiology of LVOT Obstruction

**Key Point:** In HCM, the LVOT gradient is **dynamic** — it changes with ventricular volume and contractility. The obstruction worsens when:
1. **LV volume decreases** → SAM worsens → LVOT narrower → higher flow velocity → louder murmur
2. **LV contractility increases** → more forceful systolic contraction → greater SAM

### Effect of Positional Changes

| Maneuver | LV Volume | LVOT Gradient | Murmur | Mechanism |
|----------|-----------|---------------|--------|----------|
| **Standing** | ↓ | **↑** | **↑** | ↓ preload → ↓ LV cavity → ↑ SAM → ↑ obstruction |
| **Squatting** | ↑ | **↓** | **↓** | ↑ preload → ↑ LV cavity → ↓ SAM → ↓ obstruction |
| **Hand grip** | ↔ | **↓** | **↓** | ↑ afterload → ↑ LV volume → ↓ SAM |
| **Valsalva** | ↓ | **↑** | **↑** | ↓ preload → ↓ LV volume → ↑ SAM |

**High-Yield:** The **late systolic** (or mid-to-late systolic) murmur in HCM is characteristic because SAM develops as the LV contracts and the mitral valve is pulled anteriorly by the hypertrophied septum, obstructing flow only in mid-to-late systole.

### Mechanism of Standing-Induced Worsening

```mermaid
flowchart TD
  A[Standing upright]:::action --> B[Decreased venous return]:::outcome
  B --> C[Decreased LV preload]:::outcome
  C --> D[Decreased LV end-diastolic volume]:::outcome
  D --> E[Smaller LV cavity]:::outcome
  E --> F[Increased systolic anterior motion of MV]:::outcome
  F --> G[Narrower LVOT]:::outcome
  G --> H[Higher flow velocity across LVOT]:::outcome
  H --> I[Louder murmur]:::outcome
```

**Clinical Pearl:** Squatting is a bedside maneuver to **reduce** the LVOT murmur in HCM by increasing preload and LV volume, thereby reducing SAM. This is pathognomonic for HCM and helps distinguish it from aortic stenosis (where squatting increases murmur).

### Why the Correct Answer

Standing decreases venous return → ↓ LV preload → ↓ LV volume → ↑ SAM → ↑ LVOT obstruction → ↑ murmur. The narrowed LVOT cavity increases flow velocity (Bernoulli principle), amplifying the murmur.

Answer

A. Standing increases left ventricular preload, augmenting the degree of systolic anterior motion

Answer

C. Standing increases left ventricular contractility via sympathetic activation, worsening the obstruction

Answer

D. Standing decreases aortic pressure, reducing the afterload-induced obstruction relief

Explanation

Hypertrophic Cardiomyopathy (HCM) and LVOT Obstruction Physiology

Clinical Presentation

Pathophysiology of LVOT Obstruction

Effect of Positional Changes

Mechanism of Standing-Induced Worsening

Why the Correct Answer

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Maneuver	LV Volume	LVOT Gradient	Murmur	Mechanism
Standing	↓	↑	↑	↓ preload → ↓ LV cavity → ↑ SAM → ↑ obstruction
Squatting	↑	↓	↓	↑ preload → ↑ LV cavity → ↓ SAM → ↓ obstruction
Hand grip	↔	↓	↓	↑ afterload → ↑ LV volume → ↓ SAM
Valsalva	↓	↑	↑	↓ preload → ↓ LV volume → ↑ SAM