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?

Explanation

## 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.