Hypertrophic Cardiomyopathy (HCM) Physiology

Hypertrophic Cardiomyopathy (HCM) – Study Notes

• Most common inherited cardiac defect (autosomal dominant).

• Characterized by LV hypertrophy without another cause (not due to HTN or aortic stenosis).

• Most common form: asymmetric hypertrophy of the interventricular septum.

Pathophysiology

• Abnormal myocyte arrangement: disorganized, haphazard → ineffective contraction.

• Interstitial fibrosis: abnormal collagen in interstitial space → myocardium stiff → impaired relaxation.

• Diastolic dysfunction: stiff LV, ↓ compliance, ↓ filling.

• Septal hypertrophy: disproportionate thickening of upper septum → obstructs LV outflow tract (LVOT).

• Obstruction worsens during systole as septum shifts closer to LVOT.

• Can be:

  • Obstructive HCM (with LVOT obstruction).

  • Non-obstructive HCM (no obstruction).

  • Obstruction may be dynamic (depends on contractility, preload, afterload).

Dynamic Obstruction Triggers

• ↑ Contractility (sympathetic stimulation, epinephrine, stress).

• ↓ Preload (hypovolemia, blood loss, dehydration).

• ↓ Afterload (vasodilation, low systemic BP).

• Systolic Anterior Motion (SAM) of mitral valve leaflet: Venturi effect pulls leaflet toward septum → worsens LVOT obstruction.

Hemodynamics & Coronary Perfusion

• LV must generate high pressures to overcome obstruction.

• Coronary perfusion occurs during diastole after aortic valve closes.

  • Coronary perfusion pressure = DBP – LVEDP.

  • ↓ DBP (due to ↓ stroke volume & ↓ cardiac output) → ↓ coronary supply.

  • ↑ LV mass & wall tension → ↑ myocardial O₂ demand.

• Result = supply-demand mismatch → ischemia, even without CAD.

Ventricular Function

• Diastolic dysfunction: universal in HCM (stiff ventricle).

• Systolic function:

  • EF = normal or ↑.

  • Stroke volume & cardiac output often ↓ (small cavity size + obstruction).

• Important: EF ≠ Stroke Volume ≠ Cardiac Output.

Clinical Manifestations

• Cardiomegaly.

• LV hypertrophy.

• Dilated LA (due to ↑ LV filling pressures).

• Symptoms: angina, syncope, dyspnea, arrhythmias, sudden cardiac death.

Diagnostics

• ECG: LVH, LA enlargement, high QRS voltage.

• Echo:

  • Asymmetric septal hypertrophy.

  • LVH.

  • Assess mitral valve for SAM.

  • EF often normal/high (but misleading).

Anesthesia Considerations

Goals: avoid worsening LVOT obstruction.

• Contractility:

  • Avoid ↑ contractility (avoid epinephrine, inotropes).

  • Beta-blockers (e.g., propranolol, metoprolol) helpful.

• Preload:

  • Maintain preload → avoid hypovolemia.

  • Careful fluid management (replace EBL promptly).

• Afterload:

  • Avoid ↓ afterload (hypotension, vasodilation).

  • Maintain/increase SVR if needed (phenylephrine preferred).

• Heart rate:

  • Avoid tachycardia (↓ diastolic filling, ↑ O₂ demand).

  • Prefer slow-normal HR.

• Coronary perfusion:

  • Maintain DBP to optimize coronary blood flow.

Summary Table

Factor	Effect on LVOT Obstruction	Anesthesia Goal
↑ Contractility	Worsens obstruction	Avoid; use β-blockers
↑ HR	↓ filling, ↑ O₂ demand	Avoid; keep normal/low
↓ Preload	Less LV filling, ↑ obstruction	Maintain preload
↓ Afterload	↑ velocity → worsens SAM	Maintain or ↑ SVR
Coronary supply	↓ with low DBP, high LVEDP	Maintain DBP

Key Takeaway:
HCM = stiff ventricle + dynamic LVOT obstruction.
Anesthesia management = slow, full, tight → maintain preload, maintain afterload, control HR, avoid inotropes.