What is left ventricular outlet obstruction?

Left Ventricular Outflow Tract Obstruction (LVOTO)

Left ventricular outflow tract obstruction (LVOTO) is defined as a peak instantaneous Doppler left ventricular outflow tract gradient of ≥30 mm Hg, with ≥50 mm Hg considered the threshold for invasive treatment in symptomatic patients. 1, 2

Pathophysiology and Mechanism

LVOTO is characterized by mechanical obstruction to blood flow from the left ventricle to the aorta during systole. The key pathophysiological features include:

• Typically occurs due to systolic anterior motion (SAM) of the mitral valve and mitral-septal contact
• Results in increased left ventricular systolic pressure
• Leads to a complex interplay of abnormalities:
  • Prolonged ventricular relaxation
  • Elevated left ventricular diastolic pressure
  • Mitral regurgitation
  • Myocardial ischemia
  • Decreased forward cardiac output 1

Classification of LVOTO

LVOTO can be classified based on the presence and severity of obstruction:

1. Basal obstruction: Gradient ≥30 mm Hg at rest
2. Labile obstruction: Gradient <30 mm Hg at rest but ≥30 mm Hg with physiological provocation
3. Nonobstructive: Gradient <30 mm Hg both at rest and with provocation 1

Approximately one-third of patients with hypertrophic cardiomyopathy (HCM) have obstruction under basal conditions, another third have labile obstruction, and the final third have the nonobstructive form. 1

Clinical Significance

LVOTO is clinically important because:

• It contributes significantly to heart failure symptoms in HCM
• Associated with impaired stroke volume
• Increases risk of heart failure
• Associated with reduced survival and increased risk of sudden cardiac death 2, 3
• The magnitude of LVOTO is related to a higher occurrence of sudden death 3

Management Approach

General Measures

• Avoid dehydration and excess alcohol consumption
• Encourage weight loss in overweight patients
• Avoid arterial and venous dilators (nitrates, phosphodiesterase inhibitors)
• Avoid digoxin due to its positive inotropic effects
• Promptly manage atrial fibrillation with restoration of sinus rhythm or rate control 1, 2

Pharmacological Management

1. First-line therapy: Non-vasodilating beta-blockers titrated to maximum tolerated dose

   • Example: Propranolol can reduce resting and provocable LVOTO 1, 2

2. Second-line therapy: If beta-blockers are ineffective or contraindicated

   • Disopyramide (when available) added to beta-blockers, titrated to maximum tolerated dose (usually 400-600 mg/day)
   • Verapamil (starting at 40 mg three times daily to maximum 480 mg daily) with close monitoring in patients with severe obstruction (≥100 mm Hg) or elevated pulmonary pressures 1, 2

3. Cautions with disopyramide:

   • Monitor QTc interval (reduce dose if exceeds 480 ms)
   • Avoid in patients with glaucoma, prostatism, or taking other QT-prolonging drugs
   • Use cautiously in patients with or prone to atrial fibrillation 1

Invasive Management

For patients with drug-refractory symptoms and LVOTO ≥50 mm Hg, invasive treatment options include:

1. Surgical septal myectomy:

   • Preferred option for most patients, especially younger patients with greater septal thickness
   • Contemporary early (<30 days) mortality is 1.4%
   • Contemporary late (>30 days) mortality is 0.7% 2, 4

2. Alcohol septal ablation:

   • Alternative for patients with comorbidities or advanced age 2

Special Considerations

• LVOTO can be missed if only resting echocardiography is performed; provocative testing is essential in symptomatic patients without resting gradients
• LVOTO can occur in ICU patients, particularly those with septic shock, severe hypovolemia, or hyperkinesia with or without LV hypertrophy 5
• In acute hypotension with LVOTO, intravenous phenylephrine or other vasoconstrictors without inotropic activity should be used 2
• LVOTO can also complicate the postoperative course of patients undergoing surgical aortic valve replacement 6

Prognosis

• Five-year survival from all-cause death or cardiac transplantation is lower in patients with LVOTO compared to those without
• There is a trend toward higher all-cause death and transplantation with increasing LVOTO severity
• In patients with obstruction, functional limitation (NYHA class) significantly affects survival 3
• The annual rate of sudden death in patients with LVOTO and no risk factors is approximately 0.37% 3

Understanding and properly managing LVOTO is crucial for improving symptoms, quality of life, and survival in affected patients.