Why does decreasing preload or afterload exacerbate left ventricular outflow tract (LVOT) obstruction?

From the Guidelines

Decreasing preload or afterload can worsen left ventricular outflow tract (LVOT) obstruction due to changes in cardiac chamber geometry and dynamics, as evidenced by the 2024 AHA/ACC/AMSSM/HRS/PACES/SCMR guideline for the management of hypertrophic cardiomyopathy 1. When preload decreases (from diuretics, dehydration, or blood loss), the left ventricle becomes underfilled, causing the ventricular walls to move closer together and narrowing the outflow tract. Similarly, when afterload decreases (from vasodilators like nitrates or ACE inhibitors), the ventricle contracts more vigorously and empties more completely, which can exacerbate systolic anterior motion of the mitral valve and worsen LVOT obstruction. This is particularly problematic in hypertrophic cardiomyopathy, where the interventricular septum is already thickened. In managing patients with LVOT obstruction, it's essential to maintain adequate preload through proper hydration and avoid medications that significantly reduce afterload, as recommended by the 2024 AHA/ACC/ACS/ASNC/HRS/SCA/SCCT/SCMR/SVM guideline for perioperative cardiovascular management for noncardiac surgery 1. Key considerations include:

• Maintaining adequate preload to prevent underfilling of the left ventricle
• Avoiding medications that reduce afterload, such as vasodilators
• Using beta-blockers, like metoprolol 25-100mg twice daily, as first-line therapy to reduce contractility and heart rate
• Utilizing fluid resuscitation and phenylephrine (100-180 mcg/min IV) in acute situations with hypotension and LVOT obstruction to increase preload and afterload, respectively, and improve cardiac output by reducing the obstruction, as supported by the 2020 AHA/ACC guideline for the diagnosis and treatment of patients with hypertrophic cardiomyopathy 1. The primary goal is to maintain optimal cardiac function and prevent worsening of LVOT obstruction, which can be achieved by carefully managing preload, afterload, and contractility, and using evidence-based therapies as recommended by the latest guidelines 1.

From the Research

Left Ventricular Outflow Tract Obstruction

• Left ventricular outflow tract obstruction (LVOTO) can occur in various conditions, including septic shock, apical ballooning syndrome, myocardial infarction, respiratory failure, and post-valvular surgery 2.
• Decreasing preload or afterload can increase the risk of LVOTO, as it can alter the shape of the left ventricle and induce systolic anterior motion of the mitral valve 3.
• Factors that contribute to the development of LVOTO include:
  • Decreased preload due to hypovolemia
  • Decreased afterload due to septic shock
  • Increased heart rate
  • Left ventricular hyperkinesis produced by dobutamine infusion
• The onset of LVOTO is largely unpredictable due to a complex interplay between preload, afterload, heart rhythm, and rate in susceptible patients 2.

Mechanisms and Detection

• LVOTO is usually caused by systolic anterior motion of the mitral valve, which can be detected by echocardiography 3, 4, 2.
• Echocardiography is a useful tool for assessing LVOTO and its underlying pathophysiology in critically ill patients 2.
• Detection of LVOTO is a relatively simple task using a combination of two-dimensional, M-mode, and spectral Doppler imaging by an operator alert to the possible diagnosis 2.

Treatment and Management

• Treatment of LVOTO should include discontinuation of dobutamine infusion and fluid infusion 3.
• Beta-blockers can be useful in managing LVOTO, as they can reduce the heart rate and decrease the contractility of the left ventricle 3, 5, 4.
• Avoidance of therapies that aggravate the magnitude of the LVOT obstructive gradient, including nitrates, inotropic agents, and afterload reduction, is also important 4.