Obstruction is unimportant in the pathophysiology of hypertrophic cardiomyopathy

Abstract

Summary:There has been a longstanding controversy about the significance of intracavitary pressure gradients in hypertrophic cardiomyopathy (HCM). It has been generally assumed that the gradient is the result of an 'obstruction' that impedes left ventricular outflow and which can be relieved by operative intervention. In the first decade after the discovery of HCM , the site of 'obstruction' was thought to be a muscular sphincter or contraction ring in the submitral region of the left ventricle, and operations designed to emulate pyloromyectomy (for hypertrophic pyloric stenosis) were developed. Following a challenge to the existence of the 'contraction ring' and an alternative non-obstructive explanation of the pressure gradient, the site of 'obstruction' was translocated to a point of apposition between the anterior mitral leaflet and the interventricular septum, a result of systolic anterior motion (SAM) of the mitral valve. Despite the translocation of the site and mechanism of 'obstruction', the operation for 'relief of obstruction' has not changed significantly. The newer site of 'obstruction' has been challenged on the grounds that the ventricle is not demonstrably impeded in its emptying; when a gradient is provoked, the ventricle empties more rapidly and more completely than it does without a gradient. In addition to a non-obstructive explanation of the gradient, other phenomena thought to be indicative of 'obstruction' can be explained by rapid and complete emptying of the ventricle (cavitary obliteration). Since the morbidity and mortality of symptomatic HCM patients without pressure gradients may exceed that of patients with pressure gradients, it is suggested that 'obstruction' may be unimportant in the pathophysiology of HCM and attention should be focused on abnormal diastolic function and life threatening arrhythmias.

2011 ACCF/AHA guideline for the diagnosis and treatment of hypertrophic cardiomyopathy

“…The original observations by Brock 68 and Braunwald et al 3 emphasized the functional subvalvular LVOT gradient, which was highly influenced by alterations in the load and contractility of the left ventricle. The clinical significance of the outflow tract gradient has periodically been controversial, [69][70][71][72] but careful studies have shown definitively that true mechanical obstruction to outflow does occur. 66,67 For HCM, it is the peak instantaneous LV outflow gradient rather than the mean gradient that influences treatment decisions.…”

2011 ACCF/AHA Guideline for the Diagnosis and Treatment of Hypertrophic Cardiomyopathy: Executive Summary

“…The clinical significance of the outflow tract gradient has periodically been controversial, [217][218][219][220] but careful studies have shown definitively that true mechanical obstruction to outflow does occur. 40,41 For HCM, it is the peak instantaneous LV outflow gradient rather than the mean gradient that influences treatment decisions (Table 3).…”

Emerging pharmacologic and structural therapies for hypertrophic cardiomyopathy

“…A high-velocity LVOT gradient on continuous wave Doppler with no true pressure gradient seen on catheterization has been described previously [99]. These findings may be explained by the following: (1) cavity obliteration by a hypercontractile HCM left ventricle may generate high intraventricular velocities without true impedance to flow [100][101][102]; (2) a "pressure recovery" phenomenon from a long tubular narrowing of the midventricular region may lead to overestimation of pressure gradients measured by Doppler compared to catheterization [103]; and (3) there is significant variation in LVOT pressure gradients with different pharmacologic and physiologic provocations that alter ventricular loading and contractility even during a single hemodynamic assessment, which may result in misclassification of patients [104][105][106]. Taken together, the hemodynamic effect of percutaneous mitral valve plication in obstructive HCM patients with severe MR is not entirely understood, although preliminary findings are intriguing and warrant further study.…”