Often hereditary (underlying pathophysiology is an excess of calcium channels), hypertrophic cardiomyopathy leads to decreased LVEDV secondary to the hypertrophic heart (stroke volume, by contrast, remains normal). Furthermore, during contraction the IV septum and anterior mitral leaflet may approach each other, and if the LVOT is narrow enough Venturi forces will pull the valve and septum closer together, leading to SAM (systolic anterior motion) – the timing of SAM is critical, with early, prolonged SAM leading to pressure gradients as high as 100 mm Hg across the LVOT. SAM which occurs late in the cardiac contraction cycle is relatively innocuous.
20-30% of all patients will have some degree of subvalvular obstruction, thus the question is not so much who has outflow obstruction, but how bad is it? Not all of these patients will have symptoms – in fact, most patients with HOCM are asymptomatic, although they may experience dyspnea on exertion, fatigue, syncope, near-syncope, or angina. Unfortunately, in patients less than 30 years of age, sudden cardiac death is often the first (and last) manifestation. Supraventricular and ventricular arrhythmias are common.
HOCM should be suspected if a harsh systolic murmur is noted in conjunction with left ventricular hypertrophy and deep, broad Q waves. Diagnosis can be confirmed by echocardiography. Myocardial perfusion defects (thallium-201 scans) may be found in completely asymptomatic patients.
Effect on cardiac anatomy
Hypertrophic cardiomyopathy usually results in diastolic dysfunction (elevated LVEDP) in the presence of a hyperdynamic ventricle. Hypertrophied muscle tends to be located in the upper interventricular septum (below the aortic valve), although 25% of patients will also have a dynamic obstruction of left ventricular outflow during systole (which is caused by a systolic anterior motion [SAM] of the anterior mitral valve leaflet against the hypertrophied septum). SAM may be partially due to a Venturi effect on the anterior leaflet. Obstruction due to HOCM peaks in mid-to-late systole, and is worsened by increases in contractility, decreased ventricular volume, and decreased afterload. SAM can also cause mitral regurgitation – most of these patients have some abnormalities of the mitral valve.
Can sometimes be treated with alcohol ablation of the UV septum, with dual chamber pacing, or with myectomy
Anesthetic goals should be to minimize sympathetic stimulation, expand intravascular volume, and minimize decreases in left ventricular afterload.
Volatile anesthetics are ideal in that they decrease contractility (but also decrease afterload and lead to junctional rhythms – consider a TEE probe or PA catheter with pacing capabilities). However, the tendency of volatile agents to produce junctional rhythms (HOCM depends on atrial kick) mandates caution – consideration should be given to TEE with pacing or a pacing PA catheter.
A common anesthetic regimen is to combine volatile anesthesia with opiates, and pancuronium is avoided. Hypotension is treated with phenylephrine, which increases afterload and decreases ejection fraction, as opposed to ephedrine, which has beta activity and can thus both reduce afterload and stimulate the myocardium.
In patients with significant obstruction, some degree of myocardial depression is usually desirable and can be achieved by the use of volatile anesthetic agents.
Beta-blockers are also useful in counteracting the effects of sympathetic activation and decreasing obstruction. Phenylephrine may be an ideal vasopressor in these patients because it increases SVR (afterload) without augmenting contractility.
Regional anesthesia may exacerbate left ventricular outflow obstruction by decreasing both cardiac preload and afterload.