A 28-year-old woman with hypertrophic obstructive cardiomyopathy (HOCM) presented for labor and delivery. She was initially managed by the obstetrician with intravenous butorphanol but became progressively more uncomfortable. The anesthesiologist was consulted for further management.
Describe the anatomic abnormalities in hypertrophic obstructive cardiomyopathy.
HOCM is the most common genetic cardiovascular disease, with a prevalence of approximately 1 in 500 young people in the United States. It is an autosomal dominant genetic disorder and is an important cause of heart failure at any age. HOCM is usually defined by a hypertrophied, nondilated left ventricle that occurs in the absence of other causative diseases for hypertrophy, such as chronic hypertension and aortic stenosis. It occurs in either obstructive or nonobstructive forms. The obstructive forms feature a dynamic pressure gradient across the left ventricular outflow tract (LVOT). Other conditions can produce the picture of obstructive cardiomyopathy secondary to significant infiltration of the ventricular wall, as in Pompe disease, in which a massive accumulation of cardiac glycogen in the ventricular wall produces ventricular outflow obstruction.
HOCM, hypertrophic cardiomyopathy, asymmetric septal hypertrophy, and idiopathic hypertrophic subaortic stenosis all are terms applied to the same disease process. The main anatomic feature of HOCM is a hypertrophied ventricular muscle at the septum base in the LVOT. The histologic appearance is a disorganized mass of hypertrophied myocardial cells extending from the left ventricular septal wall; the mass may involve the papillary muscles. Intramural (small vessel) coronary artery disease has been identified in autopsy specimens, especially in areas of myocardial fibrosis. This coronary artery disease may play some role in the etiology of myocardial ischemia in these patients.
Obstruction to left ventricular outflow is caused by hypertrophic muscle at the interventricular septum and systolic anterior motion (SAM) of the anterior leaflet of the mitral valve. SAM is associated with mitral regurgitation and a posteriorly directed jet. SAM previously was thought to be caused by a Venturi effect of the rapidly flowing blood in the LVOT. A more recent theory suggests that changes in the position of the leaflet coaptation zone relative to the interventricular septum and changes in blood flow caused by blood hitting the bulging septum may physically push the anterior mitral valve leaflet into the LVOT during systole. In other words, the hypertrophied ventricular septum causes the mitral valve to be positioned more anteriorly in the left ventricular cavity, bringing the leaflet coaptation point closer to the interventricular septum than normal. Excessive anterior mitral valve tissue in combination with the more anterior position of the mitral valve causes the anterior mitral valve leaflet to protrude into the LVOT. Additionally, the hypertrophied ventricular septum changes blood flow in the LVOT, redirecting it behind and lateral to the enlarged anterior mitral valve leaflet and pushing it into the septum.
Consequently, a dynamic subaortic pressure gradient is present. The outflow tract obstruction, by causing increased pressure in the ventricular chamber, can result in hypertrophy of the remainder of the ventricular muscle. As the ventricle hypertrophies, ventricular compliance decreases, and passive filling of the ventricle during diastole is limited. The ventricle increasingly depends on atrial systole (atrial kick) to maintain ventricular end-diastolic volume and, ultimately, cardiac output. Occasionally, HOCM is associated with right ventricular outflow tract obstruction as well.
What changes in preload, afterload, heart rate, and contractility optimize hemodynamic performance for patients with hypertrophic obstructive cardiomyopathy?
Determinants of the severity of the ventricular obstruction in HOCM are the following:
Systolic volume of the ventricle
Force of ventricular contraction
Transmural pressure distending the outflow tract
Large systolic volumes in the ventricle distend the outflow tract and reduce the obstruction. Paradoxically, when ventricular contractility is increased, the outflow tract is narrowed, which increases the obstruction and decreases cardiac output. When aortic pressure (afterload) is elevated, there is an increased transmural pressure distending the LVOT during systole, and this reduces the degree of obstruction. Conversely, during periods of systemic vasodilation, the outflow tract is narrowed. This results in a marked decrease in cardiac output and mitral regurgitation, as the mitral valve becomes the relief point for ventricular pressure ( Table 6-1 ).
|Heart rate||Slow||β-adrenergic blockers||Dopamine|
|Preload||Normal to high||Intravenous fluids||Nitroglycerin|
|Angiotensin-converting enzyme inhibitors|
|High-dose β-adrenergic blockers||Epinephrine |
|Heart rhythm||Normal sinus||Atrial pacing|