Aortic Stenosis

AORTIC STENOSIS

Preoperative Considerations

Valvular aortic stenosis is the most common cause of obstruction to left ventricular outflow. Left ven-tricular outflow obstruction is less commonly due to hypertrophic cardiomyopathy, discrete congenital subvalvular stenosis, or, rarely, supravalvular ste-nosis. Valvular aortic stenosis is nearly always con-genital, rheumatic, or degenerative. Abnormalities in the number of cusps (most commonly a bicuspid valve) or their architecture produce turbulence that traumatizes the valve and eventually leads to steno-sis. Rheumatic aortic stenosis is rarely isolated; it is more commonly associated with aortic regurgita-tion or mitral valve disease. In the most common degenerative form, calcific aortic stenosis, wear and tear results in the buildup of calcium deposits on normal cusps, preventing them from opening com-pletely (Figure 21–11).

Pathophysiology

Left ventricular outflow obstruction caused by val-vular aortic stenosis is almost always gradual, allow-ing the ventricle, at least initially, to compensate and maintain SV. Concentric left ventricular hypertrophy enables the ventricle to maintain SV by generating the needed transvalvular pressure gradient and to reduce ventricular wall stress.

Critical aortic stenosis is said to exist when the aortic valve orifice is reduced to 0.5–0.7 cm² (normal is 2.5–3.5 cm²). With this degree of stenosis, patients generally have a transvalvular gradient of approxi-mately 50 mm Hg at rest (with a normal cardiac output) and are unable to increase cardiac output in response to exertion. Moreover, further increases in the transvalvular gradient do not significantly increase SV. With long-standing aortic stenosis, myocardial contractility progressively deteriorates and compromises left ventricular function.

Classically, patients with advanced aortic ste-nosis have the triad of dyspnea on exertion, angina, and orthostatic or exertional syncope. A promi-nent feature of aortic stenosis is a decrease in left ventricular compliance as a result of hypertrophy. Diastolic dysfunction is the result of an increase in ventricular muscle mass, fibrosis, or myocardial ischemia. In contrast to left ventricular end-diastolic volume, which remains normal until very late in the disease, left ventricular end-diastolic pressure is elevated early in the disease. The decreased dia-stolic pressure gradient between the left atrium and left ventricle impairs ventricular filling, which becomes quite dependent on a normal atrial con-traction. Loss of atrial systole can precipitate con-gestive heart failure or hypotension in patients with aortic stenosis. Cardiac output may be normal in symptomatic patients at rest, but characteristically, it does not appropriately increase with exertion. Patients may experience angina even in the absence of CAD. Myocardial oxygen demand increases because of ventricular hypertrophy, whereas myo-cardial oxygen supply decreases as a result of the marked compression of intramyocardial coronary vessels caused by high intracavitary systolic pres-sures (up to 300 mm Hg). Exertional syncope or near-syncope is thought to be due to an inability to tolerate the vasodilatation in muscle tissue during exertion. Arrhythmias leading to severe hypoper-fusion may also account for syncope and sudden death in some patients.