Clinical features

Aortic stenosis (AS), the most common cardiac valve lesion among people living in the United States, is found in one fourth of all patients with chronic valve disease. Left ventricular outflow tract obstruction can occur in the subvalvular, valvular, and supravalvular regions. Stenosis of the aortic valve itself is most common, occurring in 75% of patients with outflow tract obstruction. The three major types of AS are (1) a congenital malformation (bicuspid instead of the normal tricuspid valve) that becomes stenotic over decades, (2) calcification or degeneration in a previously normal tricuspid aortic valve, and (3) rheumatic aortic valve disease, which usually occurs in conjunction with mitral valve abnormalities. In developed countries, the number of people with rheumatic AS has declined significantly, so that congenital stenosis (a bicuspid valve that later calcifies) and calcific stenosis of a tricuspid valve have emerged as the more common causes of AS. Between 1% and 2% of the population has a bicuspid aortic valve, which is thought to be inherited as an autosomal-dominant trait with variable penetrance. Flow through a bicuspid valve is turbulent, creating abnormal pressures on the leaflets that lead to thickening of the leaflets and, eventually, stenosis.

The risk factors for calcific degenerative AS are similar to those for atherosclerosis (e.g., older age, male sex, hypertension, hyperlipidemia with evidence of inflammation at the site of disease). However, systemic hypertension occurs in only about half of all patients with AS; a systolic pressure above 200 mm Hg is thought to generally exclude severe narrowing of the valve.

Eighty percent of patients with symptomatic AS are men, approximately 50% will have coronary artery disease, and most of these patients will be at least 70 years old (AS due to a bicuspid valve tends to occur in patients younger than 70 years of age [Figure 149-1]). Overall, however, AS is a disease of the elderly, with a prevalence of more than 4% in North American adults older than 75 years. In a study of 5201 men and women over the age of 65 years, 26% had aortic sclerosis (a thickening of the valve without hemodynamic sequelae), and 2% had AS. The prevalence of aortic sclerosis increased with age in this study: 20% in patients aged 65 to 75 years, 35% in those aged 75 to 85 years, and 48% in patients older than 85 years (AS rates were 1.3%, 2.4%, and 4%, respectively, in these groups).

Natural history

Patients with AS are at increased risk of dying suddenly (likely from cardiac arrhythmia due to ischemia from mismatching of O₂ supply and demand), yet the typical natural history of AS is a gradual onset of symptoms manifesting in the fifth to seventh decades of life. Aortic sclerosis is not an uncommon finding in patients older than 65 years, but about 16% of patients with sclerosis develop AS within 7 years. Patients with aortic sclerosis are asymptomatic, but once the pressure gradient across the valve exceeds the upper limits of normal, exertional dyspnea, angina, and syncope–the cardinal symptoms of AS–can appear within 5 years. The mortality rate is approximately 25% per year among symptomatic patients (Figure 149-2), with three quarters of those whose AS is untreated dying within 3 years of the onset of symptoms (Figure 149-3). Asymptomatic patients, on the other hand, even those with severe disease, have a more favorable outlook (risk of death <1% per year).

The typical timeframes from the onset of symptoms until death are 4.5 years for patients with angina, 2.6 years for patients with syncope, 2 years for patients with dyspnea, and 1 year for patients with congestive heart failure, with the latter being the cause of death in one half to two thirds of patients with untreated AS.

Anatomic considerations

The internal cross-sectional area of a normal aortic valve during systole is 3.0 to 4.0 cm²; significant hemodynamic obstruction does not occur until the valve area is less than 1.5 cm². Based upon measurements of valve area, peak blood flow velocity across the valve (AoVmax), mean pressure gradient, and effective orifice area, the degree of AS is categorized as mild, moderate, severe, or critical and is most commonly assessed with echocardiography (Table 149-1). The measurement of pressure gradients is accurate less than 50% of the time because the pressure gradients are flow dependent. Measuring the valve area is the most reliable method of assessing severity of AS because it depends less on ventricular contractility than do pressure gradients, but measuring valve area using two-dimensional echocardiography has several factors that may limit its usefulness, including the difficulty in obtaining the correct short-axis view, the presence of calcifications that create shadowing on the image, and the inability with a “pinhole” valve to identify the orifice during systole. Therefore, the effective valve area or the effective orifice area is calculated using the following continuity equation:

< ?xml:namespace prefix = "mml" />Aortic Valve Area = LVOTArea × LVOTVTI Aortic ValveVTI

where LVOT is the left ventricular outflow tract and VTI is the velocity time integral. Critical AS is defined as a valve area smaller than 0.8 cm² and an outflow gradient exceeding 50 mm Hg.

Surgical correction

Each year, approximately 50,000 aortic valve replacements secondary to calcific AS are performed in the United States. The timing of the operation is based on the type, duration, and severity of symptoms and the degree of valve narrowing. Bioprosthetic (tissue) valves and mechanical valves can be used to replace a diseased valve. Valve selection depends on balancing the risks associated with the use of chronic anticoagulation, the likelihood of structural failure of a bioprosthetic valve (and hence the need for subsequent replacement), and the patient’s expected longevity and functional status. An 11-year follow-up study of patients who were randomly assigned to receive either a bioprosthetic valve or a mechanical valve found no difference in survival rates between the two groups. Structural valve failure was observed in the bioprosthetic group, but this was offset by increased bleeding complications in the patients who were anticoagulated because they had a mechanical valve. In general, 10-year survival rate after aortic valve replacement is approximately 67%.

Clinical experience has quelled the initial enthusiasm that developed for the use of percutaneous transluminal aortic valvuloplasty because the associated postoperative improvements in pressure gradients across the valve and in symptoms were often only temporary and the operation did not lead to improved overall mortality rates. More recently, many centers have begun using percutaneous aortic valve replacement, otherwise known as transcatheter aortic valve replacement, in which a prosthetic valve is placed via either a transfemoral arterial approach (placing a valve retrograde across the native valve) or a transapical approach (via a thoracotomy, instrumentation of the left ventricle apex, and placement of the valve antegrade across the native valve). Patients who are poor candidates for conventional operations are considered to be the best candidates for this procedure, but choice of appropriate candidates remains to be determined once further experience is gained with this procedure.

Concomitant diseases

Patients with AS often have additional medical problems, including coronary artery disease, as mentioned earlier, with even asymptomatic patients having an incidence of coronary artery disease of up to 33%. The manifestations of Heyde syndrome, which occurs in the elderly, include AS, acquired coagulopathy, and anemia due to bleeding from intestinal angiodysplasia.

Anesthetic considerations for aortic valve replacement

Many clinicians prefer using an opioid-based technique when anesthetizing patients who require aortic valve replacement. Opioids preserve systemic vascular resistance and left ventricular contractility better than do the inhalation anesthetic agents. However, many of the concerns vis-à-vis inhalation anesthetic agents are theoretical and of little clinical consequence. In practice, most clinicians use a combination of an opioid and either an inhalation agent or an intravenously administered hypnotic to produce optimal hemodynamics and early weaning from mechanical ventilation and tracheal extubation in the intensive care unit.

In addition to routine American Society of Anesthesiologists recommended monitors, arterial and pulmonary artery catheters (the latter often inserted more for postoperative care than intraoperative care) and a transesophageal echocardiographic probe are used. Echocardiography permits assessment of preload, left ventricular function, valve gradients, and prosthetic valve function and provides real-time information to the surgical team. Arrhythmias and hypotension should be treated aggressively. Anesthetic goals are summarized in Box 149-1.

Anesthesia for patients with aortic stenosis undergoing noncardiac operations

Patients with AS who undergo noncardiac operations are also at increased risk of developing perioperative myocardial infarction, congestive heart failure, and arrhythmia. An adequate history for symptoms should be obtained and appropriate diagnostic testing should be performed before patients with AS undergo elective operations. Anesthetic goals for noncardiac operations are similar to those for aortic valve replacement (see Table 149-1). Given the potential for deleterious effects of a reduced systemic vascular resistance and reduced coronary perfusion, the use of spinal or epidural anesthesia is relatively contraindicated.