Clinical Features and Natural History

Aortic stenosis (AS) is the most common cardiac valve lesion in the United States. Approximately 1% to 2% of people are born with a bicuspid aortic valve, which is prone to stenosis with aging. Clinically significant aortic valve stenosis occurs in 2% of unselected individuals older than 65 years and in 5.5% of those older than 85 years.

Calcific AS has several features in common with coronary artery disease (CAD). Both conditions are more common in men, older people, and patients with hypercholesterolemia, and both result in part from an active inflammatory process. Clinical evidence indicates that an atherosclerotic process is the cellular mechanism of aortic valve stenosis. There is a clear association between clinical risk factors for atherosclerosis and the development of AS: increased lipoprotein levels, increased low-density lipoprotein (LDL) cholesterol, cigarette smoking, hypertension, diabetes mellitus, increased serum calcium and creatinine levels, and male sex. The early lesion of aortic valve sclerosis may be associated with CAD and vascular atherosclerosis. Aortic valve calcification is an inflammatory process promoted by atherosclerotic risk factors.

The average rate of progression is a decrease in aortic valve area (AVA) of 0.1 cm ² /year, and the peak instantaneous gradient increases by 10 mm Hg/year. The rate of progression of AS in men older than 60 is faster than in women, and it is faster in women older than 75 than in women 60 to 74 years old.

Angina, syncope, and congestive heart failure (CHF) are the classic symptoms of the disease, and their appearance is of serious prognostic significance because postmortem studies indicate that symptomatic AS is associated with a life expectancy of only 2 to 5 years.

There is evidence that patients with moderate AS (ie, valve areas of 0.7 to 1.2 cm ² ) are also at increased risk for complications, with the appearance of symptoms further increasing their risk.

Angina is a frequent and classic symptom of the disease, occurring in approximately two-thirds of patients with critical AS, and about one-half of symptomatic patients have anatomically significant CAD.

It is probably never too late to operate on patients with symptomatic AS. Unlike patients with aortic regurgitation (AR), most symptomatic patients undergo valve replacement when left ventricular function is still normal. Even when impaired left ventricular function develops in AS, the relief of pressure overload almost always restores normal function or produces considerable improvement. Morbidity rates, mortality rates, and clinical results are favorable even for the oldest surgical candidates. Advances in operative techniques and perioperative management have contributed to excellent results after aortic valve replacement (AVR) in patients 80 years of age or older, with minimal incremental postoperative morbidity.

Preoperative assessment of AS with Doppler echocardiography includes measurement of the AVA and the transvalvular pressure gradient. The latter is calculated from the Doppler-quantified transvalvular velocity of blood flow, which is increased in the setting of AS. The maximal velocity (v) is then inserted into the modified Bernoulli equation to determine the pressure gradient (PG) between the left ventricle (LV) and the aorta:

Pressure gradients determined invasively or by Doppler echocardiography correctly classify AS severity in less than 50% of cases compared with estimates of AVA. The preferred method of obtaining AVA requires only two Doppler-generated velocities: those proximal or distal to the stenotic valve. These values are inserted into the continuity equation, which relates the respective velocities and cross-sectional areas proximal and distal to a stenotic area:

Pathophysiology

The normal AVA is 2.6 to 3.5 cm ² , with hemodynamically significant obstruction usually occurring at cross-sectional valve areas of 1 cm ² or less. Accepted criteria for critical outflow obstruction include a systolic pressure gradient greater than 50 mm Hg, with a normal cardiac output, and an AVA of less than 0.4 cm ² . In view of the ominous natural history of severe AS (AVA <0.7 cm ² ), symptomatic patients with this degree of AS are usually referred for immediate AVR. A simplification of the Gorlin equation to calculate the AVA is based on the cardiac output (CO) and the peak pressure gradient (PG) across the valve.

Stenosis at the level of the aortic valve results in a pressure gradient from the LV to the aorta. The intracavitary systolic pressure generated to overcome this stenosis directly increases myocardial wall tension in accordance with Laplace’s law:

The increase of wall tension is thought to be the direct stimulus for the further parallel replication of sarcomeres, which produces the concentrically hypertrophied ventricle characteristic of chronic pressure overload. The consequences of left ventricular hypertrophy (LVH) include alterations in diastolic compliance, potential imbalances in the myocardial oxygen supply and demand relationship, and possible deterioration of the intrinsic contractile performance of the myocardium.

Fig. 15.1 shows a typical pressure-volume loop for a patient with AS. Two differences from the normal curve are immediately apparent. First, the peak pressure generated during systole is much greater because of the high transvalvular pressure gradient. Second, the slope of the diastolic limb is steeper, reflecting the reduced left ventricular diastolic compliance that is associated with the increase in chamber thickness. Clinically, small changes in diastolic volume produce relatively large increases in ventricular filling pressure.

Simultaneous left ventricular (LV) volume and pressure during one cardiac cycle. ECG, Electrocardiogram.

(From Barash PG, Kopriva DJ. Cardiac pump function and how to monitor it. In: Thomas SJ, ed. Manual of Cardiac Anesthesia. New York: Churchill Livingstone; 1984:1.)

Increased chamber stiffness places a premium on the contribution of atrial systole to ventricular filling, which in patients with AS may account for up to 40% of the left ventricular end-diastolic volume (LVEDV) rather than the 15% to 20% characteristic of the normal LV. Echocardiographic and radionuclide studies have documented that diastolic filling and ventricular relaxation are abnormal in patients with hypertrophy from a variety of causes, and significant prolongation of the isovolumic relaxation period is the most characteristic finding. This necessarily compromises the duration and amount of filling achieved during the early rapid diastolic filling phase and increases the relative contribution of atrial contraction to overall diastolic filling. A much greater mean left atrial pressure is necessary to distend the LV in the absence of the sinus mechanism. One treatment of junctional rhythm is volume infusion.

The systolic limb of the pressure-volume loop shows preservation of pump function, as evidenced by maintenance of the stroke volume (SV) and ejection fraction (EF). Use of preload reserve and adequate LVH are likely the principal compensatory mechanisms that maintain forward flow. Clinical studies have confirmed that ejection performance is preserved at the expense of myocardial hypertrophy, and the adequacy of the hypertrophic response has been related to the degree to which it achieves normalization of wall stress, in accordance with the Laplace relation. LVH can be viewed as a compensatory physiologic response; however, severe afterload stress and proportionately massive LVH could decrease subendocardial perfusion and superimpose a component of ischemic contractile dysfunction.

In AS, signs and symptoms of CHF usually develop when preload reserve is exhausted, not because contractility is intrinsically or permanently impaired. This contrasts with mitral regurgitation (MR) and AR, in which irreversible myocardial dysfunction may develop before the onset of significant symptoms.

The major threat to the hypertrophied ventricle is its exquisite sensitivity to ischemia. Ventricular hypertrophy directly increases basal myocardial oxygen demand (MV̇ o ₂ ). The other major determinants of overall MV̇ o ₂ are heart rate, contractility, and, most important, wall tension. Increases in wall tension occur as a direct consequence of Laplace’s law in patients with relatively inadequate hypertrophy. The possibility of ischemic contractile dysfunction in the inadequately hypertrophied ventricle arises from increases in wall tension, which directly parallels the imbalance between the increased peak systolic pressure and the degree of mural hypertrophy. Although there is considerable evidence for supply-side abnormalities in the myocardial supply and demand relationship in patients with AS, clinical data also support increased MV̇ o ₂ as important in the genesis of myocardial ischemia.

On the supply side, the greater left ventricular end-diastolic pressure (LVEDP) of the poorly compliant ventricle inevitably narrows the diastolic coronary perfusion pressure (CPP) gradient. With severe outflow obstruction, decreases in SV and resultant systemic hypotension may critically compromise coronary perfusion. A vicious cycle may develop because ischemia-induced abnormalities of diastolic relaxation can aggravate the compliance problem and further narrow the CPP gradient. This sets the stage for ischemic contractile dysfunction, additional decreases in SV, and worsening hypotension.

Difficulty of Low-Gradient, Low-Output Aortic Stenosis

A subset of patients with severe AS, left ventricular dysfunction, and low transvalvular gradient suffers a high operative mortality rate and poor prognosis. It is difficult to assess accurately the AVA in low-flow, low-gradient AS because the calculated valve area is proportional to forward SV and because the Gorlin constant varies in low-flow states. Some patients with low-flow, low-gradient AS have a decreased AVA as a result of inadequate forward SV rather than anatomic stenosis. Surgical therapy is unlikely to benefit these patients because the underlying pathology is a weakly contractile myocardium. However, patients with severe anatomic AS may benefit from valve replacement despite the increased operative risk associated with the low-flow, low-gradient hemodynamic state. American College of Cardiology/American Heart Association (ACC/AHA) guidelines call for a dobutamine echocardiography evaluation to differentiate patients with fixed anatomic AS from those with flow-dependent AS with left ventricular dysfunction. Low-flow, low-gradient AS is defined as a mean gradient of less than 30 mm Hg and a calculated AVA less than 1.0 cm ² .

Timing of Intervention

For asymptomatic patients with AS, it appears to be relatively safe to delay surgery until symptoms develop, but outcomes vary widely. Moderate or severe valvular calcification along with a rapid increase in aortic-jet velocity identify patients with a very poor prognosis. They should be considered for early valve replacement rather than delaying until symptoms develop.

Echocardiography and exercise testing may identify asymptomatic patients who are likely to benefit from surgery. In a study of 58 asymptomatic patients, 21 had symptoms for the first time during exercise testing. Guidelines for AVR in patients with AS are shown in Table 15.1 .

Functional outcome after AVR for patients older than 80 years is excellent, operative risk is limited, and late survival rates are good. For patients with severe left ventricular dysfunction and a low transvalvular mean gradient, the operative mortality rate was increased, but AVR was associated with improved functional status. Postoperative survival rates were best for younger patients and those with larger prosthetic valves, whereas medium-term survival rates were correlated with improved postoperative functional class.

Anesthesia Considerations

The described pathophysiologic principles dictate anesthesia management based on avoidance of systemic hypotension, maintenance of sinus rhythm and an adequate intravascular volume, and awareness of the potential for myocardial ischemia ( Box 15.1 ). In the absence of CHF, adequate premedication may reduce the likelihood of undue preoperative excitement, tachycardia, and exacerbation of myocardial ischemia and the transvalvular pressure gradient. In patients with critical outflow tract obstruction, however, heavy premedication with an exaggerated venodilatory response can reduce the appropriately increased LVEDV (and LVEDP) needed to overcome the systolic pressure gradient. In these patients, the additional precaution of administering supplementary oxygen may obviate the possibility of a similarly pronounced response to the sedative effects of the premedicant.

Box 15.1

Aortic Stenosis

• 
  

  Maintain preload and diastolic filling

  
  
• 
  

  Maintain sinus rhythm

  
  
• 
  

  Maintain or increase afterload

  
  
• 
  

  Avoid myocardial depression

  
  
• 
  

  Avoid tachycardia, hypotension, and increased myocardial oxygen demand situations

Intraoperative monitoring should include a standard five-lead electrocardiographic (ECG) system, including a V ₅ lead, because of the LV’s vulnerability to ischemia. A practical constraint in terms of interpretation is that these patients usually exhibit ECG changes because of preoperative LVH. The associated ST-segment abnormalities (ie, strain pattern) may be indistinguishable from or very similar to those of myocardial ischemia, making the intraoperative interpretation difficult. Lead II and possibly an esophageal electrocardiogram should be readily obtainable for assessing the P-wave changes in the event of supraventricular arrhythmias.

Hemodynamic monitoring is controversial, and few prospective data are available on which to base an enlightened clinical decision. The central venous pressure (CVP) is a particularly poor estimate of left ventricular filling when left ventricular compliance is reduced. A normal CVP can significantly underestimate the LVEDP or pulmonary capillary wedge pressure (PCWP). The principal risks, although minimal, of using a pulmonary artery catheter (PAC) in the patient with AS are arrhythmia-induced hypotension and ischemia. Loss of synchronous atrial contraction or a supraventricular tachyarrhythmia can compromise diastolic filling of the poorly compliant LV, resulting in hypotension and the potential for rapid hemodynamic deterioration. The threat of catheter-induced arrhythmias is significant for the patient with AS. However, accepting a low-normal CVP as evidence of good ventricular function can lead to similarly catastrophic underfilling of the LV on the basis of insufficient replenishment of surgical blood loss. To some extent, even the PCWP can underestimate the LVEDP (and LVEDV) when ventricular compliance is markedly reduced. Placement of a PAC also allows measurement of CO, derived hemodynamic parameters, mixed venous oxygen saturation (Sv o ₂ ), and possible transvenous pacing.

Intraoperative fluid management should be aimed at maintaining appropriately increased left-sided filling pressures. This is one reason why many clinicians think that the PAC is worth its small arrhythmogenic risk. Keeping up with intravascular volume losses is particularly important in noncardiac surgery, in which the shorter duration of the operation may make inhalation or potentially vasodilating regional anesthesia preferable to a narcotic technique.

Patients with symptomatic AS are usually encountered only in the setting of cardiovascular surgery because of their ominous prognosis without AVR. Few studies have specifically addressed the response of these patients to the standard intravenous and inhalation induction agents; however, the responses to narcotic and nonnarcotic intravenous agents are apparently not dissimilar from those of patients with other forms of VHD. The principal benefit of a narcotic induction is assurance of an adequate depth of anesthesia during intubation, which reliably blunts potentially deleterious reflex sympathetic responses capable of precipitating tachycardia and ischemia.

Many clinicians also prefer a pure narcotic technique for maintenance. The negative inotropy of inhalation anesthetics is a theoretical disadvantage for a myocardium faced with the challenge of overcoming outflow tract obstruction. A more clinically relevant drawback may be the increased risk for arrhythmia-induced hypotension, particularly that associated with nodal rhythm and resultant loss of the atrium’s critical contribution to filling of the hypertrophied ventricle.

Occasionally, surgical stimulation elicits a hypertensive response despite the impedance posed by the stenotic valve and a seemingly adequate depth of narcotic anesthesia. In these patients, a judicious trial of low concentrations of an inhalation agent, used purely for control of hypertension, may prove efficacious. The ability to concurrently monitor CO is useful in this situation. The temptation to control intraoperative hypertension with vasodilators should be resisted in most cases. Given the risk for ischemia, nitroglycerin seems to be a particularly attractive drug. Its effectiveness in relieving subendocardial ischemia in patients with AS is controversial; however, there is always the risk for transient episodes of overshoot. The hypertrophied ventricle’s critical dependence on an adequate CPP may be unforgiving of even a momentary dip in the systemic arterial pressure.

Intraoperative hypotension, regardless of the primary cause, should be treated immediately and aggressively with a direct α-adrenergic agonist such as phenylephrine. The goal should be to immediately restore the CPP and then to address the underlying problem (eg, hypovolemia, arrhythmia). After the arterial pressure responds, treatment of the precipitating event should be equally aggressive, but rapid transfusion or cardioversion should not delay the administration of a direct-acting vasoconstrictor. Patients with severe AS in whom objective signs of myocardial ischemia persist despite restoration of the blood pressure should be treated extremely aggressively. This may mean the immediate use of an inotropic agent or accelerating the institution of cardiopulmonary bypass (CPB).