Valvular Heart Disease: Replacement and Repair

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Chapter 14 Valvular Heart Disease: Replacement and Repair

Valve surgery is very different from coronary artery bypass grafting (CABG). Over the natural history of valvular heart disease (VHD), the physiologic characteristics change markedly and, in the operating room, physiologic and hemodynamic conditions are quite variable and are readily influenced by anesthetic interventions. For some types of valve lesions it can be relatively difficult to predict preoperatively how the heart will respond to the altered loading conditions associated with valve repair or replacement.

It is essential to understand the natural history of each of the major adult-acquired valve defects and how the pathophysiologic conditions evolve. Surgical decision making regarding valve repair or replacement must also be understood, because a valve operated on at the appropriate stage of its natural history will have a good and more predictable outcome than one operated on at a late stage, when the perioperative result can be quite poor. Because pathophysiologic conditions are dynamic and differ significantly among valve lesions, understanding the physiology and natural history of individual valve defects is the foundation of developing an anesthetic plan that includes various requirements for pacing rate and rhythm, use of inotropes (or negative inotropes), and use of vasodilators or vasoconstrictors to alter loading conditions.

Although valvular lesions impose various physiologic changes, all VHD is characterized by abnormalities of ventricular loading. The status of the ventricle changes over time as ventricular function and the valvular defect are influenced by the progression of volume or pressure overload. The clinical status of patients with VHD therefore can be complex and dynamic. It is possible to have clinical decompensation in the context of normal ventricular contractility or ventricular decompensation with normal ejection indices. The altered loading conditions characteristic of VHD may result in a divergence between the function of the heart as a systolic pump and the intrinsic inotropic state of the myocardium. This divergence between cardiac performance and inotropy occurs as a result of compensatory physiologic mechanisms specific to each of the ventricular loading abnormalities.

AORTIC STENOSIS

Clinical Features and Natural History

Aortic stenosis is the most common cardiac valve lesion in the United States. One to 2 percent of the population is born with a bicuspid aortic valve, which is prone to stenosis with aging. Calcific aortic stenosis 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. There is clinical evidence of an atherosclerotic hypothesis for the cellular mechanism of aortic valve stenosis. There is a clear association between clinical risk factors for atherosclerosis and the development of aortic stenosis: elevated lipoprotein levels, increased low-density lipoprotein (LDL) cholesterol, cigarette smoking, hypertension, diabetes mellitus, increased serum calcium and creatinine levels, and male gender.1 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 rate of progression is on average a decrease in aortic valve area (AVA) of 0.1 cm2/yr, and the peak instantaneous gradient increases by 10 mmHg/yr. The rate of progression of aortic stenosis in men older than age 60 years is faster than in women, and it is faster in women older than age 75 years 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 aortic stenosis is associated with a life expectancy of only 2 to 5 years. There is evidence that patients with moderate aortic stenosis (i.e., valve areas of 0.7 to 1.2 cm2) are also at increased risk for the development of 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 aortic stenosis; and about one half of symptomatic patients are found to have anatomically significant CAD.

The preoperative assessment of aortic stenosis 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 presence of aortic stenosis. This maximal velocity (v) is then inserted in the modified Bernoulli equation to determine the pressure gradient (PG) between the left ventricle and the aorta:

image

Pathophysiology

The normal AVA is 2.6 to 3.5 cm2, with hemodynamically significant obstruction usually occurring at cross-sectional valve areas of 1 cm2 or less. Generally accepted criteria for critical outflow obstruction include a systolic pressure gradient greater than 50 mmHg, with a normal cardiac output and an AVA of less than 0.4 cm2. In view of the ominous natural history of severe aortic stenosis (AVA < 0.7 cm2), symptomatic patients with this degree of aortic stenosis are generally referred for immediate aortic valve replacement. 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:

image

An obvious corollary of the previously described relationship is that “minimal” pressure gradients may actually reflect critical degrees of outflow obstruction when the cardiac output is significantly reduced (i.e., the generation of a pressure gradient requires some finite amount of flow). Clinicians have long recognized this phenomenon as a “paradoxical” decline in the intensity of the murmur (i.e., minimal transvalvular flow) as the aortic stenosis worsens.

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

image

in which P is the intraventricular pressure, R is the inner radius, and h is the wall thickness.

This elevation of wall tension is believed 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 this LV hypertrophy 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.

Figure 14-1 shows a typical pressure-volume loop for a patient with aortic stenosis. Two differences from the normal curve are immediately apparent. First, the peak pressure generated during systole is much higher because of the high transvalvular pressure gradient. Second, the slope of the diastolic limb is steeper, reflecting the reduced left ventricular (LV) diastolic compliance that is associated with the increase in chamber thickness. Clinically, this means that small changes in diastolic volume produce relatively large increases in ventricular filling pressure.

image

Figure 14-1 Pressure-volume loop in aortic stenosis. LV = left ventricular.

(From Jackson JM, Thomas SJ, Lowenstein E: Anesthetic management of patients with valvular heart disease. Semin Anesth 1:239, 1982.)

This increased chamber stiffness places a premium on the contribution of atrial systole to ventricular filling, which in patients with aortic stenosis may account for up to 40% of the LV end-diastolic volume (LVEDV), rather than the 15% to 20% characteristic of the normal left ventricle. Echocardiographic and radionuclide studies have documented that diastolic filling and ventricular relaxation are abnormal in patients with hypertrophy from a variety of causes, with significant prolongation of the isovolumic relaxation period being 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 higher mean left atrial (LA) pressure is necessary to distend the left ventricle 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). It is likely that use of preload reserve and adequate LV hypertrophy are 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 relationship. LV hypertrophy can be viewed as a compensatory physiologic response; however, severe afterload stress and proportionately massive LV hypertrophy could decrease subendocardial perfusion and superimpose a component of ischemic contractile dysfunction.

In aortic stenosis, signs and symptoms of CHF usually develop when preload reserve is exhausted, not because contractility is intrinsically or permanently impaired. This contrasts to mitral and aortic regurgitation, 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 elevates basal myocardial oxygen demand (image). The other major determinants of overall image are heart rate, contractility, and, most important, wall tension. Increases in the latter 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 elevated 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 aortic stenosis, clinical data also support increased image as important in the genesis of myocardial ischemia.

On the supply side, the higher LV 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 aortic stenosis, LV dysfunction, and low transvalvular gradient suffers a high operative mortality rate and poor prognosis.2 It is difficult to accurately assess the AVA in this low-flow, low-gradient aortic stenosis 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 aortic stenosis 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 aortic stenosis may benefit from valve replacement despite the increased operative risk associated with the low-flow, low-gradient hemodynamic state. Guidelines from the American College of Cardiology (ACC) and American Heart Association (AHA) call for a dobutamine echocardiography evaluation to distinguish patients with fixed anatomic aortic stenosis from those with flow-dependent aortic stenosis with LV dysfunction. Low-flow, low-gradient aortic stenosis is defined for a mean gradient of less than 30 mmHg and a calculated AVA less than 1.0 cm2.

Timing of Intervention

In asymptomatic patients with aortic stenosis, it appears to be relatively safe to delay surgery until symptoms develop, but outcomes vary widely. The presence of moderate or severe valvular calcification along with a rapid increase in aortic-jet velocity identifies patients with a very poor prognosis. These patients 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.3 In a study of 58 asymptomatic patients, 21 had symptoms for the first time during exercise testing. Guidelines for AVR in patients with aortic stenosis are shown in Table 14-1.

Table 14-1 Recommendations for the Use of Aortic Valve Replacement in Patients with Aortic Stenosis

Replacement Indicated

Replacement Possibly Indicated

Adapted from the American Heart Association web site (www.americanheart.org).

Functional outcome after aortic valve replacement in patients older than 80 years is excellent, operative risk is limited, and late survival rates are good. In patients with severe LV dysfunction and low transvalvular mean gradient, operative mortality is increased, but aortic valve replacement was associated with improved functional status. Postoperative survival was best in younger patients and with larger prosthetic valves, whereas medium-term survival was related to improved postoperative functional class.

Anesthetic Considerations

The foregoing pathophysiologic principles dictate that anesthetic management be based on the avoidance of systemic hypotension, maintenance of sinus rhythm and an adequate intravascular volume, and awareness of the potential for myocardial ischemia (Box 14-1). In the absence of CHF, adequate premedication may reduce the likelihood of undue preoperative excitement, tachycardia, and the resultant potential for exacerbating myocardial ischemia and the transvalvular pressure gradient. In patients with truly critical outflow tract obstruction, however, heavy premedication with an exaggerated venodilatory response can reduce the appropriately elevated LVEDV (and LVEDP) needed to overcome the systolic pressure gradient. In these patients in particular, the additional precaution of administering supplementary oxygen may provide worthwhile insurance.

BOX 14-1 Aortic Stenosis

Preload: Increased
Afterload: Increased
Goal: Sinus rhythm
Avoid: Hypotension, tachycardia, bradycardia

Intraoperative monitoring should include a standard five-lead ECG system, including a V5 lead, because of the left ventricle’s vulnerability to ischemia. A practical constraint in terms of interpretation is that these patients usually exhibit ECG changes because of preoperative LV hypertrophy. The associated ST-segment abnormalities (i.e., strain pattern) may be indistinguishable from or at least very similar to those of myocardial ischemia, making the intraoperative interpretation difficult. Lead II 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 LV filling when LV 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 (PA) catheter in the patient with aortic stenosis are arrhythmia-induced hypotension and ischemia. Loss of synchronous atrial contraction or a supraventricular tachyarrhythmia can compromise diastolic filling of the poorly compliant left ventricle, resulting in hypotension and the potential for rapid hemodynamic deterioration. The threat of catheter-induced arrhythmias is significant for the patient with aortic stenosis. However, accepting a low-normal CVP as evidence of good ventricular function can lead to similarly catastrophic underfilling of the left ventricle 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 PA catheter also allows for measurement of cardiac output, derived hemodynamic parameters, mixed venous oxygen saturation, and possible transvenous pacing.

Intraoperative fluid management should be aimed at maintaining appropriately elevated left-sided filling pressures. This is one reason why many clinicians believe that the PA catheter is worth its small arrhythmogenic risk. Keeping up with intravascular volume losses is particularly important in noncardiovascular surgery.

Patients with symptomatic aortic stenosis are usually encountered only in the setting of cardiovascular surgery because of their ominous prognosis without aortic valve replacement. Few studies have specifically addressed the response of these patients to the standard intravenous and inhalation induction agents; however, the responses to narcotic and non-narcotic intravenous agents are apparently not dissimilar from those of patients with other forms of VHD. The principal benefit of a narcotic induction is the 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 the 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 of 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 such 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 cardiac output is useful in this situation. The temptation to control intraoperative hypertension with vasodilators should be resisted in most cases. Given the risk of ischemia, nitroglycerin seems to be a particularly attractive drug. Its effectiveness in relieving subendocardial ischemia in patients with aortic stenosis is controversial; however, there is always the risk of even transient episodes of “overshoot.” The hypertrophied ventricle’s critical dependence on an adequate CPP may be very 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 (e.g., 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 aortic stenosis 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 simply accelerating the institution of cardiopulmonary bypass (CPB).

HYPERTROPHIC CARDIOMYOPATHY

Hypertrophic cardiomyopathy (HCM, formerly known as hypertrophic obstructive cardiomyopathy) is a relatively common genetic malformation of the heart with a prevalence of approximately 1 in 500. The hypertrophy initially develops in the septum and extends to the free walls, often giving a picture of concentric hypertrophy. Asymmetric septal hypertrophy leads to a variable pressure gradient between the apical LV chamber and the LV outflow tract (LVOT). The LVOT obstruction leads to increases in LV pressure, which fuels a vicious cycle of further hypertrophy and increased LVOT obstruction.4 Various treatment modalities include β-adrenoceptor antagonists, calcium channel blockers, and surgical myectomy of the septum. For more than 40 years, the traditional standard treatment has been the ventricular septal myotomy-myomectomy of Morrow, in which a small amount of muscle from the subaortic septum is resected. Two new treatment modalities have gained popularity in recent years: dual-chamber pacing and septal reduction (ablation) therapy with ethanol.

Pathophysiology

In HCM, the principal pathophysiologic abnormality is myocardial hypertrophy. The hypertrophy is a primary event in these patients and occurs independently of outflow tract obstruction. Unlike aortic stenosis, the hypertrophy begets the pressure gradient, not the other way around. Histologically, the hypertrophy consists of myocardial fiber disarray, and, anatomically, there is usually disproportionate enlargement of the interventricular septum.

A consensus exists that the disease is characterized by a wide spectrum of the severity of obstruction. It is totally absent in some patients, may be variable in others, or may be critically severe. Its most distinctive qualities are its dynamic nature (depending on contractile state and loading conditions), its timing (begins early, peaks variably), and its subaortic location. Subaortic obstruction arises from the hypertrophied septum’s encroachment on the systolic outflow tract, which is bounded anteriorly by the interventricular septum and posteriorly by the anterior leaflet of the mitral valve. In most patients with obstruction, exaggerated anterior (i.e., toward the septum) motion of the anterior mitral valve leaflet during systole accentuates the obstruction. The cause of this systolic anterior motion (SAM) is unclear. One possibility is that the mitral valve is pulled toward the septum by contraction of the papillary muscles, whose orientation is abnormal because of the hypertrophic process. Another theory is that vigorous contraction of the hypertrophied septum results in rapid acceleration of the blood through a simultaneously narrowed outflow tract. This could generate hydraulic forces consistent with a Venturi effect whereby the anterior leaflet of the mitral valve would be drawn close to or within actual contact with the interventricular septum (Fig. 14-2). This means that after the obstruction is triggered the mitral valve leaflet is forced against the septum by the pressure difference across the orifice. However, the pressure difference further decreases orifice size and further increases the pressure difference in a time-dependent amplifying feedback loop. This analysis is also consistent with observations that the measured gradient is directly correlated with the duration of mitral-septal contact. There appears to be good correlation between the degree of SAM and the magnitude of the pressure gradient. The SAM-septal contact also underlies the severe subaortic obstruction characteristic of HCM of the elderly, although the narrowing is usually more severe and the contribution of septal movement toward the mitral valve is usually greater.

In addition to SAM, approximately two thirds of patients exhibit a constellation of structural malformations of the mitral valve. These malformations include increased leaflet area and elongation of the leaflets or anomalous papillary muscle insertion directly into the anterior mitral valve leaflet. HCM is not a disease process confined to cardiac muscle alone, because these anatomic abnormalities of the mitral valve are unlikely to be acquired or secondary to mechanical factors.

Three basic mechanisms—increased contractility, decreased afterload, and decreased preload—exacerbate the degree of SAM-septal contact and produce the dynamic obstruction characteristic of patients with HCM. The common pathway is a reduction in ventricular volume (actively by increased contractility, directly or reflexly in response to vasodilation, or passively by reduced preload), which increases the proximity of the anterior mitral valve leaflet to the hypertrophied septum. Factors that usually impair contractile performance, such as myocardial depression, systemic vasoconstriction, and ventricular overdistention, characteristically improve systolic function in patients with HCM and outflow tract obstruction. Diagnostically, these paradoxes are exploited by quantifying the degree of subaortic obstruction after isoproterenol (e.g., increased inotropy, tachycardia, and decreased volume) and the Valsalva maneuver (e.g., decreased venous return and ventricular volume), both of which reliably elicit increases in the pressure gradient. In the operating room, catheter-induced ectopy or premature ventricular contractions resulting from cardiac manipulation may also transiently exacerbate the gradient by increased inotropy from postextrasystolic potentiation. Therapeutically, volume loading, myocardial depression, and vasoconstriction should minimize obstruction and augment forward flow.

Poor diastolic compliance is the most clinically apparent manifestation of the relaxation abnormalities. LV filling pressures are markedly elevated despite enhanced systolic ejection and the normal or subnormal end-diastolic volume. This reduced ventricular volume reemphasizes the pivotal role played by the hypertrophied but intrinsically depressed myocardium. Reductions in afterload, mediated by hypertrophy, support the ventricle’s systolic performance, resulting in increased emptying and a small diastolic volume. However, hypertrophy also impairs relaxation, resulting in poor diastolic compliance and an elevated ventricular filling pressure. The key point is that the high filling pressure does not reflect distention of a failing ventricle, even though stress-volume relationships suggest that its contractility is intrinsically depressed. This disease is characterized by systolic and diastolic dysfunction.

As in patients with valvular aortic stenosis, relatively high filling pressures reflect the LVEDV (i.e., degree of preload reserve) needed to overcome the outflow obstruction. Intervention with vasodilators is therefore inappropriate. The poor ventricular compliance also means that patients with HCM depend on a large intravascular volume and the maintenance of sinus rhythm for adequate diastolic filling. The atrial contribution to ventricular filling is even more important in HCM than in valvular aortic stenosis, and it may approach 75% of total SV.

Another similarity between HCM and valvular aortic stenosis is that the combination of myocardial hypertrophy, with or without LVOT obstruction, may precipitate imbalances in the myocardial oxygen supply and demand relationship. Angina-like discomfort is one of the classic symptoms of patients with HCM, and its pathogenesis has been attributed to increases in image, specifically the increased overall muscle mass and the high systolic wall tension generated by the ventricle’s ejection against the dynamic subaortic obstruction. However, as in patients with aortic stenosis, there is also evidence of a compromise in myocardial oxygen supply.

Hemodynamic derangements peculiar to the disease may aggravate the ventricle’s anatomic vulnerability to ischemia. The increased LVEDP for any LVEDV (i.e., poor compliance) inevitably narrows the diastolic CPP gradient. This may precipitate subendocardial ischemia in some patients with HCM, particularly those faced with the increased oxygen demand of overcoming late-systolic obstruction. There is evidence that hypertrophy-induced myocardial ischemia may underlie the diastolic dysfunction characteristic of HCM. As in patients with valvular aortic stenosis, ischemia-induced abnormalities of diastolic calcium sequestration may further exacerbate relaxation abnormalities, initiating a vicious cycle.

β-Blockers and calcium channel blockers form the basis of medical therapy for HCM. β-Blockade is most useful for preventing sympathetically-mediated increases in the subaortic gradient and for the prevention of tachyarrhythmias, which can also exacerbate outflow obstruction. Disopyramide has also been used to reduce contractility and for its antiarrhythmic properties. Calcium channel blockers often prove clinically effective in patients with HCM regardless of the presence or absence of systolic obstruction. The mechanism of action involves improvement in diastolic relaxation, allowing an increase in LVEDV at a relatively lower LVEDP. The negative inotropy may attenuate the subaortic pressure gradient, although in selected patients, the gradient may worsen because of pronounced and unpredictable degrees of vasodilation.

Surgery—a septal myotomy or partial myomectomy by the aortic approach—is reserved for those patients who remain symptomatic despite maximal pharmacologic therapy. In a long-term retrospective study, the cumulative survival rate was significantly better in surgically than in pharmacologically treated patients. However, it is quite likely that pharmacologic therapy may be more appropriate for the patient with a dynamic component to their degree of subaortic obstruction. Further improvement in the clinical outcome of surgically treated patients may be achieved with the addition of verapamil, presumably reflecting a two-pronged attack on the systolic (myomectomy) and diastolic (verapamil) components of the disease. Enthusiasm continues for the therapeutic use of dual-chamber pacing in this disease, with some patients demonstrating reductions in their subaortic gradients. It is not an option for patients in atrial fibrillation.