Progressive LVH (which may be attended by fibrosis) leads to impaired LV compliance, resulting in diastolic dysfunction that eventually leads to elevated mean left atrial pressures. Moreover, as the disease progresses, the increase in wall thickness may be insufficient to offset the rise in pressure with afterload mismatch, resulting in a rise in wall stress and decline in left ventricular systolic function. True depression of myocardial contractility also occurs in the presence of aortic stenosis for unknown reasons, again leading to a decline in ventricular systolic function. A decline in LV systolic function also leads to elevated mean left atrial pressure and dyspnea. Over time, increased mean left atrial pressure induces left atrial dilation, which may result in atrial arrhythmias. Impaired filling also limits LV preload at rest and with exercise, thereby limiting stroke volume which compounds diminished cardiac output attributable to the obstruction itself.
Augmented atrial contractile function also plays an important compensatory role in AS. Under conditions of abnormal LV compliance, the atrial booster pump function disproportionately contributes to filling of the stiff LV chamber at a lower mean left atrial pressure, thereby allowing better functional capacity. Conversely, loss of this “atrial kick” due to atrial fibrillation may lead to clinical decompensation characterized by pulmonary congestive symptoms (dyspnea) and impaired output (fatigue).
Prolonged LV Ejection Time: Compensation and Insight into Severity of Obstruction
The second mechanism by which the heart compensates for AS is through prolongation of the LVET. In patients with normal valvular function, aortic valve flow peaks in mid-systole. Under conditions of outflow obstruction, the LVET prolongs in order for the LV to more fully empty and generate forward SV. This compensatory mechanism is detectable on physical exam by the pattern and timing of the peak of the systolic murmur, and the behavior of S2.
Historical Features of Aortic Stenosis
Survival in aortic stenosis is nearly normal until symptoms develop (Fig. 2.1). Symptoms typically develop only with at least moderate AS. Once symptoms occur, the prognosis varies according to the clinical presentation. Classic symptoms associated with AS include exertional dyspnea, angina, and syncope. Patients with known AS may also complain of progressive fatigue and decreased exercise tolerance. As the severity of aortic stenosis progresses, heart failure will progress and left ventricular function will eventually be compromised. Other associated findings with AS include gastrointestinal bleeding and infective endocarditis [2].
Fig. 2.1
Average survival and symptom onset in patients with aortic stenosis (From Ross and Braunwald [1] with permission)
In an echocardiographic study of 498 patients with severe AS, Park et al. divided patients into four groups depending on the presentation; asymptomatic, syncope, dyspnea, and chest pain [3]. Despite similar valve area and gradient, symptomatic patients were older and had lower cardiac output, and a higher E/e’ ratio (an echocardiographic correlate of left atrial pressure). Moreover, patients with syncope displayed smaller LV dimension, stroke volume, cardiac output, left atrial volume index, and E/e’ ratio. Conversely, patient with dyspnea were found to have worst diastolic dysfunction with largest left atrial index and E/e’ ratio.
Angina
Angina in the setting of aortic stenosis is multifactorial, and differs between those with and without coronary artery disease. As LV thickness increases as a compensatory mechanism secondary to chronic pressure overload, there is a reduction of oxygen delivery due to compression of the coronary vessels. Additionally, with increased ventricular end-diastolic pressure, and impaired relaxation, diminished diastolic coronary filling occurs leading to decreased coronary supply to the myocardium.
On the other hand, the hypertrophied myocardium has an increased oxygen requirement contributing to the mismatch between oxygen supply and demand and causing angina. An alternate mechanism may be seen in patients with CAD, where coronary obstruction may lead to angina. In these patients, angina may be exacerbated by periods of decreased cardiac output as well as during exercise to the fixed obstruction of aortic stenosis again due to an imbalance between supply and demand, respectively.
Syncope
Syncope may be attributed to several etiologies. The predominant mechanism relates to reduced cerebral perfusion, usually occurring during exertion. In the presence of a fixed cardiac output, systemic arterial vasodilation results in reduced blood pressure. Malfunction of the baroreceptor mechanism and a vasodepressor response in the setting of severe AS can also lead to syncope. At rest, syncope may result from multiple transient mechanisms. Transient ventricular fibrillation may cause reduced perfusion. Atrial fibrillation may impair LV filling, leading to a reduced cardiac output and subsequent decrease in cardiac output. The extension of calcification into the conduction system may cause transient AV block leading to syncope.
Dyspnea
Exertional dyspnea may be caused by several factors. First, a rise in LV end-diastolic pressure may result from LV diastolic dysfunction, leading to pulmonary vascular congestion. Second, an inability to augment cardiac output in the setting of a fixed obstruction may lead to exertional symptoms. Patients may develop heart failure symptoms including orthopnea, paroxysmal nocturnal dyspnea, and pulmonary edema as the AS severity increases, leading to pulmonary venous hypertension.
Gastrointestinal Bleeding
Less commonly, GI bleeding develops with severe AS, and is related to AV malformations or angiodysplasia, a condition known as “Heyde’s syndrome.” Bleeding results from an acquired type IIA von Willebrand’s syndrome, caused by a deficiency of high-molecular-weight multimers of von Willebrand factor. These abnormalities may be correctable with AVR.
Infective Endocarditis
Infective endocarditis is a complication of aortic stenosis, generally more prevalent in young rather than older individuals. These patients may develop cerebral emboli, TIAs, or loss of vision due to calcific embolic occlusion of the central retinal artery.
Frailty Assessment
Assessment of frailty has emerged as a tool to help guide the candidacy of patients for surgical versus transaortic valve replacement. This assessment includes dominant hand grip strength (in kg), 15-ft walk duration (seconds), Katz activities of daily living (which includes degree of independence in bathing, dressing, toileting, continence, and feeding), independence in ambulation and the serum albumin (g/dl).
Physical Exam
The physical signs of AS follow from the pathophysiologic mechanisms previously described. The cardinal features of the AS clinical examination include changes in the pulse waveform, precordial examination, and auscultation.
Pulse Waveform
AS inscribes a classic pulse wave abnormality which is palpable in the brachial arteries, but best appreciated in the carotid artery. As valve obstruction progresses from mild to severe, the carotid pulse demonstrates progressive alterations in upstroke, peak, and amplitude. In severe AS, the expected carotid waveform is characterized by a slow-rising, late peaking, low-amplitude pulse (Fig. 2.2). In Latin, this is described as “pulsus parvus et tardus,” which translates to “slow and late.” Caution must be employed in ascribing a diminutive carotid pulse to AS alone, for severe depression of SV due to cardiomyopathy may mimic the pulse of AS (but not the murmur). Conversely, in those with very stiff arterial systems, which amplify pulses, severe AS may be present despite a seemingly normal carotid waveform. This should be kept in mind in elderly patients with symptoms and a systolic murmur consistent with AS. Such patients often have inelastic arterial vessels due to calcification, and may have normal or even increased carotid upstrokes due to increased reflected waves in the aorta. Similarly, patients with systemic hypertension or concurrent aortic insufficiency may also have a normal or increased carotid impulse. In the face of severe AS, echocardiography successfully adjudicates such cases.
