Hensley's Practical Approach to Cardiothoracic Anesthesia

16. Anesthetic Considerations for Surgical Myectomy in Patients With Hypertrophic Cardiomyopathy 347

B. Pathophysiology Patients with HCM have diastolic dysfunction driven by reduced LV compliance due to LV hypertrophy and fibrosis. This results in increased left atrial and pulmonary venous pressures, which contribute to dyspnea on exertion and limited aerobic capacity. LVOT obstruction lengthens ejection time and reduces LV stroke volume. With worsening LVOT obstruction and diastolic dysfunction, LV filling becomes more dependent on atrial contraction. Atrial ar rhythmias, especially atrial fibrillation (AF), may cause an acute decrease in cardiac output and exacerbation of symptoms. Dynamic LVOT obstruction is an important cause of symptoms in ∼ 70% of patients with HCM. Accelerated blood flow near the hypertrophied basal septum leads to SAM of the mitral valve. Thus, leaflet coaptation is reduced, producing variable degrees of MR characteristically directed posteriorly (Figure 16.3A, B). An anteriorly directed color jet on Doppler echocar diography should raise the possibility of primary mitral valve diseases, such as a flail or prolaps ing segment. MR is an important pathophysiologic component of HCM that contributes to the symptoms of dyspnea and fatigue. C. Clinical Presentation and Diagnosis The clinical course of HCM is variable, but the onset of symptoms (dyspnea, fatigue, chest pain, syncope) often correlates with the development of LVOT obstruction, an independent predictor of heart failure, stroke and death. It is uncertain why initially asymptomatic patients without obstruction may develop obstruction in adulthood. AF occurs in nearly one in five patients with HCM, and the onset of AF can precipitate symptoms and predispose to sys temic thromboembolism. 14 Infective endocarditis rarely occurs with HCM, with a reported incidence of 1.4 cases per 1,000 person-years. 15 1. Echocardiography Two-dimensional and Doppler transthoracic echocardiography (TTE) is the primary imaging modality for diagnosing HCM, providing information on ventricular morphol ogy, hemodynamics, and valve function. 15 mm septal wall thickness is a commonly used threshold for the diagnosis of HCM. In 5-10% of patients, LV wall thickness is massively increased, measuring > 30 mm (up to 50 mm). Morphology of the septum varies, with the most common being the sigmoid configuration (Figure 16.1B). On continuous-wave Dop pler echocardiography, LVOT obstruction is seen as a high-velocity, late peaking, “dagger shaped” signal (Figure 16.3C). In patients with a low velocity at rest ( < 3 m/s), Valsalva maneuver (decreases venous return), squat-to-stand (decreases preload and afterload), in halation of amyl nitrite (vasodilator), exercise or isoproterenol administration (vasodilation and tachycardia) may unveil latent obstruction. The presence and severity of MR can be determined by Doppler color-flow imaging. It is essential to differentiate the true outflow tract velocity from the MR jet, which can be challenging given spatial proximity and similar timing. MR that results from SAM is eccentric and classically directed posterolaterally dur ing late systole (Figure 16.3B). A centrally or anteriorly directed jet should raise suspicion of a primary leaflet abnormality. Outpatient preoperative transesophageal echocardiography (TEE) is unnecessary in most patients, but TEE is important in assessing the extent of hy pertrophy and the results of myectomy in the operating room. 2. Cardiac magnetic resonance imaging Cardiac MRI is synergistic with echocardiographic imaging. Cardiac MRI helps identify regions of LV hypertrophy not easily recognized by TTE, like the anterolateral free wall and the apex. A unique feature of cardiac MRI is the ability to detect the presence, distribution, and severity of late gadolinium enhancement (myocardial fibrosis), which can play a role in sudden cardiac death risk stratification. 3. Cardiac catheterization In current practice, cardiac catheterization is rarely necessary to diagnose HCM. Coronary angiography is indicated for patients with symptoms of angina and in patients at risk for coronary artery disease (CAD) who undergo myectomy. A hemodynamic study with iso proterenol provocation can help identify patients with occult labile obstruction that cannot be elicited during echocardiography.

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