Valve Surgery: Aortic/Pulmonary



Valve Surgery: Aortic/Pulmonary


Douglas R. Johnston



I. INTRODUCTION

The aortic and pulmonic valves share similar structure and functional anatomy. Both prevent regurgitation of blood into the ventricle during diastole, and in normal function provide an unimpeded flow of blood from the ventricular outflow tract to the aorta or pulmonary artery (PA). Both are trileaflet in normal configuration, and rely on adequate coaptation of three similarly sized semilunar valve cusps for valve competence. Normal valve function depends on the shape and flexibility of the leaflets, size and orientation of the annulus and sinuses, and the orientation of the commissures between leaflets. Surgical therapy for the aortic and pulmonic valves may be appropriately categorized based on the indication—stenosis versus regurgitation, whether repair or replacement is feasible, whether any adjunctive procedures are required, and the surgical approach. Although the valves share similar anatomy, the pulmonic valve does not share a fibrous skeleton with adjacent valves as does the aortic valve (Fig. 15.1). This has important considerations for evaluation of sizing and concomitant valve procedures.






FIGURE 15.1 Relations of the aortic and pulmonic valves. Note the aortic, mitral, and tricuspid valves share a fibrous skeleton, whereas the pulmonic valve annulus shares a muscular connection with the other three valves.


II. AORTIC VALVE INDICATIONS

A. Aortic stenosis

The most common indication for operation on the aortic valve is aortic stenosis (AS), and the most common cause of AS is degenerative valve calcification. Although small amounts of collagen disruption and calcific deposits on aortic valves are common in people without clinically evident aortic valve disease, significant aortic valve calcification is rare before the age of 30 years in patients with tricuspid aortic valves. Accelerated calcification may occur in patients with bicuspid or unicuspid valves. The histopathology of calcific AS is similar to that of atherosclerotic coronary disease, and shares similar risk factors at least in patients with tricuspid valve anatomy. This has prompted research into the potential for lipid-lowering therapy to alter the progression of calcific AS.

Rheumatic disease of the aortic valve is characterized by an early inflammatory phase consisting of edema, lymphocyte infiltration, and neovascularization. This is followed by a proliferative phase in which leaflets become thickened and retracted, with rolled edges and commissural fusion. Valve leaflets may become severely calcified, although annular calcification is rare.

Congenital alteration in the number and orientation of the aortic valve leaflets may result in bicuspid (Fig. 15.2), unicuspid, or quadricuspid morphology. Of these, bicuspid valves are most common, and are present in approximately 2% of the general population, with a male to female ratio of 2:1. They are more common in first-degree relatives with bicuspid valves.

The calcification present in stenotic aortic valves may be limited to the leaflets, or may extend into the annulus, interventricular septum, or anterior leaflet of the mitral valve. Calcium may exist primarily as surface deposits or extend deep into the surrounding tissues. Even mature lamellar bone formation may occur in very calcified aortic valves. Removal of invasive calcium requires care to avoid injuring surrounding structures, and when injury occurs reconstruction of the annulus with autologous pericardium or other material may be necessary. The distribution of calcification has implications for surgical approach and choice of valve prosthesis. Figure 15.3 shows the computed tomographic (CT) appearance of a heavily calcified unicuspid valve, with calcium invading into the ventricular septum.

B. Aortic insufficiency

Derangement in the integrity or morphology of the annulus, leaflets, or sinotubular junction may result in aortic insufficiency (AI). Dilatation of the aortic root is the most common cause of AI in North America. Dilatation of the sinotubular junction,
with relative sparing of the leaflets and annulus, is associated with atherosclerotic ascending aortic aneurysm and AI in older patients.






FIGURE 15.2 Bicuspid aortic valve morphology. A: The common finding of fusion of the left and right coronary cusps, separated by a raphe. B: The uncommon “true” bicuspid valve with equal length cusps and only two commissures.






FIGURE 15.3 Computed tomographyic appearance of severe aortic valve calcification, in this case associated with a unicuspid valve, invasive into the ventricular septum.

Patients with degenerative calcific aortic valve disease often have a mixture of AS and AI; as leaflets become stiffened, fixed, and retracted, coaptation is impaired. In a similar fashion, rheumatic heart disease leads to AI through fixation of the leaflets.

Congenital bicuspid valve disease may produce insufficiency via fibrosis and calcification in a manner similar to a degenerated calcified tricuspid valve. In addition, distortion and stretching of the leaflets may result in malcoaptation, in which one leaflet overrides another leading to an eccentrically directed regurgitant jet.

Aortic dissection often produces severe regurgitation in an otherwise anatomically normal valve when the commissures and associated intima are detached from the aortic adventitia and prolapse inward.

Bacterial endocarditis may account for 10% or more of AI. Healed endocarditis lesions may present as an isolated leaflet perforation in an otherwise normal-appearing valve.

Less common etiologies of regurgitation include injury to the valve from blunt or penetrating chest trauma, iatrogenic injury to the leaflets during catheter procedures, and suture injuries during mitral valve surgery.

C. Pulmonic stenosis

Pulmonic stenosis in developed countries occurs most commonly as a result of congenital heart defects, prior surgical or endovascular intervention, or rarely rheumatic disease.


D. Pulmonic insufficiency

Pulmonary insufficiency occurs mostly as a result of annular enlargement, congenital malformations, prior intervention, or pulmonary hypertension.

III. EVALUATION

A. Aortic stenosis

Noninvasive measurement of aortic valve gradients extrapolated from aortic jet velocity on two-dimensional echocardiography has been shown to be the most reproducible and accurate method of grading AS, such that catheter measurement of aortic valve gradients is uncommon except in select cases (see low-gradient AS, later). Recent reports have suggested that gated cardiac CT may provide more precise measurements of aortic valve morphology and area than echocardiography. Evaluation of AS by gated CT may be of particular value in patients being considered for transcatheter aortic valve implantation, because it provides data on the anatomic relationship between leaflet calcification and the coronary ostia (Fig. 15.4). Cardiac CT, cardiac magnetic resonance imaging (MRI), and three-dimensional (3D) echocardiography all have the potential to provide direct (anatomic) measurement of aortic valve area (AVA). However, they may all underestimate the functional severity of AS, which depends on the dynamic interaction between the ejecting ventricle, outflow tract, and aortic
valve orifice. Thus, a decision on timing for surgery in AS must take into account the etiology, chronicity, the condition of the ventricle, degree of concomitant AI, and the presence of associated valve or coronary lesions.






FIGURE 15.4 Computed tomographic evaluation of aortic valve calcium location and extent in relation to coronary ostia.

1. AS grading table















Grade


AVA (cm2)


Mild


>1.5


Moderate


1-1.5


Severe


<1


2. Natural history

Much has been learned about the natural history of AS since the landmark study published by Ross and Braunwald in 1968. The classic symptom triad of angina, syncope, and dyspnea has since served as a hallmark for the evaluation of patients with AS. These investigators reported a survival of 3 years with angina and syncope, 2 years with dyspnea, and 1.5 years with heart failure. These dismal survival statistics in untreated symptomatic AS, corroborated by a number of subsequent studies, have since driven recommendations for aggressive operative therapy in patients with symptomatic severe AS. The appropriate treatment of asymptomatic patients is more controversial, however. Estimates of the rate of sudden death in asymptomatic AS are generally in the range of 1% per year. In addition, one-third of asymptomatic patients with severe AS will become symptomatic within 2 years. Two-thirds of patients will proceed to aortic valve replacement (AVR) or cardiac death within 5 years of diagnosis. A significant proportion of “asymptomatic” patients have markers of more advanced disease such as severe left ventricular hypertrophy (LVH) or decreased ejection fraction (EF), and many have a positive stress test. Exercise stress testing is likely underutilized in this population. Asymptomatic patients with a positive stress test have a prognosis similar to that of symptomatic patients and should be offered early surgery.

3. Medical treatment

Given the pathologic link between AS and coronary disease, there has been enthusiasm for using lipid-lowering drugs to slow the progression of leaflet disease. Despite high expectations, results have been mixed. The SEAS study, a randomized controlled trial of 1,873 patients with mild-moderate asymptomatic AS receiving simvastatin and ezetimibe or placebo, failed to show any reduction in AS-related events in the lipid-lowering arm. Efforts to find a medical treatment that is effective in reducing severity or progression of AS are ongoing. At present, medical treatment in AS is aimed at hemodynamic stabilization in asymptomatic patients and control of comorbid conditions.

Hypertension control may help alleviate LVH; however, overmedication raises the risk of decompensation in patients who are dependent on already reduced diastolic pressure for coronary perfusion in a hypertensive ventricle. Angiotensin-converting enzyme inhibitors have been shown to provide short-term benefit in hypertensive patients with AS. Atrial fibrillation may have a significant adverse impact in patients with AS who are particularly dependent on atrial contraction for diastolic filling; therefore, cardioversion and rhythm control are important considerations both before and after AVR. Endocarditis prophylaxis is indicated in all patients with AS.

4. Timing of intervention for AS

Published guidelines support AVR in symptomatic patients with severe AS. Patients without clear symptoms, or with symptoms but equivocal echo findings present
a clinical challenge. In these patients, the surgeon must weigh the individual risks of AVR with watchful waiting, which may include a potential for sudden cardiac death, as well as ongoing LV remodeling. LVH as measured on routine electrocardiogram is an independent predictor of symptom development in severe AS, albeit with low sensitivity. Development of symptoms during exercise stress testing is a predictor of symptom development within 12 months. Onset of symptoms in a patient with severe AS who has been previously asymptomatic portends a poor prognosis; however, patients often subconsciously reduce their activity levels and may therefore fail to report “symptoms.” Exercise testing may be particularly valuable in these patients.

Iung et al. estimate that at least one-third of patients with symptomatic, severe isolated AS do not undergo surgical repair. These data have been confirmed in multiple studies; however, the number of untreated patients may decrease with wider application of transcatheter aortic valve replacement (TAVR). The fact that even elderly patients benefit from AVR in almost all cases and a large number of “asymptomatic” patients have markers for worse surgical outcome argues for an aggressive approach to early surgery.

5. Low gradient-low flow AS

In this subset of patients with anatomic AS who do not demonstrate increased gradients at rest (<40 mm Hg), operative mortality may be as high as 18%, with a 3-year survival of only 57%. Essential to the work-up of these patients is determining whether the etiology of ventricular dysfunction is secondary to ischemic scar or cardiomyopathy, or excessive afterload. Low-dose dobutamine stress echocardiography serves to diagnose those patients with adequate contractile reserve (CR), defined as a ≥20% increase in stroke volume from baseline. Absence of adequate CR is predictive of perioperative mortality. The TOPAS multicenter study of low flow-low gradient AS suggests that patients with this syndrome may benefit from AVR, with AVA as high as 1.2 cm2, suggesting that even moderate AS may be poorly tolerated in the setting of lower LV CR. Most significant risk factors for poor outcome were impaired functional capacity as measured by Duke activity status index or 6-minute walk test distance, more severe stenosis, and reduced peak stress left ventricular ejection fraction (LVEF). However, both functional status and LVEF improved in patients surviving AVR. Properly selected patients fare better with AVR than with medical therapy.

B. Aortic insufficiency

Evaluation of AI should aim to determine the mechanism, morphology of the leaflets, and concomitant aortic root and ascending aortic disease that guide the pathway for surgical therapy.

Echocardiographic evaluation should focus on leaflet morphology, thickness, mobility, and coaptation. In addition to severity of AI, echocardiographic evaluation is essential to make a preoperative assessment of valve repairability.

With the exception of aortic dissection, trileaflet valves amenable to repair exhibit normal leaflet morphology and mobility, with malcoaptation caused by dilatation of the aortic root and/or sinotubular junction. Bileaflet valves may exhibit eccentric regurgitation related to relative prolapse of the conjoined cusp in addition to dilatation. Thickening, calcification, fenestrations, or other leaflet anomalies suggest a lower probability of successful repair.

Three-dimensional imaging of the aortic root and ascending aorta by CT or MRI is essential in evaluating bicuspid valve patients or those with trileaflet valves with root or ascending aortic enlargement. Wide availability of desktop programs for multiplanar 3D reconstruction has allowed for detailed evaluation of the root and ascending aorta from the surgeon’s perspective.


IV. PERIOPERATIVE MANAGEMENT

A. Preoperative work-up

1. Transthoracic echocardiography. For aortic valve disease, evaluation of gradients, valve area, degree of regurgitation, valve morphology, ascending aortic dilatation/PA dilatation, concomitant valve disease, and ventricular function is necessary. For pulmonic valve disease, particular attention should be paid to estimation of pulmonary hypertension.

2. Coronary angiography. This is done for evaluation of coronary anatomy, presence/absence of coronary anomalies, and coronary disease requiring concomitant bypass.

3. Routine laboratory evaluation includes complete blood count, electrolytes, blood urea nitrogen/creatinine ratio, liver function tests, urinalysis, urine culture, prothrombin time/partial thromboplastin time, and a current blood bank sample.

4. Chest and abdominal CT angiography. Chest CT is indicated in patients with aortic dilatation by echocardiography, or for evaluation of ascending and arch calcification, and in the case of TAVR or peripheral cannulation, descending aorta and iliac and femoral vessel diameter and quality. CT is also indicated in patients with prior chest surgery for planning safe reentry, and in patients considered for minimally invasive approaches for planning (Fig. 15.5).

5. Electrocardiogram. This is essential for baseline rhythm analysis and important to use for comparison to detect postoperative ischemia.

6. Ancillary studies for risk stratification. Pulmonary function testing is indicated in patients with known pulmonary disease or extensive smoking history and in the setting of radiation heart disease. Carotid Doppler studies and lower extremity noninvasive studies may be indicated in patients with relevant history or findings but are not routinely indicated.

B. Contraindications to surgery

1. Absolute contraindications include extreme frailty with poor life expectancy, untreatable malignancy, active cerebral hemorrhage, and Childs C cirrhosis.

2. Relative contraindications are active infection, Childs B cirrhosis, ascending aortic calcification, multiple reoperations, poor ventricular function, patient comorbidities, patient frailty, and patient limitations determined more by comorbidities than aortic valve disease.

3. Perioperative medical management

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Oct 4, 2018 | Posted by in CARDIOLOGY | Comments Off on Valve Surgery: Aortic/Pulmonary
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