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18 февраля 2002 00:00   |   GRAEME L. HAMMOND

Surgery for aortic and pulmonary valve disease

The most common causes of aortic valve disease are rheumatic fever, endocarditis, congenital abnormalities, and collagen vascular disorders. The physiological sequelae and symptomatic profile of aortic stenosis, aortic insufficiency, or combined stenotic and insufficient lesions are remarkably similar, at least initially. This chapter, however, will deal only with the acquired variety as seen in adults and will not deal with hypertrophic cardiomyopathies, which may also mimic aortic valve disease.
Aortic stenosis
Aortic stenosis is characterized by a long, latent period, when the patient is asymptomatic, even in the presence of a clinically detectable, systolic murmur in the aortic area. The late onset of symptoms is due to the great ability of the heart to compensate for increasing pressure gradients across the aortic valve, principally through the development of myocardial hypertrophy. Hypertrophy can progress for many years before the ratio of muscle fibre volume to capillaries exceeds that required for oxidative metabolism. Even at this point, myocardial cells develop subtle biochemical changes, such as shifts in their lactate dehydrogenase isozyme profiles, that allow the cell to contract under relatively anaerobic conditions for many years. The development of hypertrophy is also associated with unremitting fibrosis that, although insidious, eventually results in a stiff, non-compliant ventricle that is unable to pump blood effectively. The classical symptoms of aortic stenosis, such as angina, syncope, and congestive heart failure, usually begin to appear by the age of about 60. Such symptoms are expected to result from progressive myocardial ischaemia (due to an increase in workload and increased ratio of muscle mass to blood supply), and cardiac fibrosis. Although the symptoms usually appear separately, towards the end of the natural history of the disease the three major symptoms often appear together. The onset of symptoms carries a poor prognosis: almost all patients die within 3 years unless the obstruction is removed. Of the three symptoms, congestive heart failure carries the worst prognosis and angina the least severe prognosis (Fig. 1) 1806. Nevertheless, the onset of any symptom is an indication for evaluation and, usually, aortic valve replacement. A loud aortic ejection murmur alone occasionally prompts referral of minimally symptomatic patients. Such individuals should undergo either cardiac catheterization or two-dimensional echo Doppler examination to determine whether critical aortic stenosis is present. The normal aortic valve has an area of 2.5 to 3.5 cm²; an area of less than 0.7 cm² represents critical aortic stenosis.
Critical aortic stenosis is associated with a high incidence of sudden death, and surgery should be undertaken even in individuals with minimal or no symptoms. A minimally symptomatic patient who does not have a critical aortic valve area, should be monitored carefully for the onset or progression of symptoms, hypertrophy, or left ventricular strain by electrocardiography. Heart size should be monitored by chest radiography, and left ventricular wall thickness and contractility by two-dimensional echocardiography. If these studies show evidence of progressive change, surgery should be considered.
Aortic insufficiency
Unlike aortic stenosis, chronic aortic insufficiency can be tolerated for many years. In contrast to the 100 per cent 3−year mortality rate of aortic stenosis, aortic insufficiency carries a 50 per cent mortality rate over a 10−year period (Fig. 2) 1807. Like aortic stenosis, chronic aortic insufficiency also leads to left ventricular hypertrophy; however, the volume overload also produces distension and progressive ventricular dilatation. The insidious dilatation and fibre stretching produces the progressive, irreversible loss of myocardial fibres that is characteristic of aortic insufficiency. When fibre loss has reached a critical point, surgical intervention will do little to alter the patient's clinical state. It is therefore important to perform aortic valve replacement before the onset of clinically apparent congestive heart failure, manifest by pulmonary oedema. A change in exercise tolerance or the onset of shortness of breath are important premonitory signs. In addition to accurate history taking and careful physical examination, the timing of surgery is also facilitated by monitoring changes in cardiac silhouette by chest radiography and following changes in chamber size by two-dimensional echocardiography. If the end-systolic dimension is greater than 55 mm, the patient should be catheterized. Patients with aortic insufficiency also develop angina and syncope, but these symptoms do not carry the ominous prognosis associated with aortic stenosis.
Acute aortic insufficiency such as that produced by endocarditis or collagen vascular disorders (i.e. isolated detached cusp) carries a much higher early mortality rate as there are no compensatory adaptive measures in place. Accordingly, while patients with acute endocarditis should ideally receive a full 6−week course of antibiotics, this is often impossible. The timing of aortic valve replacement secondary to acute aortic insufficiency depends primarily upon clinical symptoms. The onset of orthopnoea or shortness of breath will invariably proceed within hours to acute pulmonary oedema, and should be considered as an important signal for the need for surgery. Electrocardiographic changes of volume overload such as ST-T wave changes, atrial arrhythmias, and axis changes also invariably proceed clinical deterioration by 6 to 12 h and are very helpful indicators for moving ahead rapidly with surgery. Patients with aortic insufficiency secondary to acute type A thoracic dissections should undergo emergency surgery, but this is for other reasons.
Technical considerations
Valve repair has not yet gained general acceptance as a form of surgical treatment for aortic valve disease: in most cases, present management calls for valve replacement. There are many prosthetic devices available, each of which has its own advantages and disadvantages. In general, the use of tissue valves is restricted to older patients (≥70 years) due to their higher incidence of failure in younger patients, and to patients who cannot receive anticoagulants. Low-profile mechanical valves are ideally suited for the narrow aortic root and the Starr-Edwards (ball and cage) valve is an excellent all-purpose valve that can be used in most situations. The technique of aortic valve replacement for insufficiency or stenosis is very similar except for the considerations listed below.
Heavy calcification is invariably associated with aortic stenosis rather than aortic insufficiency. A calcified annulus may obliterate landmarks which are necessary for careful debridement, such as the aortic leaflet of the mitral valve and the atrioventricular node area. Pieces of calcium may fall into the coronary orifices or become trapped in trabeculae, with subsequent cerebral or systemic embolization. Perivalvular leaks secondary to inadequate annular decalcification or iatrogenic perforations of the root of the aorta from excessive decalcification may occur.
Abscess formation
This is often associated with acute endocarditis. Abscesses can burrow into the myocardium, with destruction of the annulus and valve leaflets. It is often necessary to place Teflon backed sutures from outside the aorta into the aortic lumen and through the valve sewing ring, or to use the medial wall of the pulmonary artery to buttress the suture line and ensure a competent valve–annulus alignment.
Narrow aortic root
This is invariably associated with aortic stenosis or mixed stenotic-insufficient lesions. Tertiary orifice obstruction from ball and cage valves precludes the use of ball valves for the narrow aortic root. Numerous operations have been developed to overcome this problem. Annular enlargement procedures such as those described by Manouguian and Konno-Rastan have been used with varying degrees of success. These operations tend to be technically difficult, carry a high incidence of mortality and morbidity, and are not always successful. Problems with the small aortic annulus have now been largely resolved with development of newer valves with greater effective orifice to annulus ratios. A 19 mm St Jude valve will fit most narrow aortic annuli and provides satisfactory relief of outflow obstruction, even in large adults. In order to place a 19 mm valve, it may occasionally be necessary to seat the valve at an angle to the plane of the annulus. In other words, the sutures are placed in the annulus in the usual way, but the valve is lowered into place so that the sewing ring is below the right and left coronary arteries but angled into the non-coronary sinus. The left and right coronary annular sutures are tied first, thereby abutting the sewing ring to the annulus below the coronary ostia. The non-coronary sutures are tied last: this allows the valve to ride above the annulus in this portion of the aorta. At the present time, annulus enlargement procedures should be reserved for those rare patients who cannot accept a 19 mm St Jude valve.
Pulmonic stenosis
With the advent of balloon valvulotomy for pulmonic stenosis, surgery for pulmonary valve disease is now reserved almost exclusively for the treatment of obstructive pulmonary artery abnormalities or the secondary pulmonic insufficiency that results from surgery upon the right ventricular outflow tract. Most patients with right ventricular outflow tract obstruction are children with conditions such as hypoplastic right ventricular outflow tract, absent pulmonary valve syndrome (invariably associated with annular stenosis), and double outlet right ventricle. Symptoms normally include dyspnoea, fatigue, congestive heart failure, cyanosis caused by shunting through atrial septal defects or a patent foramen ovale and, in infants, poor feeding. Patients rarely become symptomatic when the peak outflow tract gradient is less than 50 mmHg.
Occasionally patients may be referred for pulmonic surgery because of residual infundibular gradients following balloon valvuloplasty. Approximately 20 per cent of patients with pulmonic stenosis have secondary infundibular obstruction. In one study, five of 13 infundibular gradients disappeared following valvuloplasty in 62 patients; five patients developed new infundibular gradients. Patients that had systemic or super-systemic infundibular gradients following valvuloplasty responded well to propranolol and, as is the case with surgical pulmonic valvulotomy, infundibular stenosis regressed. Accordingly, surgery is not indicated for infundibular stenosis following balloon valvuloplasty.
Pulmonary insufficiency
For all intents and purposes, pulmonary insufficiency is not caused by rheumatic fever, endocarditis, or collagen vascular disorders. It is iatrogenic, and follows transannular repair of tetralogy of Fallot, or repair of other surgical disorders involving the right ventricular outflow tract. It may be serious or inconsequential but affected patients should be monitored carefully for the onset of right ventricular deterioration. Major pulmonary regurgitation is often associated with peripheral pulmonary artery stenosis. The most important symptom of a deteriorating right ventricle is fatigue, and the key signs are a murmur of tricuspid insufficiency and its associated systemic physical findings. Important radiological and angiographic signs include progressive cardiomegaly, a regurgitant fraction above 30 per cent and a transannular patch to descending aorta ratio of at least 2.5. Patients with these signs should undergo surgery promptly. In a study performed by Ilbawi et al., 42 patients with radiographic findings of pulmonary insufficiency following tetralogy of Fallot repair underwent pulmonic valve replacement. At subsequent catheterization of 18 patients, right ventricular function became normal in 83 per cent of those that had undergone surgery within 2 years of tetralogy repair, while right ventricular function remained abnormal in all 12 patients who underwent valve replacement more than 2 years following tetralogy repair. Although we recommend that pulmonary competency should be restored early after the documentation of significant pulmonary insufficiency, a review of the literature indicates that satisfactory results are obtainable for many years following the surgical creation of pulmonary insufficiency.
Surgery for pulmonary insufficiency
Surgical treatment of pulmonary insufficiency falls into three major categories: valve replacement with a mechanical or biological valve prosthesis, conduit replacement with aortic or pulmonary artery homografts or Dacron-valve conduits, and single cusp repair with a cusp bearing transannular patch. Valve replacement must often be performed in a main pulmonary artery that is too small to accept a valve capable of allowing flow without a significant gradient. As a consequence, valve insertion into the pulmonary artery is usually performed with a Dacron patch angioplasty (Fig. 11) 1816.
Despite the well known rapid degradation of biological valves in the aortic and mitral positions in children, they appear to provide excellent long-term results on the right side of the heart. Mechanical valves placed in the tricuspid and mitral positions are associated with a high rate of thrombosis or dysfunction caused by tissue ingrowth. Fleming et al. reported that of 18 biological valves inserted on the right side of the heart, only two had to be replaced over a 9−year follow-up period. On the other hand, three of four patients required replacement of St Jude valves after 36 to 56 months.
Replacement of the pulmonary valve with homografts or valve-conduits is generally reserved for conditions in which right ventricular to pulmonary artery continuity must be established, such as pulmonary atresia and truncus arteriosis. As most of these operations are performed in infants or adolescents, reoperation is often required due to growth. In a series of conduit operations at the Mayo Clinic, 44 per cent of patients required reoperation after 10 years: this must be considered when choosing the type of conduit. Dacron graft-valve conduits (Fig. 12) 1817 have the advantage of being readily available and, because they develop a surrounding fibrous peel, are easily redissected at the time of reoperation. Their primary disadvantage is suture line bleeding at the time of implantation, stiffness which limits their versatility and increases the possibility of kinking, and a tendency of the xenograft valve to calcify.
Fresh or cryopreserved aortic or pulmonic homografts are more versatile, provide good functional results, are technically easy to use, and have a low incidence of calcification. Due to the shortage of homograft material, both the aorta and pulmonary arteries should be harvested from donor hearts. The advantage of the pulmonary homograft is that it is thin-walled and supple, thereby allowing a technically simple distal anastomosis to the recipient pulmonary artery and minimizing distal suture line bleeding. The proximal portion of the pulmonary homograft, however, lacks tough fibrous tissue at the base of the valve; therefore donor outflow tract muscle tissue must be incorporated into this portion of the homograft. Muscle tissue does not cryopreserve well and, when thawed, sutures tend to tear through the friable muscle, with the potential for false aneurysm development at the proximal suture line. Aortic homografts, on the other hand, have a substantial fibrous ring at the base of the valve and have the added advantage of the attachment of the anterior leaflet of the mitral valve to the non-coronary annulus of the aortic valve. Positioning the aortic homograft allows for the insertion of a transannular patch, using the anterior leaflet of the mitral valve as the patch. The disadvantage of the aortic homograft is that the distal anastomosis is somewhat more difficult to construct due to the thicker aortic wall. The general method of insertion is shown in Fig. 13 1818. At the present time, the greatest disadvantage of the aortic and pulmonary homografts is their limited availability and their tendency to become very thin-walled and densely adherent to surrounding tissues, which increases operative mortality during reoperation due to haemorrhage.
Single cusp angioplasty attempts to create some degree of pulmonary competence when the pulmonary annulus has been divided. One approach is to insert a transannular patch consisting of an aortic homograft with one cusp preserved or bovine or porcine pericardial patches with a monocusp. A second approach is to fold the inferior margin of the incised native pulmonary artery into the outflow tract and then reconstruct the pulmonary artery with pericardium. The portion of pulmonary artery wall in the lumen acts as a valve cusp. Both procedures seem to help in the postoperative period by decreasing pulmonic regurgitation, but their long-term results remain to be evaluated.
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