surgical intervention and support for cardiomyopathies of childhood

Surgical Intervention and Support for Cardiomyopathies of Childhood

WILLIAM G. WILLIAMS, M.D. IVAN M. REBEYKA, M.D.

Department of Surgery The University of Toronto Faculty of Medicine

Toronto, Ontario, Canada

DEFINITION Hypertrophic obstructive cardiomyopathy (HOCM) is a primary myocardial disorder of unknown etiol- ogy resulting in marked ventricular septal hypertrophy leading to obstruction of left (and occa- sionally right) ventricular ejection, It may occur in sporadic or familial forms.

MECHANISM OF OBSTRUCTION Thickening of the ventricular septum in the subaortic area adjacent to the mitral valve apparatus nar- rows the left ventricular outflow tract. As a conse- quence of this narrowing, there is a localized increased velocity of ejection, resulting in a Venturi effect which draws the mitral leaflets and chordae towards the septum in systole. This systolic anterior motion (SAM) of the mitral apparatus produces anterior leaflet-septal contact, thereby add- ing to the outflow tract obstruction. The SAM distorts mitral leaflet coaptation, thereby creating mitral insufficiency which is typically late in systole and posteriorly directed. l

Once left ventricular outflow tract obstruction develops, the increased systolic pressure load stim- ulates further ventricular hypertrophy, leading in turn to more obstruction and greater hypertrophy. Diastolic dysfunction, consisting of delayed ven-

Address correspondence to William G. Williams, M.D., Chief, Cardiovascular Surgery, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada MSG 1X8.

tricular relaxation and decreased compliance, is also present and related to the degree of myocardial hypertrophy.2

The obstruction is dynamic and factors which increase contractility (i.e., all inotropes, such as digoxin and dopamine) or decrease afterload (i.e., nitroglycerin, sodium nitroprusside) will worsen the degree of obstruction and are therefore contra- indicated. The obstruction may occur at rest or intermittently either with provocation (latent) or, rarely, with no apparent precipitating cause (la- bile). While the obstruction is typically in the imme- diate subaortic area, less commonly it may occur as mid-ventricular obstruction or, rarely, at apical level.jp4

Surgical intervention should alleviate symptoms by removing enough of the ventricular septum to allow normal left ventricular ejection and restora- tion of normal mitral leaflet coaptation.5

NATURAL HISTORY Patients with unoperated hypertrophic obstructive cardiomyopathy have a 3 % to 4% per year mortality. 6,7 Maron et a1.8 believe the risk is increased in young patients; in their series, 23 of 26 sudden deaths occurred in patients <25 years of age. Fiddler and associates” experience was similar and Mc- Kenna and Camm’O estimated the incidence of sudden death to be 2% to 4% per year in adults and 4% to 6% per year in children.

Prog Pediatr Cardiol 1992; 1(4):61-71 Copyright 0 1992 by Andover Medical

Progress in Pediatric Cardiology

A family history of sudden death is a poor prog-

nostic sign.” Arrhythmias also indicate poor prog- nosis but they occur uncommonly in young pa-

tients who are nonetheless prone to sudden death.” The occurrence of ventricular tachycardia is more common with severe ventricular hypertr0phy.l’ Treatment of ventricular tachycardia with amiodarone improved symptoms and tread-mill exercise; however, it did not prevent sudden death.13

Myocardial perfusion is decreased at rest and during exercise in hypertrophic obstructive cardiomy-

opathy.14 Subaortic myectomy improves myocardial oxygen consumption and metabolism and also

reduces or eliminates symptoms and probably improves longevity.‘5-‘8

Data on the natural history of children with hy-

pertrophic obstructive cardiomyopathy are limited. Schaeffer et all9 reported the cases of 13 infants presenting at <2 years of age, 3 treated with myectomy and 6 with propranolol. There were no

deaths. Meyer et al.” reviewed 10 infants <2 years

of age; there were two deaths. Calcium channel

blockers and S-blockers were used in 4 of these

children; none underwent surgery. McKenna et a1.21 showed that arrhythmias are uncommon in children but sudden death is not.

INDICATIONS FOR SURGERY Obstruction

The main indication for surgical intervention in ob-

structive cardiomyopathy is failure to control

symptoms by medication or, alternately, failure to tolerate or comply with medical treatment. A persistent pressure gradient >60 mm Hg despite medi-

cal treatment is also an indication for surgery. The angiographic and echocardiographic dem-

onstration of obstruction are illustrated in Figures 1 and 2. In the young patient, severe hypertrophy with the septum enlarged to 30 to 40 mm may be a relative indication for surgery even in the absence of symptoms. Asymptomaticpatients with high left ventricular outflow tract pressure gradient despite medication should be considered for surgical inter-

vention,

FIGURE 1. Left ventricular angiogram in long axis and oblique projection illustrates the marked increase in septal thickness (arrows). The severe systolic anterior motion of the thickened translucent mitral valve (arrow SAM) produces ex- treme narrowing of the left ventricular outflow tract. (A0 = aorta; MV = mitral valve; LV = left ventricle; HOCM = hypertrophic obstructive cardiomyopathy; SAM = systolic anterior motion.)

Surgical Intervention and Support 63

A B

FIGURE 2. (A) Transesophageal echocardiography demonstrating the left ventricular outjlow tract. In late systole, the mitral valve apparatus is pulled toward the thickened interventricular septum. Color Doppler imaging illustrates the turbulent flow starting at the mitral chordal-septal contact. The posteriorly directed jet of mitral insufficiency occurs because of the distorted position of the leaflets. (LA = left atrium; LV = left ventricle.) (B) F o 11 owing myectomy (arrow), the plane of closure of the mitral valve is normally positioned, and the width of the outflow tract is widened. The shelflike projection (arrow) is due to the plane of the echo crossing at an angle to the surgically created tunnel. The residual septal thickness in the tunnel is 6 mm from the aortic valve to the body of the ventricle. Color Doppler imaging (right panel) illustrates a trivial, more centrally directed jet of mitral insufficiency and no turbulence in the outflow tract.

Mitral Insufficiency The presence of mitral insufficiency is not an indication for valve surgery. Because mitral insufficiency is dynamic in HOCM, it can be improved or allevi- ated by relieving the left ventricular obstruction. Only mitral insufficiency associated with intrinsic mitral valve disease and unresponsive to manipula- tion that lowers the pressure gradient should be considered for mitral valve replacement or repair.

Arrhythmias Ventricular arrhythmias, unresponsive to medical treatment, may require implantation of an auto- matic internal defibrillator.

Coronary Artery Stenosis

Coronary artery stenosis is more critical in the presence of severe ventricular hypertrophy and should be treated at the time of subaortic myectomy. In children, coronary atherosclerotic lesions are un- likely, but myocardial bridges overlying a coronary

artery can produce stenosis and ischemia. A patient with angina and HOCM should have selective coronary angiography regardless of age.

MEDICAL MANAGEMENT Left Ventricular Outflow Tract Obstruction The drug of choice for decreasing left ventricular outflow tract obstruction and controlling mitral insufficiency is disopyramide.22 The negative inotro- pit effect will often dramatically lower the pressure gradient. At the same time, the abolition of systolic anterior motion of the mitral valve allows it to close in a normal plane and decreases or eliminates the mitral insufficiency.

The loading dose for adults of short-acting disopyramide is 100 mg PO qid. It may be increased up to a maximum of 600 mg/day, while observing the effect on left ventricular outflow tract obstruction. If the heart rate does not decrease to the mid 60s (below 70), a P-blocker is added (Atenolol, 50 to

64 Progress in Pediatric Cardiology

100 mg/day or Propanalol, 2 to 10 m&kg/day, titrated to a heart rate of 60 to 70 beats per minute)

(E. D. Wigle, written communication, May, 1992).

These doses must be adjusted for the size of the child. Duncan et aLz3 used disopyramide at a dose of 6 to 12 mg/kg/day given every 8 hours.

Diastolic Dysfunction

While the obstructive element of hypertrophic cardiomyopathy dominates, patients with nonobstruc- tive hypertrophic cardiomyopathy and a few treated for obstruction and followed many years may demonstrate symptoms related to diastolic dysfunction.

Impaired relaxation results in a high left ventricular end diastolic pressure causing dyspnea and fatigue. Calcium channel blockers, specifically verapamilz4 5 mg/kg/day (140 mg/MZ) given every 8 hours is

useful in managing these patients, although higher dosage may be required.

Arrhythmias

It seems probable, though unproven, that sudden death in hypertrophic cardiomyopathy is due to ventricular arrhythmias. 25,26 Alternately, hemodynamic instability from any cause, including atria1

arrhythmias, may precipitate myocardial ischemia and thereby fatal ventricular’ arrhythmias. Stewart and McKennal’ believe that low-dose amiodarone may be protective against sudden death. Amioda- rone (10 mg/kg/day loading dose X 7 days, then 5 mg/kg/day maintenance) may also be beneficial in both paroxysmal and chronic atria1 fibrillation,

although both are uncommon arrhythmias in children. Long-term toxicity of amiodarone, causing skin discoloration and thyroid dysfunction, is well known. Sudden death may occur despite amioda-

rone therapy. 13,26

SURGICAL TECHNIQUE Subaortic Myectomy

The heart is arrested with warm-induction blood cardioplegia2’ and then cooled with cardioplegia to a myocardial temperature of 15°C. Topical cooling is used to maintain a consistent cold temperature. An oblique aortotomy into the noncoronary sinus of Valsalva is used to expose the subaortic area through the retracted aortic valve. Palpation of the septal thickness and extent of hypertrophy is a useful guide to determine the size of the resection.

FIGURE 3. Surgical view of the left ventricular outflow tract seen through an oblique aortotomy into the noncoronary sinus. The line of the 3 incisions is shown (arrow).

A rectangular block of myocardium is excised from the ventricular septum. The initial incision is placed 3 to 4 mm to the right of the middle of the right coronary cusp and 3 mm below the aortic annulus (Figure 3). It is directed toward the apex of the heart, generally 3 to 4.5 cm long and deep enough to leave a residual septal thickness of 5 to 8 mm. The thickness of the anterior left ventricle

(LV) free wall below the septum (the septum appears to cross the left ventricular outflow tract obliquely from the surgeon’s perspective) is used as

a guide to estimate the residual septal thickness.

A second incision, parallel to the first, is placed 3 mm below the left aortic cusp and 2 or 3 mm anterior to the mitral valve apparatus on the pa-

tient’s left side. The anterior papillary apparatus is retracted with a sucker to prevent inadvertent damage.

The third incision is placed 2 mm below and par-

allel to the aortic annulus to join the cephalic end of the two previous incisions. The resultant rectangular block of myocardium is dissected distally, using frequent palpation of the septum to estimate residual septal thickness. Resection is completed when the lower end of the specimen breaks continu- ity with the anterior free wall. The resection on average is 2.5 cm wide, 4 cm long, and 1.5 cm deep, but varies considerably with the size of the patient and thickness of the septum.


Preoperative and Postoperative l-lemodynamics of Adults and Children

Preoperative

LVOT LVEDP MR

Postoperative

n % LVOT LVEDP MR n %

Children n 20 16 20

=vg 80 10 None 12 60 7 7 8 Mild 6 30 18 13 None 7 88 Moderate 2 10 Mild 1 13

Adults n 114 90 129 20 =vg 80 21 None 21 17 19 15 None 12 48

Mild 51 40 Mild 11 44

Moderate 50 40 Moderate 2 8

Severe 4 3 Severe 0 0

LVOT, Left ventricular outflow tract pressure gradient; LVEDP, left ventricular end-diastolic pressure; MR, mitral regurgitation.

INTRAOPERATIVE MONITORING Transesophageal echocardiography (TEE) during surgery, both before and after myectomy, is ex- tremely useful in planning the resection and con- firming its success. (L. Grigg et al., unpublished data, 1992). The plane of mitral valve closure should become normal with loss of SAM and LV outflow turbulence (Figure 1). The outflow tract pressure gradient should be abolished. Manual induction of an extrasystole is used to observe the first postextrasystolic beat to demonstrate absence of pressure gradient and SAM with provocation.

If TEE is not available, a pullback pressure trac- ing with a catheter inserted before closing the aorta should be recorded to confirm the absence of a pressure gradient at rest and postextrasystole.

SURGICAL RESULTS Subaortic Myectomy

Twenty-five children, ages 2 months to 17.4 years, have undergone subaortic myectomy for hypertrophic obstructive cardiomyopathy. Most children were asymptomatic but had left ventricular outflow tract pressure gradients >60 mm Hg despite treatment; a few were severely symptomatic. Arrhyth- mias were uncommon.

Preoperative and postoperative hemodynamics are shown in the Table. Compared with adults, the children had lower left ventricular end diastolic pressure and less mitral insufficiency despite similar left ventricular outflow tract gradients.

Actuarial survival following myectomy in children is 100% at 5 years and 86% ( +13%) at 10 years (one late death at reoperation). Postoperative survival is not different than in 139 adults (Fig- ure 4).

Although survival is not different, children ap- pear to have an increased risk of reoperation for left ventricular outflow tract obstruction. Three of the 25 children required reoperation between 10

POST-MYECTOMY SURVIVAL IN HOCM

FIGURE 4. Actuarial survival of 25 children and 139 adults after subaortic myectomy for hypertrophic obstructive cardiomyopathy. The survival rate at 10

years - 86% for children, 79% for adults-is not statis- tically different. The crosshatches on the survival lines indicate the current end of follow-up for each patient.


RE-OPERATION AFTER MYECTOMY FOR HOCM

T T

FIGURE 5. Reoperation rates for recurrent (persistent) left ventricular outflow tract obstruction in children (3 of 25) is significantly higher than for the adult group (I of 139).

months and 9 years postmyectomy. Reoperation rates are compared to 139 adults in Figure 5.

Associated Procedures

Twenty-eight percent of the children, compared with 21% of adults, had concomitant procedures at the time of myectomy. Four children required resection of right ventricular outflow tract obstruction, one had an aortic valve repair, and four had miscellaneous procedures.

Other Surgical Procedures

Implantable Defibrillator. Four children with non- obstructive hypertrophic cardiomyopathy have undergone implantation of a cardioverterdefibrillator for potentially fatal arrhythmias. All had syncope or had been resuscitated from “sudden death.” Elec- trophysiologic study demonstrated polymorphic ventricular tachycardia in two and was negative in the others. Sotalol and amiodarone were prescribed in addition to the defibrillator; if sinus tachycardia was present, it was controlled with B-blockers. The first three children have all had clinically appropriate successful cardioversions without sequelae.

Coronary Stenosis from Myocardial Bridges.

Atherosclerosis causing coronary stenosis is not uncommon in adults with hypertrophic cardiomyopathy but was not seen in the children. However, coronary stenosis may also be caused by myocardial bridges over the coronary artery. It was present

in six adults and was recently seen in a teenager without obstruction. He had an episode of sudden death and by history had classic angina. Selective coronary angiography (Figure 6A, B) demonstrated a very long myocardial compression of the left anterior descending coronary artery. At operation, the myocardium overlying the artery was divided (Fig- ure 7) and a cardioverter-defibrillator was im- planted. He is currently asymptomatic 2 months after surgery, and his exercise thallium scan is normal.

Hypertrophic obstructive cardiomyopathy can be safely managed by subaortic myectomy with good long-term results in children in whom control of the left ventricular outflow tract by medical means has been unsuccessful. Associated lesions may require surgical management and do not affect the long-term result. However, the incidence of reoperation for recurrent (or persistent) obstruction may be more frequent than in adults.

SURGICAL THERAPIES FOR THE FAILING HEART

Since the 198Os, cardiac transplantation has become an accepted therapeutic modality in the treatment of end-stage cardiac failure that is not amena- ble to standard forms of medical and surgical treatment. Although current 5-year actuarial survival rates for cardiac transplantation exceed 80 % , donor availability has become a major limiting fac- tor; 20% to 30% of potential transplant candidates die before a suitable donor heart becomes available. For most patients, the risk of mortality during the preoperative waiting phase exceeds the risk of the transplant procedure. The scarcity of donors com- bined with an increasing demand for organs has compelled investigators to explore methods of mechanical circulatory support in an attempt to maintain adequate systemic perfusion until a suitable donor is found or native cardiac function improves. 28,29 Although the vast majority of experience with these devices has been directed toward the adult with end-stage cardiac failure, potential application to the pediatric population is currently being investigated. Several alternative forms of cardiac support are presently available as therapeutic options, including the intraaortic balloon pump, ventricular assist devices, and biomechanical cardiac assist.


FIGURE 6. (A) Preoperative tine angiogram of the left anterior coronary compression within a long myocardial tunnel (arrow) during systole (left panel). In diastole the caliber of the artery is normal (right panel). (B) Postoperative angiogram in the same patient shows relief of the previous compression both in systole (left panel) and diastole (right panel).


FIGURE 7. Operative photograph of the left anterior descending coronary artery of a p-year-old unroofed by incising the overlying myocardium. Four centime- ters of the artery was deep to myocardium. The maximum depth was 9 mm.

lntruaortic Balloon Pump

The intraaortic balloon pump (IABP) provides left ventricular assistance using the principle of internal counterpulsation whereby aortic pressure is low- ered immediately before left ventricular ejection (systolic unloading) and raised during diastole (diastolic augmentation). The IABP has been used extensively in the adult population for temporary circulatory support in various forms of cardiomyopathy. Although only a marginal increase in cardiac output with the IABP may be anticipated and long-term cardiac support is unrealistic, the beneficial effects on myocardial energy consumption and performance may be sufficient to support the patient until intrinsic myocardial function improves or a suitable heart becomes available for

transplantation. The hemodynamic effects vary from patient to patient and typically, in the adult, an increase in cardiac index from 10% to 40% can be anticipated.

Accurate synchronization of the IABP with the native cardiac cycle is necessary to achieve satisfac- tory systolic unloading and diastolic augmentation. Optimal counterpulsation occurs at a heart rate of 90 to 100 beats per minute while, at rates >120 beats per minute, the effect on systolic unloading and diastolic augmentation decreases. At heart rates >120 beats per minute, assist ratios of 1:2 or 1:3 may result in more effective circulatory support. Current technology allows the IABP to be triggered by the arterial pressure waveform, the R-wave of the native ECG, and ventricular or atrio- ventricular sequential pacer spikes, as well as asyn- chronously at preset rates. Balloon volume should be -40% to 60% of the patient’s normal stroke volume.

In the past, pediatric intraaortic balloon pumping has been primarily confined to children >5 years of age.3o Although successful use in a 2-kg infant with a 2.5ml balloon has been reported, patient size undoubtedly presents important limitations.3’ The greater elasticity and compliance of the infant or child’s aorta limits the degree of systolic unloading and diastolic augmentation that can be ob- tained. A beneficial hemodynamic effect conceiv- ably correlates with use of the IABP in the older child. In addition, the rate of significant complications including renal failure, mesenteric vascular obstruction, and especially leg ischemia may be higher in the younger. age group. Accordingly, IABP insertion in the infant and young child is often performed through the external iliac artery rather than via the femoral approach. The external iliac artery is easily accessible through an anterior flank incision and balloon fixation can be achieved with a small purse string suture or through a graft placed in an end-to-side fashion. Despite these precau- tions, severe leg ischemia is not infrequent and may require removal of the IABP or placement on the opposite side.

Despite the limitations of the IABP in the pediatric population, the availability of specially designed pediatric balloons in a wide selection of sizes and improvements in compressor response times have allowed this device to be successfully used in younger patients, including infants. Patients in


severe low cardiac output will likely require addi- tional support beyond that provided by the IABP.

Ventricular Assist Devices and Extracorporeal Membrane Oxygenation Approximately 30 centers in the United States currently have FDA approval to investigate several types of implantable ventricular assist devices (VAD; NOVACOR, Thoractec Pierce-Donachy, Therm0 Cardiosystems Pusher-Plate) to temporar- ily support the circulation as a bridge to transplantation. The indications for the various devices are dictated by protocol guidelines but, in general, include patients who are dependent on maximum inotropic support and show early evidence of pro- gressive end-organ failure. Despite the number of complications associated with circulatory assist devices, including bleeding, hemolysis, embolism, infection, and end-organ failure, the overall results of more recent clinical trials have been encourag- ing. 32-35 Approximately 50% to 60% of patients placed on VADs have survived the bridge-to-transplant phase and subsequent cardiac replacement. The University of Pittsburgh has supported patients for (10 months before successful transplantation; 12 patients have been supported for >60 days (R. L. Kormos, oral communication, June, 1992). Unfortunately, these devices are not suitable for pediatric use as they are designed for large patients weighing 840 kg.

The application of ventricular assist in the pediatric population has focused primarily on patients with profound cardiopulmonary failure following open-heart repair. 36 The centrifugal pump (Bio- Medicus, Inc., Eden Prairie, Minn.) appears best suited to smaller patients as it is more responsive to changes in the peripheral circulation (constant pressure) than the roller pump (constant flow) and may have a lower rate of hemolysis during prolonged periods of support. In the absence of severe right ventricular dysfunction or respiratory failure, left heart assist with left atria1 and aortic cannulation may be sufficient to support the circulation and has the advantage of simplicity and reduced need for heparin anticoagulation. Although left ventricular support alone may be adequate for most chron- ically failing hearts, including patients with dilated cardiomyopathies, a subgroup estimated at 25% will require right ventricular support in addition.37

Incorporation of a membrane oxygenator into

the circuit for extracorporeal membrane oxygenation (ECMO) allows single systemic venous cannulation of the superior vena cava or right atrium for VAD support.38 However, complete left ventricular failure may necessitate decompression of the left heart, which can be accomplished either by di- rect left atria1 cannulation through a recent open sternotomy or by transatrial blade and balloon sep- tostomy.

The primary issues that must be considered with the use of ventricular assist in children are similar to those in the adult, namely, the risk of infection and thromboembolism, prolonged maintenance of normal end-organ function, and recipient selection for a decreasing donor organ ~001.~~ Whether transplantation should be performed in a suboptimal recipient, given the scarcity of available donor hearts, remains a difficult question to answer. A growing experience and refinement of techniques will allow better definition of appropriate indications for ventricular assist in the pediatric population.

Biomechanical Cardiac Assist Biomechanical cardiac assistance involves the use of electrically stimulated autologous skeletal muscle to augment or partially replace heart muscle function in the setting of advanced cardiac failure. Two forms of biomechanical assist are presently being investigated. Dynamic cardiomyoplasty re- fers to the placement of electrically stimulated skeletal muscle directly onto the heart for cardiac assistance. Skeletal muscle-powered assist entails the creation of an auxiliary chamber, usually placed in parallel with the aorta, that is powered by skeletal muscle as an external counterpulsator.

The procedure of dynamic cardiomyoplasty used clinically involves wrapping a pedicled skeletal muscle, usually the latissimus dorsi, around the heart. Following transformation of fatigue-resis- tant muscle over 6 to 8 weeks, it is paced synchronously with the inherent cardiac rhythm to augment myocardial contractility. Since its first clinical application by Carpentier and Chachque#“ in 1985, several groups have reported the use of cardiomyoplasty for the treatment of left ventricular tumors and aneurysms and in patients with dilated and ischemic cardiomyopathy.41r” Although objective evidence of significant improvement in clinical improvement has been modest, the best hemody-


namic results have been in patients with dilated

cardiomyopathies.43 Jatene et a1.44 have recently reported results in 11 patients with idiopathic dilated

cardiomyopathy demonstrating significant improvement in heart function indices at 3 to 12

months’ follow-up. Although the youngest patient

in this series was 16 years of age, the role of cardiomyoplasty in the pediatric age group has not been addressed. Thus, the limited clinical experience

with dynamic cardiomyoplasty precludes definitive conclusions regarding its long-term role in the treat-

ment of severe cardiac failure. Despite the beneficial physiologic effects of skeletal muscle assist demonstrated in several laboratory models, its clin-

ical value is not yet established and awaits further experimental and clinical study.

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surgical intervention and support for cardiomyopathies of childhood

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