Transposition of great vessels

Study objective: This study reports on the cardiorespiratory response to graded exercise in patients after venous switch operation for transposition of the great arteries. Design: Several small studies have documented a diminished exercise tolerance after Mustard repair for transposition of the great arteries; little information exists, however, about long-term cardiorespiratory exercise performance in patients who have had the Senning procedure. Patients: This prospective study reports on the serial long-term (mean, 11[+ or -]2.8 years) cardiopulmonary exercise performance of 43 patients (age, 12[+ or -3.1 years) who underwent a Senning procedure, with no significant postoperative abnormalities. Forty-three matched healthy children were also studied as a control group. Measurements and results: All underwent exercise testing (Bruce protocol) with metabolic gas exchange to determine parameters at 3 min, anaerobic threshold, similar heart rate (150 beats/min), and peak exercise. Time of exercise was 10.5[+ or -]1.9 min in patients and 13.4[+ or -]2 min in control subjects (p=0.0001). Overall, patients reached 73% of peak oxygen uptake achieved by control subjects (32.6[+ or -]5.6 vs 44.7[+ or -]6 mL/kg/min). Chronotropic response (188[+ or -]15.7 vs 166.5[+ or -]19.6 beats/min [p=0.0001]) and oxygen pulse (7.4[+ or -].9 vs 10.7[+ or -]4.2 mL/beat [p=0.0002]) were lower in patients at peak exercise. Patients had a greater respiratory response to exercise: both respiratory rate and ventilatory equivalent for carbon dioxide were significantly higher at all stages of exercise. Exercise capacity assessed by peak oxygen uptake was correlated with time elapsed since surgical repair (r=0.48; p=0.001). Conclusions: It is concluded chat even in asymptomatic patients, exercise endurance and respiratory response are generally altered as much as 11[+ or -]2.8 years after venous switch operation, although early surgical repair is predictive of a better long-term functional result.

(CHEST 1997; 111:23-29J

Key words: exercise testing; Senning; transposition of great vessels

Abbreviations: ASO=atrial switch operation; AT=anaerobic (ventilatory) threshold; RR=respiratory rate; TGA=transposition of great arteries; [VCO.sub.2]=carbon dioxide output; VE=minute ventilation; [VO.sub.2]=oxygen uptake; VT=tidal volume

Although the arterial switch operation is now frequently the procedure of choice for correcting transposition of the great arteries (TGA), most adolescents and young adults born with TGA owe their survival to the atrial switch operation (ASO).[1, 2] Mustard and Senning procedures result in low early morbidity and mortality, but long-term results have shown an increased incidence of arrhythmia,[3-6] sudden deaths, and a decrease in functional capacity,[7-18] probably because the morphologic right ventricle may be unable to function as successfully as a morphologic left ventricle in the systemic position for long periods of time.[19-23] Previous investigators have described a limited cardiorespiratory response to exercise in patients who have had AS0.[6, 9, 14-16] However, these series were limited in number, did not always analyze gas exchange, and included patients especially operated on with the Mustard technique. Therefore, the present study was undertaken to evaluate exercise tolerance in a larger series of 43 patients who had undergone a Senning procedure, with special attention to cardiopulmonary response.

MATERIALS AND METHODS

Subjects

The forty-three patients (11 girls, 32 boys) underwent ASO at our cardiologic hospital department between 1977 and 1988; after operation, they had yearly clinical, ECG, and echocardiographic examinations. All patients had at least one postoperative catheterization. Twenty-six patients had simple transposition, defined as TGA with an otherwise normal heart, whereas the remainder had additional associated defects, including ventricular septal defect (seven patients) or ventricular outflow obstruction (ten patients). At the time of the study, 11 patients had mild pulmonary stenosis ([is less than]40 mm Hg), 5 had atrial baffle obstruction, and 1 had important tricuspid regurgitation. Two patients had permanent pacemakers. All considered themselves asymptomatic and most of them took part in gym at school. Mean age at surgical repair was 9.6[+ or -]5.6 months, and the postoperative interval ranged from 6.5 to 18.3 years (mean, 11[+ or -]2.8 years). Two patients were taking cardiac medications at the time of their exercise study; these were diuretics (one patient) and beta-blockers (one patient). As a control group, 43 healthy voluntarily enrolled children from local schools were also studied; none had a sport activity elsewhere. They were matched for age, sex, height, and weight to TGA patients (Table 1).

Mean heart rates at rest on ECG were 87[+ or -]15.1 beats/min in patients and 99.2[+ or -]14.9 beats/min in the normal group (p=0.0001). All normal subjects and 36 of 43 patients were in sinus rhythm at rest; 5 patients had intermittent junctional rhythm; 2 patients were paced because of sinus node dysfunction. All patients but one developed sinus tachycardia with exercise; 6 patients had significant atrial (n=3) or ventricular (n=3) arrhythmias during exercise. At peak exercise, control subjects achieved a significantly higher heart rate (187.8[+ or -]16.1 vs 166.5[+ or -]19.6 beats/mini p=0.0001). Twenty-one patients were unable to reach 80% of their theoretical maximal heart rate, and 6 patients did not achieve 150 beats/min at the end of exercise; the relationship between chronotropic response and [VO.sub.2] was y=3.1 x+47.2 in control subjects and y=4.2 x+31.3 in patients, where y=heart rate (beats/min) and x=[VO.sub.2] in mL/min/kg (Fig 1). Maximal exercise heart rate showed no correlation with either age at the time of operation or age at the time of exercise testing. However, maximal heart rate was weakly correlated with peak [VO.sub.2] in patients (r=0.29, p=0.05). Systolic BP was also different at rest (patients, 100[+ or -]10 mm Hg; control subjects, 109[+ or -]13 mm Hg; p=0.002) and at peak exercise (patients, 122[+ or -]15 mm Hg; control subjects, 140[+ or -]23 mm Hg; p=0.0001).

Exercise duration, [VO.sub.2], and work capacity were significantly greater in control subjects than in patients (Table 2). Overall, patients achieved 73% of peak [VO.sub.2] obtained by control subjects (32.6[+ or -]5.6 vs 44.7[+ or -]6 mL/kg/min; p=0.0001). Time of exercise was 10.5[+ or -]1.9 min in patients and 13.4[+ or -]2 min in control subjects (p=0.0001). The anaerobic (ventilatory) threshold (AT) could be determined in 38 of the 43 patients and in 39 control subjects. [VO.sub.2] at AT was 24.6[+ or -]4.2 mL/kg/ min in patients and 34.7p[+ or -]6.1 mL/kg/min in control subjects (p=0.0001). Compared with normal control subjects, the AT was surpassed sooner (6[+ or -]1.8 vs 8.6[+ or -]2 mini p=0.0001). The mean values of [VO.sub.2] at AT were 75[+ or -]11% of peak [VO.sub.2] in patients and 78[+ or -]8% in control subjects (p=NS). [VO.sub.2] was also lower in patients at all levels of exercise: 19.5[+ or -]3.6 vs 24.6[+ or -]5.9 mL/kg/min; p=0.0001; at 3 min, 26.6[+ or -]5.2 vs 33.1[+ or -]7.3 mL/kg/ mini p=0.0003; at an identical heart rate of 150 beats/min (Fig 2).

[Figure 2 ILLUSTRATION OMITTED]

(*)VE/[VCO.sub.2]=respiratory equivalent for carbon dioxide; [O.sub.2] pulse=oxygen pulse. [dagger]p[is less than]0.001. [double dagger]p[is less than]O.0001. [sections]p[is less than]0.01. [parallel]p[is less than]0.05.

Exercise capacity assessed by peak [VO.sub.2] was correlated with time elapsed since surgical repair (r=0.48; p=0.001) (Fig 3).

[Figure 3 ILLUSTRATION OMITTED]

As a result of [VO.sub.2] and heart rate decreases at each stage of exercise, the oxygen pulse was lower in patients at the third minute of exercise (6.7[+ or -]2.6 vs 8.6[+ or -]3 mL/beat; p=0.001), at AT (7.1[+ or -]2.6 vs 10.2[+ or -]4.1 mL/beat; p=0.0007), and at the end of exercise (7.4[+ or -]2.9 vs 10.7[+ or -]4.2 mL/beat; p=0.0002) (Fig 4).

[Figure 4 ILLUSTRATION OMITTED]

As in normal subjects (r=0.85; p=0.001), oxygen pulse at the end of exercise was strongly correlated with body surface area in the patient group (r=0.89; p=0.0001) but with a lower slope relationship (control subjects, y=11.2 x-4.78; patients, y=8.3 x-2.8, where y=oxygen pulse and x=body surface area).

During exercise, the patient group had a VE lower than control subjects (46[+ or -]18.7 vs 69.3[+ or -]30.1 L/min; p[is less than]0.0001) at the end of exercise. However, the ventilatory pattern was very different between the two groups: the increase of tidal volume (VT) was lower during exercise in patients. Patients had a greater respiratory response to exercise with RR/[VCO.sub.2] and VE/[VCO.sub.2] showing significant increases at all stages of exercise. Figure 5 shows the VT-RR relationship for each group. We found no significant correlation between VE/[VCO.sub.2] and [VO.sub.2] at the end of exercise.

[Figure 5 ILLUSTRATION OMITTED]

All these differences in cardiopulmonary parameters were also present for the Senning subgroup with or without additional defects.

DISCUSSION

Comparisons of cardiopulmonary function in patients having undergone venous switch operation with normal subjects have indicated different maximum performances and adaptations to exercise.[7, 8] However, most studies have focussed on patients who had the Mustard procedure. Reybrouck et al[15] alone recently reported a small series of 20 patients followed up 7.3 years after a Senning operation. The follow-up for the Senning procedure is shorter simply because it is more recent. Better long-term performance capacity could theoretically be expected with the Senning operation, because it uses exclusively viable atrial tissue. We report herein the largest series of operated on TGA patients (n=43) exclusively including patients operated on with the Senning procedure with a follow-up of more than 10 years (from 6.5 to 18.3; mean, 11[+ or -]2.8).

Compared to matched control subjects, our population had a reduced aerobic capacity with shorter exercise time ( - 22%), peak [VO.sub.2] ( - 27%), or [VO.sub.2] at aerobic threshold (-29%). Although our patients were asymptomatic, the slope of [VO.sub.2] vs workload increase was reduced as in patients with different stages of heart failure.[24] The results from the present study are in agreement with the previous report of Reybrouck et al[16] and with similar cardiopulmonary alterations observed after Mustard operations.[9, 14, 15] Exercise performance limitation after atrial repair has been attributed to an impaired chronotropic response.[6] Our patients, as a combined group, had a mean resting heart rate that was significantly less than normal; on exercise stress testing, the rhythm reverted to sinus in all patients but remained lower at the end of exercise (166[+ or -]20 vs 188[+ or -]16 beats/mini p=0.0001). Twenty-one patients were unable to reach 80% of their theoretical maximal heart rate. We found a weak correlation between maximal heart rate and peak [VO.sub.2] in patients (r=0.29; p=0.05) that was absent in normal subjects. However, peak [Vo.sub.2] decrease (-27%) is only partially explained by chronotropic alteration (-12%) after atrial repair. This chronotropic limitation may not be specific to atrial repair but might be a common denominator to all intracardiac repair, as the cardiopulmonary bypass procedure has been associated with surgical fibroelastosis that could directly or indirectly disrupt the integrity of the sinus mode.[14]

The other cause suggested for limited capacity after venous switch operation is an inadaptation of the right ventricle in the systemic position. Several echocardiographic, angiographic, and especially isotopic exercise studies have shown ventricular hypokinesia and a decrease in stroke volume.[19-23] Although right ventricle dilatation may be a compensatory mechanism for chronotropic insufficiency,[6] [CO.sub.2] rebreathing and isotopic measurements of cardiac flow showed a decrease in stroke volume during exercise testing under constant maximal load.[9] In our study, oxygen pulse, an indirect indicator of the stroke volume, was decreased at the end of testing (7.4 [+ or -] 2.9 vs 10.7 [+ or -] 4.2 mL/beat; p [is less than] 0.001) and at submaximal loads.

In normal children and adolescents, oxygen pulse correlates better than peak [Vo.sub.2] with morphometric parameters, particularly body surface.[25] In our patients and control subjects, these correlations were excellent but with different coefficients.

Although our study unfortunately confirms that children treated with the Senning technique have a long-term functional limitation close to that in children operated on a few years earlier with the Mustard operation, it does show for the first time an inverse relationship between time since intervention and ulterior aerobic capacity (r=0.48; p=0.001), while there is no significant correlation between the age of patients (or control subjects) and peak [Vo.sub.2]. We observed no significant relationship between age at surgery and maximal heart rate or [O.sub.2] pulse at peak effort. Unlike previous series in which no association was noted between time since intervention and exercise capacity, it seems that the earlier the children are operated on with the Senning technique, the better their ultimate effort capacity.

Our patients were all operated on by the same surgeon, and were operated on as quickly as possible when functional tolerance was poor (persistent cyanosis despite an efficient atrioseptostomy of Rashkind-Miller). It may be that submitting the right ventricle early to conditions of systemic load leads to better long-term tolerance, with exercise capacities similar to those in patients operated on with an arterial switch technique.[26]

The ventilatory adaptation of our patients during exercise testing was also quite different from that of normal patients and is reminiscent of the modifications described in cardiac insufficiency.[27,28] There is, in fact, an excess ventilatory response as measured by the respiratory frequency (or overall ventilation) compared to the production of [CO.sub.2].[29] Such anomalies have also been described more recently after Fontan procedures and repair of tetralogy of Fallot.[30]

However, we found no correlation between respiratory equivalents for [CO.sub.2] measured at the end of exercise and peak [Vo.sub.2] in patients or control subjects, contrary to a previous report.[24] The relationship VT-RR, which is all the more disturbed as the limitation of effort capacity increases, was also abnormal in our patients. This disturbance is associated more with a decrease in VT at the end of effort than with an acceleration in respiratory frequency. However, the mechanisms involved in this abnormal ventilatory response are not well understood. Unfortunately our patients did not have additional respiratory function tests (including vital capacity, pulmonary diffusing properties, lung distensibility, etc) and our study cannot elucidate this discussion. But as in patients with cardiac insufficiency, the relationship between ventilation and [CO.sub.2] production was not linear in our patients, so there is growing support for another stimulus being involved, particularly a muscular ergoreflex.[27] This could explain the benefits of physical training observed in recent years in patients with cardiac insufficiency,[31] and also in children operated on by venous switch operation.[32]

Our study in a large series of patients mainly operated on with the Senning technique and with a long follow-up confirms that atrial repair is associated with a limitation in maximal aerobic capacity with an early anaerobic AT. Although patients may be asymptomatic, they have an inadapted ventilatory response to exercise. Chronotropic insufficiency and especially the already documented relative inadaptation of the right ventricle to exercise should make clinicians vigilant about the long-term outcome of these patients. However, it would seem that interventions performed earlier with the Senning technique lead to better long-term exercise capacity, thus reopening the debate concerning the respective advantages of the arterial[26-33] with venous switch operation.

ACKNOWLEDGMENT: We thank Dr. Ray Cooke for his linguistic assistance.

REFERENCES

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(*)From the Cardiology Department of Hopital Cardiologique Haut Leveque, Pessac, France. Manuscript received February 14, 1996; revision accepted July 29. Reprint requests: Dr. Douard, Service des Epreuves d'Effort, Hopital Cardiologique, 33604 Pessac, France.

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