Ann Thorac Surg 2006;81:988-991
© 2006 The Society of Thoracic Surgeons
Original article: Cardiovascular
Elastic Properties of the Reconstructed Aorta in Hypoplastic Left Heart Syndrome
Brian M. Cardis, MD,
Derek A. Fyfe, MD, PhD,
William T. Mahle, MD
*
Sibley Heart Center Cardiology, Children's Healthcare of Atlanta, Emory University School of Medicine, Atlanta, Georgia
Accepted for publication September 26, 2005.
* Address correspondence to Dr Mahle, 1405 Clifton Rd NE, Atlanta, GA 30322 (Email: mahlew{at}kidsheart.com).
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Abstract
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BACKGROUND: Patients with repaired coarctation of the aorta retain abnormal elastic properties of the aorta. It is not known whether patients with hypoplastic left heart syndrome also manifest abnormal elastic properties after palliative surgery. The presence of such abnormalities may have important clinical implications as reduced aortic compliance might adversely impact single right ventricular function.
METHODS: We prospectively evaluated the elastic properties of the aorta in a cohort of patients with hypoplastic left heart syndrome who had undergone the Norwood procedure with aortic arch reconstruction and subsequent bidirectional Glenn or Fontan procedure. The hypoplastic left heart syndrome patients (n = 20) were compared with single-ventricle patients (n = 18) without history of arch reconstruction and patients with double-ventricular lesions (n = 22). Aortic elastic function was quantified by distensibility index and stiffness index. M-mode measurements of the transverse aortic arch were obtained with transesophageal echocardiography under general anesthesia. Patients were evaluated at a median age of 22.2 months with no age difference between patient subgroups.
RESULTS: Distensibility index was significantly less (p = 0.007) and stiffness index greater (p = 0.005) in the reconstructed arch of hypoplastic left heart syndrome patients compared with single-ventricle and double-ventricle patients.
CONCLUSIONS: Patients with hypoplastic left heart syndrome after Norwood palliation have increased aortic stiffness and decreased distensibility in the reconstructed transverse arch. As previous studies in adults have shown that decreased aortic compliance increases the energy cost of cardiac ejection, examination of modifications to the surgical technique that might improve elastic properties is warranted.
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Introduction
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Elastic properties of the arterial tree are known to have profound influence on ventricular function [1–4]. Arterial elasticity may be of particular importance to those patients with hypoplastic left heart syndrome (HLHS) and its variants. This patient population has a hypoplastic aortic arch at birth, and surgical palliation, the Norwood procedure, involves reconstruction of the aortic arch with homograft—usually of pulmonary artery origin [5]. An understanding of the elastic properties of the reconstructed aorta is vital given that ventricular failure is common after the Fontan procedure and that patients with HLHS may be at particular risk [6, 7]. In the present study we sought to examine the elastic properties of the reconstructed aortic arch in subjects with HLHS and to compare these data to subjects with other forms of single ventricle and subjects with simple shunt lesions who served as controls.
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Material and Methods
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With the approval of the Institutional Review Board of Emory University (October 15, 2003) we examined the elastic properties of the aorta at the time of routine preoperative transesophageal echocardiography. A total of 60 patients were enrolled into the study. There were 20 patients with HLHS who had previously undergone Norwood palliation and subsequent bidirectional Glenn surgery. One patient had a modified Norwood operation using a right ventricle to pulmonary artery conduit whereas the remainder had classic Norwood operations using the Blalock-Taussig shunt. We also evaluated 18 age-matched patients with single-ventricle lesions without arch obstruction or hypoplasia. The anatomic diagnoses of these subjects included pulmonary atresia with intact ventricular septum (n = 6), tricuspid atresia (n = 4), double-outlet right ventricle with atrioventricular canal defect (n = 2), and other (n = 6). A control group of 22 patients with two-ventricular lesions and no aortic arch hypoplasia or obstruction were also studied. Diagnoses in this group included atrial septal defect (n = 10), ventricular septal defect (n = 8), balanced atrioventricular septal defect (n = 3), and total anomalous pulmonary venous return (n = 1). The three patient groups were comparable with respect to age and body surface area (Table 1).
The surgical technique of arch reconstruction in HLHS involved first extending an incision on the medial portion of the descending thoracic aortic distally beyond the insertion of the ductus arteriosus. The incision was extended proximally along the lesser curvature of the arch to the level of the transected pulmonary artery. A cryopreserved homograft patch was sutured using continuous polypropylene suture along the entire arch. The ductal tissue was excised. The pulmonary artery was then anastomosed in a side-to-side fashion with the ascending aorta using running polypropylene suture. Either aortic (n = 1) or pulmonary (n = 19) homograft material was used in all of the Norwood arch reconstructions. One patient in the HLHS group required balloon angioplasty for recoarctation within 1 month of the Norwood operation. This subject had no residual gradient in the reconstructed arch at the time of acquisition of elasticity data. All other HLHS patients had no residual coarctation noted on cardiac catheterization or echocardiography. Qualitative right ventricular systolic function was mildly diminished in 2 patients in the HLHS group and normal in the remainder. Tricuspid regurgitation in the HLHS group was none to mild in 17 patients, moderate in 2 patients, and severe in 1 patient. Medication usage at the time of hospital admission is shown in Table 1.
Hemodynamic Monitoring
All patients had simultaneous blood pressure recording using invasive arterial monitoring at the time of the echocardiogram. Systolic, diastolic, and mean arterial blood pressure was recorded at the time of M-mode interrogation. The heart rate was also recorded from a surface electrocardiogram.
Echocardiography
Images of the aortic arch were obtained by preoperative transesophageal echocardiography under general anesthesia. Echocardiograms were performed using either a Philips Sonos 5500 or 7500 (Andover, MA) with standard 4- to 7-MHz multiplane transesophageal probes. Short-axis (cross-sectional) views of the distal reconstructed transverse arch (between the origin of the left carotid and left subclavian arteries) were obtained. To obtain theses images the transesophageal probe tip was positioned just above the level of the transverse arch, and the probe tip was retroflexed. M-mode measurements were used to determine the end-systolic as well as end-diastolic diameter. End systole and diastole were determined by the largest and smallest diameter of the aorta, respectively, and confirmed by correlating measurements to the surface electrocardiogram.
Aortic Elastic Function
The aortic elastic function was quantified using the distensibility index, where distensibility index = 2 x [change in aortic diameter/(diastolic diameter x pulse pressure)] and the stiffness index (ß) where ß = [ln(systolic blood pressure/diastolic blood pressure)]/(change in aortic diameter/diastolic aortic diameter). Both distensibility index and stiffness index have previously been extensively used to evaluate arterial compliance [8–13]. The latter measure is influenced less by changes in blood pressure [8].
All continuous variables are reported as mean ± standard deviation. We used Fisher's exact test or analysis of variance, with post hoc Tukey test, to determine significant group differences. Significance was set at the 0.05 
level. Analysis was performed with STATA 6.0 (College Station, TX).
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Results
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There was no difference among groups with regard to median age, mean blood pressure, heart rate, or body surface area (Table 1). The HLHS group had more angiotensin-converting enzyme inhibitor and diuretic usage when compared with the single- and two-ventricle groups (p = 0.02; Table 1). The distensibility index in the transverse (reconstructed) arch was 7.2 ± 4.7 x 10–3/mm Hg for the HLHS group versus 10.1 ± 3.1 x 10–3/mm Hg and 10.7 ± 2.6 x 10–3/mm Hg in the single- and two-ventricle groups, respectively (Fig 1). Stiffness index (ß) within the transverse (reconstructed) arch was 6.8 ± 4.0 for the HLHS group versus 4.1 ± 1.4 and 4.9 ± 1.9 for the single- and two-ventricle groups, respectively (Fig 2). When compared with the other single-ventricle and two-ventricle patients, the HLHS patients had a significantly lower distensibility index and higher stiffness index (p = 0.007 and p = 0.005, respectively). There was no significant difference in measures of elasticity between the non-HLHS single-ventricle and two-ventricle groups. Transverse arch diastolic dimension indexed to body surface area was 2.3 ± 0.6 in HLHS patients versus 1.8 ± 0.4 in other single-ventricle patients and 1.5 ± 0.3 in two-ventricle patients. The transverse arch diameter of the HLHS patients was significantly larger than other single-ventricle and two-ventricle patients (p = 0.01 and p < 0.001, respectively).
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Fig 1. Distensibility index (10–3/mm Hg) in the transverse aortic arch in the three patient groups. The hypoplastic heart syndrome (HLHS) group demonstrated a lower index than either of the other two groups, non-HLHS single-ventricle lesions or two-ventricle lesions (*p = 0.007).
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Fig 2. Stiffness index (ß) in the transverse aortic arch in the three patient groups. The hypoplastic heart syndrome (HLHS) group with aortic arch reconstruction demonstrated a significantly higher stiffness index than the two other patient groups, non-HLHS single-ventricle lesions or two-ventricle lesions (*p = 0.005).
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Comment
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In the present study, we have demonstrated a decrease in elasticity within the reconstructed aortic arch after stage I Norwood palliation of HLHS. This impairment may be related in part to abnormal elastic properties in the native aortic tissue in these patients with aortic arch hypoplasia. In patients with coarctation of the aorta, aortic tissue proximal to the obstruction has been shown to contain more collagen and less smooth muscle than the tissue distally [14]. A persistent increase in stiffness within the aorta after successful repair of coarctation of the aorta is known to exist [8, 15]. As opposed to adults with hypertension in whom the elastic properties of the entire arterial bed are thought to be affected, in coarctation of the aorta the abnormalities in elastic properties are thought to occur proximal to the area of coarctation [8, 16–18]. In addition, it is known that the myocardium of patients with HLHS contains less collagen matrix than that of normal hearts [19]. It is possible that this abnormality involves the aorta as well in these patients. Abnormalities in the fibrous matrix of the aorta may in fact be present in a spectrum of congenital heart diseases including seemingly "minor" ones such as restrictive ventricular septal defect [20]. Another factor possibly contributing to impaired elasticity within the reconstructed arch is the incorporation of homograft material. The Norwood palliation commonly uses pulmonary homograft material in the neoaortic reconstruction, and the homograft often comprises more than 50% of the reconstructed segment in many patients [5]. Several groups have suggested that aortic arch reconstruction in the Norwood should be performed primarily, thereby avoiding the use of homograft material [21, 22]. The impetus for this approach has been to avoid aneurysmal changes to the aorta in adolescence and adulthood. Although this surgical modification may increase the risk for early residual coarctation, the long-term effects of this strategy on arch growth, potential for dilation, and elasticity are not known. In addition, it is possible that technical changes could be made so that the caliber of the reconstructed aorta would be smaller, which would result in homograft material comprising less of the overall dimension.
A number of investigators have directly studied the elastic properties of aortic and pulmonary homograft as these materials are often used in aortic reconstruction in adults. In these studies cryopreservation has been shown to impair endothelial function. However, elastic properties and breaking stress are thought to be maintained after cryopreservation [23]. In addition, aortic and pulmonary homografts display similar mechanics after cryopreservation [24]. Furthermore, Melina and colleagues [25] have demonstrated normal distensibility within aortic root homografts up to 1 year after replacement. As such, one could speculate that in HLHS patients who have undergone the Norwood procedure it is abnormal native aortic tissue, rather than the inclusion of homograft material, that may play a greater role in the impairment in elasticity in the short term. Although the reconstructed arch in HLHS patients is known to grow similarly to normal aortic growth during childhood, it is not known whether the homograft material affects the elastic properties of the aorta long term [5].
The use of angiotensin-converting enzyme inhibitors has been shown to improve aortic elastic properties in a number of disease states [26, 27]. Patients with HLHS in this study had diminished elasticity measures despite increased usage of angiotensin-converting enzyme inhibitors compared with the other two groups. To date, no benefit of routine angiotensin-converting enzyme inhibitor use has been demonstrated in children with single-ventricle Fontan physiology. Further studies will be needed to determine what role such medications have in altering aortic elastic properties in the HLHS population. Moreover, studies linking abnormal arterial elastic properties with measures of ventricular compliance or mass will be needed.
In conclusion, we observed that aortic stiffness and distensibility in the reconstructed aortic arch are impaired after the Norwood operation for HLHS—a finding not seen in other patients with single-ventricle lesions. Given the variety of approaches of arch reconstruction in HLHS, ongoing evaluation will be helpful in understanding whether these strategies may alter elastic properties after Norwood palliation.
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Abstract
Introduction
