The natural and modified history of the patient with congenitally corrected transposition, more recently referred to by some as double discordance because of the discordant connections found at both the atrioventricular and ventriculoarterial junctions, is indeed a guarded and perilous one.^1–7 From a clinical perspective, the outcomes of these patients are closely related to the frequent co-existing cardiac anomalies, particularly ventricular septal defect and obstruction of the sub-pulmonary outflow tract from the morphologically left ventricle, and the anatomy of the specialized conduction tissue, which underscores the predisposition to spontaneous and acquired complete heart block.^1–8 Perhaps the “Achilles Heels” of this malformative complex, nonetheless, are the form and function of the systemic morphologically right ventricle and the morphologically tricuspid valve.^{1–5, 7} Because of many clinical observations documenting attrition of patients with congenitally corrected transposition both before and after surgery, indeed worsening in some even after physiological surgical repair,^{1–4, 7, 9–22} it is not surprising that many have questioned the long-term ability of the morphologically right ventricle to function as the systemic ventricle in these patients and have thus sought alternatives to physiological repair. The surgical innovation of repositioning the morphologically left ventricle into the systemic circulation, and the morphologically right ventricle to the lesser circulation, of the patient with double discordance began with the experience of Ilbawi and colleagues in 1990.²³ Their novel approach was the first to achieve a type of double switch operation at both atrial and ventricular levels. Similar concerns about the right ventricle, of course, are germane to the patient with concordant atrioventricular but discordant ventriculoarterial connections who has undergone an atrial form of physiological repair.²⁴ Since the systemic morphologically right ventricle in patients with double discordance, or indeed in “regular” transposition after physiological repair, is vulnerable or susceptible to early failure, what are the underlying mechanisms contributing to this?

Considerable data has been marshalled to either support or refute, indeed largely to refute, the ability of the systemic morphologically right ventricle, the lesser ventricle, to function normally in the greater systemic circulation of the patient with double discordance. A number of accounts have been given of patients with double discordance surviving to the 7th, 8th and 9th decades of life,^25–34 the inference of these observations being that the morphologically right ventricle can indeed function long-term as the systemic ventricle. Yet, despite these anecdotal cases of longevity, there is increasing clinical evidence, though not unanimity, that the morphologically right ventricle does not perform well in the medium-to-long-term as a systemic ventricle in relatively large cohorts of patients with double discordance. These observations are predicated on the many longitudinal studies of these patients published primarily in the past two and a half decades.^{1, 7, 9–21} Furthermore, most such longitudinal studies indicate that both moderate-to-severe tricuspid regurgitation, and poor right ventricular function, are arbiters of poor outcome amongst patients with double discordance.^{1, 7, 9–21, 35, 36} The exact prevalence of abnormalities of the morphologically tricuspid valve in these patients is subject to discussion and debate. Some anatomical studies report a prevalence of abnormalities in between half and nine-tenths of patients, while in other clinical studies, the prevalence was lower at 44%.^{2, 3, 35–37} Acar and colleagues have found that important functional and anatomical abnormalities of the tricuspid valve are more common in patients with double-discordance who have either an intact ventricular septum or associated ventricular septal defect when compared to a group with associated ventricular septal defect and pulmonary stenosis.³⁶ Van Praagh and his colleagues have placed into perspective those anatomic features of the morphologically right ventricle and the tricuspid valve in the setting of double discordance that jeopardize these patients.³⁸ These include the nature of the coronary circulation, the disposition of the papillary muscles, and the geometric integrity of the tricuspid as opposed to the mitral valve when guarding a circular systemic ventricular orifice.³⁸ Some investigations of outcome suggest that the functional deterioration of the systemic right ventricle begins with chronic volume loading from tricuspid regurgitation, and that treatment should be aimed at maintaining tricuspid valvar competence.³⁵ Other studies are less certain or persuasive about the relationship between right ventricular dysfunction and tricuspid regurgitation, raising the spectre of the “chicken and egg” phenomenon.^{14, 17–19}

Acknowledging that systemic right ventricular dysfunction and tricuspid regurgitation are interlinked, indeed comorbid, and that both adversely affect the outcomes of these patients, it becomes important further to define the etiology of the systemic right ventricular dysfunction in these patients. Thus, there has been a long interest in the myocardium of the systemic right ventricle in patients with double discordance, with many studies using radionuclide imaging.^39–45 Some years ago, Hornung and colleagues,⁴⁶ using Sestamibi scanning, reported that patients with double discordance have a high prevalence of myocardial perfusion defects, with consequent regional abnormalities of motion of the ventricular walls, and hypertrophy and impaired ventricular performance. They went on to conclude that ischemia and infarction are important arbiters of systemic right ventricular failure in these patients. Espinola-Zavaleta and colleagues extend these observations in this issue of Cardiology in the Young.⁴⁷ They studied a modest-sized cohort of adults with double discordance using both contrast echocardiography and equilibrium radionuclide ventriculography and gated single-photon emission computed tomography with Technetiumc-99 m sestamibi. Like Hornung and his colleagues,⁴⁶ they also found persistent and ischemic perfusion defects, suggesting that these defects, together with chronic volume overload from tricuspid regurgitation, are the determining factors of right ventricular dysfunction in these patients. Tulevski and his colleagues,⁴⁸ also in this issue, have studied global and regional right ventricular function in patients with double discordance using cardiac magnetic resonance imaging, at rest and during dobutamine stress. In this study of again only a modest cohort of patients, they found that ischemia of the right ventricular myocardium contributes to the development of right ventricular dysfunction.⁴⁸ These authors found that the morphologically right ventricle, when subjected to systemic pressures in patients with double discordance, shows extensive and global hypertrophy compared to the thin wall of the right ventricle when functioning under normal pressure. Hypertrophy places additional demand on the supply through the right coronary artery, and progressive ischemia likely develops, leading to ventricular dysfunction.⁴⁹ If myocardial ischemia and infarction of the abnormally hypertrophied systemic right ventricle contribute to the dysfunction of the right ventricle, then the observations of Hauser and colleagues⁵⁰ are equally germane. They have shown that patients with double discordance have impaired myocardial blood flow and coronary flow reserves of the systemic morphologically right ventricle.⁵⁰ It is hardly surprising in the light of these observations concerning anatomical and clinical issues relative to longitudinal outcome and the state of the ventricular myocardium and its arterial supply that the desirable evolution from physiological to anatomic repair of patients with double discordance has now been widely adopted.^{23, 51–56} We should not be surprised that, amongst patients with a concordant atrioventricular but discordant ventriculoarterial connections who have undergone a physiological atrial repair by means of the Mustard or Senning procedures, findings of myocardial ischemia and infarction using similar methodologies have also been identified, providing one explanation for the right ventricular dysfunction and failure also seen in these patients.^{24, 57–63} With these increasingly sophisticated observations, it becomes clear why the “lesser” ventricle does not fare well in the “greater” circulation. They support strongly the notion that, at least for some patients with double discordance, surgical strategies to reposition or restore the lesser ventricle into the lesser pulmonary circulation should likely be employed, or at least considered.^64–67