Electrophysiologic delineation of the specialized atrioventricular conduction system in two patients with corrected transposition of the great arteries in situs inversus (I,D,D)

Step-By-Step Segmental Analysis of the Fetal Heart with an Uncommon Situs Anomaly

Congenital heart diseases are the most common congenital anomalies during the fetal period. The diagnosis of complex congenital heart diseases in the fetal period is not always easy. While evaluating cardiac abnormalities, performing segmental analysis and step-by-step evaluation will facilitate the diagnosis of prenatal cardiac diseases. Here, we report a rare variant AV (atrioventricular) and VA (ventriculoarterial) discordance pattern detected in utero and identified based on step-by-step segmental analysis.

The systemic right ventricle in adult congenital heart disease: why is it still such a challenge and is there any hope on the horizon?

PURPOSE OF REVIEW

Adult congenital heart disease patients with systemic right ventricle comprise a distinctly clinically challenging group of patients with increased morbidity and mortality. This article aims to review the different subgroups, most common complications and different treatment strategies.

RECENT FINDINGS

Most commons long-term complications include heart failure and arrhythmias. Heart failure medical therapy treatments include several new agents, which show promise in systemic right ventricle patients. In addition, interventional therapies to mitigate atrioventricular valve regurgitation, baffle/conduit stenosis are discussed. Furthermore, several electrophysiological approaches to manage tachyarrhythmias as well as bradycardias are discussed. There is ongoing excitement on the new medical as well as interventional therapies that could provide benefit in additional to standard goal-directed medical therapy.

SUMMARY

There is an array of medications as well as interventions aimed to treat patients with systemic right ventricle with limited benefits. A multidisciplinary approach with a prudent combination of such therapies to maximize benefit is imperative. This article reviews the data supporting such therapies.

The assessment of patients with situs inversus and corrected transposition of the great arteries

INTRODUCTION

Congenitally corrected transposition of the great arteries (ccTGA) is a rare congenital heart defect. There are different subgroups according to the location of the heart in the thorax, apical position and situs.

OBJECTIVES

The purpose of this study was to assess pediatric patients with situs inversus (SI) ccTGA (SI-ccTGA), a rare subgroup of this condition, in detail.

METHODS

The records of patients with SI-ccTGA followed between January 1, 2010 and January 1, 2019 in our clinic were analyzed retrospectively. Demographic features, associated cardiac defects, arrhythmias and follow-up data were recorded.

RESULTS

Twenty-one out of 120 ccTGA patients had SI. The median age was 30 months (4 days-18 years). There were hemodynamically significant associated lesions in 85.7% (n=18) of the patients. A large ventricular septal defect was found in 16 patients (76.2%), severe pulmonary stenosis in 11 (52.4%), pulmonary atresia in six (28.5%), and severe tricuspid regurgitation in two (9.5%). Eleven out of 21 patients had biventricular physiology and eight had single-ventricle physiology. Bidirectional cavopulmonary anastomosis followed by a hemi-Mustard-Rastelli operation were planned for the remaining two patients. Twelve out of 18 patients with associated defects (66.6%) were operated and surgery was planned for three more patients (16.6%). The remaining three patients were scheduled for clinical follow-up. Arrhythmias developed in two (9.5%) patients on follow-up; ablation was performed in one of them and pacemaker implantation followed by cardiac resynchronization therapy was performed in the other. Two patients died during follow-up, one after a central shunt operation and the other preoperatively due to pneumonia and sepsis.

CONCLUSION

SI-ccTGA is not a mirror image of situs solitus ccTGA (SS-ccTGA) due to important anatomic and physiologic differences between them. SI-ccTGA patients have a lower risk of tricuspid valve regurgitation than SS-ccTGA patients. The timing of clinical presentation of these patients mainly depends on the type and severity of the associated lesions, as in all subtypes of ccTGA.

Toward detection of conduction tissue during cardiac surgery: Light at the end of the tunnel?

Postoperative conduction block requiring lifetime pacemaker placement continues to be a considerable source of morbidity for patients undergoing repair of congenital heart defects. Damage to the cardiac conduction system (CCS) during surgical procedures is thought to be a major cause of conduction block. Intraoperative identification and avoidance of the CCS is thus a key strategy to improve surgical outcomes. A number of approaches have been developed to avoid conduction tissue damage and mitigate morbidity. Here we review the historical and contemporary approaches for identification of conduction tissue during cardiac surgery. The established approach for intraoperative identification is based on anatomic landmarks established in extensive histologic studies of normal and diseased heart. We focus on landmarks to identify the sinus and atrioventricular nodes during cardiac surgery. We also review technologies explored for intraoperative tissue identification, including electrical impedance measurements and electrocardiography. We describe new optical approaches, in particular, and optical spectroscopy and fiberoptic confocal microscopy (FCM) for identification of CCS regions and working myocardium during surgery. As a template for translation of future technology developments, we describe research and regulatory pathways to translate FCM for cardiac surgery. We suggest that along with more robust approaches to surgeon training, including awareness of fundamental anatomic studies, optical approaches such as FCM show promise in aiding surgeons with repairs of heart defects. In particular, for complex defects, these approaches can complement landmark-based identification of conduction tissue and thus help to avoid injury to the CCS due to surgical procedures.

Introduction to the Congenital Heart Defects: Anatomy of the Conduction System

The position and course of the conduction system in congenital heart disease are intricately tied to the underlying congenital malformation. Although only subtle differences exist between the anatomy of the conduction axis for simple congenital heart lesions and normal anatomy, almost every patient with congenital heart disease harbors some important anatomic variation. This article summarizes the body of literature by retaining original classical concepts and by attempting to translate the available knowledge into useful points for the congenital heart disease specialist. This discussion spans the entire spectrum of simple to complex congenital heart disease.

Problems in the diagnosis of discordant atrioventricular with concordant ventriculo-arterial connections: anatomical considerations, surgical management, and long-term outcome

BACKGROUND

Discordant atrioventricular with concordant ventriculo-arterial connections is a rare cardiac defect. When isolated, the haemodynamics resemble transposition of the great arteries. In complex heart defects such as heterotaxy, haemodynamics guide the surgical approach.

OBJECTIVE

To report a series of eight patients with discordant atrioventricular and concordant ventriculo-arterial connections focussing on anatomical and diagnostic difficulties, surgical management, and follow-up.

METHODS

A retrospective review was carried out from 1983 to 2013. Anatomical description was based on segmental analysis. Emphasis was placed on the venoatrial connections.

RESULTS

Segmental arrangement was {I, D, S} in six patients, all with spiralling great vessels. There were two patients with parallel great vessels of whom one had {S, L, D} and the other had {S, L, A} arrangement. Of eight patients, five had heterotaxy syndrome. Median age at repair surgery was 1.4 years (with a range from 1.1 months to 8.1 years). The repair surgery finally performed was the atrial switch procedure in seven out of eight patients. The main post-operative complications were two cases of baffle obstruction and one sick sinus syndrome needing pacemaker implantation. There were two early post-operative deaths and six late survivors. Median follow-up was 4.2 years (with a range from 3.9 to 26.7 years) with good functional status in all survivors. Discussion Diagnosing discordant atrioventricular with concordant ventriculo-arterial connections remains challenging. There are ongoing controversies about the definition of atrial morphology and heterotaxy syndrome animating the anatomic discussion of these complex heart defects. Haemodynamically, the atrial switch procedure is the surgical method of choice with an encouraging long-term follow-up despite rhythm disturbances and baffle obstruction.

Research accomplishments in pediatric electrophysiology: a historical review

Arrhythmia management in young patients plays a small but important role within the broader discipline of cardiac electrophysiology (EP). By virtue of its sharp focus on developmental cardiology, congenital heart defects, and hereditary channelopathies, pediatric EP has contributed in significant ways to improved understanding of rhythm disorders in patients of all ages. Scientific progress in the field was recently reviewed at the 2012 meeting of the Heart Rhythm Society, where historical details of pioneering investigations in pediatric EP were highlighted. In this article, we hope to summarize those details and acknowledge the roles of individuals and organizations that were instrumental in developing the subspecialty.

Comparison of outcomes in adults with congenitally corrected transposition with situs inversus versus situs solitus

The long-term outcome of patients with congenitally corrected transposition of the great arteries is mainly determined by progressive morphologically tricuspid valve regurgitation, heart block, atrial arrhythmias, and/or systemic ventricular dysfunction. Situs abnormalities have been reported in ≤34% of cases, but whether clinical differences exist between a situs inversus and situs solitus arrangement has not yet been studied. The clinical records of 38 adults with congenitally corrected transposition of the great arteries (mean age 40 ± 15 years) followed for a mean period of 7.4 years were reviewed. Of these 38 patients, 8 presented with situs inversus and 30 with situs solitus. No significant differences were found between the 2 groups in age, gender, ventricular septal defect, pulmonary tract stenosis, previous surgical repair, or duration of follow-up. However, none of the patients with situs inversus presented with an Ebstein-like anomaly of the morphologically tricuspid valve and none developed nonoperative-related complete atrioventricular block compared to 15 (50%; p = 0.013) and 11 (42%; p = 0.032) of the patients with situs solitus, respectively. At follow-up, 2 patients with situs inversus (25%) presented with sustained atrial arrhythmia, severe tricuspid regurgitation, or severe systemic right ventricular systolic dysfunction compared to 22 (73%) of 30 those with situs solitus (p = 0.034). No patient with situs inversus presented with cardiac death or severe heart failure compared to 12 (40%) of 30 with situs solitus (p = 0.038). In conclusion, Ebstein-like anomaly or spontaneous complete atrioventricular block are rare in patients with congenitally corrected transposition of the great arteries with situs inversus, and late complications are uncommon. The long-term outcome of patients with situs inversus was significantly better than that for patients with situs solitus.

Twin atrioventricular nodes connecting to sling of conduction tissue in congenitally corrected transposition associated with straddling tricuspid valve

This report describes a case of reentrant supraventricular tachycardia relating to "twin" atrioventricular nodes in a 6-year-old female patient with congenitally corrected transposition of the great arteries associated with straddling tricuspid valve. Straddling and overriding of the tricuspid valve literally or by definition indicate malalignment of the inlet part of the ventricular septum and atrial septum by an abnormally large ventriculoatrial septal angle. Therefore, anterior atrioventricular conduction tissues are expected in this setting; however, the present report describes an exception to the rule that combination of straddling tricuspid valve is unlikely to be associated with twin atrioventricular nodes/sling. The patient underwent successful ablation of one atrioventricular node, eliminating the tachycardia, and subsequently Fontan-type palliation.

Corrected transposition of the great arteries involving situs inversus {I, D, D} and mild pulmonary stenosis: conduction system identified during preoperative investigations for a double-switch operation

The atrioventricular (AV) conduction system in corrected transposition of the great arteries (cTGA) involving situs inversus remains unclear. A case of cTGA involving situs inversus, mild pulmonary stenosis, and ventricular septal defect (VSD) is reported. An electrophysiologic study using the CARTO system and angiography was performed to identify the AV conduction system before a double-switch operation with VSD enlargement. The His bundle was located at the anteroseptum of the left-sided RA. It descended along the posteroinferior margin of the VSD with cTGA involving situs inversus and a normal-sized pulmonary trunk. No AV block occurred after surgery. The combination of the CARTO system and angiography could be useful for identifying the conduction system.

The diverse cardiac morphology seen in hearts with isomerism of the atrial appendages with reference to the disposition of the specialised conduction system

Congenital cardiac malformations which include isomerism of the atrial appendages are amongst the most challenging of problems for diagnosis and also for medical and surgical management. The nomenclature for pathological description is controversial, but difficulties can be overcome by the use of a segmental approach. Such an approach sets out the morphology and the topology of the chambers of the heart, together with the types and modes of the atrioventricular, ventriculo-arterial, and venous connections. We have applied this method to a study of 35 hearts known to have isomerism of the atrial appendages. We have already published accounts of 27 of these cases, but these were reviewed for this study in the light of our increased awareness of the implications of isomerism, and 8 new cases were added. After examining, or re-examining, the morphology of every heart in detail, we grouped them together according to their ventricular topology and modes of atrioventricular connection. Then we studied the course of the specialised conduction system, by the use of the light microscope, first in each individual case, and then together in their groups. We conclude that the pathways for atrioventricular conduction in hearts with isomerism of the atrial appendages are conditioned both by ventricular topology, and by the atrioventricular connections. Based on our experience, we have been able to establish guidelines that direct the clinician to the likely location of the conduction tissues.

Congenitally corrected transposition: size of the pulmonary trunk and septal malalignment

BACKGROUND

In hearts with congenitally corrected transposition of the great arteries, the atrial and ventricular septums are usually malaligned. This is associated with an abnormal location of the atrioventricular conduction system. However, well-aligned septums and normally positioned atrioventricular conduction tissues have been found in a few cases, some of which also happened to have a small or atretic pulmonary trunk. Our aim was to determine whether septal alignment could be predicted on the basis of the size of the pulmonary trunk. Potentially this could provide a guide to the disposition of the atrioventricular conduction tissues.

METHODS

We examined 14 hearts. In all cases there was usual atrial arrangement. We carried out histologic examination of the sites of the atrioventricular conduction tissues in 1 heart.

RESULTS

We found a statistically significant correlation between the size of the pulmonary trunk and the degree of septal malalignment. In the presence of a small or atretic pulmonary trunk, the septums were well aligned. Our histologic study of one such specimen revealed dual atrioventricular nodes connecting to a sling of conduction tissue. This suggests that presence of a postero-inferiorly situated atrioventricular conduction bundle in addition to an anteriorly located bundle may be anticipated when the septums are well aligned.

CONCLUSIONS

The presence of a small or atretic pulmonary trunk in congenitally corrected transposition of the great arteries is associated with good septal alignment. This should alert the surgeon to the possibility of a slinglike arrangement of the atrioventricular conduction system.

Double switch for congenitally corrected transposition of the great arteries

OBJECTIVE

To evaluate the early and medium term results of operations for congenitally corrected transposition of the great arteries (CCTGA) wherein the left ventricle becomes the systemic ventricle.

METHODS

Fourteen patients with CCTGA who underwent anatomic repair from 1994 to 1998 were placed in one of two groups: those without pulmonic stenosis (PS) (N = 7) and those with PS (N = 7). Main associations in the group without PS were: severe left atrioventricular (AV) valve regurgitation (N = 4), ventricular septal defect (VSD) (N = 4), criss-cross AV connection (N = 1), and hypoplasia of the morphologic RV (N = 1). Main associations in the group with PS were: VSD (N = 7), anomalous pulmonary venous drainage (N = 2) and major aorto-pulmonary collaterals (N = 1). All patients in the group without PS underwent an arterial switch operation and all except one had a modified senning repair for atrial rerouting for anatomic correction. All seven patients with PS underwent a Rastelli repair and all except one needed a modified senning repair.

RESULTS

Early survivorship in both groups was 6/7. Follow-up ranges from 1 month to 48 months. The mean left ventricular ejection fraction (LVEF) in the arterial switch and senning patients was 65% and in the Rastelli and atrial rerouting was 52%. None of the survivors except one have any significant mitral regurgitation (MR). The one exception resulted from iatrogenic damage to a chorda during VSD closure. All except two patients are class I symptomatic and all are in normal sinus rhythm.

CONCLUSION

Satisfactory early and mid term results of anatomic repair support the double switch option as the procedure of choice for patients of CCTGA amenable to biventricular repair.

Congenitally corrected transposition with atrioventricular septal defect

We describe 4 cases of congenitally corrected transposition associated with atrioventricular septal defect, diagnosed by echocardiography and angiocardiography. Two had usual atrial arrangement and two had mirror imaged atrial arrangement . All cases were associated with subpulmonary valvar stenosis. All patients presented with cyanosis and were in sinus rhythm. Atrioventricular septal defect with common atrioventricular junction was easily diagnosed on the basis of a common atrioventricular valve permitting interatrial and interventricular communications. All patients had balanced right and left ventricles. The echocardiographic recognition of the ventricles was based on the presence of the moderator band within the morphologically right ventricle, the characteristics of the apical septal trabeculations, and the shape of the ventricles. Angiocardiographic recognition of the ventricles was achieved on the basis of right and left ventriculography. In one case with usual atrial arrangement, we recorded two His bundle potentials, one anteriorly and another posteriorly. Atrial stimulation revealed blocked atrioventricular conduction at the level of the posterior bundle, and normal atrioventricular conduction through the anterior bundle. In both cases with atrial mirror-imagery, only a posterior His bundle potential was found, with normal atrioventricular conduction revealed by atrial stimulation The clinical course with this combination depends on the other lesions present in addition to the common atrioventricular valve. Our electrophysiological studies show that the conduction system in presence of a common atrioventricular valve is as expected for congenitally corrected transposition with two atrioventricular valves.

Surgical Repair of Corrected Transposition of the Great Arteries with Cardiac Anomalies: A Report of Fifty-Three Cases

Between February 1964 and June 1994, we operated on 53 patients with corrected transposition of the great arteries associated with cardiac anomalies. Their ages ranged from 2.5 to 37 years (mean 15.9 years). There were 36 patients with type SLL and 17 with type IDD. Forty-three patients had ventricular septal defect with pulmonary stenosis, 7 had VSD with pulmonary hypertension and only 3 patients had atrioventricular valve incompetence without ventricular septal defect. Atrial septal defects were found in 13 cases and patent ductus arteriosus in 2. The main operative procedures were closure of ventricular septal defect (49), closure of atrial septal defect (13), resection of pulmonary stenosis (43) and pulmonary annulus enlargement (3). Additional procedures were bypass between the morphological left ventricle and the pulmonary artery using valved external conduit (4), tricuspid valve repair (4), tricuspid valve replacement (4) and one Fontan operation. Nine patients died (17%) within 30 days of operation. The mortality rate decreased from 29.2% to 6.9% after 1988. The main cause of early death was low cardiac output syndrome. The most common perioperative complications were complete heart block (5) and residual tricuspid valve incompetence (4). Forty of the 44 survivors were followed up from 2 months to 5 years. There were 2 late deaths due to tricuspid incompetence. Our surgical experience in the prevention of operative complications are discussed.

Late results of systemic atrioventricular valve replacement in corrected transposition

From December 1964 to October 1993, 40 patients (aged 5 months to 70 years, mean 21.8 years, median 13.6 years) with corrected transposition and systemic atrioventricular valve insufficiency underwent replacement (n = 39) or repair (n = 1) of the systemic atrioventricular valve. Thirty-nine patients had situs solitus and 1 had situs inversus. Associated anomalies included Ebstein's malformation of the systemic atrioventricular valve (n = 22), ventricular septal defect (n = 19), and pulmonary stenosis (n = 14). Preoperatively, 16 patients (40.0%) had complete heart block and 27 patients (67.5%) were in New York Heart Association functional classes III and IV. The early mortality was 10.0% (n = 4) and 8 patients died subsequently. The principal cause of death in all 12 patients was systemic ventricular failure. Overall survival including early mortality was 78.0% at 5 years and 60.7% at 10 years; survival excluding early mortality was 86.7% at 5 years and 67.5% at 10 years. Survivorship correlated with preoperative systemic ventricular ejection fraction of 44% or more (p < 0.001) and later interval of operation (9 deaths in 15 patients before 1981 versus 3 deaths in 25 patients subsequently) (p = 0.06). There were no cases of surgically induced complete heart block. Two patients underwent late reoperations related to the systemic atrioventricular valve prosthesis. Follow-up extended to 26.0 years (median 4.7 years). At last follow-up, 18 of the 28 survivors were in New York Heart Association functional class I, 9 were in class II, and 1 was in class III. We conclude that the results of systemic atrioventricular valve replacement in corrected transposition have improved significantly during the past decade. To preserve systemic ventricular function, we suggest operation be considered at the earliest sign of progressive ventricular dysfunction as assessed by serial clinical evaluation and echocardiography.

The surgical anatomy of ventricular septal defect

There is still no consensus as to how best to categorize and describe interventricular communications. In a series of three reviews, a system will be described showing how the anatomical criteria chosen for categorization will also serve as a guide for surgeons as to the location of the axis responsible for atrioventricular conduction tissue. In this first review, the defects described are not complicated by overriding of arterial or atrioventricular valves and are present in hearts that have basically normal segmental connections, or have some discordant connections (complete transposition or congenitally corrected transposition). The rims of the defect categorize the boundaries to which a surgeon may place a patch. Variations in these rims produce three classes of defect: perimembranous; muscular; and doubly committed and juxtaarterial (subarterial). The second part of the classification recognizes the further variation existing with respect to the component of the morphologically right ventricle into which the defect predominantly empties. Deficient atrioventricular septation can also lead to interventricular shunting in isolation, but the morphology is then quite different from hearts with simple deficiencies of the ventricular septum. We emphasize the abnormal location of the atrioventricular node in hearts with atrioventricular, as opposed to ventricular, septal defects.

Congenitally corrected transposition with mirror-image atrial arrangement

Congenitally corrected transposition (discordant atrioventricular and ventriculoarterial connexions) with mirror-image atrial arrangement is much less frequent than the same chamber combinations in the setting of usual atrial arrangement. This scarcity of cases has made their analysis difficult. In this study we have compared the anomalies found in 19 patients with congenitally corrected transposition in mirror-image arrangement with those in 39 patients having usual atrial arrangement. Absolute, relative and attributable risks were calculated for the presence of subvalvar pulmonary stenosis, ventricular septal defect, tricuspid regurgitation, atrioventricular block or the absence of these anomalies for each of the two groups. A greater absolute risk was found for both subvalvar pulmonary stenosis (68.4 vs. 43.6%, respectively) and ventricular septal defect (63.2 vs. 58.9%, respectively). The absolute risk for tricuspid regurgitation was 25.6% in those with usual arrangement as against 21.1% in those with mirror-image arrangement. Atrioventricular block was also more frequent in those with usual arrangement (25.6 vs. 10.5%) as was absence of associated cardiac defects (17.9 vs. 10.5%, respectively). The relative risks were greater for subvalvar pulmonary stenosis and ventricular septal defect in the patients with mirror-image as opposed to usual atrial arrangement (1.57: 1.00 and 1.07: 1.00, respectively), contrasting with greater relative risk for tricuspid regurgitation (1.26: 1.00), atrioventricular block (2.43: 1.00) and absence of associated anomalies (1.70: 1.00) in the patients with usual arrangement. The attributable risk for subvalvar pulmonary stenosis was estimated to be 24.8% and ventricular septal defect 4.3% for those having mirror-image atrial arrangement.(ABSTRACT TRUNCATED AT 250 WORDS)

Surgical treatment of double-outlet left ventricle in situs inversus [I,D,D]

A case of double-outlet left ventricle [I,D,D] with a subaortic ventricular septal defect and pulmonary stenosis is reported. The diagnosis was made at operation and was reconfirmed postoperatively by two-dimensional echocardiography. Repair was achieved by patch closure of the ventricular septal defect and enlargement of the functional right ventricular outflow tract using a valved patch. The postoperative course was uneventful except for a transient complete heart block. To our knowledge, this represents the second instance of surgical treatment of double-outlet left ventricle [I,D,D] reported in the literature.

Congenital atrioventricular block in right atrial isomerism (asplenia). A case due to atrionodal discontinuity

The association of complex congenital heart disease with asplenia syndrome and congenital AV block is described in a newborn. Cardiovascular lesions were right atrial isomerism, d-loop, discordant ventriculoarterial connection, pulmonary stenosis, total anomalous pulmonary venous drainage, absent right pulmonary artery, and bilateral superior vena cava. The ECG recording disclosed complete AV block with narrow QRS (suprahisian block). The histology of the conduction system revealed two AV nodes, one anterior and one posterior, none of which connected with the atrial myocardium; only the posterior node was continuous with a penetrating and bifurcating bundle. The histologic findings were consistent with the suprahisian AV block due to lack of connection between the atria and the regular posterior AV specialized junction.

Intraoperative recording of specialized atrioventricular conduction tissue electrograms in 47 patients

Intraoperative mapping of the specialized atrioventricular conduction system was performed in 47 patients during cardiac surgery. Specialized conduction tissue electrograms were identified in 37, and atrioventricular conduction preserved in 92%. Specialized conduction tissue was identified in 27 patients with atrioventricular canal defect: complete heart block was avoided in 25. Conduction tissue was located in six of 12 patients with complex transpositions; atrioventricular conduction was preserved in all six. Other lesions in which the technique was useful were Ebstein's anomaly and single atrium. Limitations to the technique are 1) deep hypothermia and circulatory arrest; 2) interruption in atrioventricular conduction during mapping; 3) inadequate exposure and access to probable sites of conduction tissue; 4) variation of size and spatial relations of individual malformations; and 5) limited time for identification of unusually located conduction tissue. Indications for use of this technique include patients with both forms of atrioventricular canal, complex transpositions, atrioventricular discordance, single ventricle and single atrium.

Conducting tissues in congenitally corrected transposition with situs inversus

Three cases of congenitally corrected transposition in situs inversus individuals were characterised by visceroatrial situs inversus, atrioventricular discordance, and ventiruloarterial discordance: one case was studied clinically, and the other 2 were necropsy specimens. The disposition of the atrioventricular conducting tissues was established in each case, in the living patient by intraoperative mapping, and in the necropsy specimens by histopathological investigation. In all, the connecting atrioventricular bundle arose from a normally situated posterior atrioventricular node, normally related to the landmarks of the atrial septum. Though anterior nodes were identified in the necropsy specimens, as reported in congenitally corrected transposition in situs individuals, they differed from those in the latter situation in that they made no atrioventricular connection. These findings have obvious surgical importance. It is suggested that the posterior connection is related to the good septal alignment in these cases which lacked significant septal defects. Posterior connections are not necessarily present in all situs inversus individuals with corrected transposition, particularly when there are malalignment ventricular septal defects: further studies are required in such cases.