Cardiac rhythms in dextrocardia

Cardiac Rhythms in Dextrocardia

KAZUO and

MOMMA,

LEONARD

LosAngeles,

MD* M.

LINDE,

MD

California

From the Division of Cardiology, Department of Pediatrics, UCLA School of Meditine, Los Angeles, Calif. 90024. Supported by funds from the Los Angeles County Heart Association, Los Angeles, Calif. Manuscript received February 14, 1969, accepted July 15, 1969. * Present address: Department of Pediatrics, Japan Heart Institute, Tokyo Women’s Medical College, Tokyo, Japan. Address for reprints: Leonard M. Linde. MD, Department’ of Pediatrics, University of California at Los Angeles School of Medicine, The Center for the Health Sciences, Los Angeles, Calif. 90024.

420

The P waves of 40 patients with dextrocardia were analyzed. The usual clinical ranges of the frontal plane axes of the P wave were between +30” and +lOO” in dextroversion and between + 120” and +150” in mirror-image dextrocardia. Exceptions occurred mainly in association with anomalies of the venae cavae, asplenia and poly splenia syndrome. Left atrial rhythms were associated with bilateral superior vena cava, and coronary sinus rhythms occurred with polysplenia syndrome or absent inferior vena cava. The clinical and embryologic aspects of these findings are discussed. Electrocardiographic and vectorcardiographic studies have concentrated on analysis of ventricular depolarization. This study indicates that valuable anatomic information can also be obtained by analysis of atrial depolarization as represented by the P wave axis and contour. Analysis of the P waves of the electrocardiogram is important in the diagnosis of atria1 situs in dextrocardia. An upright P wave in lead I in dextrocardia is associated with atria1 situs solitus, and an inverted P wave in lead I is associated with atria1 situs inversus.1-5 This rule has two exceptions. An ectopic pacemaker may occur in the left (arterial) atrium,sJ-7 or the P wave in lead I may be flat.0v8 This paper presents the results of analysis of P waves in 40 patients with dextrocardia. All but 4 patients had associated cardiac defects; the 36 with cardiac defects were studied at autopsy or cardiac catheterization and, in some cases, at operation. The results of the electrocardiographic analysis, especially the frontal plane axes of the P wave, were correlated with the visceroatrial situs and anomalies of the venae cavae. In this study dextrocardia means right-sided heart. Visceroatrial situs may be inverted (mirror-image dextrocardia) or may not be inverted (dextroversion) . Patients with dextrocardia due to extrinsic causes were excluded from this study. Visceroatrial situs was classified as situs solitus or inversus according to these criteria”rR*g: (1) position of stomach bubble and liver shadow on chest X-ray examination ; (2) position of superior and inferior vena cava at cardiac catheterization; and (3) position of atria and abdominal organs at autopsy. Absent inferior vena cava indicates infrahepatic interruption of the inferior vena cava and azygos continuation.10 The terms “right atrium” and “left atrium” denote anatomic right and left atrium.g

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CARDIAC RHYTHMS

TABLE

IN DEXTROCARDIA

I

Twenty-Four

Cases with Dextrocardia

and Visceroatrial

Situs Solitus -----

Case no.

Age (yr)

Persistent

1

3

M

Means of Diagnosis

Congenital Heart Disease & Other Anomalies

Sex

Left Superior

P-R Interval

c; 0

IVC

+f30 +170

c; 0

0.14 0.12

100 70

+30 -60

+10 +60

C

0.12

100

-60

+50

C

Variable*

+30 -70

+20

3

6

F

Single ventricle; absent incomplete diagnosis

4

l/12

F

ASD; VSD; PS; absent

5

l/180

F

Tricuspid atresia; PDA; PS; left SVC via coronary sinus to RA

C;

6

19

M

VSD; absent

7

25

M

Pentalogy

8

4/12

F

VSD; persistent

left SVC+

Normal

Vena Cava -60

ASD: absent

IVC

Transverse Plane

-130

F

of Fallot; LSVC to LA

Frontal

120 120

7

IVC

P Wave Axis __--.._-.

0.18 0.20

2

IVC;

(per min)

(set)

Vena Cava or Absent Inferior -

Complete A-V canal; absent IVC; bilateral SVC (LSVC to LA)

Heart Rate

0; A

80* 90*

+40

0.09

140

+60

+20

C

0.14

75

-50

+50

c; 0 c; 0

0.20

85

+80

+30

0.10 0.08 0.12

100 150 115

+60 +90 -120

+96 +9fJ :

Venae Cavae

9

3112

M

Single atrium;

C

0.11

130

+90

+90

10

1

F

Single atrium; VSD; infundibular PS; atresia of gallbladder; inperforate anus

C;0; A

0.10

170

+30

+50

11

l/12

M

Complete transposition vessels; ASD

of great

C

0.10

180

12

49

M

Corrected vessels;

of great

C; A

0.16

2

13

VSD; hemivertebrae

transposition VSD

F

VSD

14

l/180

M

Pulmonary corrected

15

5112

M

ASD; VSD; incomplete

16

l/180

F

Trisomy valve

atresia; ASD; VSD; transposition diagnosis

18; VSD; bicuspid

pulmonic

+1m

+100

80

+60

+10

c; 0

0.12

140

+60

+70

C;O; A

0.10

100

+90

+50

C

0.09

180

+80

+90

A

0.14

140

+60

+W

+50

+10

c; 0

0.18

115

c;0

0.13

150

+70

+70

C

0.12

100

+60

+90

C

0.14

80

+100

+90

Acyanotic CHD; incomplete diagnosis

C

0.08

170

+90

+130

F

ASD

c; 0

0.14

95

+40

-50

F

ASD; mild PS

C

0.10

110

+60

+100

M

ASD; PS; corrected

c;0

0.18

80

+80

+10

17

3

M

VSD; ASD; PS

18

l/12

M

VSD; PS; corrected

19

5

F

(Functional

20

3

M

Tetralogy

21

l/12

M

22

7

23

5

24

15

transposition

murmur) of Fallot

transposition

* Complete heart block. + Persistent left superior vena cava was found during banding of the pulmonary artery. Draining site was not ascertained. * P wave axis in transverse plane was difficult to estimate because pacemakers wandered in different chest leads. A = autopsy; ASD = atrial septal defect; AV canal = atrioventricular canal; C = cardiac catheterization; CHD = congenital heart disease; IVC = inferior vena cava; LA = left atrium; LSVC = left superior vena cava; 0 = operation; PDA = patent ductus arteriosus; PS = pulmonary stenosis; RA = right atrium; SVC = superior vena cava; VSD = ventricular septal defect.

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421

MOMMA AND LINDE

FRONTAL

Spatial P wave vectors in dextroversion with norFigure 1. mal venee cavae (vectors 1 and Z), absent inferior vena cava (vector 3, coronary sinus rhythm) and persistent left superior vena cava (vector 4, left atrial rhythm). Vectors l’, 2’, 3’, and 4’ represent frontal projections and vectors I”, 2”, 3” and 4” represent the transverse projections of the respective spatial vectors.

genograms wrre rrviewed to check thr position of the heart, anomalies of the lung or diaphragm and the position of the stomach and liver. All avaihble electrocardiograms obtained before cardiac operation and before administration of digitalis were reviewed. The heart rate, P-R interval, frontal and transverse plane axes of the P wave, and the height and contour of the P wave in leads I, Vc and VaR were measured. We used the established electrocardiographic criteria for heart rate, P-R interval for age, coronary sinus rhythm, atrioventricular block and left atria1 rhythm.ll-l4 Frontal plane P wave axis was determined with use of the triaxial reference frames,ll and the transverse plane P wave axis with use of the reference frames of Mirowski.14 Results were compared with those obtained in a control group of 66 patients with various congenital heart diseases, normal cardiac position and normal vena caval drainage.l” Visceroatrial situs was determined according to the three criteria noted. There were 24 cases of visceroatrial situs solitus (dextroversion) and 13 cases of visceroatrial situs inversus (mirror-image dextrocardia) . Two subjects had polysplenia syndrome 16, and another had asplenia syndr0me.l’ These 3 cases were classified in a third group.

Results Dextroversion

Materials and Methods Forty cases of dextrocardia were collected from cardiac catheterization, autopsy and medical records at the University of California at Los Angeles Medical Center. Autopsy data were available in 9 cases, and cardiac catheterization data in the remaining cases except for 3 in which the subjects had no cardiac defect. The subjects’ ages ranged from 2 days to 49 years; 32 were under 15 years of age. Chest roent-

Clinical Data

Twenty-four patients had dextrocardia and visceroatria1 situs solitus. Age, sex, congenital cardiac anomabdominal visceral anomalies, methods of alies, diagnosis, F-R interval, heart rate, frontal and transverse plane axis of the P wave in these patients are listed in Table I. There were 13 male and 11 female patients from 2 days to 49 years of age. Chest roentgenogram showed right-sided heart, stomach bubble on the left side and hepatic shadow on the right side. A defect in the atria1 or ventricular septum was present in all but Patient 19. Pulmonary stenosis or atresia was present in 10, corrected transposition in 4 and anomalies of the venae cavae in 8 patients. These anomalies included absent inferior vena cava with azygos continuation in 5 patients and persistent left superior vena cava in 4 patients. Electrocardiographic

Data

Normal venae cavae :

Electrocardiogram of a 3 year old boy Figure 2. Case 20. with dextroversion and tetralogy of Fallot (clinical diagnosis). P wave is diphasic (-+) in lead I. Frontal plane P wave axis is about +90”. Leads marked x are recorded at half-standardization.

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Two types of Y waves were noticed in the subjects with normal venae cavae. Spatial vectors of these two P waves are shown as vectors 1 and 2 in Figure 1. Upright P waves in leads I, II and III were the most commonly observed (vector 1, Fig. 1). In these cases, the frontal plane P wave axis was distributed in a normal range ( + 15” to -t-75’), and P waves were upright in leads V, to

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V,. The rtlrt of those with dextroversion and normal venae cavac showed flat or diphasic (- +) P waves in lead I with upright P waves in leads II and III (v&or 2, Fig. 1 and 2). Frontal plane axis was between +80° and +lOO”. No subject showed signs of ectopic rhythm. Heart rate and P-R interval were usually normal. The P wave was always upright in lead V,;. In lead V,R, the dire&ion of the P wave was variable. Absent inferior vena cava: Coronary sinus rhythm was common in these cases. A typical electrocardiogram is shown in Figure 3A, and the spatial P vector is shown as vector 3 in Figure 1. The P wave was upright. in lead I and inverted in leads II and III ; it was always upright in V6, as expected. The frontal plane P wave axis was about -60”. In these cases of coronary sinus rhythm, the heart rate and P-R interval were usually normal, but a degree of heart block was present in some. In some cases, coronary sinus rhythm occasionally changed to sinus rhythm.

Persistent

left superior vena cava to left atrium:

Transient left atria1 rhythm was frequently observed in these cases (Fig. 3B). The spatial P vector of the left atria1 rhythm is shown as vector 4 in Figure 1. The P wave was inverted in leads I, II and III. In chest leads, the P wave was upright in lead VsR, inverted in V,; and showed dome and dart P waves in VI. The heart rate was normal, and the P-R interval was either normal or slightly prolonged.

Mirror-image Dextrocardia Clinical

Data

Thirteen patients had dextrocardia with visceroatria1 situs inversus (Table II) ; 7 were male and 6 were female. Their ages ranged from 1 month to 42 years. All except 3 had associated congenital cardiac defects. Chest roentgenograms showed right-sided heart, right-sided stomach bubble and left-sided liver shadow. A defect in the atria1 or ventricular septum was present in all patients with heart disease. Two subjects had ahsent inferior vena cava with azygos continuation, and 2 had bilateral superior ven’ae cavae. The left-sided superior and inferior venae cavae were diagnosed by cardiac catheterization in all patients with cardiac defects. Electrocardiographic

I

II

RHYTHMS

m

AVR

IN

DEXTROCARDIA

AVL

AVF

Case 1. Electrocardiograms of a 3 year old boy Figure 3. with dexfroversion, complete A-V canal, bilateral superior vena cava and absent inferior vena cava. Two cardiac rhythms are observed. Leads marked x are recorded at halfstandardization. A, so-called coronary sinus rhythm. The P-R interval is prolonged (0.18 set). 6, inverted P waves in leads I, II and III. Chest leads show dome and dart configuration of the P wave in lead V, and inverted P waves in leads K, and Vn (left atria1 rhythm).

tributed mostly between heart rate and P-R interval

+120” and were normal.

$150”.

The

Bilateral superior venae cavae: In 1 of the 2 cases with bilateral superior venae cavae, upright P waves were recorded in leads I, II and III. The spatial P vector is shown in Figure 5 (vect;or 3). This was interpreted as left atria1 rhythm in mirror-image dextrocardia. Absent inferior vena cava: In 1 of these 2 cases

Data

Normal venae cavae (left-sided) was inverted right in the verted in V, and represents dextrocardia. in Figure 5.

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25,

: The P wave in lead I, upright in leads II and III, upright chest leads including VaR and in(Fig. 4). This is the reciprocal of normal normal sinus rhythm in mirror-image The spatial P vector is shown as vector 1 The frontal plane P wave axis was dis-

APRIL

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Figure 4. Case 32. Electrocardiogram of an 8 year old girl with mirror-image dexfrocardia, abdominal situs inversus, ventricular septal defect, pulmonic stenosis and corrected transposition. The P wave is inverted in lead I. The P-R interval is prolonged (0.20 set). P waves are upright in the right chest leads and inverted in the left chest leads (mirror image of the normal P waves).

423

MOMMA

TABLE Thirteen

Case no.

AND

LINDE

II Cases with Dextrocardia

Age (yr) --

Sex

and Visceroatrial

Situs lnversus

Congenital Heart Disease & Other Anomalies

Means of Diagnosis

Persistent 14

25

F

P-R Interval

c; 0

P Wave Axis _-_~_-_ Transverse Plane -___ __~

(per min)

Frontal

0.16 0.16 0.18

70 70 60

+150 +190 +240

+140 +140

(set) ~_.____

Left SVC or Absent

VSD; PDA; absent IVC; bilateral SVC (right SVC via coronary sinus to anatomic RA)

Heart Rate

IVC *

26

1

M

Single ventricle?; bilateral SVC?; incomplete diagnosis

C

0.10

120

+70

+80

27

31

F

ASD; partial anomalous pulmonary venous drainage; absent IVC

C

0.26 t

70 50

+140 -80

+150

Normal

+190

Venae Cavae

28

19

M

Complete transposition of great vessels; VSD; PS

c; 0

0.13

70

+210

+120

29

16

M

Cyanotic CHD; incomplete diagnosis

c; 0

0.18

80

+120

+150

30

6

F

VSD; PS

c; 0

0.14

85

+150

+130

31

5

M

Absence ear

0.16

110

+150

+110

32

8

F

VSD; PS; corrected transposition

c; 0

0.20

110

+120

+130

of right external

33

3/12

F

VSD; PS; partial anomalous pulmonary venous drainage; incomplete diagnosis

C

0.14

150

+90

+70

34

l/12

M

Single atrium; double outlet inverted right ventricle; total anomalous pulmonary venous drainage

C; A

0.09

160

+130

+150

35

42

F

No heart disease

0.16

85

+130

+180

36

3

M

No heart disease

0.10

120

+120

+110

37

l/12

M

PDA; PS

0.11

150

+130

+180

* P wave axis in transverse olane was difficult t Lower nodal rhythm. P wave followed QRS. Abbreviations as in Table I.

c; 0 to estimate

because

without inferior vena cava, the P waves were inverted in leads I, II and III. The spatial P vector is shown in Figure 5 (vector 2). The P-R interval and heart rate were normal. This is coronary sinus rhyt.hm in mirror-image dextrocardia. This rhythm occasionally changed to sinus rhythm. Atrioventricular junctional rhythm with an inverted P wave after the QRS complex (so-called lower nodal rhythm) was also transiently observed.

pacemakers

wandered

Clinical

Asplenia

424

Asplenia

Data

was diagnosed

in 1 case and

polysplenia

chest leads.

in 2 cases. (Table III). The child with asplenia syndrome and another wit,h polysplenia syndrome had left superior and inferior venae cavae, suggesting atria1 situs inversus. The other child with polysplenia syndrome had a right superior vena cava and a persistent left superior vena cava draining to the right atrium through the coronary sinus, suggesting atria1 situs solitus. Electrocardiographic

Dextrocardia with Splenic Syndrome

in different

Data

syndrome :

Two different types of P waves were recorded in the child with asplenia syndrome. In one type the P wave was flat in lead I and

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TABLE Three

RHYTHMS

IN DEXTROCARDIA

III Cases with

Dextrocardia

Case No.

Age (yr)

Sex

38

5/12

M

and Splenic

Syndrome

Congenital Heart Disease & Other Anomalies

Means of Diagnosis

Single ventricle; pulmonary atresia; PDA: left-sided SVC and WC; asplenia ASD (cushion defect type); !-transposition; total anomalous pulmonary venous return; partial abdominal situs inversus

A

P-R Interval

Heart Rate

P Wave Axis ~--Transverse Plane

(set)

(par min)

0.14 0.16

150 130

+180 +m

+w

Frontal

+120 ___

39

2112

M

ASD; VSD; partial anomalous pulmonary venous drainage; leftsided SVC and IVC: polysplenia; biliary atresia; symmetric liver

C; 0; A

0.12

100

+230

+170

40

l/12

F

VSD; hypoplastic left ventricle; anomalous return of right pul-. monary veins; absent IVC; left SVC via coronary sinus to RA; normal right SVC; polysplenia

C; 0; A

0.08

120

-30

+80

Abbreviations

as in Table I.

FRONTAL

FRONTAL

aVL

oVF

RI

II

/

I

Figure 5. Spatial cardia with normal vena cava (vector superior vena cava 2’ and 3’ represent 2” and 3” represent

1 P wave vectors in mirror-image dextrovenae cavae (vector l), absent inferior 2, coronary sinus rhythm) and bilateral (vector 3, left atrial rhythm). Vectors l’, the frontal projections and vectors l”, the transverse projections. I

\

\

\

upright in leads II and III, and the heart rate was normal. The spatial P vector is shown in Figure 6 (vector 1). This is typical of asplenia syndrome.l* In another record the pacemaker shifted to a different focus in which the P wave became inverted in lead I, as ex-

VOLUME 25, APRIL 1970

/

/iir.z+ 3

Figure 6. Spatial P wave plenis syndrome (vector 1) tors 2 and 3). Vectors l’, 2’ jections and vectors l”, 2” projections of the respective

vectors in dextrocsrdia with asand polyspknie syndrome (vecand 3’ represent the frontal proand 3” represent the transverse spatial vectors.

petted in situs inversus. The P-R prolonged in both instances. Polysplenia syndrome: In splenia syndrome, coronary sinus The P wave was inverted in leads

interval

was slightly

both cases of polyrhythm was present. II and III. In lead I

425

MOMMA

AND

LINDE

the P wave was upright in situs solitus and inverted in situs inversus. The spatial P vector in these coronary sinus rhythms is shown in Figure 6 (vectors 2 and 3). The P-R interval and heart, rate were normal.

Discussion In mirror-image dextrocardia the P wave vector is clearly rightward in most cases, whereas the vector in dextroversion is vertically downward in about one third of cases and clearly leftward in the remaining cases. A high degree of correlation was present between vena caval anomalies and abnormal atria1 rhythms. Left atria1 rhythms were observed only in those patients with bilateral superior venae cavae and coronary sinus rhythms with absent inferior vena cava or polysplenia syndrome. Left atria’1 rhythm in, dextrocardia ‘i and frequent association. of bilatera.1 superior vena,e cavale with dextroversion 9 have been well documented. Bilateral sinoatrial node tissue has been noted histologically in patient.s with asplenia syndrome and bilateral superior vena cava. lg In the heart of the early embryo, pacemaking tissue develops bilaterally at the right and left sinus horns.“0 The right one becomes the sinoatrial node, whereas the left one may become inactive.20 Thus, an embryologic basis may exist for the association of dextrocardia, bilateral superior venae cavae and persistence of bilateral active sinoatria1 nodes. All instances of coronary sinus rhythm in dextroversion were associated with absent inferior vena cava in this series. Therefore, coronary sinus rhythm in a patient with dextroversion is highly suggestive of absent inferior vena cava and possible associated complicated cardiac anomalies. The exact position of the pacemaker in these coronary sinus rhythms is not clarified in this study, although a lower right atrium focus including the orifice of the coronary sinus is assumed.*l Some coronary sinus rhythms may origi-

nate from the left at,rium,’ but no signs of left atria1 rhythm were apparent in our cases. Coronary sinus rhythm is frequently associated with such congenital heart lesions as sinus venosus type atria1 septal defect,“” single atrium,“” persistent left superior vena cava,*:! polysplenia syndrome 18~21 or partial situs inversus. I* This study shows a frcquent association of dextrocardia, absent inferior vena cava, atria1 or ventricular septal defect and coronary sinus rhythm. Mild right axis deviation of the P u’a.ve (+80” y +lOO”) was observed in about one third of the patients with dextroversion in this study. These are probably sinus rhythms in dextroversio,n, because no other sign of ectopic rhythm other than mild right axis deviation is present.. The latter is explained by rightward rotation of the atria fi~ciand associated right atria1 enlargement. e.zB Frontal P wave axis in this range was observed in only 1 patient with mirrorimage dextrocardia in this series. Therefore, for practical purposes, a frontal P wave axis between +80° and +lOO” in a case of dextrocardia with congenital heart disease suggests dextroversion rather than mirror-image dextrocardia. The same holds true for a small negative wave followed by a small positive wave in lead I 5~8 or a positive wave followed by a negative wave in the right chest leads.R In our series of 6 pa.tients with asplenia syndrome and normally situa.ted heart, wandering of the pacemaker and vertical frontal P wave axis (+90”) were common.‘” Both features were present in a case with asplenia syndrome reported in this study. In our earlier series of 4 patients with polysplenia syndrome and a normally situated heart,l* all showed so-called coronary sinus rhythm. Coronary sinus rhythm was also present in 2 cases reported here. Therefore a high degree of correlation between the splenic syndrome and abnormal rhythm is documented in both levocardia and dextrocardia.

References 1. Campbell M, Reynolds G: Significance of the direction of P-waves in dextrocardia and isolated levocardia. Brit Heart J 14:481, 1952 Uncomplicated isolated dextro2. Burchell HB, Pugh DG: cardia (“dextroversio cordis” type). Amer Heart J 44:196, 1952 3. Keith JD, Rowe RD, Vlad P: Heart Disease in Infancy and Childhood. New York, Macmillan, 1958, p 535 Isolated congenital dextrocardia. 4. Lichtman SS: Arch Intern Med (Chicago) 48:683, 1931 5. Portillo B, Anselmi G, Sodi-Pallares D, et al: Importance of the unipolar leads in the diagnosis of dextrolevocardias, dextropositions and dextrorotacardias, tions. Amer Heart J 57:396, 1959 6. Arcilla RA, Gasul BM: Congenital dextrocardia. J Pediat 58:39, 251, 1961

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7. Mirowski M, Neil1 CA, Taussig HB: Left atrial ectopic rhythm in mirror-image dextrocardia and in normally placed malformed hearts. Report of twelve cases with “dome and dart” P waves. Circulation 27:864, 1963 8. Van Praagh R: Malposition of the heart, in Heart Disease in Infants, Children and Adolescents (Moss AJ, Adams FH, ed). Baltimore, Williams & Wilkins, 1968, p 602 9. Van Praagh R, Van Praagh S, Vlad P, et al: Anatomic types of congenital dextrocardia. Amer J Cardiol 13:510, 1964 10. Anderson RC, Adams P, Burke B: Anomalous vena cava with azygos continuation (infrahepatic interruption of the inferior vena cava). Report of 15 cases. J Pediat 59:370, 1961 11. Lipman BS, Massie E: Clinical Scalar Electrocardio-

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12. 13. 14.

15.

16.

17.

18.

graphy, 5th edition. Chicago, Yearbook Medical Publishers, 1965 Electrocardiography. Ref. 8, p 183 Liebman J: Clinical Disorders of the Heart Beat. PhilaBellet S: delphia, Lea & Febiger, 1963, p 318, 391 Left atrial rhythm. Diagnostic criteria and Mirowski M: differentiation from nodal arrhythmias. Amer J Cardiol 17:203, 1966 Abnormal rhythms asociated Momma K, Linde LM: wit.h persistent left superior vena cava. Pediat Res 3:210, 1969 Moller JH. Nakib A, Anderson RC, et al: Congenital cardiac disease associated with polysplenia: a develop. mental complex of bilateral “left-sidedness.” Circulation 36:789, 1967 Ruttenberg HD, Neufeld HN, Lucas RV, et al: Syndrome of congenital cardiac disease with asplenia. Amer J Cardiol 13:387, 1964 Momma K, Linde LM: Abnormal cardiac rhythms in congenital heart disease associated with asplenia and polysplenia syndrome. In preparation

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19. Van Mierop LHS, Patterson PR, Reynolds RW: Two cases of congenital asplenia with isomerism of the cardiac atria and the sinoatrial nodes. Amer J Cardiol 13:407, 1964 20. Patten BM: The development of the sino-ventricular conduction system. Univ Michigan Med Bull 22:1, 1956 21. Hoffman BF, Cranefield PF: The physiological basis of cardiac arrhythmias. Amer J Med 37:670, 1964 22. Hancock EW: Coronary sinus r.hythm in sinus venosus defect and persistent left superior vena cava. Amer J Cardiol 14:608, 1964 23. Munoz-Armas S, Gorrin JRD, Anselmi G, et al: Single atrium. Amer J Cardiol 21:639, 1968 24. Ongley PA, Titus JL, Khoury GH, et al: Anomalous connection of pulmonary veins to the right atrium associated with anomalous inferior vena cava, situs inversus and multiple spleens. Mayo Clin Proc 40:609, 1965 25. Burch GE, DePasquale NP: Electrocardiography in the Diagnosis of Congenital Heart Disease. Philadelphia, Lea & Febiger, 1967

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