Anatomical Problems with Identification and Interruption of Posterior Septa1 Kent Bundles

Anatomical Problems with Identification and Interruption of Posterior Septa1 Kent Bundles Will C. Sealy, M.D., and Eileen M. Mikat, Ph.D. ABSTRACT To gain insight into the cause of the complex anatomical problems associated with posterior septal Kent bundles, 20 cadaver hearts were carefully examined and the operative results in 22 patients analyzed. The following important anatomical relationships were noted. The posterior right atrium overlies the left ventricle and muscular septum. The coronary sinus wall contains myocardium continuous with both atria. The posterior superior process of the left ventricle connects the mitral annulus to the muscular septum. The epicardium of the crux can be 2.5 to 3.5 cm from the right fibrous trigone. A Kent bundle can originate in either atrium, the atrial septum, or the coronary sinus and connect with the left ventricle or muscular septum. At operation, antegrade and retrograde activation sequences were used for identification. Antegrade maps could not be recorded in 4 patients. Two operations were used, right atrial and left atrial. The initial right atrial operation was successful in 12 patients-all 7 with earliest antegrade activation over the midpart of the muscular septum or its right side and 3 with activation on its left side. Among the 6 patients with more than one operative approach, 5 had Kent bundle division. One of the patients probably had a left free wall pathway. Two pathways thought to be free wall turned out to be septal. The Kent bundles were divided in 18 patients and missed in 4, 2 of the latter having His interruption. There were no deaths. The conclusions are that the right atrial operation is reliable when the pathways are clearly posterior septal. Surgical problems occur because Kent bundles in the posterior left free wall sometimes cannot be separated from Kent bundles in the posterior septal area. Both right atrial and left atrial operations are needed if there is doubt about the location of a pathway. From the Departments of Surgery and Pathology, Duke University Medical Center, Durham, NC. Accepted for publication Sept 28, 1982. Address reprint requests to Dr. Sealy, Professor of Thoracic Surgery, Box 3093, Duke University Medical Center, Durham, NC 27710. Posterior septal Kent bundles are the most difficult of the accessory pathways of atrioventricular (AV) conduction to identify and interrupt. In order to gain insight into the cause of this difficulty, two studies were done. In the first, 20 human hearts were examined, with the dissection following the steps of the operative procedure [l] for posterior septal pathways used at Duke University Medical Center. In the second study, the cases of the last 22 patients to undergo operation with either a preoperative or postoperative diagnosis of posterior septal pathways were reviewed to determine if the failure to find and divide the Kent bundle was caused by a lack of understanding of some of the subtle morphological characteristics of the area. In a previous report [l], the posterior septal area was described as a toppled pyramid enclosing a fat-filled space. In this analogy, the apex of the pyramid was described as the right fibrous trigone (made up of the confluence between the membranous ventricular septum and the tricuspid, atrial, and aortic annuli), while the base was the epicardium over the area of the crux. Two of the sides were the right and left atria, respectively, which fused at the superior angle into the atrial septum. The third side was a combination of the posterior superior process of the left ventricle and the muscular ventricular septum. The interior of the pyramid contained fat and branches of the coronary arteries, and the undersurface of the coronary sinus with its branches to the right and left ventricles. The major components of the normal conduction system were noted as being adjacent to or in the walls of the pyramid. Material and Methods Anatomical Studies The 20 human hearts were dissected in a way that simulated the operation for posterior septal pathways, referred to hereafter as the right atrial operation. On the posterior aspect of the heart, the septal area is identified by the crux. 584

585 Sealy and Mikat: Identification and Interruption of Posterior Septa1 Kent Bundles Fig 1. Posterior aspect of the heart showing the area of the crux (adapted from McAlpine [Zl). (A) The right atrium (RA) and the termination of the coronary sinus (CS) are superior to the left ventricle (LV). The interatrial groove is to the left of the interventricular groove. The atrioventricular junction on the right is lower than that on the left. (B, C) Variations in this relationship. A catheter electrode is shown in the coronary sinus to emphasize the problems with preoperative and intraoperative localization of pathways in this region. See Surgical Experience in text for more details. (LA = left atrium; IVC = inferior vena cava; RV = right ventricle.) (Used by permission of Duke University Medical Center.) This contains the termination of the coronary sinus in the right atrium and the junctions of the two atria and the two ventricles (Fig 1A). According to McAlpine [2], the term crux refers to this as an area rather than just the point where the four heart chambers meet. In fact, a true cross, the literal meaning of crux, is not present. As shown in Figure 1, the conformation of the junction of the four chambers of the heart varies. In 3 of the 20 hearts, the left AV junction was markedly superior to the right by as much as 20 mm (Table 1; Fig 1B). In the other extreme, as noted in 1 heart, the AV junction of the right and left heart made almost a straight line (Fig 1C). A remarkable feature of the crux, which influences the interpretation of the electrophysiological study, is that the junction between the two atria, which is more of a wraparound of the right over the left than a square abutment, is far to the left of the interventricular groove. This places both the terminal portion of the coronary sinus and a part of the right atrium over the left Table 1. Height of Mitral Annulus above the Tricuspid Annulus in 3 Normal Hearts At 0 s of At Right At Crux Coronary Sinus Fibrous Trigone Specimena (mm) (mm) (mm) A 16 9 5 B 25 15 10 C 8 3 1 "See Figure 1 for description of A, 6, and C.

586 The Annals of Thoracic Surgery Vol 36 No 5 November 1983 Fig 2. The right atrium (RA) has been opened, thereby exposing the fossa ovalis (FO), corona ry sinus (CS), right fibrous trigone (RFT), and tricuspid valve (TV). An incision has been made above the tricuspid annulus and will extend from the RFT to the right free wall as shown by the dashed lines. (Used by permission of Duke University Medical Center.) Fig 3. The fat that fills the pyramidal space is dissected from the edges of the muscular ventricular septum (MVS) and posterior from the posterior superior process of the left ventricle, which cannot be seen. This exposes the annuli of the left atrium (LA) and the right atrium (RA). The RA is shown superiorly on the left exposing the undersurface of the coronary sinus (CS). The atrioventricular nodal artery (AVNA) here enters the muscular septum posterior to the right fibrous trigone (RFT), although in some specimens it entered the septum just back of the RFT. In all specimens, the annuli, represented by the heavy white lines, were easy to find. (TV = tricuspid valve.) (Used by permission of Duke University Medical Center.) ventricle. In the posterior septal area, McAlpine [2] described the ventricular connection of the mitral valve annulus to the muscular septum as the posterior superior process of the left ventricle. This, of course, varies in size and was found to be 3 to 20 mm at its widest point. As in the right atrial operation, the right atrium was opened through its lateral wall. The 0s of the coronary sinus, the atrial extension of the membranous ventricular septum that forms the inferior part of the right fibrous trigone, the fossa ovalis, and the tricuspid annulus were identified first (Fig 2). The tendon of Todaro, a fibrous structure 2 to 3 mm in diameter originating from the superior lip of the 0s of the coronary sinus (eustachian valve) and running to the posterior aspect of the membranous septum just anterior to the AV node, was sought for in 20 hearts but could be found in only 13. This can be used as a marker of the AV node-his bundle junction. The membranous ventricular septum visible above the tricuspid valve usually measured 8 to 10 x 10 to 12 mm. An atrial endocardia1 incision was begun 3 to 4 mm above the tricuspid ring at the level of the middle of the 0s of the coronary sinus. The latter usually was about 18 mm from the posterior edge of the right fibrous trigone. The incision was extended posteriorly beyond the junction of the muscular ventricular septum and right free wall. Then it was continued anteriorly to the atrial septum, the latter containing and protecting the AV node. The atrial septum was found to be attached to the right fibrous trigone in an oblique manner with entry of the left annulus to the right fibrous trigone higher by 5 mm or so than the left. The septal attachment measured 8 to 10 mm in the sagittal plane and 3 to 4 mm in the transverse plane. If the insertion of the atrial septum into the right fibrous trigone is divided, the AV node-his bundle junction will be interrupted. The floor of the toppled pyramid, composed of the posterior superior process of the left ventricle and the muscular ventricular septum covered by a fat pad, was now exposed (Fig 3). The space also contains the AV nodal artery and a small septal artery. The pyramidal space fat was

587 Sealy and Mikat: Identification and Interruption of Posterior Septa1 Kent Bundles easily separated from the edges of the muscular ventricular septum, the posterior aspect of the exposed atrial septum, the tricuspid annulus, and the right atrial wall, deflecting the AV nodal artery buried in the fat toward the left. The undersurface of the coronary sinus, as it crossed the base of the pyramid, was identified and carefully freed from the fat, thereby exposing the epicardium of the crux. The insertion of the mitral annulus into the right fibrous trigone was identified next. Then the pyramidal space fat was separated from the left atrial wall, the mitral annulus, the edge of the muscular septum, and the posterior superior process of the left ventricle. This exposure demonstrates one of the important aspects of the morphology of this area. The mitral annulus usually inserts into the right fibrous trigone as much as 5 mm superior to the insertion of the tricuspid annulus. At the level of the 0s of the coronary sinus, the mitral annulus was often 9 mm above the tricuspid annulus, while at the crux the difference was 15 mm. The greater the elevation of the mitral above the tricuspid annulus, the larger the posterior superior process of the left ventricle. The last step in the dissection was the division of the epicardium of the crux beginning at its attachment to the right atrial free wall at the level of the termination of the right atrial endocardial incision (Fig 4). The incision was extended across the crux to the left free wall after ligation of the middle cardiac vein ascending from the ventricular apex usually to the left of the interventricular groove. The small cardiac vein coming from the right was not always present and when found, did not always have to be divided. The coronary sinus now was reflected upward. By keeping the dissection close to the sinus wall, the coronary arteries were displaced downward with the coronary sulcus fat and, unless sought for, remained buried in the fat. Although there are variations in the coronary artery in this area, if the dissection stays close to the sinus, the arteries, when present, can be deflected downward with the sulcus fat. In most hearts, the undersurface of the coronary sinus was 15 mm above the summit of the left ventricle and about 10 to 12 mm above the annulus. Fig 4. The epicardial incision shown here begins on the right free wall and extends across the crux area well onto the left free wall. If present, the small cardiac vein from the right rarely has to be divided; however the middle cardiac vein and the left ventricular branches are always ligated. The coronary sulcus fat is separated from the coronary sinus (CS), which then can be retracted superiorly, and the sulcus fat is pushed inferiorly. (LA = left atrium; RA = right atrium; LV = left ventricle; RV = right ventricle.) (Used by permission of Duke University Medical Center.) With this dissection completed, the entire septal portion of the left atrium as well as the posterior part of the left ventricular free wall was easily exposed. This was much simpler to do when there was no branch of the coronary artery in this part of the sulcus, as was found in 12 specimens. Although it has not been done in patients, it would be possible to enter the left atrium through this approach by an incision above the annulus fibrosus. This exposure opened to view the floor of the pyramid (Fig 5). In 1 heart, the distance from the apex of the pyramid, the right fibrous trigone, to the most posterior extent of the right and left valve annuli measured 35 mm and 28 mm, respectively, whereas, the distance posterior to the middle of the muscular septum was 30 mm. Another measurement of interest was the distance from the insertion of the atrial septum into the right fibrous trigone to the right ventricle in the anterior septal area. This was 21 mm in the heart just described. The measurement was along the posterior edge of the membranous ventricular septum to its inferior aspect and then anterior to the right ventricle.

588 The Annals of Thoracic Surgery Vol 36 No 5 November 1983 A Fig 5. (A) Floor of the pyramidal space. Measurements to the annuli and the epicardium in the middle of the crux in one heart show the distance from the right fibrous trigone, the point at which the atrial extension of the membranous ventricular septum (AMS) joins the two atrioventricular valve annuli. (5') Sagittal section of the left ventricle (LV) at the junction of the free wall and septum. The coronary sinus (CS) wall contains myocardium that is continuous with the atrial myocardium. (A0 = aorta; MV = mitral valve; TV = tricuspid valve; LA = left atrium.) (Used by permission of Duke University Medical Center.) The dissection just completed exposed the undersurface of the atrial septum along its entire course from the right fibrous trigone superiorly to the coronary sinus. The latter structure was fused with the atrial septum superiorly and anteriorly. The coronary sinus usually ended in the right atrium as a gaping orifice 8 to 12 mm in diameter. When it was traced retrograde from the right atrium, the terminal 5 to 10 mm was still attached to the left atrium; however, on the epicardial side this was covered with right atrial muscle. The coronary sinus is not only a blood conduit but an interatrial myocardial connection as well, since the muscular coat in its wall is myocardium. In these studies, the distances between landmarks in the posterior septal area were B measured. Since the 20 hearts did vary in age, size, and sex of the cadaver, average measurements have little import. Instead, three sets of measurements are given (Table 2; see Table 1). By the dissection described, pathways within the posterior septal area would be interrupted if their routes were as follows: 1. From the coronary sinus to a. The posterior superior process of the left ventricle, b. To the summit of the left ventricle at the junction with the free wall, and to c. The muscular ventricular septum 2. From the left atrium and adjacent atrial septum to a. The posterior superior process of the left ventricle or to b. The muscular ventricular septum 3. From the posterior aspect of the atrial septum at the right fibrous trigone to a. The muscular ventricular septum adjacent to the His bundle or 4. From the right atrium to a. The muscular ventricular septum Surgical Experience The 22 patients were operated on for posterior septal pathways at the time when the complex- Table 2. Floor of Pyramidal Space in 3 Normal Hearts Right Fibrous Right Fibrous Trigone to Mitral Annulus Right Fibrous Trigone to Tricuspid Annulus Specimen Trigone to IV Groove (mm) at Free Wall Junction (mm) at Free Wall Junction (mm) D 20 15 20 E 30 28 35 F 35 25 35 IV = interventricular.

589 Sealy and Mikat: Identification and Interruption of Posterior Septa1 Kent Bundles ity of the courses of the pathways in this area was being recognized. Extensive preoperative electrophysiological evaluation, as described in another report [3], was performed on all patients. A posterior septal pathway was thought to be present before operation in 20 patients; in the other 2, the preoperative diagnosis was made of a left free wall pathway. One comment is appropriate concerning these studies. The coronary sinus (see Fig 1) is easily entered with a catheter electrode. Its four electrodes permit relative timing of retrograde conduction to the myocardium of the sinus wall during reentry tachycardia. The problem is that the catheter electrode cannot separate a left free wall pathway from a posterior septal one. Even the os of the coronary sinus can be over the posterior superior process of the left ventricle. The electrodes are 1 cm apart. The most proximal electrode in the orifice of the coronary sinus could record on retrograde mapping a pathway entering the atrial muscle in the coronary sinus wall from the posterior superior process of the left ventricle. The second electrode could be over the left free wall, as are the more distal ones in Figure 1. During the operation, by methods described elsewhere 141, identification of a pathway was done by epicardial activation sequences of the ventricle and the atrium with the earliest point of activation considered to be the connection point. In most patients, atrial pacing was used for antegrade recordings. The retrograde activation sequence of the atrium was recorded when possible during reentry tachycardia; however, if ventricular pacing had to be used, fusion occurred with impulses conducted by the His bundle, thereby making interpretation subject to error. After the atriotomy, retrograde activation times were frequently recorded on the endocardium of the right atrium. In the antegrade maps of the ventricle, the early areas were described as being over the crux, meaning the groove between the two ventricles that was approximately the mid part of the septum, to the left crux, meaning over the muscular ventricular septum at its approximate junction with the left ventricle, or to the right crux indicating the approximate junction of the right ventricle with the muscular septum. On retrograde maps of the epicardial surface of the atrium, the early points were described, respectively, as being over the crux, which was the groove where the two atria joined; over the coronary sinus, meaning just proximal to its entry into the right atrium; or over the right or left posterior atrial wall. Endocardially, the atrial early points were noted as the right atrial septal area, 0s of the coronary sinus, or within the coronary sinus. In 2 patients, the endocardium of the septal area of the left atria was examined. The procedure used for the 22 patients is designated in Tables 3, 4, and 5 as right or left atrial operation, listed in the order in which done. The right atrial operation followed the steps given in the anatomical dissection. The left atriotomy incision shown in Figure 6, which could be done easily, particularly when no arteries are in the area, has not been done at operation. The left atrial operation reported here included a left atriotomy similar to the approach for a mitral valve replacement and described elsewhere for left free wall pathways [5]. The incision began in the left atrial septal area at the right fibrous trigone. Results In 12 of the 22 patients the Kent bundles were successfully interrupted at one operation using the right atrial approach (see Table 3). Antegrade mapping in 4 patients showed the earliest ventricular activation to be to the right of the crux; in 3, to be over the crux; and in 3, to be to the left of the crux. In 1 patient the pathway would conduct only retrograde, and in another, the antegrade study was unsatisfactory. On retrograde mapping, 3 patients were considered to have an unsatisfactory study because the reentry tachycardia could not be induced. One patient s pathway had only antegrade conducting capacity. Epicardial and endocardia1 retrograde mapping was done in 8 patients; the early areas were found on the right atrium, crux, right atrial septum, and os of the coronary sinus. The next group, consisting of 6 patients (see Table 4), had to have two or more attempts at interruption of the Kent bundles before success was achieved. Patient 14 had two separate operations; the right atrial approach was inadequate the first time. Although preoperative and operative studies pointed to a septal pathway, Patient 15 had a left free wall pathway, as shown by the

590 The Annals of Thoracic Surgery Vol 36 No 5 November 1983 Table 3. Successful lnterruption of Kent Bundles with Opie Approach Map at Operation Surgical Patient No. Antegrade Retrograde Approach 1 Left crux RA, AS Right 2 Right crux 0s" Right 3b Right crux Not applicable Right 4 Crux Not satisfactory Right 5 Crux Not satisfactory Right 6 Crux A5 Right 7 Left crux RA Right 8 Right crux Not satisfactory Right 9' Not applicable Crux, AS Right 10 Right crux Crux, AS Right 11 Not satisfactory Crux, osa Right 12 Left crux RA, osa Right "0s of coronary sinus. bantegrade conduction only. 'Retrograde conduction only. RA = epicardium posterior right atrial wall; AS = right atrial septal surface. Table 4. lnterruption of Kent Bundles with More than One Approach Map at Operation Patient Surgical No. Antegrade Retrograde Approaches Comment 13 Left crux" Endocardium, Right + left + right; Posterior septal pathway next left atrium His interrupted to His bundle 14b Not applicable Crux, AS Right, right' Inadequate approach at first opera tion 15 Left crux Left atrium, AS Right, leftd Probably left free wall pathway; no maps at second operation; divided by left atrial incision 16 Left crux 0s of coronary sinus Left + right + left; Posterior septal pathway next His interrupted to His bundle 17 Left crux Not satisfactory Left + right Error in localization to left free wall 18 Left crux Crux, AS Left + right,c Error in localization right,' to left free wall left + right,g on first and third cryothermia operations successful "Early breakthrough; also anterior septal area bretrograde conduction only. 'Second operation ten days later. dsecond operation 12 hours later. 'First operation. 'Second operation, 24 hours later. "Third operation, six months later. AS = right atrial septal area.

591 Sealy and Mikat: Identification and Interruption of Posterior Septa1 Kent Bundles Table 5. Failure to Interrupt Kent Bundles Map at Operation Surgical Patient No. Antegrade Retrograde Approaches Comment 19" Not applicable Crux, RAS Left + right; 0s of His interruptedb coronary sinus 20 Left crux Crux Right + left' 21 22 Left crux Endocardium, Right + left; left atrium His interrupted" Left crux Crux Left with cryothermia Posterior septal pathway Probably left free wall pathway Probably left free wall pathway Inadequate operation "Retrograde conduction only. bcryothermia applied too early area beneath coronary sinus. His bundle interrupted at another hospital a year later. 'Second operation 24 hours later. 'Second operation a month later. RAS = right anterior septal. Fig 6. (A) The right atrium is held wide open with a retractor in the atrial endocardia1 incision made as the first step in the operation. An incision is being made in the exposed left atrial wall. Note the coronary sinus (CS) well above the incision. The sinus frequently is damaged during the dissection, but is easily repaired with the epicardial closure. The left coronary artery branch was present in the sulcus supplying the septum from which the drawing was made, although a branch of the right coronary artery was in the interventricular groove. (B) The superficial fibers entering the mitral annulus are divided as they will be on the right. (C) Closure of the incision. (LA = left atrium; LV = left ventricle; RV = right ventricle.) (Used by permission of Duke University Medical Center.)

592 The Annals of Thoracic Surgery Vol 36 No 5 November 1983 second operation. The preoperative studies were of interest in that retrograde early activation was found 1 to 2 cm from the coronary sinus orifice. Two patients had pathways adjacent to the His bundle, and the Kent bundles were not interrupted until the His bundles were ablated. The first patient (No. 13) had two activation sites occurring at about the same time, one over the right ventricular infundibulum suggesting an anterior septal pathway and the other to the left crux suggesting a posterior septal pathway. A careful dissection as described for anterior septal pathways was done, followed by the right atrial approach and then the left atrial approach. The Kent bundle was not divided. The Kent bundle and the His bundle were both divided when the atrial septum was detached from the right fibrous trigone. The second patient (No. 16) in whom the Kent bundle and the His bundle were adjacent to each other had a less than satisfactory map in that reentry tachycardia was hard to induce. Mapping the antegrade activation sequence showed two points of early activation, one to the left of the crux and one on the left free wall. On the assumption that two pathways were present, the suspected left free wall pathway was approached through a left atriotomy. The suspected septal pathway was next approached from the right. The cannulas for bypass were removed, but the patient still was noted to have preexcitation. The left atrium was opened a second time. With the cryothermia unit, three freeze lesions were made beginning at the right fibrous trigone and extending to the left free wall. The Kent and His bundles were both interrupted. After preoperative and intraoperative electrophysiological studies, 2 patients were thought to have left free wall pathways. In preoperative evaluation of Patient 17, the earliest area of retrograde activation was estimated to be 2.5 cm within the coronary sinus. Mapping at operation was not entirely satisfactory, as function of the pathway was easily obtunded by almost any manipulation and reentry tachycardia could not be induced. The antegrade activation sequence maps that were recorded showed the early area to be to the left crux. The left atrium was opened and an incision made beginning posterior to the right fibrous trigone and including most of the left free wall. This did not interrupt the pathway. Antegrade activation sequences again showed the early area over the left crux. After operation using the right atrial approach, the pathway was interrupted. In Patient 18, three separate operations were required before the posterior septal pathway was interrupted. Studies before operation indicated that the pathway was in the posterior onethird of the left free wall. At operation, antegrade activation sequences showed the early breakthrough to be in the posterior left free wall, while retrograde maps during reentry revealed a site on the left atrium opposite that on the ventricle. A generous dissection, one used for left free wall pathways, was begun at the septal part of the left annulus fibrosus and extended laterally to the orifice of the atrial appendage. This technique appeared to have interrupted the pathway. About 24 hours later, the electrocardiogram showed preexcitation and runs of supraventricular tachycardia. The patient was operated on again. Retrograde maps at that time implicated the posterior septal area as shown by the endocardia1 maps, where the earliest activation site was on the right atrial septal wall. After the right atrial operation, the pathway was thought to be interrupted. At discharge from the hospital, the patient was again found to have preexcitation on the ECG. Approximately six months later, she was readmitted for further consideration of operation because of the incessant nature of the tachycardia. Preoperative electrophysiological studies at this time showed early retrograde activation 1.5 to 2 cm from the coronary sinus orifice. At the third operation, antegrade maps again indicated that the pathway was in the posterior one-third of the left free wall. However, extensive dissection in the left free wall area failed to interrupt the pathway. The right atrium was opened, and retrograde maps pointed to the orifice of the coronary sinus as the pathway site. Because the postoperative changes made sharp dissection difficult, three

593 Sealy and Mikat: Identification and Interruption of Posterior Septa1 Kent Bundles freeze lesions were placed posterior to the atrial septum. The pathway was successfully interrupted, sparing the AV node. This patient could have had two pathways; however, the mapping at operation was never entirely satisfactory. It is more likely that only one pathway, a posterior septal one, was present and was missed at the second operation because of an inadequate dissection. The patient was only 4 years of age with a hypertrophied heart due to the incessant tachycardia. This could have altered the surface maps. Four patients (see Table 5) did not have the Kent bundles interrupted, although 2 had reentry tachycardia abolished by ablation of the His bundle. Patient 19 had a pathway with only retrograde function thought to be a "left" posterior septal pathway based on preoperative studies as well as the activation sequences measured at operation. The approach was first by the left atrium where a long endocardia1 incision was made starting at the right fibrous trigone and extending to the left free wall area; this failed. Then the right atrium was opened where the activation sequences during reentry tachycardia showed the early area to be on the right atrium just below the coronary sinus orifice. Using the cryothermia probe the temperature of the area was reduced to less than PC, and conduction was blocked. Three contiguous freeze lesions were made. After the patient was discharged from the hospital, the reentry tachycardia returned. His bundle interruption was performed at another hospital. The patient's pathway was undoubtedly a posterior septal one, and failure was due to inadequate dissection from the right atrial approach. The second patient (No. 21) who had His interruption because of failure to find the Kent bundle, had an extensive dissection on both the right and left sides. In spite of this, the pathway was missed. Finally, the His bundle was interrupted, but conduction over the pathway continued. This patient's pathway was probably in the left free wall. Another patient (No. 22) had what was described as a "left" posterior septal pathway. From the left approach, freeze lesions were made along the mitral annulus and this brought temporary interruption of the Kent bundle. After the patient left the hospital, preexcitation returned. There was confusion at operation about whether or not this was a left free wall pathway. It was probably a posterior septal one, and the operation was a failure because of inadequate dissection. Patient 20 had both the right and left approaches, in two attempts. In retrospect, the pathway could have been a left free wall pathway or, a remote possibility, the Kent bundle coursed with the His bundle. Comment Relating the atrial and ventricular connections of posterior septal Kent bundles to the early epicardial activation sequences obtained by mapping is complex because of the possibility of multiple combinations of connections at a distance from access by the mapping probe. In free wall Kent bundles, the electrode usually can be placed within 1 cm of the entry point of a pathway to either the atria or the ventricles. In posterior septal pathways, the ventricular entry from one of the atria, if close to the right fibrous trigone, can be 28 mm from the left crux, 3 mm from the crux, and as much as 35 mm from the right crux as shown in Figure 5. Another example of the identification problem is the relationship of the coronary sinus and right atrium to the posterior superior process of the left ventricle and even the left free wall. Pathways between these structures can cause early antegrade activation either to the left of the crux or to the free wall; yet on retrograde conduction, the right atrium or termination of the coronary sinus will be activated the earliest. The right atrial operation was always successful when the antegrade epicardial activation was over the crux or to the right of the crux. Antegrade activation earliest to the left of the crux was found in 11 of the 18 patients with antegrade conducting Kent bundles. Success at the first operative approach occurred in only 3 of these 11 patients with a fourth success at a second operation. These 4 had only the right atrial approach. Three patients probably had a left free wall rather than the suspected septal pathway, while 2 thought to have left free wall

594 The Annals of Thoracic Surgery Vol 36 No 5 November 1983 pathways turned out to have posterior septal ones. Explanations for the confusion in localization in the 5 could be the short span of the septal left atrium between the right fibrous trigone and the left free wall and the coronary sinus and right atrium overlying the left ventricle. In the detection of Kent bundles in locations other than the posterior septal area, the retrograde activation sequences of the atria are of great localizing value. The problems in the posterior septal area occur from the many possible points of atrial origin of Kent bundles including the coronary sinus and the fact that the free wall of the left ventricle and the posterior superior process both may be underneath the right atrium. Endocardia1 mapping of the right atrial side has not been helpful because of the myocardial continuity of the right and left atria, atrial septum, and coronary sinus as well as the overlap of the right over the left atrium. Localization of posterior septal Kent bundles by mapping to the degree achieved in other pathway sites will never be possible, as can be seen readily in the Figures and previous discussion. In addition to localization of pathways within the boundaries of the large posterior septal area, problems were encountered with separating the septal pathways from ones on the free wall of the posterior left ventricle. In an effort to be more precise, posterior septal pathways, when early activation was over the left crux, were sometimes labeled after the electrophysiological study as "left" posterior septal. After careful review of the anatomy of the area, it is obvious that such a separation is not feasible. Assuring that the pathway is in the posterior septal area is about as accurate a localization as will be possible. The surgical procedure described in the anatomical studies, the right atrial approach, should be effective in interrupting any posterior septal pathways other than those that course with the His bundle, and even they can be interrupted by extending the incision anteriorly through the atrial septum. However, there may be one exception, for the left atrial wall is not divided. There is a possibility that the pathways might follow the route of a right free wall pathway described by Lev and colleagues [6], which was intramural in both its atrial and ventricular courses; however, the ventricle and atrium were continuous only on the epicardial side of the annulus. Exposure of the annulus should divide a pathway with a course similar to the one in the description of Lev and associates. Success did follow a right atrial operation alone in 13 of the patients; when it failed, the cause was missed localization or inadequate dissection. What should surgeons do when confronted with posterior septal pathways? They should realize that localization is imprecise and that the Kent bundle connections can be anywhere in a very large area. They should also be aware of the problems of separating left free wall from posterior septal pathways. When the pathway is clearly posterior septal as shown by antegrade activation earliest over the crux or right crux, then only the right atrial approach should be used. If the early area is over the left crux, the right atrial operation still should be done first, followed by the left atrial operation. When the left atrial operation is used, the endocardia1 incision should include at least one-half of the left free wall as well as the septal area. The last consideration is that if conduction over the Kent bundle is still present after both right and left atrial operations are done, the surgeon should consider the possibility that the Kent and His bundles are adjacent to each other. If the Kent bundle has the potential to cause sudden death, both the His and Kent bundles will have to be interrupted. The 22 patients in this report were operated on after one of us (W. C. S.) had had experience with 100 patients with all varieties of Kent bundles. Four of the 22 posterior septal Kent bundles were missed, if the three probable left free wall pathways are included in the 22. Five patients underwent several approaches at one operation or had a second operation, and 1 had three separate operations. The causes of problems were difficulties in localization in most of the patients and an inadequate dssection in a few. Supported by the John Klein Heart Research Fund. References 1. Sealy WC, Gallagher, JJ: The surgical approach to the septal area of the heart based on the experiences with forty-five patients with Kent bundles. J Thorac Cardiovasc Surg 79:542, 1980

595 Sealy and Mikat: Identification and Interruption of Posterior Septa1 Kent Bundles 2. McAlpine WA: Heart and Coronary Arteries. New York, Springer-Verlag, 1975, pp 68, 69 3. Gallagher JJ, Gilbert M, Svenson RH, et al: Wolff- Parkinson-White syndrome: the problem, evaluation and surgical correction. Circulation 51:767, 1975 4. Gallagher JJ, Kasell J, Sealy WC, et al: Epicardial mapping in the Wolff-Parkinson-White syndrome. Circulation 575354, 1978 5. Sealy WC, Gallagher JJ: Surgical treatment of left free wall accessory pathways of atrioventricular conduction of the Kent type. J Thorac Cardiovasc Surg 81:698, 1981 6. Lev M, Sodi-Pallares D, Friedland C: A histopathologic study of atrioventricular communications in a case of WPW with incomplete left bundle branch block. Am Heart J 66:399, 1963