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The Conduction System of the Swine Heart
The Conduction System of the Swine Heart* Saroja Bharati, M.D., F.C.C.R; Marc Levine, M.D.; Shoei K. Stephen Huang, M.D.; Broce Handler; M.D.; Grant \( S. Ibn; M.D., F.C.C.R; Robert Bauernfeind, M.D.; and Maurice Lev, M.D., F.C.C.P. Although the pig has been used as an experimental model for ischemic heart disease and sudden death, relatively little is known about the anatomy of the conduction system (CS) of this animal. We attempted to correlate electrophysiologicand anatomic differences between the pig and human cs. Invasive electrophysiologic studies were performed in 6ve healthy anesthetized pigs. In contrast to the adult human, the pig has sinus tachycardia, shortened PR and H-V intervals, and a relatively short sinoatrial conduction time. Compared with the human CS, serial sections of the CS of pig hearts showed the following differences: (1) the atrioventricular node is located more to the right of the summit of the ventricular septum; (2) the penetrating bundle is very short, and the bifurcation of the bundle into bundle branches occurs more proximally; (3) there is more
swine has often been used as an experimental T hemodel for ischemic heart disease and sudden death.>" The anatomy of the conduction system, however, has been infrequently studied. ~12 Therefore, it seemed appropriate to delineate the course of the conduction system in this animal and compare it with that of a man. *From the Congenital Heart and Conduction System Center, The Heart Institute for Children, Christ Hospital and Medical Center, Oak Lawn, Illinois; Department of Pathology, Rush Medical College, Chicago; Department of Pediatrics, Medical College of Ohio, Toledo, Ohio; and University of Massachusetts Medical Center, Worcester, Massachusetts. Reprint requests: Dr. Bharati, Congenital Heart and Conduction System Center; 11475 Southwest Highway, ltJlos Hts; IL 60453
connective tissue and less elastic tissue; and (4) there is a copious amount of nerve fibers (about 50 percent throughout the CS). The presence of the abundant neural tissue implies that there is an important neurogenic component to conduction in the pig. Because of the above differences from the human, the pig should be used with caution as an experimental model in ischemic heart disease and sudden death where arrhythmias are studied. (Cheat 1991; 100:207-12) AERP = atrial effective refractory period; AV= atrioventricular; AVERP=AV node effective refractory period; CSNRT= corrected sinus node recovery time; H = His bundle; HRA = high right atrium; H- V His bundle to ventricle interval; IHR = intrinsic heart rate; LRA = low right atrium; SA = sinoatrial; SACT = sinoatrial conduction time
=
MATERIAL AND METHODS
Five pigs ranging in weight from 30 to 40 kg and from 10 to 15 weeks of age were studied. The animals were sedated with ketamine 15 to 20 mg intramuscularly (1M), and then intubated. They were fully ventilated on room air (tidal volume, 10 mllkg) and light anesthesia maintained using 0.25 to 0.75 percent halothane, to effect. Routine electrophysiologic studies were performed according to the methods outlined by Gillette and Carson." Quadripolar catheters were inserted percutaneously from the right and left groins and positioned at the superior vena cava-right atrial junction and across the tricuspid valve for pacing and intracardiac recording (Fig 1). Pacing was performed using a pacemaker (Grass S-88) with a custom-built programmable timing device. Intracardiac recordings were obtained on an eight-channel recorder (Beckman R6I2). Six hearts of various ages were compared with adult human hearts at a gross level and the conduction systems of two of the pig hearts
FIGURE 1. His bundle recording of the pig. Intervals: HRA-LRA = 10 ms; LRA-H =65 ms; H -V =25 ms. HRA = high right atrium; HBE = His bundle electrocardiogram; A = atrial wave; H = His bundle spike; V = ventricular wave; and LRA = low right atrium. CHEST I 100 I 1 I JUl"+(, 1991
207
Table 1- Ekctrophysrology Results Surface Electrocardiograms
Normaillumans
Avera~e
Ran~e
lIeart rate, heats per minute PR interval, ms
132:t32
91-167
60-100
94:t 27
50-120
(3-5 Y/O) 110-150 (5-9 Y/O) 120-160
QT interval, ms
256:t69
150-340
Intracardiac studies RR interval, ms HRA-LRA, illS
457:t 62 1O:t0
370-510 10
63:t2
60-65
25:t7
20-35
LRA-H,
illS
II-V, illS CSNRT, illS SACT, illS (Strauss) SAC.., illS (Narula) AERp, illS AVERp, ms
118 :t48 64:t 17 64:t1O 165:t 27 <210
60-180 36-80
47-73 130-200
(IIR (IIR
= 150) 210-280 = 1(0) 260-350
(2-5 Y/O) 6-38 (6-10 Y/O) 0-41 (2-5 Y/O) 45-101 (6-10 Y/O) 40-124 (2-5 Y/O) 27-59 (6-10 Y/O) 28-52
31-275 48-200 217:t45 (RR = 450-599) 135-295
were examined in the following manner. The sinoatrial and atrioventricular nodes, the atrioventricular bundle and bundle branches were serially sectioned and every tenth section was retained . These sections were consecutively stained with hematoxylin-eosin, Weigert-van Gieson, and Gomori trichrome stains for a total of 610 and 618 sections, respectively. In addition, selective sections were stained with Holmes silver stain for nerve fibers . FINDINGS
Comparison of the Electrophysiology of the Pig with That ofa Man The average heart rate of these animals was 132 ± 32 (mean ± SO) (range , 91 to 167 beats per minute); the PR interval was 94 ± 27 ms (range, SO to 120 ms), and the QT interval was 256 ± 69 ms (range, ISO to 340 ms). Intracardiac recording at an average RR interval of 457 ms (range, 370 to 510 ms) demonstrated a high right atrium to low right atrium interval (HRA-LRA) of 10 ms, a low right atrium to His bundle interval (LRA-H) of 63 ms and a His bundle to ventricle interval (H-V) of 25 ms (Fig 1). The maximal corrected sinus node recovery time (CSNRT) was 118 ms and the sinoatrial conduction time (SAC[) by both the Strauss and Narula methods was 74 ms. The atrial effective refractory period (AERP) was 165 ms and the AV node effective refractory period (AVERP) was less than 210 ms (See Table 1). Comparison of these values with the normal adult human reveals that the heart rates are clearly faster in the pig. Since many of the other parameters are related to heart rate and age, it seems more appropriate to use normals in children for comparison. Thus, the PR interval is shorter than expected, but the QT interval is appropriate for the heart rates in these 208
AO
LBB
Frc uue 2. Gro ss anatomy of the pi ~ heart with diagru mmatlc outline of the course of the conduction system . (a, upper), Right atrial and right ventricular view. (b, lower), Left ventricular view. CS = coronary sinus ; RA = right atrium; RV= right ventricle; N = AV node ; BII = Bundle of His; RBB = right bundle branch; AO = aorta; LBB = left bundle branch; AR = anterior radiation; PR = posterior radiation; and LV = left ventricle.
animals. HRA-LRA and LRA-H intervals are in the low normal range of children; however, the H-V interval is definitely shorter. The CSNRT was similar to that in humans. The SAC[ was in the lower range of normal. AERP and AVERP were both similar to the values seen in the human. Conduction System 01Swine Heart (Sharati at a/)
FIGURE 3. Sinoatrial (SA) node (Weigert-Van Gieson stain x 30). A= atrial muscle; Ne= nerve fiber on periphery of SA node . Arrows point to SA node .
COURSE OF THE CONDUCTION SYSTEM IN THE PIC HEART
Histologic Findings of the Conduction System in the Pig Sinoatrial (SA) Node The SA node is situated in the usual position, at the junction of the superior vena cava and the right atrial appendage, as in man (Fig 2, top). The diameter of the parenchymal cells is about the same as that of atrial muscle cells. The cytoplasm is very pale with few myo6brils as compared to atrial "working" cells. Striations and intercalated disks are not seen at the light level. There is an increase in collagenous connective tissue and elastic fibers as compared with that of atrial working cells. There are numerous ganglion cells and nerve fibers on the periphery of the node (Fig 3) and occasionally within the node.
Atrioventricular (AV) Node The AV node (Fig 2, top, and 4) lies on the right side of the crest of the ventricular septum . It lies lower down on the septum than in man . The parenchymal cells are arranged in a plexiform manner. The diameter of the cells is about the same as that of atrial cells, and distinctly less than that of ventricular cells. The cytoplasm stains more lightly than that of atrial cells, with a fewer number of myofibrils, Striations are occasionally seen, but intercalated disks are not seen at the light level. There is a distinct increase in collagenous and elastic tissue as compared with that of the ventricular myocardium . Nerve fibers are found in the periphery and in the node itself (Fig 4). No nerve cells are seen in the node.
AV Bundle-Penetrating Portion The AV node climbs to the right side of the summit of the ventricular septum, thus entering the central fibrous body (Fig 2, top, and 5, left). The parenchymal cells now become larger in diameter and are almost
FIGURE 4. Approaches to the AV node and AV node (Weigert-Van Gieson stain x 45). Ap = approaches to the AV node ; N = AV node . Arrows point to nerve fibers.
the size of the ventricular working cells. Here again the connective and elastic tissues are increased as compared with atrial and ventricular working cells. The cytoplasm stains lightly with few myo6brils. The bundle is now full of nerve fibers (Fig 5), but no ganglion cells. Some of the parenchymal cells are somewhat greater in diameter than ventricular myocardial cells. The penetrating bundle is very short and there is no branching bundle. The penetrating portion divides into the right and left bundle branches.
Left Bundle Branch This passes between the left infundibular and ventricular muscle (Fig 2, bottom, and 6). During its passage, its cells are smaller in diameter than ventricular myocardial cells. It again shows an increase in collagenous, connective, and elastic tissue as compared with the ventricular myocardial cells. It thus reaches the endocardium of the left ventricle where it remains a discrete large bundle. Its parenchymal cells become larger in diameter than ventricular working cells, resembling typical Purkinje cells. This bundle then breaks up into anterior and posterior radiations, which in turn break up into smaller fasciculi supplying the left ventricle. Throughout its course, the left bundle CHEST I 100 I 1 I JULY, 1991
209
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FIGURE 5. Bundle of His. (a. left), Weigert- Van Gieson stain X 45. (h, right), Holmes silver stain x 150. C =central fibrous body ; V = ventricular muscle; B =bundle. Arrows point to nerve fibers.
branch is full of nerve fibers, but no nerve cells.
Right Bundle Branch Most of the right bundle branch (Fig 2, top) is intramyocardial. In the beginning, it lies adjacent to the annulus of the tricuspid valve. The cells in this part have the same or a smaller diameter than the working myocardial cells. Here, again, there is a marked increase in collagenous connective tissue and elastic tissue as compared with the ventricular myocardium. Gradually, as it progresses downward, the cells equal in diameter the ventricular myocardial cells. As they reach the endocardium of the right ventricle, they have a larger diameter than the working cells and may be called Purkinje cells. There is a distinct increase in collagenous connective and elastic tissue throughout the right bundle branch as compared with the working myocardium (Fig 1). Numerous nerve fibers accompany the right bundle branch throughout its course (Fig 1). DISCUSSION
The fundamental difference between the conduction system of man and that of the pig lies in its nerve 6ber content. In man, some nerve 6bers are present 210
in the AV node and bundle and less in the bundle branches. In the pig, however, there are an immense number of nerve trunks distributed throughout the bundle and bundle branches. According to recent work, these fibers in the pig are both cholinergic and adrenergic. Further investigation concerning the effects of sympathetic and vagal innervation on heart rate, conduction, velocity, and arrhythmia thresholds would certainly be important. The quantity of nerve fibers in the pig conduction system has also raised the question as to whether conduction in this animal is neuromyogenic." It is basically myogenic in man. Previous work on the microscopic anatomy of the conduction system at the light level was done by Meyling and TerBorgO (1951), Wensing lO (1964), and Truex and Smythe" (1965). Their findings did not differ from ours. In addition, the ultrastructure of the SA node was examined by Tranum-jensen'> (1978). He found that the SA nodal cells were in the central portion with the transitional cells in the periphery. The cells showed no T tubular systems. The cell coat consisted of glycoprotein and mucosubstances. The nodal cells showed the contractile elements to stain poorly. The sarcoplasm did not have the degree of organization seen in working cardiocytes. There was a ConductionSystemof Swine Hearl (Sharali et ell
FIG Ullt: 6 (at left ). Le ft bundl e brun ch pass ing between conal and ventric ular muscle (C omori trich ro me stain X 45). C = conal mu scle; V = ventri cular muscle , Arrows point to left bundle branch ,
random di stribution of mito ch ondria that were lesser in volume . Th e am ount of glycoge n varied from cell to ce ll, and the lipid conte nt was less in am ount as compared with regu lar ca rdiocy tes . Th ere was a pau cit y of th e int ercellular junctions consisting of fasciae adh erentes, small desm osom es, and sparse nexus. Th e nerve supply was ab undant , as seen at the light level. Th e myofibe rs were both ad re ne rgic and choline rgic. Th e Golgi corn pl xes were small. The elec trophys iology of th pig differs from that of the human in seve ral ways: (I) the sin us heart rat e is mark ed ly increased , (2) th e PH inte rval is sho rte ne d, and (3) th e SACf is sho rt ened . We will tr y to pro vide possibl e explanations for each of th ese differen ces by conside ring th e ph ysiology of man . In man , sinus hear t rat es reflect an int erplay of three factors: (1) intrinsic heart rate, (2) sympathe tic tone , and (3) vagal ton e . IS Intrinsic hea rt rate is defin ed as the rate of the sinus node whe n isolated from th e influences of the autonomic n rvous syste m. Jos and FI G UR E i (below). Hight bundle branch . (a, left ). \\c igert-Van Cieson stain X ISO. (b, right), Holmes silver stain X 4SO. V = ven tricular mu scle ; RBB = right bundle branch . Arrows poi nt to nerve fibers.
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CHEST I 100 I 1 I JULY, 199 1
211
Collison" measured intrinsic heart rate in a series of volunteers by administering large intravenous doses of both propranolol and atropine. 16 They found that intrinsic heart rate (IHR) could be described by the equation IHR (beats/minute) = 118 - 0.57 x age (years), revealing that intrinsic heart rates are generally faster than resting heart rates. This reflects the fact that there is relatively little resting sympathetic tone'? and a relatively great resting vagal tone. 18 An increased sinus heart rate (as seen in the pigs) could be explained by an increased intrinsic heart rate, increased sympathetic tone, or decreased vagal tone. Our studies reveal that AV conduction is more rapid in the pig. The shorter PR interval reflects a significantly reduced H-V interval, as well as low normal HRA-LRA and LRA-H intervals. The shorter length of the penetrating AV bundle undoubtedly results in the shorter H-V interval, although conduction velocity through the atrial myocardium, AV node, and AV bundle may be increased as well. The relatively shorter SACT could also reflect increased conduction velocity through the sinoatrial junction. The similarities between the pig and human of CSNRT, AE~ and AVERP suggest no differences in the intrinsic function of the structures. The nerve content of the conduction system varies in mammals. There are three types." In ruminants (sheep, cattle), the atrioventricular node, bundle, and bundle branches all contain ganglion cells and nerve fibers. 19,10 In the hog and horses, nerve cells are lacking in the bundle and bundle branches. However, nerve fibers are copiously present throughout the bundle and bundle branches in these animals. In man, baboon, dog, and cat, ganglia are not present in the AV node, AV bundle, and bundle branches. However, some nerve fibers are present in the AV node, a lesser number in the AV bundle, and a few or no nerve fibers in the bundle branches. The findings of the present study lead to the basic question as to whether the pig can be used in experiments aimed at the assessment of the conduction system and production of arrhythmias to extrapolate these data to humans. It would seem that the cerebral genesis of arrhythmias might better be studied in this animal than in the dog or other animals with a lesser component of nerve tissue in the conduction system. 21-f3 However, it is clear that conclusions about arrhythmias in ischemic experiments in pigs should be extended with great caution to ischemic heart episodes in man. REFERENCES 1 Janse MJ, Morena H, Cinca J, Fiolet}Wf, Krieger WJ, Durrer D. Electrophysiological, metabolic and morphological aspects of acute myocardial ischemia in the isolated porcine heart: characterization of the border zone. J Physiol Paris 1980; 76: 785-90
212
2 Kleber AG, Janse MJ, Van Capelle FJL, Durrer D. Mechanism and time course of S-T and T-Q segment changes during acute regional myocardial ischemia in the pig heart determined by extracellular and intra-eellular recordings. Cire Res 1978; 42: 603-13 3 Cinca J, Janse MJ, Morena H, Candell J, Valle ~ Durrer D. Mechanism and time course of the early electrical changes during acute coronary artery occlusion: an attempt to correlate the early electrocardiographic changes in man to the cellular electrophysiology in the pig. Chest 1980; 77:499-505 4 Skinner VE, Lie JT, Entman ML. Modification of ventricular fibrillation latency following coronary artery occlusion in the conscious pig: the effects of psychological stress and betaadrenergic blockage. Circulation 1975; 51:656-67 5 Pashkow F, Holland R, Brooks H. Early changes in contractility and coronary blood flow in the normal areas of the ischemic porcine heart. Am Heart J 1977; 93:349-57 6 McCallister L~ Liedtke AJ, Hughes HC. Ischemic injury to the conducting system of the heart; involvement of myocardial lysosomes. J Thorac Cardiovasc Surg 1979; 77:647-61 7 Verdouw PD, Deckers ~ Conard GJ. Antiarrhythmias and hemodynamic actions of flecainide acetate (R818)in the ischemic porcine heart. J Cardiovasc Pharmacoll979; 1:473-86 8 Janse MJ, Van Capelle FJL. Electronic interactions across an inexcitable region as a cause of ectopic activity in acute regional myocardial ischemia: a study in intact porcine and canine hearts and computer models. Circ Res 1982; 50:527-37 9 Meyling HA, TerBorg H. The conducting system of the heart in hoofed animals. Cornell Veterinary 1957; 47:419-47 10 Wensing CJG. The conductive system and its nervous component in the pig's heart. Thesis, G. Van Dijk, N.~ Breukelen, 1964:9-78 11 Truex RC, Smythe MQ. Comparative morphology of the cardiac conduction tissue in animals. Ann NY Acad of Sci 1965; 127: 19-23 12 Tranum-Jensen J. The fine structure of the sinus node. In: Bonke FIM, ed. The sinus node. Boston: Martinus NijhofF Medical Division; 1978:149-65 13 Gillette PC, Garson A Jr, eds. Pediatric cardiac dysrhythmias. New York: Grune & Stratton; 1981 14 Bojsen-Meller F, Tranum-Jensen J. Whole-mount demonstration of cholinesterase-containing nerves in the right atrial wall, nodal tissue and atrioventricular bundle of the pig heart. J Anat 1971; 108:375-86 15 Bauernfeind RA, Amat-y-Leon F, Dhingra RC, Kehoe R, Wyndham C, Rosen KM. Chronic nonparoxysmal sinus tachycardia in otherwise healthy persons. Ann Intern Med 1979; 91: 702-10 16 Jose AD, Collison D. The normal range and determinants of the intrinsic heart rate in man. Cardiovasc Res 1970; 4:160-67 17 Eckberg DH, Drabinsky M, Braunwald E. Defective cardiac sympathetic control in patients with heart disease. N Eng} Med 1971; 285:877-83 18 Goldstein RE, Beiser GD, Stampfer M, Epstein SEe Impairment of autonomically mediated heart rate control in patients with cardiac dysfunction. Cire Res 1975; 36:571-78 19 Davies F, Francis ETB, King TS. Neurological studies of the cardiac ventricles of mammals. J Anat 1952; 86:130-43 20 Boisen-Meller F, Tranum-Jensen J. On nerves and nerve endings in the conductive system of the moderator band (septo-marginal trabecula). J Anat 1971; 108:387-95 21 Lown B. Mental stress, arrhythmias, and sudden death. Am J Med 1982; 72:177-80 22 DeSilva RA, Lown B. The role of the central nervous system in fatal arrhythmias. JSC Med Assoc 1979; 75:567-71 23 Taylor AL, Fozzard HA. Ventricular arrhythmias associated with CNS disease. Arch Intern Med 1982; 142:232-33 Conduction System of Swine Heart(BharatJ et aJ)
swine has often been used as an experimental T hemodel for ischemic heart disease and sudden death.>" The anatomy of the conduction system, however, has been infrequently studied. ~12 Therefore, it seemed appropriate to delineate the course of the conduction system in this animal and compare it with that of a man. *From the Congenital Heart and Conduction System Center, The Heart Institute for Children, Christ Hospital and Medical Center, Oak Lawn, Illinois; Department of Pathology, Rush Medical College, Chicago; Department of Pediatrics, Medical College of Ohio, Toledo, Ohio; and University of Massachusetts Medical Center, Worcester, Massachusetts. Reprint requests: Dr. Bharati, Congenital Heart and Conduction System Center; 11475 Southwest Highway, ltJlos Hts; IL 60453
connective tissue and less elastic tissue; and (4) there is a copious amount of nerve fibers (about 50 percent throughout the CS). The presence of the abundant neural tissue implies that there is an important neurogenic component to conduction in the pig. Because of the above differences from the human, the pig should be used with caution as an experimental model in ischemic heart disease and sudden death where arrhythmias are studied. (Cheat 1991; 100:207-12) AERP = atrial effective refractory period; AV= atrioventricular; AVERP=AV node effective refractory period; CSNRT= corrected sinus node recovery time; H = His bundle; HRA = high right atrium; H- V His bundle to ventricle interval; IHR = intrinsic heart rate; LRA = low right atrium; SA = sinoatrial; SACT = sinoatrial conduction time
=
MATERIAL AND METHODS
Five pigs ranging in weight from 30 to 40 kg and from 10 to 15 weeks of age were studied. The animals were sedated with ketamine 15 to 20 mg intramuscularly (1M), and then intubated. They were fully ventilated on room air (tidal volume, 10 mllkg) and light anesthesia maintained using 0.25 to 0.75 percent halothane, to effect. Routine electrophysiologic studies were performed according to the methods outlined by Gillette and Carson." Quadripolar catheters were inserted percutaneously from the right and left groins and positioned at the superior vena cava-right atrial junction and across the tricuspid valve for pacing and intracardiac recording (Fig 1). Pacing was performed using a pacemaker (Grass S-88) with a custom-built programmable timing device. Intracardiac recordings were obtained on an eight-channel recorder (Beckman R6I2). Six hearts of various ages were compared with adult human hearts at a gross level and the conduction systems of two of the pig hearts
FIGURE 1. His bundle recording of the pig. Intervals: HRA-LRA = 10 ms; LRA-H =65 ms; H -V =25 ms. HRA = high right atrium; HBE = His bundle electrocardiogram; A = atrial wave; H = His bundle spike; V = ventricular wave; and LRA = low right atrium. CHEST I 100 I 1 I JUl"+(, 1991
207
Table 1- Ekctrophysrology Results Surface Electrocardiograms
Normaillumans
Avera~e
Ran~e
lIeart rate, heats per minute PR interval, ms
132:t32
91-167
60-100
94:t 27
50-120
(3-5 Y/O) 110-150 (5-9 Y/O) 120-160
QT interval, ms
256:t69
150-340
Intracardiac studies RR interval, ms HRA-LRA, illS
457:t 62 1O:t0
370-510 10
63:t2
60-65
25:t7
20-35
LRA-H,
illS
II-V, illS CSNRT, illS SACT, illS (Strauss) SAC.., illS (Narula) AERp, illS AVERp, ms
118 :t48 64:t 17 64:t1O 165:t 27 <210
60-180 36-80
47-73 130-200
(IIR (IIR
= 150) 210-280 = 1(0) 260-350
(2-5 Y/O) 6-38 (6-10 Y/O) 0-41 (2-5 Y/O) 45-101 (6-10 Y/O) 40-124 (2-5 Y/O) 27-59 (6-10 Y/O) 28-52
31-275 48-200 217:t45 (RR = 450-599) 135-295
were examined in the following manner. The sinoatrial and atrioventricular nodes, the atrioventricular bundle and bundle branches were serially sectioned and every tenth section was retained . These sections were consecutively stained with hematoxylin-eosin, Weigert-van Gieson, and Gomori trichrome stains for a total of 610 and 618 sections, respectively. In addition, selective sections were stained with Holmes silver stain for nerve fibers . FINDINGS
Comparison of the Electrophysiology of the Pig with That ofa Man The average heart rate of these animals was 132 ± 32 (mean ± SO) (range , 91 to 167 beats per minute); the PR interval was 94 ± 27 ms (range, SO to 120 ms), and the QT interval was 256 ± 69 ms (range, ISO to 340 ms). Intracardiac recording at an average RR interval of 457 ms (range, 370 to 510 ms) demonstrated a high right atrium to low right atrium interval (HRA-LRA) of 10 ms, a low right atrium to His bundle interval (LRA-H) of 63 ms and a His bundle to ventricle interval (H-V) of 25 ms (Fig 1). The maximal corrected sinus node recovery time (CSNRT) was 118 ms and the sinoatrial conduction time (SAC[) by both the Strauss and Narula methods was 74 ms. The atrial effective refractory period (AERP) was 165 ms and the AV node effective refractory period (AVERP) was less than 210 ms (See Table 1). Comparison of these values with the normal adult human reveals that the heart rates are clearly faster in the pig. Since many of the other parameters are related to heart rate and age, it seems more appropriate to use normals in children for comparison. Thus, the PR interval is shorter than expected, but the QT interval is appropriate for the heart rates in these 208
AO
LBB
Frc uue 2. Gro ss anatomy of the pi ~ heart with diagru mmatlc outline of the course of the conduction system . (a, upper), Right atrial and right ventricular view. (b, lower), Left ventricular view. CS = coronary sinus ; RA = right atrium; RV= right ventricle; N = AV node ; BII = Bundle of His; RBB = right bundle branch; AO = aorta; LBB = left bundle branch; AR = anterior radiation; PR = posterior radiation; and LV = left ventricle.
animals. HRA-LRA and LRA-H intervals are in the low normal range of children; however, the H-V interval is definitely shorter. The CSNRT was similar to that in humans. The SAC[ was in the lower range of normal. AERP and AVERP were both similar to the values seen in the human. Conduction System 01Swine Heart (Sharati at a/)
FIGURE 3. Sinoatrial (SA) node (Weigert-Van Gieson stain x 30). A= atrial muscle; Ne= nerve fiber on periphery of SA node . Arrows point to SA node .
COURSE OF THE CONDUCTION SYSTEM IN THE PIC HEART
Histologic Findings of the Conduction System in the Pig Sinoatrial (SA) Node The SA node is situated in the usual position, at the junction of the superior vena cava and the right atrial appendage, as in man (Fig 2, top). The diameter of the parenchymal cells is about the same as that of atrial muscle cells. The cytoplasm is very pale with few myo6brils as compared to atrial "working" cells. Striations and intercalated disks are not seen at the light level. There is an increase in collagenous connective tissue and elastic fibers as compared with that of atrial working cells. There are numerous ganglion cells and nerve fibers on the periphery of the node (Fig 3) and occasionally within the node.
Atrioventricular (AV) Node The AV node (Fig 2, top, and 4) lies on the right side of the crest of the ventricular septum . It lies lower down on the septum than in man . The parenchymal cells are arranged in a plexiform manner. The diameter of the cells is about the same as that of atrial cells, and distinctly less than that of ventricular cells. The cytoplasm stains more lightly than that of atrial cells, with a fewer number of myofibrils, Striations are occasionally seen, but intercalated disks are not seen at the light level. There is a distinct increase in collagenous and elastic tissue as compared with that of the ventricular myocardium . Nerve fibers are found in the periphery and in the node itself (Fig 4). No nerve cells are seen in the node.
AV Bundle-Penetrating Portion The AV node climbs to the right side of the summit of the ventricular septum, thus entering the central fibrous body (Fig 2, top, and 5, left). The parenchymal cells now become larger in diameter and are almost
FIGURE 4. Approaches to the AV node and AV node (Weigert-Van Gieson stain x 45). Ap = approaches to the AV node ; N = AV node . Arrows point to nerve fibers.
the size of the ventricular working cells. Here again the connective and elastic tissues are increased as compared with atrial and ventricular working cells. The cytoplasm stains lightly with few myo6brils. The bundle is now full of nerve fibers (Fig 5), but no ganglion cells. Some of the parenchymal cells are somewhat greater in diameter than ventricular myocardial cells. The penetrating bundle is very short and there is no branching bundle. The penetrating portion divides into the right and left bundle branches.
Left Bundle Branch This passes between the left infundibular and ventricular muscle (Fig 2, bottom, and 6). During its passage, its cells are smaller in diameter than ventricular myocardial cells. It again shows an increase in collagenous, connective, and elastic tissue as compared with the ventricular myocardial cells. It thus reaches the endocardium of the left ventricle where it remains a discrete large bundle. Its parenchymal cells become larger in diameter than ventricular working cells, resembling typical Purkinje cells. This bundle then breaks up into anterior and posterior radiations, which in turn break up into smaller fasciculi supplying the left ventricle. Throughout its course, the left bundle CHEST I 100 I 1 I JULY, 1991
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FIGURE 5. Bundle of His. (a. left), Weigert- Van Gieson stain X 45. (h, right), Holmes silver stain x 150. C =central fibrous body ; V = ventricular muscle; B =bundle. Arrows point to nerve fibers.
branch is full of nerve fibers, but no nerve cells.
Right Bundle Branch Most of the right bundle branch (Fig 2, top) is intramyocardial. In the beginning, it lies adjacent to the annulus of the tricuspid valve. The cells in this part have the same or a smaller diameter than the working myocardial cells. Here, again, there is a marked increase in collagenous connective tissue and elastic tissue as compared with the ventricular myocardium. Gradually, as it progresses downward, the cells equal in diameter the ventricular myocardial cells. As they reach the endocardium of the right ventricle, they have a larger diameter than the working cells and may be called Purkinje cells. There is a distinct increase in collagenous connective and elastic tissue throughout the right bundle branch as compared with the working myocardium (Fig 1). Numerous nerve fibers accompany the right bundle branch throughout its course (Fig 1). DISCUSSION
The fundamental difference between the conduction system of man and that of the pig lies in its nerve 6ber content. In man, some nerve 6bers are present 210
in the AV node and bundle and less in the bundle branches. In the pig, however, there are an immense number of nerve trunks distributed throughout the bundle and bundle branches. According to recent work, these fibers in the pig are both cholinergic and adrenergic. Further investigation concerning the effects of sympathetic and vagal innervation on heart rate, conduction, velocity, and arrhythmia thresholds would certainly be important. The quantity of nerve fibers in the pig conduction system has also raised the question as to whether conduction in this animal is neuromyogenic." It is basically myogenic in man. Previous work on the microscopic anatomy of the conduction system at the light level was done by Meyling and TerBorgO (1951), Wensing lO (1964), and Truex and Smythe" (1965). Their findings did not differ from ours. In addition, the ultrastructure of the SA node was examined by Tranum-jensen'> (1978). He found that the SA nodal cells were in the central portion with the transitional cells in the periphery. The cells showed no T tubular systems. The cell coat consisted of glycoprotein and mucosubstances. The nodal cells showed the contractile elements to stain poorly. The sarcoplasm did not have the degree of organization seen in working cardiocytes. There was a ConductionSystemof Swine Hearl (Sharali et ell
FIG Ullt: 6 (at left ). Le ft bundl e brun ch pass ing between conal and ventric ular muscle (C omori trich ro me stain X 45). C = conal mu scle; V = ventri cular muscle , Arrows point to left bundle branch ,
random di stribution of mito ch ondria that were lesser in volume . Th e am ount of glycoge n varied from cell to ce ll, and the lipid conte nt was less in am ount as compared with regu lar ca rdiocy tes . Th ere was a pau cit y of th e int ercellular junctions consisting of fasciae adh erentes, small desm osom es, and sparse nexus. Th e nerve supply was ab undant , as seen at the light level. Th e myofibe rs were both ad re ne rgic and choline rgic. Th e Golgi corn pl xes were small. The elec trophys iology of th pig differs from that of the human in seve ral ways: (I) the sin us heart rat e is mark ed ly increased , (2) th e PH inte rval is sho rte ne d, and (3) th e SACf is sho rt ened . We will tr y to pro vide possibl e explanations for each of th ese differen ces by conside ring th e ph ysiology of man . In man , sinus hear t rat es reflect an int erplay of three factors: (1) intrinsic heart rate, (2) sympathe tic tone , and (3) vagal ton e . IS Intrinsic hea rt rate is defin ed as the rate of the sinus node whe n isolated from th e influences of the autonomic n rvous syste m. Jos and FI G UR E i (below). Hight bundle branch . (a, left ). \\c igert-Van Cieson stain X ISO. (b, right), Holmes silver stain X 4SO. V = ven tricular mu scle ; RBB = right bundle branch . Arrows poi nt to nerve fibers.
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Collison" measured intrinsic heart rate in a series of volunteers by administering large intravenous doses of both propranolol and atropine. 16 They found that intrinsic heart rate (IHR) could be described by the equation IHR (beats/minute) = 118 - 0.57 x age (years), revealing that intrinsic heart rates are generally faster than resting heart rates. This reflects the fact that there is relatively little resting sympathetic tone'? and a relatively great resting vagal tone. 18 An increased sinus heart rate (as seen in the pigs) could be explained by an increased intrinsic heart rate, increased sympathetic tone, or decreased vagal tone. Our studies reveal that AV conduction is more rapid in the pig. The shorter PR interval reflects a significantly reduced H-V interval, as well as low normal HRA-LRA and LRA-H intervals. The shorter length of the penetrating AV bundle undoubtedly results in the shorter H-V interval, although conduction velocity through the atrial myocardium, AV node, and AV bundle may be increased as well. The relatively shorter SACT could also reflect increased conduction velocity through the sinoatrial junction. The similarities between the pig and human of CSNRT, AE~ and AVERP suggest no differences in the intrinsic function of the structures. The nerve content of the conduction system varies in mammals. There are three types." In ruminants (sheep, cattle), the atrioventricular node, bundle, and bundle branches all contain ganglion cells and nerve fibers. 19,10 In the hog and horses, nerve cells are lacking in the bundle and bundle branches. However, nerve fibers are copiously present throughout the bundle and bundle branches in these animals. In man, baboon, dog, and cat, ganglia are not present in the AV node, AV bundle, and bundle branches. However, some nerve fibers are present in the AV node, a lesser number in the AV bundle, and a few or no nerve fibers in the bundle branches. The findings of the present study lead to the basic question as to whether the pig can be used in experiments aimed at the assessment of the conduction system and production of arrhythmias to extrapolate these data to humans. It would seem that the cerebral genesis of arrhythmias might better be studied in this animal than in the dog or other animals with a lesser component of nerve tissue in the conduction system. 21-f3 However, it is clear that conclusions about arrhythmias in ischemic experiments in pigs should be extended with great caution to ischemic heart episodes in man. REFERENCES 1 Janse MJ, Morena H, Cinca J, Fiolet}Wf, Krieger WJ, Durrer D. Electrophysiological, metabolic and morphological aspects of acute myocardial ischemia in the isolated porcine heart: characterization of the border zone. J Physiol Paris 1980; 76: 785-90
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2 Kleber AG, Janse MJ, Van Capelle FJL, Durrer D. Mechanism and time course of S-T and T-Q segment changes during acute regional myocardial ischemia in the pig heart determined by extracellular and intra-eellular recordings. Cire Res 1978; 42: 603-13 3 Cinca J, Janse MJ, Morena H, Candell J, Valle ~ Durrer D. Mechanism and time course of the early electrical changes during acute coronary artery occlusion: an attempt to correlate the early electrocardiographic changes in man to the cellular electrophysiology in the pig. Chest 1980; 77:499-505 4 Skinner VE, Lie JT, Entman ML. Modification of ventricular fibrillation latency following coronary artery occlusion in the conscious pig: the effects of psychological stress and betaadrenergic blockage. Circulation 1975; 51:656-67 5 Pashkow F, Holland R, Brooks H. Early changes in contractility and coronary blood flow in the normal areas of the ischemic porcine heart. Am Heart J 1977; 93:349-57 6 McCallister L~ Liedtke AJ, Hughes HC. Ischemic injury to the conducting system of the heart; involvement of myocardial lysosomes. J Thorac Cardiovasc Surg 1979; 77:647-61 7 Verdouw PD, Deckers ~ Conard GJ. Antiarrhythmias and hemodynamic actions of flecainide acetate (R818)in the ischemic porcine heart. J Cardiovasc Pharmacoll979; 1:473-86 8 Janse MJ, Van Capelle FJL. Electronic interactions across an inexcitable region as a cause of ectopic activity in acute regional myocardial ischemia: a study in intact porcine and canine hearts and computer models. Circ Res 1982; 50:527-37 9 Meyling HA, TerBorg H. The conducting system of the heart in hoofed animals. Cornell Veterinary 1957; 47:419-47 10 Wensing CJG. The conductive system and its nervous component in the pig's heart. Thesis, G. Van Dijk, N.~ Breukelen, 1964:9-78 11 Truex RC, Smythe MQ. Comparative morphology of the cardiac conduction tissue in animals. Ann NY Acad of Sci 1965; 127: 19-23 12 Tranum-Jensen J. The fine structure of the sinus node. In: Bonke FIM, ed. The sinus node. Boston: Martinus NijhofF Medical Division; 1978:149-65 13 Gillette PC, Garson A Jr, eds. Pediatric cardiac dysrhythmias. New York: Grune & Stratton; 1981 14 Bojsen-Meller F, Tranum-Jensen J. Whole-mount demonstration of cholinesterase-containing nerves in the right atrial wall, nodal tissue and atrioventricular bundle of the pig heart. J Anat 1971; 108:375-86 15 Bauernfeind RA, Amat-y-Leon F, Dhingra RC, Kehoe R, Wyndham C, Rosen KM. Chronic nonparoxysmal sinus tachycardia in otherwise healthy persons. Ann Intern Med 1979; 91: 702-10 16 Jose AD, Collison D. The normal range and determinants of the intrinsic heart rate in man. Cardiovasc Res 1970; 4:160-67 17 Eckberg DH, Drabinsky M, Braunwald E. Defective cardiac sympathetic control in patients with heart disease. N Eng} Med 1971; 285:877-83 18 Goldstein RE, Beiser GD, Stampfer M, Epstein SEe Impairment of autonomically mediated heart rate control in patients with cardiac dysfunction. Cire Res 1975; 36:571-78 19 Davies F, Francis ETB, King TS. Neurological studies of the cardiac ventricles of mammals. J Anat 1952; 86:130-43 20 Boisen-Meller F, Tranum-Jensen J. On nerves and nerve endings in the conductive system of the moderator band (septo-marginal trabecula). J Anat 1971; 108:387-95 21 Lown B. Mental stress, arrhythmias, and sudden death. Am J Med 1982; 72:177-80 22 DeSilva RA, Lown B. The role of the central nervous system in fatal arrhythmias. JSC Med Assoc 1979; 75:567-71 23 Taylor AL, Fozzard HA. Ventricular arrhythmias associated with CNS disease. Arch Intern Med 1982; 142:232-33 Conduction System of Swine Heart(BharatJ et aJ)