This study was supported by the National Institutes of Health, Grant HE Received for publication December 6,

Transcription

1 Mechanisms of Ventricular Fibrillation Yoshio WATANABE, M.D. and Leonard S. DREIFUS, M.D. T was pointed out more than two decades ago, by Wegria and Wiggers, that any satisfactory theory of fibrillation should explain (1) the initiation of a premature contraction and a series of reentrant waves and (2) the factors which favor their transformation into true fibrillation.1) Numerous experimental studies utilizing electrocardiographic2),3) as well as cinematographic4) technics have attempted to clarify these conditions. However, arguments based on recordings of gross electrical or mechanical events may be inadequate if ventricular fibrillation is truly a complete disorganization of the ventricles at the cellular level.5) Several investigators5)-7) have demonstrated the cellular electrophysiologic events during ventricular fibrillation, utilizing glass microelectrode technics. Sano et al.5) thus concluded that multiple reentries constantly changing their routes were most likely responsible for the maintenance of fibrillation. Unfortunately, these previous studies were carried out only during established fibrillation. Hence, the present study was designed mainly to observe the transition from a regular supraventricular or ventricular mechanism to fibrillation with two simultaneously impaled microelectrodes under several different conditions. METHODS Mongrel dogs weighing 15 to 20Kg. were used in the first group of experiments. Animals were placed under hypothermia with the aid of cardiopulmonary bypass. The heart was cradled in the pericardial sac. A small bipolar electrode was attached to the epicardial surface of the ventricle to record an electrogram. Esophageal temperature was continuously monitored and the myocardial temperature at various recording sites was frequently checked by the use of thermisters. Ventricular fibrillation was produced on 16 occasions by focal heating of the myocardium with warm saline. A second series of experiments was carried out on isolated, spontaneously beating rabbit hearts, utilizing an Anderson heart perfusion apparatus. The heart was perfused with modified Chenoweth's solution. The temperature of the perfusate was maintained at 37 }1 Ž. A bipolar electrocardiogram was recorded between a needle electrode inserted into a ventricular cavity and a surface electrode From the Cardiovascular Section, Department of Medicine, Hahnemann Medical College, Philadelphia, Pennsylvania, U.S.A. This study was supported by the National Institutes of Health, Grant HE Received for publication December 6,

2 Vol. 7 No. 2 MECHANISMS OF VENTRICULAR FIBRILLATION 111 attached to the epicardium of the contralateral ventricle. Ventricular fibrillation was induced by a perfusion of desacetyl lanatoside C (0.2mg./L.), with or without concomitant lowering of potassium concentration in the perfusate (from 4.5 to 1.5mEq./L.), or by a subepicardial local injection of aconitine solution. In both groups, 2 flexibly mounted glass microelectrodes were impaled into subepicardial ventricular fibers (1-5mm. apart) to record transmembrane potentials. Additional observations were made on isolated, perfused puppy hearts. Following right ventriculotomy, the anterior papillary muscle was severed from the interventricular septum. Care was taken to avoid injury to the attached strand of Purkinje fibers. The papillary muscle was thus connected to the entire heart only through the free-running Purkinje fibers and was bathed with the perfusate in the ventricular cavity. During spontaneous ventricular fibrillation, intracellular recordings were made from a Purkinje fiber and a papillary muscle fiber adjacent to the Purkinje papillary junction. Transmembrane potentials were amplified by a neutralized input capacity amplifier8) and Tektronix type D and M amplifiers, and displayed on 2 Tektronix type 532 oscilloscopes. The polarity of one of the action potentials was inverted for ease of distinguishing the 2 curves. Photographs were taken from one oscilloscope with a DuMont type 321-A oscillographic camera, at a paper speed of either or 44.4 inches per min. RESULTS I. Observations in Hybothermic, in-situ Dog Hearts A record of the onset of ventricular fibrillation is shown in Fig. 1 A. Slight irregularities of the basic supraventricular mechanism are noted. Fiber 2 (inverted action potentials) shows a longer action potential duration than fiber 1 (upright action potentials). This variation in the action potential Fig. 1. A. The action potential duration is longer in fiber 2 than in fiber 1. This variation of action potential duration is clearly demonstrated as a premature systole (beat 4) normally activates fiber 1 but is blocked in fiber 2. Progressive asynchrony between fibers 1, 2 and the electrocardiogram (ECG) culminates in fibrillation within several beats. B. Recorded several min. prior to strip A. In spite of a similar heart rate, the action potential duration was relatively long and local myocardial temperature differences produced only occasional premature systoles (beat 4) which failed to result in fibrillation.

3 112 WATANABE AND DREIFUS Jap. Heart J. M arch, 1966 duration was clearly demonstrated as an extremely premature systole (beat 4) fully activated fiber 1 but only incompletely depolarized fiber 2 late in its repolarization phase. Apparently, ventricular tachycardia or flutter was initiated by this premature beat. The presence of a significant time difference between the normally inscribed QRS complex in the electrocardiogram and the premature action potentials in these fibers could be due to fibrillatory activity in one or more focal regions of the ventricle. Disorganization of the entire ventricle was completed within several sec. On the other hand, occasional premature systoles with relatively longer coupling intervals were recorded several min. prior to strip A (Fig. 1 B, beat 4). In spite of the presence of a similar heart rate, the action potential duration was longer at this time, and focal warming of the epicardial surface failed to produce ventricular fibrillation. II. Observations in Isolated, Perfused Rabbit Hearts Ventricular tachycardia produced by a subepicardial local injection of aconitine in the perfused rabbit heart was usually sustained, often lasting more than 30min. before terminating in fibrillation. During the later period of tachycardia, the rate of depolarization decreased and electrical alternation often developed. Unless a significant change in the heart rate occurred, the relative timing of depolarization of individual fibers was distinctively fixed with reference to the electrocardiogram (Fig. 2 A). A second injection of Fig. 2. Ventricular tachycardia produced by subepicardial focal injection of aconitine in the isolated perfused rabbit heart. A. Electrode 1 was in the aconitine site and electrode 2, 5mm. apart. The rate of depolarization was slower in fiber 1, with electrical alternation. B. Ventricular tachycardia with a different rate and QRS configuration following a second injection of aconitine near electrode 2. Electrode 1 was impaled in another fiber near fiber 1, strip A. Note that the activation of fiber 2 now occurs in the middle of 2 QRS complexes but the timing of activation of fibers 1 and 2 remains fixed.

4 Vol. 7 No. 2 MECHANISMS OF VENTRICULAR FIBRILLATION 113 aconitine in another region of the ventricle usually induced a different ventricular rate and QRS configuration (Fig. 2 B). In this instance, depolarization of fiber 2 occurred either 102 msec. after or 95 msec. prior to the R wave of the electrocardiogram. Again, the timing of depolarization of the 2 fibers remained fixed and showed a constant relationship to the electrocardiogram. Ventricular fibrillation did not occur in this particular heart. The transition from ventricular tachycardia to fibrillation engendered by aconitine is illustrated in Fig. 3. Strips A, B and C represent 3 different portions of a long continuous record where microelectrodes 1 and 2 remained in their respective fibers for several min. In strip A, marked electrical alternation was seen in the electrocardiogram as well as in action potentials from the fiber adjacent to the site of the aconitine injection (fiber 1). Intervals between successive action potentials were regular, and the amplitude of the alternating smaller action potentials was constant. At the left hand of strip B, intervals between successive action potentials became irregular, as Fig, 3. Three different portions of a long continuous strip. A. Ventricular tachycardia produced by focal injection of aconitine. A slower rate of depolarization and a shorter action potential duration are evident. Electrical alternation is seen in fiber 1 as well as in the electrocardiogram. B. The time interval between fibers 1 and 2 is no longer fixed and the height of the alternating smaller action potentials (or local response) fluctuates from beat to beat. Fibrillation rapidly ensued. C. Spontaneous recovery from fibrillation. Several action potentials which are relatively regular in timing and configuration with a higher upstroke velocity and smaller time difference between fibers 1 and 2, and a longer pause are seen immediately prior to the re-establishment of a supraventricular mechanism (extreme right).

5 114 WATANABE AND DREIFUS Jap. Heart J. M arch, 1966 evidenced by the timing of fiber 1. The amplitude of the alternating smaller action potentials in fiber 1 was further decreased and varied from beat to beat. Finally, regular alternation was disrupted, by 3 successively smaller action potentials. Ventricular fibrillation ensued in less than a second, as recorded by the electrocardiogram and the action potential of fiber 1, although the activity of fiber 2, which was more distant from the aconitine site, remained relatively regular during this short period. Fig. 3 C shows spontaneous recovery from ventricular fibrillation. Reestablishment of a regular rhythm was preceded by a run of several action potentials showing greater amplitude as well as a more rapid upstroke velocity. The intervals between successive action potentials were gradually prolonged, and depolarization of the 2 fibers became more synchronous. A longer pause followed this run of action potentials, which, in turn, was terminated by a regular supraventricular mechanism. Prolonged perfusion of the heart with desacetyl lanatoside C, with or without concomitant lowering of the potassium concentration, frequently produced ventricular fibrillation. Digitalis glycoside caused a significant shortening of the action potential, a finding in keeping with previous observations of Woodbury and Hecht.9) In addition, a marked difference in the action potential duration of individual fibers was common prior to the initiation of fibrillation (Fig. 4 A). Even after the onset of ventricular fibrillation, some fibers showed a longer action potential duration while others showed spike potentials (Fig. 4 B). One experiment of this group deserves special attention. Frequent periods of ventricular fibrillation were produced by mechanical stimulation of the ventricle after 45min. of perfusion with 0.6mg./L. of desacetyl lanatoside C in the presence of a lowered potassium concentration. (K= Fig. 4. A. Prolonged perfusion of desacetyl lanatoside C produced asynchronous repolarization of fibers. The action potential duration of fiber 1 is 74 msec., and that of fiber 2, 152 msec. B. Variation of the action potential duration is often maintained after the onset of ventricular fibrillation. Fiber 2 shows spike potentials while fiber 1 has relatively normal action potential duration.

6 Vol. 7 No. 2 MECHANISMS OF VENTRICULAR FIBRILLATION 115 Fig. 5. Repetitive episodes of ventricular fibrillation following excessive dose of lanatoside C (0.6mg./L.) and low extracellular potassium concentration (1.5 meq./l.) in a rabbit heart. S=supraventricular beat. V=ventricular beat. F= flutter waves. Note similar coupling interval and QRS configurations between V2, F1 and F2 in strip A and V, F1 and F2 in strip B. Detailed description in text. 1.5mEq./L.). Each episode was followed by a spontaneous restoration of supraventricular rhythm (Fig. 5). Strips A, B and C show the onset of fibrillation in the first, third and the fifth episodes, respectively. In strip A, one supraventricular beat (S) was followed by a ventricular systole (V1) after a long coupling interval (1,600 msec.). In contrast to the supraventricular beat, the ventricular systole was characterized by a bizarre and wide QRS configuration. Fibers 1 and 2 were activated later in the QRS duration. The first ventricular systole was followed by a second ventricular beat (V2), showing a similar QRS contour. However, the coupling interval became shorter (298 msec.). A propagated response was suggested although the time difference between depolarization of fibers 1 and 2 was greater and the rate of depolarization slower in these 2 beats. With the occurrence of the third QRS (F1), fiber 1 was activated before the completion of repolarization and resulted in a local response. Asynchronous depolarization of these fibers rapidly developed and ventricular fibrillation was evident from the electrocardiogram. On the other hand, ventricular fibrillation in strip B was initiated by a supraventricular beat (S2), which was followed by a ventricular premature systole (V). The coupling interval of this premature systole (292 msec.) was similar to the coupling interval V1-V2 in strip A (298 mesc.). Furthermore, the succeeding 2 QRS complexes (F1 and F2, strips A and B) were almost identical in coupling and configuration during these 2 separate periods of fibrillation. Similar relationships were also demonstrated in the second and the fourth episodes (not shown). Depolarization of fibers 1 and 2 (strip B)

7 116 WATANABE AND DREIFUS Jap. HeartJ. M arch, 1966 remained rather synchronous, even after the establishment of fibrillation, when the interelectrode distance was small (1mm.). The fifth episode of ventricular fibrillation (Fig. 5 C) was again initiated by a ventricular premature systole. In this instance, the supraventricular rhythm was more rapid than in the previous episodes, and the premature action potential was terminated late in its repolarization phase by a nonpropagated local response (especially in fiber 1). Disorganization of the entire ventricle was already evident by this time. A similar rapid transition to fibrillation was observed also in the sixth episode. III. Observation in Isolated, Perfused Puppy Hearts Records obtained in this group of experiments are reproduced in Fig. 6. In strip a, one microelectrode (inverted action potentials) was impaled into a Purkinje fiber adjacent to the Purkinje-papillary junction while the other microelectrode (upright action potentials) was in a papillary muscle fiber approximately 3mm. distal to this junction. A regular and rapid mechanism was seen in the Purkinje fiber at the rate of 400/min. The papillary muscle fiber showed irregular response following Purkinje action potentials with varying time intervals (at the left hand). A local response was seen in beat 8. The action potential duration was shorter in muscle fibers. At the right hand of the strip, the papillary muscle potential showed a 2:1 response. Hence, varying degrees of conduction block are suggested between Purkinje and papillary muscle fibers. On the other hand, when 2 microelectrodes were impaled both in the papillary muscle (4mm. apart), the action potentials from Fig. 6. Spontaneous ventricular fibrillaion in an isolated puppy heart. a. Inverted action potentials from a Purkinje fiber near the Purkinje papillary junction. Upright action potentials from a ventricular muscle fiber 3mm. distal to the junction. Note regular Purkinje potentials and variable timing of the papillary muscle fiber response with occasional local responses. 2:1 Purkinje-papillary conduction develops at the right hand of the strip. b. Transmembrane potentials from 2 papillary muscle fibers (4mm. apart). 1:1 propagation is seen in the papillary muscle despite its irregular response to Purkinje fiber potentials. The action potential upstroke retouched.

8 Vol. 7 No. 2 MECHANISMS OF VENTRICULAR FIBRILLATION 117 the 2 muscle fibers were synchronous (strip b). Thus the spread of excitation through the papillary muscle appeared uniform despite its irregular response to the Purkinje action potentials. DISCUSSION Observations concerning the initiation of a premature contraction are beyond the experimental design of this study. However, factors favoring the onset of ventricular fibrillation can be discussed from the present results. The onset of ventricular fibrillation observed in this study demonstrated 2 distinct characteristics: Rapid initiation by 1 or 2 premature systoles early in the repolarization phase of a previous ventricular excitation (type A) and gradual development following a sustained period of ventricular tachycardia (type B). The electrophysiologic events prevalent in A were seen in the hypothermic dog heart experiments as well as in the digitalis experiments of the isolated perfused rabbit hearts. Those of type B were observed in rabbit heart experiments following the focal injection of aconitine. Since an earlier report by Williams et al.10) several experimental studies have stressed the role of the gvulnerable period h in the production of ventricular fibrillation by a premature impulse.11),12) Furthermore, Smirk and Palmer13) presented cases of sudden death following premature systoles interrupting T waves. One of them suggested that gthe propagation front of a very premature systole may meet refractory tissue which may be bypassed and the stage be set for re-entry and the precipitation of chaotic rhythm. h11) The importance of a very early premature systole in the genesis of ventricular fibrillation has been illustrated in these experiments (Figs. 1, 5). In these instances, ventricular fibrillation was triggered by a premature systole falling late in the repolarization phase of a preceding action potential. Variations of the action potential duration in different fibers and the presence of local refractory tissues were prevalent in these studies (Fig. 1A). Rapid development of asynchrony between fibers following an early premature systole suggests a slow, irregular spread of excitation due to local block. The inhomogeneity of action potential duration in the hypothermic dog heart can easily be explained by local temperature difference in the myocardium, which were produced by rapid non-uniform cooling of the heart during cardiopulmonary bypass and focal warming of the ventricle. The observed relationship between the rate of fibrillation in a single fiber and the myocardial temperature14) may well support this explanation. On the other hand, the mechanism producing asynchronous repolarization of different fibers in the presence of digitalis (Fig. 4) is unknown.

9 118 WATANABE AND DREIFUS Jap. Heart J. March, 1966 Nevertheless, digitalis and hypothermia produced variations in the action potential duration or a non-uniform recovery of excitability within the ventricles. Under these conditions the occurrence of an early premature systole engendered local block and irregular spread of excitation. Rapid disorganization of the ventricle quickly ensued. Hence, the vulnerable phase could be defined as the period during which fiber excitability differs in various portions of the ventricle. A similar explanation has been postulated by Palmer.11) When ventricular tachycardia was produced by focal injection of aconitine, its transformation into fibrillation was rather gradual. In the presence of rapid ventricular tachycardia, the rate of depolarization in individual fibers was often significantly decreased, suggesting a decrease in conduction velocity across the myocardium (Fig. 2). This finding as well as the development of electrical alternation in some of the ventricular fibers are in full agreement with an earlier report.7) The fact that electrical alternation is not seen in all the fibers may suggest some variations of the excitability and/or the conduction velocity in different portions of the ventricle. However, the present experiments showed that, as long as the timing of depolarization in individual fibers was fixed in relation to the electrocardiogram, transition to fibrillation did not occur (Figs. 2 and 3A). Furthermore, a second injection of aconitine could produce a change in both rate and order of ventricular excitation without precipitating fibrillation (Fig. 2B). These observations suggest that the spread of excitation during a particular episode of tachycardia is constant, even though the order of fiber discharge is quite abnormal. On the other hand, the transition from ventricular tachycardia to fibrillation was immediately preceded by (1) the development of a more prominent electrical alternation accompanied by further decrease in the rate of depolarization in the majority of fibers, and (2) fluctuations of the relative timing of depolarization in individual fibers. The first observation suggests the development of local block, and the second implies variation in the spread of excitation from beat to beat. Finally, regular alternation is disrupted by a series of action potentials of intermediate size, terminating in a complete disorganization of the ventricles (Fig. 3B). From the above discussion, several common factors may be apparent at the onset of ventricular fibrillation. These are the presence of local block and a resultant irregular spread of excitation. In type A, following 1 or 2 premature systoles, local block appeared to be caused by an extremely premature impulse during the vulnerable period. In type B, progressive inhomogeneity, as evidenced by electrical alternation in some fibers and a generalized decrease in conduction velocity, engendered local block and ir-

10 Vol. 7 No. 2 MECHANISMS OF VENTRICULAR FIBRILLATION 119 regular spread of excitation. A similar conclusion was also reached by Burn, in the explanation of atrial as well as ventricular fibrillation.15) Furthermore, evidence has been presented that the junction between peripheral Purkinje fibers and ordinary ventricular muscle fibers could possibly be one of the sites of conduction failure in the genesis of local block and irregular spread of ventricular excitation (Fig. 6). In this instance, the action potential duration was shorter in ventricular fibers than in Purkinje fibers, as was reported earlier by Hoffman.16) Hence, block cannot be due to refractoriness of the papillary muscle fibers. Although it has been known that fiber diameter is smaller and conduction velocity slower in ventricular muscle fibers than in peripheral Purkinje fibers,16)-18) membrane discontinuity between these 2 tissues was ruled out as a possible obstacle for transmission of the impulse.19) Hence, elucidation of the mechanism awaits further studies. On the other hand, the exact mode of continuation of the irregular spread of excitation remains obscure. It should be pointed out, however, that a similar and almost superimposable sequence of electrocardiographic changes was repeatedly observed during the onset of 4 consecutive episodes of fibrillation (Fig. 5). From this fact, it is most tempting to say that a similar series of irregular pathways of conduction was utilized in the initial cycles of each episodes, as it is possible that the gross geographical distribution of tissue excitability and conductivity was retained during this period of observation. Naturally the electrophysiologic characteristics of minute portions of the myocardium may not remain unaltered after each episode of fibrillation. Hence, the nature of numerous gdaughter waves h produced during the initial 2 or 3 cycles of irregular excitation should differ from episode to episode (Fig. 5A and B). Finally, it is of no importance whether ventricular inhomogeneity is manifested by early premature systoles or by gradual deterioration of the electrophysiologic events following sustained ventricular tachycardia. Variation in the action potential duration with the development of local block and irregular spread of excitation always heralded the onset of ventricular fibrillation under these experimental conditions. SUMMARY Transition from a regular supraventricular or ventricular mechanism to fibrillation was recorded utilizing 2 ultramicroelectrodes. Fibrillation in the hypothermic dog heart was preceded by asynchronous repolarization of individual fibers. An early premature beat caused incomplete depolarization of some fibers (local block) and irregular spread of excitation,

11 120 WATANABE AND DREIFUS Jap. Heart J. M arch, 1966 rapidly disorganizing the ventricles. A similar sequence of events was demonstrated in the development of ventricular fibrillation due to desacetyl lanatoside C. Focal injection of aconitine engendered a sustained, regular ventricular tachycardia characterized by a decreased rate of depolarization and electrical alternation in some fibers. A gradual transition to fibrillation occurred when the timing and configuration of individual action potentials became irregular. Common factors observed at the onset of ventricular fibrillation were the development of local block and an irregular spread of excitation, demonstrating electrophysiologic inhomogeneity of the ventricles. During spontaneous ventricular fibrillation in isolated puppy hearts, the Purkinje-ventricular muscle junction was shown to be a possible site of conduction failure in the genesis of local block and irregular spread of excitation. REFERENCES 1. Wegria, R. and Wiggers, C. J.: Am. J. Physiol. 131: 119, Harris, A. S. and Guevara Rojas, A.: Exper. Med. Surg. 1: 105, Wiggers, C.J.: Am. Heart J. 20: 399, Wiggers, C. J.: Am. Heart J. 5: 351, Sano, T., Tsuchihashi, H., and Shimamoto, T.: Circulat. Res. 6: 41, Hoffman, B. F. and Suckling, E. E.: Am. J. Physiol. 179: 644, Hogancamp, C. E., Kardesch, M., Danforth, W. H., and Bing, R. J.: Am. Heart J. 57: 214, Amatniek, E.: I. R. E. Trans. Med. Electronics 10: 3, Woodbury, L. A. and Hecht, H. H.: Circulation 6: 172, Williams, H. B., King, B. G., Ferris, L. P., and Spence, P. W.: Proc. Soc. Exp. Biol. Med. 31: 873, Palmer, D. G.: Am. Heart J. 63: 367, Wiggers, C. J. and Wegria, R.: Am. J. Physiol. 128: 500, Smirk, F. H. and Palmer, D. G.: Am. J. Cardiol. 6: 620, Watanabe, Y. and Dreifus, L. S.: Genesis of ventricular arrhythmias. in gcardiovascular Drug Therapy h, Edit. by Brest, A. N. and Moyer, J. H., Grune and Stratton, New York, Burn, J. H.: Canad. M. A. J. 84: 625, Hoffman, B. F. and Cranefield, P. F.: Electrophysiology of the Heart, McGraw-Hill, New York, Lewis, T.: The Mechanism and Graphic Registration of the Heart Beat (3rd Ed.), Shaw, London, Glomset, D. J. and Glomset, A. T.A.: Am. Heart J. 20: 677, Alanis, J. and Pilar, G.: Am. J. Physiol. 199: 775, 1960.