INTRODUCTION

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REPERFUSION OF ACUTELY ISCHEMIC myocardium can result in the development of ventricular tachycardia (VT) within seconds (7, 10, 17). This VT can degenerate rapidly to ventricular fibrillation (VF) and this may be more commonly associated with reperfusion than with coronary artery occlusion (16). There appears to be an important relationship between the time of ischemia and the development of reperfusion arrhythmia, with the incidence of VT being highest when the ischemia duration is ~20 min (8).

Although evidence for slow conduction in the epicardium (3) indirectly suggests that conditions for reentry may be present during reperfusion, a study by Pogwizd and Corr (12), using a high-resolution mapping system, showed that nonreentrant mechanisms may account for the majority of reperfusion VT. In that study, nonreentrant mechanisms arising in the area of the endocardium were responsible for 75% of all reperfusion VT. Pogwizd and Corr, however, did not record signals from Purkinje tissue. It is unknown whether the Purkinje system contributes to genesis or maintenance of reperfusion VT. On the other hand, we (1) reported that Purkinje tissue may be important in the genesis of spontaneous VT when it occurs acutely after coronary artery occlusion. The goal of this study was to evaluate the contribution of the Purkinje system to the initiation and maintenance of early reperfusion VT, given the previous implications that the endocardium appears to be of significant importance in this type of VT.

	METHODS

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Twenty-three healthy adult mongrel dogs were used for this study. The protocol was approved by the University of Iowa Animal Use and Care committee and conforms to the position of the American Physiological Society on research animal use.

Surgical preparation. Anesthesia was induced with the use of thiopental sodium (500 mg) and -chloralose (100-200 mg/kg iv) as a bolus and maintained with a continuous intravenous infusion of -chloralose dissolved in polyethylene glycol at 8 mg · kg¹ · h¹. The animals were intubated and ventilated on a ventilator (Harvard Apparatus) with settings adjusted to achieve a physiological arterial PCO₂ (25-35 Torr) and to maintain normal PO₂ (80-150 Torr). An infusion of NaHCO₃ was used to maintain the pH within physiological range (7.30-7.45) if necessary. Serum electrolytes K⁺ (3.6-5.0 meq/l), Mg²⁺ (1.5-3.0 mg/dl), and Ca²⁺ (8.5-10.5 mg/dl) were intermittently measured. Arterial pressure was continuously monitored via a femoral arterial line and the femoral vein was cannulated for infusion of drugs and saline.

Electrophysiological measurements. A bipolar electrode was used to pace the right atrium at two times diastolic threshold with pulses of 2-ms duration at a cycle length (CL) of 300 ms. To control the heart rate, atrial pacing at this CL and sinus node clamping was performed. While surface electrocardiographic leads II and V₅R were continuously monitored, all six limb leads (I, II, III, aVR, aVL, and aVF) and lead V₅R were recorded. Before coronary artery occlusion, 21 multipolar plunge needles were inserted into and surrounding the risk zone of the LAD coronary artery occlusion as previously described in detail (1). Needles were placed in five rows of one to six electrodes. The first row was placed diagonally into the septum along the right side of the anterior descending artery and the last row at the end of the first regional branch (Figs. 2 and 3). Each needle recorded six bipolar electrograms from circumferential electrodes made from Teflon-insulated tungsten wire was 1 mm apart, enabling recordings from 126 sites. Spacing between the needles was ~10 mm (1).

Definitions. VT was defined as at least three or more premature ventricular complexes in sequence. VT was considered sustained after >30 s or because either cardioversion or pace termination was required before 30 s due to hypotension. VT lasting <30 s and spontaneously terminating before that time was considered nonsustained. If VT degenerated to VF it was classified as such.

Experimental protocol. After instrumentation of the myocardium with the multipolar plunge electrodes, dogs were observed for spontaneous VT for ~45-60 min before coronary artery occlusion to exclude VT that might occur from mechanical artifact due to insertion of the needles. The LAD was then occluded with a snare and an occluder, which allowed for easy restoration of flow in the vessel after 20 min. All spontaneous VTs occurring in the first 20 min after occlusion were recorded and stored on computer. After 20 min, the occluder was released and all spontaneous VT occurring in the first 2 min after occlusion was recorded and stored. Restoration of blood flow was assessed by both visual reactive hyperemia of the risk zone and changes in the transmural activation times.

Statistics. Data are expressed as means ± SE. Student's t-test was used for comparison between groups. A value of P < 0.05 was considered significant.

	RESULTS

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Of the 23 dogs studied, VT occurred within 1 min of reperfusion in 19 (83%). Of these 19 VTs, 11 (58%) degenerated to VF usually within seconds whereas 8 were nonsustained. All dogs had visual reactive hyperemia of the ischemic risk zone on reperfusion. The mean number of multipolar needles recording Purkinje signals at the beginning of each experiment was 9 ± 0.7.

Transmural activation times. Before coronary artery occlusion, the mean transmural activation time was 37 ± 1 ms. After 20 min of coronary artery occlusion, the transmural activation time had increased to 62 ± 8 ms (P < 0.05 vs. baseline), consistent with conduction delay caused by ischemia. Immediately after reperfusion, the activation times were 54 ± 5 ms (P = not significant vs. 20 min of occlusion), but at 5 min after reperfusion, the transmural activation times had decreased to 41 ± 2 ms (P < 0.05 vs. early reperfusion). These conduction data support myocardial ischemia that occurs during occlusion and subsequent restoration of blood flow to the myocardium during the reperfusion period.

Mechanism and sites of origin of reperfusion VT. The mean CL of the reperfusion VT was 214 ± 2 ms. The 11 VTs that degenerated to VF had a CL of 194 ± 3 ms compared with a CL of 236 ± 3 in the 8 nonsustained VTs (P < 0.05).

View larger version (28K): [in this window] [in a new window]   Fig. 1.   Electrogram showing spontaneously occurring reperfusion ventricular tachycardia (VT) of focal Purkinje origin that subsequently degenerated to ventricular fibrillation (VF). An atrial paced complex is followed by 3 VT complexes. On the top are surface electrocardiogram (ECG) leads V5R and II. Electrogram E-F is from the site that was the focus of origin of the two of three VT complexes shown. Others are located in the same plane surrounding the focus except O-F, which is from the myocardium immediately overlying the focus, and midwall-Northeast (M-NE), which is towards the base and left lateral from the focus. South (S), North (N), and West (W) are located accordingly. During atrial pacing, clear high-frequency, low-amplitude Purkinje potentials (lines pointing to E-F and E-S) are seen preceding the muscle activity. During the first VT complex (the onset of the surface QRS is indicated by vertical lines), the earliest activity seen is the Purkinje potential in electrogram E-S (arrow) with subsequent activation of surrounding Purkinje and muscle. In the next two complexes, the earliest activity recorded in the Purkinje potential is E-F. The vertical lines again indicate the onset of the surface VT complex. In VT complexes 2 and 3, the muscle potential in electrogram M-NE is early in relationship to the surface QRS, despite the Purkinje potential being the earliest activity. This suggests a second independent focus of early activity.

View larger version (56K): [in this window] [in a new window]   Fig. 2.   Activation map of the first VT complex in Fig. 1. Each rectangle represents a layer in the anterior wall of the heart extending from epicardium (EPI) through to Purkinje (PURK). Activation is shown in colored isochrones. Colors indicate 20-ms isochrones with yellow the earliest, then red, green, sky blue, purple, navy blue, and white. The earliest activity seen is in the Purkinje layer (17), consistent with the observations from the electrogram in Fig. 1. This indicates that the onset of electrical activity in the Purkinje system occurred 17 ms before the onset of the surface QRS. The activation proceeds from the focus of origin to surrounding and overlying tissue without conduction delay, suggesting a focal origin. Some evidence of conduction delay is seen in the epicardium although the onset of this is clearly later than the early activity in the Purkinje layer.

View larger version (55K): [in this window] [in a new window]   Fig. 3.   Activation map of second VT complex of electrogram shown in Fig. 1. The earliest focus of activity (31 ms) is again seen in the Purkinje layer with an appearance of a focal origin. Early activation (10) is also seen in the epicardium and this activity is earlier than expected from transmural activation, suggesting a secondary independent focus of early activity. Immediately next to the early epicardial site is the conduction block and the development of and conduction delay around the site of block. Color code is the same as in Fig. 2.

Maintenance of VTs. The majority of VTs seen after reperfusion were also maintained by complexes of focal origin. Macroreentry was rare during maintenance complexes of the VTs (Table 2). However, conduction delay was frequently seen in the epicardium-creating conditions, such as unidirectional block, which might promote reentry (Fig. 4). The VTs that subsequently degenerated to VF were in 7 of 11 cases maintained by complexes where more than one independent site of early focal activity was seen (Table 2), examples of which are shown in Figs. 3 and 4. In Fig. 3, the earliest focus of activity is in the Purkinje layer (31 ms). However, there is also early activity in the epicardial layer (10 ms) that appears to be independent of the activity in the Purkinje layer; the transmural conduction being too slow to result in the early epicardial activation time seen. In Fig. 4, potentially three separate sites of early activity, including focal Purkinje, focal subendocardial, and subepicardial reentry are observed in a late complex before occurrence of VF. These are examples of the multiple sites of independent early electrical activity, which could facilitate degeneration to VF. Complexes with more than one site of early activity were seen only on a single occasion during maintenance of VTs, which were self-terminating and did not degenerate to VF.

View larger version (48K): [in this window] [in a new window]   Fig. 4.   Activation map of late VT complex just before occurrence of VF. Inset: the VT complex depicted is indicated by the vertical line in V5R. Earliest activation occurs in the subepicardium (72 ms), most likely the result of reentrant excitation from adjacent late-activated site. Other independent sites occur in subendocardium at 44 ms and in Purkinje layer at 32 ms. The latter is activated 8 ms before the earliest activation in the adjacent endocardium, consistent with an independent Purkinje site of early activity. Color code is the same as for Fig. 2, with the exception that the very early site in the subepicardium has a white isochrone.

Relationship to occlusion VT. Because it has been described that the presence of VT during reperfusion is more common if VT occurs during coronary occlusion, we also recorded all occlusion VT during the first 20 min of the experiment with the purpose to compare reperfusion and occlusion VT. Eight of the 19 dogs (42%) that had reperfusion VT also had VT occurring during the 20-min period of coronary artery occlusion. The mean CL of the occlusion VTs was 251 ± 1 ms (P = not significant vs. CL of reperfusion VT) and all the VTs were nonsustained (3-7 complexes). As we previously reported (1), the occlusion VTs frequently took origin from Purkinje, although not the same Purkinje sites as with reperfusion. The 4 dogs of the total 23 studied without reperfusion VT also had no VT during the occlusion phase.

	DISCUSSION

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The main findings of this study are twofold. First, these data imply that the Purkinje system might be importantly involved in the genesis of reperfusion VT. The results presented herein are consistent with previous data suggesting that focal mechanisms arising in the subendocardial area may be a common source of early reperfusion VT. However, a prior study did not differentiate between subendocardial myocardium and Purkinje tissue immediately overlying the endocardium. Second, we observed that subsidiary sites of VT origin per complex of VT were associated with degeneration to VF. These results may further add to our understanding of the origin of early reperfusion arrhythmias.

Reperfusion VT. The occurrence of VT and VF after reperfusion of ischemic myocardium is somewhat time dependent, that is, the peak incidence of arrhythmias occurs after ~20-30 min of coronary artery occlusion and decreases thereafter. In this study, the incidence of reperfusion VT after 20 min of ischemia was 75% and the VTs commonly had a focal Purkinje origin. Previously, Janse et al. (6) have shown that, whereas complete and incomplete reentrant waveforms were seen during transition of reperfusion VT to VF, no evidence of reentry was seen during the initiation of VT in isolated canine and porcine hearts. Ideker et al. (5) suggested that during transition of reperfusion VT to VF the complexes were initiated by a focal source in the ischemic border zone in the dog. These studies were somewhat limited in that the number of recording sites did not allow for exact delineation of the mechanism of the arrhythmia. In a more recent study, Pogwizd and Corr (11), using a detailed mapping system with 232 recording sites in the feline heart, demonstrated that nonreentrant mechanisms involving the endocardium were the source of 75% of all reperfusion VT seen. The data presented in our study, showing a preponderance of focal Purkinje VT after reperfusion, are not in conflict with the results published by Pogwizd and Corr. Their mapping system, although with higher spatial resolution in the cat model than the one used in the present dog model, did not record Purkinje potentials and was therefore not able to distinguish between Purkinje tissue and muscle as the source of focal VT originating from the endocardium. The activation maps of the VTs originating from Purkinje tissue in this study showed that the earliest focus of activity was in the Purkinje layer. There was rapid activation of surrounding Purkinje tissue before activation of overlying muscle, which was later than that observed with atrial pacing. This is consistent with a focal origin of VT within the Purkinje layer. We did observe evidence of slow conduction and even conduction block in the epicardium, but this activity was more often than not significantly later than the focus of origin. Our data may therefore complement the findings of Pogwizd and Corr by further characterizing the contribution of the Purkinje system and myocardium to VT of focal endocardial origin. Although reentry has been described as causing reperfusion VT, such was not seen during this study.

Possible mechanisms of reperfusion VT. Kaplinsky et al. (7) have described that reperfusion VT tends to occur in two distinct phases: immediate and delayed. It has also been proposed that different mechanisms may contribute to these two phases of arrhythmias. In this study, we studied only the immediate phase reperfusion arrhythmias. The early phase correlates with fragmentation of local electrograms and continuous diastolic activity, which suggests that conditions for reentry are present. Evidence from mapping studies has, however, suggested that nonreentrant mechanisms may be important in the genesis of reperfusion VT. In this study, evidence of conduction delay and perhaps incomplete reentrant waveforms were seen in the epicardium, although in most cases, the VT complex had been initiated by a focal source in the Purkinje or endocardium.

Occurrence of VF. VF was seen in 58% of all cases where reperfusion VT occurred. VF occurs more commonly with reperfusion than coronary occlusion (16). In Pogwizd and Corr's (11) study on reperfusion arrhythmias, VF occurred in 33% of animals studied. The CL of the VTs, which subsequently degenerated to VF in this study, had a shorter CL than those who were self-terminating. This is in part due to acceleration of the VT CL seen just before degeneration to VF. In 7 of 11 cases where VTs degenerated to VF, more than one independent site of early activity was noted during one or more of the maintenance beats, whereas this was seen in only one of eight of the nonsustained VTs. The observation of more than one independent site of early activity immediately after reperfusion likely reflects the disorganized recovery of myocardial tissue from ischemia. The recovery of ischemic tissue during reperfusion is nonhomogeneous as evidenced by areas of slow conduction in this study, especially in the epicardium. The preponderance of multiple independent sites of early activity among those animals where VT degenerated to VF suggests that the relationship is causal.

Limitations. The recording of electrical activity was limited to the risk zone of the proximal LAD coronary artery occlusion and areas immediately surrounding the risk zone. This does not exclude that some of the activity seen could have originated from remote areas, although such remote activity could hardly be related to ischemia or reperfusion. However, in the reperfusion VT in this study, pre-QRS activity was fully surrounded by electrodes in the ischemic and border areas in all instances, strongly suggesting that the focus of origin was within the anticipated risk zone from the LAD occlusion.