Effect of coronary perfusion of heptanol or potassium on conduction and ventricular arrhythmias DAVID J. CALLANS, ROBERT S. KIEVAL, E. NEIL MOORE, AND JOSEPH F. SPEAR
BRUCE
G. HOOK,
Department of Animal Biology, School of Veterinary Medicine, and the Division of Cardiology, Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Callans, David J., Robert S. Kieval, Bruce G. Hook, E. Neil Moore, and Joseph F. Spear. Effect of coronary perfusion of heptanol or potassium on conduction and ventricular arrhythmias. Am. J. Physiol. 263 (Heart Circ. Physiol. 32): H1382-H1389, 1992.-Abnormalities in cellular coupling, modulated in part by intracellular gap junctions, have an important role in the genesis of reentrant arrhythmias in the setting of chronic myocardial infarction. The effects of heptanol, which has a relatively selective action on gap junctional resistance at low concentrations, and potassium, which primarily affects active membrane properties, were assessed using a localized intracoronary infusion system in 11 normal dogs in vivo. Both agents caused a dose-related slowing of conduction. Programmed stimulation during potassium infusion resulted in ventricular fibrillation in two of six animals treated with a low dose (5.0-5.5 meq/l) and five of six animals treated with a high dose (X0-7.5 meq/l). During the infusion of 1.0 mM heptanol, uniform ventricular tachycardia was induced in four of eight animals. Infusion of heptanol, but not potassium, increased the susceptibility to presumably reentrant ventricular tachycardia in normal myocardium. This suggests that agents that affect cellular coupling may have markedly different arrhythmogenic consequences than agents that primarily alter active membrane properties. cellular coupling; ventricular tachycardia; myocardial conduction
forslowingcardiacconduction velocity and producing conduction block include depression of active membrane properties, i.e., a reduction in fast sodium conductance or decreased excitability, and alterations in passive cell-to-cell electrotonic coupling (13). Recently, cell coupling, modulated primarily via intracellular gap junctional conductance, has been recognized as playing an important role in arrhythmogenesis (10, 26, 27). Abnormal, dissociated conduction is an important constituent of the electrophysiological substrate responsible for reentrant arrhythmias in the setting of healed myocardial infarction (10, 12, 22, 25, 27, 28). In addition, there is increasing evidence that myocardial tissue anisotropy may also play a role in arrhythmogenesis by contributing to slow conduction and influencing the location of conduction block (9, 10, 26). Heterogeneously distributed abnormalities in cell-to-cell electrical coupling have been implicated as a primary mechanism for the slow dissociated conduction associated with healed myocardial infarction (27). Even under normal conditions, the architecture of the myocardial cell determines the distribution of gap junctions such that effective axial resistance to current flow is greatest transverse to myocardial fiber orientation (7, 36). This anatomic anisotropy in turn influences conduction (24, 25). Systemic hyperkalemia can profoundly depress cardiac conduction. However, the cardiac arrhythmias as-
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sociated with hyperkalemia are relatively benign up to the point of “sine wave” ventricular tachycardia (VT) or fibrillation (VF) (8, 35). The difference between the arrhythmias associated with hyperkalemia and chronic myocardial infarction may relate to the global depression in conduction associated with hyperkalemia versus the more discrete nature of infarction. It is also possible that the nature of the conduction abnormality caused by cellular uncoupling is intrinsically more arrhythmogenic. To test this hypothesis we designed a system to deliver agents at precise, constant concentrations to localized regions of the canine left ventricle in vivo. This permitted testing for susceptibility to arrhythmias during the infusion of agents, which cause conduction slowing and block by different mechanisms. An added advantage of this system is that infused agents are diluted in the venous circulation and produce minimal systemic effects. Although there are no known agents that solely affect gap junctional conductance, heptanol has been reported to slow conduction by decreasing gap junctional conductance and to have minimal effects on action potential rate of depolarization at low concentrations (3, 9). In isolated ventricular myocytes and myocyte pairs the concentration of heptanol required to produce half block of fast sodium conductance is an order of magnitude greater than that which produces half block of gap junctional conductance (20, 23). Our results show that in animals infused with heptanol, programmed ventricular stimulation was more likely to result in the induction of uniform VT than in animals infused with potassium even though the degree of myocardial slowing was comparable. METHODS General Experiments were performed on 11 adult mongrel dogs of either sex, weighing 9-15 kg. The dogs were anesthetized with pentobarbitol sodium (30 mg/kg iv), intubated, and ventilated with room air using a volume-cycled, positive pressure ventilator. A 2-mm catheter was placed into each carotid artery; the right carotid was used for arterial pressure monitoring, the left carotid was used as the blood supply for the coronary bypass system. A 1.5-mm catheter was placed in the right internal jugular vein for administration of saline and additional anesthesia as necessary. Body temperature was monitored and maintained by use of a heated table. Exhaled carbon dioxide concentration, arterial blood pressure, and a lead II surface electrocardiogram (ECG) were monitored continuously. The heart was approached through a left lateral thoracotomy and suspended in a pericardial sling. Animals were killed at the conclusion of the experiments with an overdose of pentobarbital sodium. All experiments were performed in accordance with the
0 1992 the American
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EFFECTS
OF HEPTANOL
AND
K ON
NIH “The Guiding Principles in the Care and Use of Laboratory Animals” [DHEW Publication No. (NIH) 80-23, Revised 1978, Office of Science and Health Reports, DRR/NIH, Bethesda, MD 202051 and approved by the Council of the American Physiological Society. Coronary
Perfusion
System
A coronary bypasssystemwasconstructed using a peristaltic pump (Masterflex) to supply arterial blood to a reservoir so that pulsatile pressurewas dampedand reservoir output wasat constant flow. The details of this system are schematizedin Fig. 1. The blood supply for the perfusion system originated from the left carotid arterial cannula and perfused the left anterior descendingcoronary artery (LAD). Potassiumor heptanol could be introduced into the perfusate at known, constant concentrations through a syringe pump attached to a sidearm of the system; the volume infused through the sidearm at maximum flow rates contributed insignificantly (~5%) to total coronary flow. The bypass system contained a stainlesssteel coil positioned inside the chest cavity, which allowed passivereheating of the perfusate to body temperature. A l.O-mm OD stainless steel cannula formed the distal end of the perfusion system. The LAD coronary artery was dissectedfree proximal to the first diagonal branch. After systemic anticoagulation (heparin sodium,200 U/kg iv followed by 100U *kg-‘. h-i), the proximal LAD wasoccluded,and the cannula from the bypasssystemwas introduced through an arteriotomy and securedby a snare so that coronary flow wastotally determined by the perfusion system. The total time of LAD flow interruption was 150 min. More specifically, changesin epicardial activation times were~5% over this time period, and no arrhythmias were induced in any animal with repeatedprogrammed stimulation every 30 min (5).
ARRHYTHMIA
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Electrophysiological
Recordings
A 2 x 1 cm plaque consisting of 50 silver electrodesarranged as40 bipolar pairs (10 x 4 array, l-mm interbipole distance) was sutured to the epicardium in the LAD perfusion field. The plaque waspositioned with its long axis parallel to the longitudinal axis of the musclefibers that form the superficial epicardial layer, perpendicular to the LAD. Bipolar electrodeswere positioned at the borders of the plaque for pacing to establish conduction in either the longitudinal or transverse directions relative to fiber axis. Bipolar stainlesssteel plunge electrodes wereplacedin the left atrium for atria1pacing, the anterior right ventricle (outside the perfusion field) to serve as a reference recording for mapping, and at the left ventricular apex for programmedstimulation. Electrograms were filtered at 40-150 Hz band pass,displayedon a memory oscilloscope,and stored on an electrostatic strip-chart recorder at a paper speedof 250 mm/s and on magnetic tape. The meanepicardial activation time, that is, the averagetime required from the onset of ventricular activation to the intrinsic deflection at each of the 40 bipolar recording sites, was measured during sinus rhythm. The time of intrinsic activation at each site was defined as the peak of a monophasicrecording or the baselinecrossingof a biphasic recording. Activation times were referenced relative to the activation time recorded from the anterior right ventricular plungeelectrode (i.e., a site outside the perfusion field), which ultimately wasreferencedto the QRS onset of the surface ECG lead II. Activation time reflected conduction through the Purkinje system,Purkinje-musclejunction, and endocardium to the epicardium where the recording was made. A typical sinus activation map exhibiting nearly simultaneousactivation acrossthe entire area is shown in Fig. 2. Measurementswere made using a manual digitizing tablet (GTCO) interfaced with a computer (Hewlett-Packard 9836). The time resolution of this system is +O.l ms. In a similar manner, isochronalmapsof activation times were constructed during conduction longitudinal or transverse to fiber axis. Typical longitudinal and transverse conduction maps are shown in Fig. 3, A and B. Local activation times were referenced from the pacing artifact. The isochronal activation mapswere analyzed to estimatethe axis of fiber orientation. For longitudinal conduction (Fig. 3A) the major axis of the isothrones was visually determined (solid line). This was confirmed by inspecting the isochronesthat were distant from the pacing site during transverse conduction (Fig. 3B), which parallel the fiber axis and correspond to the axis determined in Fig. 3A. Using a linear regressionanalysis, conduction velocity during longitudinal and transverse propagation was calculated
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Fig. 1. Schematicview of coronaryperfusionsystem.Arterial bloodis suppliedfrom left carotidartery to a roller pumpwith a reservoirsystem. Heptanolor potassiumcan be introducedusinga syringepump thorougha sidearmin the system.Bloodisdeliveredat constantflowto the fieldof the left anteriordescending coronaryartery, whichhasbeen occludedproximally.Seetext for further discussion.
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Fig. 3. Example of activation maps during epicardial pacing longitudinal (A) and transverse (B) to fiber axis. Numbers refer to local activation times at each bipolar recording site, referenced to pacing artifact. Computer-generated isochronal lines are displayed at 2-ms intervals. Solid lines in A and B represent axis of longitudinal and transverse conduction, respectively. Activation time measurements and distances from pacing site at points along these lines were used to compute conduction velocity. Points used for determination of electrogram duration are those within dotted lines, which radiate at angles t10” from longitudinal axis and k30” from transverse axis.
from the slope of a plot of activation times versus distance from the pacing stimulus taken from sites on the longitudinal or transverse axis (solid lines in Fig. 3, A and B). The duration and morphology of the local bipolar electrogram were also characterized at each site. The duration of the electrogram was defined as the time between the first component with an amplitude >O.l mV to the last deflection above 0.1 mV. All 40 sites were included in the calculation of electrogram duration during sinus rhythm. Recording sites activated by propagation in the longitudinal direction, defined as those located within t 10” of parallel to fiber orientation (dashed lines in Fig. 3A), were included for determination of electrogram duration during longitudinal conduction. Similarly, sites within t30" of perpendicular to fiber axis (dashed lines in Fig. 3B) were included for the determination of electrogram duration during transverse conduction. Experimental Protocols Control experiments. In a separate series of experiments in which the LAD bypass was performed in six animals, repeated measurements of activation time during sinus rhythm, epicardial conduction velocity, and response to programmed ventricular stimulation remained constant over a 15O-min period (5). Infusion experiments. The effects of infusion of known constant concentrations of potassium and/or heptanol were studied in 11 animals. The serum potassium concentration was mea-
K ON
ARRHYTHMIA
INDUCTION
sured at the beginning of each experiment and averaged 3.8 t 0.2 meq/l. After baseline conduction measurements and programmed stimulation were performed in the control state, isotonic potassium chloride was infused through the sidearm of the perfusion system to bring the total serum potassium concentration inside the system to 5.0-5.5 meq/l. This group is referred to as the K, or 5.5 meq/l group. Conduction measurements and programmed stimulation were performed after a 5-min period of infusion, based on pilot experiments that indicated a steadystate effect at this time period. The potassium infusion was then discontinued, and a 15-min washout period followed the conclusion of the stimulation protocol. When conduction properties returned to baseline, the experiment continued, and similar determinations were made during the infusion of isotonic KC1 to reach a concentration of 7.0-7.5 meq/l (K, or 7.5 meq/l group) within the perfusion system. Conduction properties were again measured after the washout phase of the final potassium infusion; this determination served as the control for the heptanol infusion experiments. Heptanol (60 mM solution in 50% ethanol) was infused to attain serum concentrations of 0.5 and 1.0 mM sequentially (H,, HZ), and measurements of conduction and programmed stimulation were performed in a similar manner at each concentration, but after a IO-min infusion period. Separate pilot experiments documented the absence of an effect of the ethanol vehicle on conduction or susceptibility to inducible arrhythmias. This order of infusion experiments was used because of concern over the lack of complete reversibility of heptanol’s effect on conduction. The entire protocol, with infusion of all substances at each concentration, was not performed in all animals. There were six animals in the K, group, six in the K2 group, six in the H, group, and eight in the H2 group. Programmed Stimulation Constant current pulses were generated by a custom-made programmable stimulator and delivered as 2.0-ms square wave pulses at twice diastolic threshold. The pacing protocol included single, double, and triple ventricular extrastimuli, scanning diastole, delivered after eight beat-paced drive trains at a cycle length just below the sinus cycle length. The end point of stimulation was either the induction of a ventricular arrhythmia or completion of the protocol. Any repetitive activity lasting more than three beats was characterized according to the following schema: 1) uniform VT, defined by the presence of a regular repetitive electrogram morphology on the epicardial and surface ECG recordings with a cycle length ~100 ms; 2) polymorphic VT (PMVT), defined by the presence of continuously changing epicardial electrogram morphology, surface ECG lead axis, and cycle length; or 3) VF, defined by chaotic appearing electrograms on both the surface and epicardial leads with irregular R-R intervals 400 ms. Statistical Analysis Results are presented as means * SD where appropriate. The effects of potassium and heptanol infusion on conduction and electrogram characteristics are presented as percent change from control values. Differences in measured variables between control and high-dose potassium and heptanol groups were compared by two-tailed Student’s t test for paired data. A P value < 0.05 was considered statistically significant. RESULTS
Effect of Potassium and Heptanol Infusion on Electrogram Duration and Morphology
The mean electrogram durations in the control groups were 16.2 t 2.7 ms during sinus rhythm, 22.0 t 3.1 ms during epicardial conduction longitudinal to fiber axis,
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EFFECTS
OF HEPTANOL
AND
and 30.8 t 2.8 ms during conduction transverse to fiber axis. Infusion of 5.5 meq/l potassium caused no change in electrogram duration. During infusion of 7.5 meq/l potassium, significant increases in electrogram duration of 69.1 t 39.4% (P = 0.01) during sinus rhythm and 31.1 t 16.7% (P = 0.02) during longitudinal conduction were observed. This increase was not associated with fractionation of the waveform but with a decrease in the amplitude and attenuation of the rapid components. A nonsignificant increase in electrogram duration of 9.7 t 19.2% was seen in the K2 group during transverse conduction. The electrogram duration during longitudinal and transverse conduction could not be determined in experiment 9 because of conduction block during the K2 infusion. Heptanol infusion did not cause significant changes in electrogram duration at either concentration. In addition, heptanol infusion did not produce fractionated electrograms or cause the disappearance of individual electrogram components. There was no significant change in the QRS duration in surface lead II with either potassium or heptanol infusion. Changes in the S-T segment and T wave of the ECG consistent with systemic hyperkalemia were not observed during potassium infusions. Effect of Potassium and Heptanol Infusion on Epicardial Activation in Sinus Rhythm
The mean epicardial activation time during sinus rhythm in the control state was 26.2 t 5.4 and 26.8 t 4.5 ms for the potassium and heptanol control groups, respectively. Infusion of 5.5 meq/l potassium caused a decrease of 5.2 t 2.7% in the mean activation time during sinus rhythm (Table 1). Mean activation time increased markedly with infusion of 7.5 meq/l potassium, with a change from control of 33.2 t 23.8% (P = 0.03). Heptanol infusion caused a concentration-dependent increase in activation time of 3.2 t 5.0% at 0.5 mM and 15.3 t 7.9% at 1.0 mM (P = 0.003; Table 2). Effect of Potassium and Heptanol on Conduction Velocity During Epicardial Pacing
The control values for conduction velocity longitudinal and transverse to fiber orientation in the potassium exTable 1. Effects of potassium Sinus Expt
No.
1 2 3 4 5 6 7 8 9 10 Mean &SD P value
Control, ms
30.7 19.1 17.8 27.1 33.0 24.4 21.5 30.7 31.8 25.8 26.2 5.4
on epicardial
Activation 5.5 meq/l (%change)
-5.2 -9.9 -6.4 -3.9 -6.0 ND ND ND -2.7 -2.1 -5.2 2.7
CV (L), longitudinal conduction velocity; block. Negative changes in sinus activation velocity.
activation
K ( 3N ARRHYTHMIA
H1385
INDUCTION
periments were 0.456 t 0.045 and 0.153 t 0.030 m/s, respectively. Infusion of 5.5 meq/l potassium caused a slight increase in epicardial conduction velocity, resulting in a change of 2.9 t 6.1% during longitudinal and 3.5 t 9.7% during transverse conduction (Table 1). A similar potassium-induced acceleration in conduction velocity has been reported in vitro (11, 30). Conduction velocity decreased substantially during infusion of 7.5 meq/l potassium; longitudinal conduction velocity slowed by 42.2 t 30.8% (P = 0.04) and transverse conduction velocity slowed by 17.8 t 31.1% (P = NS). Epicardial conduction block preventing calculation of longitudinal and transverse conduction velocity occurred in one experiment in the K2 group. The control longitudinal and transverse conduction velocities in the heptanol experiments were 0.432 t 0.031 and 0.135 t 0.041 m/s. Infusion of 0.5 mM heptanol had little effect on epicardial conduction velocity, causing a slowing of 4.4 t 7.3% longitudinal and 5.0 t 4.2% transverse to fiber orientation (Table 2). In the 1.0 mM heptanol group, longitudinal and transverse conduction velocity decreased 12.7 t 11.3 and 16.4 t 10.5% compared with control (P < 0.05 for both comparisons). Conduction block during epicardial pacing was not observed with heptanol infusion. Although heptanol caused conduction slowing during impulse propagation in both directions relative to fiber axis, a preferential effect on transverse conduction was not observed. Effects of Potassium and Heptanol Infusion on Induction of Ventricular Arrhythmias
There was no significant change in the ventricular effective refractory period during any of the infusions. Programmed stimulation in the control state resulted in the initiation of VF in 1 of 11 animals with no inducible arrhythmias in the remaining 10. There were no changes in ventricular effective refractory period or in the sinus cycle length with either potassium or heptanol infusion. Repeated programmed stimulation during potassium infusion initiated VF in two of six K1 experiments and in five of six K2 experiments (Table 3). In the other four K1 experiments there were no inducible arrhythmias, and in
time and conduction cv UJ
Time 7.5 meq/l (%change)
ND 22.8 ND ND ND 26.6 56.8 43.8 55.1 ND 33.2 23.8 0.03
Control, m/s
5.5 meq/l (%change)
0.470 0.475 0.526 0.451 0.444 0.484 0.460 0.485 0.372 0.396 0.456 0.045
ND, not determined; time denote improved
0.4 6.3 -3.2 -3.8 2.5 ND ND ND 5.6 -2.9 2.9 6.1
CV CT) 7.5 meq/l (%change)
Control, 4s
5.5 mey/l (%change)
7.5 mey/l (%change)
ND -23.8 ND ND ND -56.2 -61.3 -72.1 NC ND -42.2 30.8 0.04
0.177 0.144 0.145 0.129 0.134 0.225 0.145 0.169 0.160 0.122 0.155 0.030
16.9 6.9 2.8 7.0 ND ND ND ND -12.5 11.9 3.5 9.7
ND -7.6 ND ND 13.4 0.4 66.9 27.2 NC ND -17.8 31.1 0.25
CV (T), transverse conduction conduction; negative changes
velocity; NC, not calculated because of conduction in CV (L) or CV (T) denote decreased conduction
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H1386
EFFECTS
OF HEPTANOL
Table 2. Effects of heptanol on epicardial Sinus Expt
No.
Control, ms
1 2 3 4 5 6 7 8 Mean +SD P value See Table
25.0 23.6 32.9 24.7 20.5 30.4 32.4 25.1 26.8 4.5 1 for abbreviations
Activation
No.
Group
Coupling
activation
0.5 mM (Y&change)
K ON
ARRHYTHMIA
cv 1.0 mM (S&change)
ND ND 6.5 2.3 -2.0 11.0 3.5 -2.1 3.2 5.0
5.1 7.2 15.4 8.3 17.7 23.4 27.1 18.4 15.3 7.9 0.003
INDUCTION
time and conduction
Time Control, m/s
0.42 1 0.432 0.49 1 0.444 0.438 0.440 0.381 0.413 0.432 0.03 1
CV (T)
(11)
0.5 mM (%change)
1.0 mM (Y&change)
Control, m/s
0.5 mM (%change)
1.0 mM (Ghange)
ND ND -4.9 -2.9 4.3 -17.0 -6.6 0.5 -4.4 7.3
-3.3 -23.8 -19.3 -23.1 -2.1 -26.6 -20.5 1.0 -12.7 11.3 0.02
0.171 0.127 0.134 0.251 0.146 0.175 0.138 0.134 0.153 0.041
ND ND -3.7 -6.8 -2.7 -10.9 -7.2 1.5 -5.0 4.2
-15.8 -3.2 -10.4 -15.1 -10.3 -30.3 -34.1 -11.9 -16.4 10.5 0.004
and description.
Table 3. Results of programmed Expt
AND
Intervals,
stimulation ms
Arrhyt,hmia
Control 300/140/100/100 None 280/140/130 VT (CL 130, 21 beats) H2 Control 350/190/140/120 None 350/170/150/120 None Kr Control 350/170/120/100 None 350/170/120/100 None Kr 330/170 VF K2 330/170 PMVT (CL 150, 4 beats) Hz Control 290/140/110/160 VF 350/160/170 VF Kr Control 350/170/130/100 None 350/150/140 VF Kr Control 350/180/150/130 None 350/l 70/l 70 VF K2 310/170/150/120 None K H, 320/200/170/140 None 7 Control 320/170/130/120 None 320/190 VF & 320/180/140/120 None HI 320/190/150/140 VF Hz 8 Control 330/200/170/140 None 300/150/140 VF K2 300/190/180/170 None HI 300/190/200/170 None H2 9 Control 250/170/140/120 None 280/170/140 PMVT (CL 130, 6 beats) K2 280/170/130/120 None HI 280/170/130 VT (CL 135, 12 beats) Hz 10 Control 250/150/130/110 None 250/140/130/110 None K, K, 250 (drive) VF 250/170/130 VT (CL 140, 11 beats) HI 250/160/140 VT (CL 135, 6 beats) 5 11 Control 350/180/140/150 None 330/180/150/130 None K* 350/190/170/150 None HI H, 320/190/140/120 VT (CL 130, 27 beats) K, and K%, potassium infusion, 5.0-5.5 and 7.0-7.5 meq; H, and Hg, heptanol infusion, 0.5 mM and 1.0 mM; VT, ventricular tachycardia; PMVT, polymorphic ventricular tachycardia; CL, cycle length. * Data on coupling interval refer to the stimulation that resulted in arrhythmia induction or the closest coupling intervals attained if no arrhythmias were induced.
the remaining Kz experiment PMVT (mean cycle length, 130 ms), which lasted six beats before degenerating to VF, was induced. A typical example of arrhythmia induction during infusion of 7.5 meq/l potassium is shown in Fig. 4. VF was initiated with single or double ventricular extra-
stimuli in four of five cases in the K2 group and during the drive train in the remaining experiment. During infusion of heptanol at a concentration of 0.5 mM, no arrhythmias were induced in five of six experiments; an episode of uniform, nonsustained VT with a cycle length 140 ms, lasting 11 beats before converting spontaneously to sinus rhythm, was initiated in experiment 10 (Table 3). Programmed stimulation during the infusion of 1.0 mM heptanol resulted in the induction of uniform VT in four of eight animals, with a mean cycle length of 133 ms (range 130-135 ms). VT episodes lasted 6-27 beats before degenerating to VF (Table 3). An example of the induction of uniform VT during 1.0 mM heptanol infusion is shown in Fig. 5. For the remaining experiments, no arrhythmia was induced in two cases, PMVT degenerating to VF after four beats, and VF were initiated in one experiment each. DISCUSSION
The major finding of this study is the differential response to programmed stimulation during the infusion of heptanol and potassium. Uniform VT (6-27 beats) was induced in four of eight animals in the H2 group. Only polymorphic VT and VF, which are generally considered nonspecific arrhythmias (4), were induced during the infusion of potassium. The type of arrhythmia induced was not related to the degree of conduction slowing produced by the various infusions because heptanol caused an intermediate amount of slowing compared with the two doses of potassium. Therefore heptanol produced a qualitatively different effect on arrhythmia inducibility than potassium. Importantly, the induction of ventricular arrhythmias cannot be explained as a “side effect” of prolonged perfusion by the LAD bypass system. In a separate series of experiments (5), over a period of 2 h (which was greater than the amount of time required for the completion of the drug infusion experiments), control animals consistently had no inducible arrhythmias with serial programmed stimulation. In addition, activation times during sinus rhythm and conduction velocity during epicardial pacing remained constant over a similar time period within the perfusion field. Thus this model
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EFFECTS
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G. HOOK,
Department of Animal Biology, School of Veterinary Medicine, and the Division of Cardiology, Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Callans, David J., Robert S. Kieval, Bruce G. Hook, E. Neil Moore, and Joseph F. Spear. Effect of coronary perfusion of heptanol or potassium on conduction and ventricular arrhythmias. Am. J. Physiol. 263 (Heart Circ. Physiol. 32): H1382-H1389, 1992.-Abnormalities in cellular coupling, modulated in part by intracellular gap junctions, have an important role in the genesis of reentrant arrhythmias in the setting of chronic myocardial infarction. The effects of heptanol, which has a relatively selective action on gap junctional resistance at low concentrations, and potassium, which primarily affects active membrane properties, were assessed using a localized intracoronary infusion system in 11 normal dogs in vivo. Both agents caused a dose-related slowing of conduction. Programmed stimulation during potassium infusion resulted in ventricular fibrillation in two of six animals treated with a low dose (5.0-5.5 meq/l) and five of six animals treated with a high dose (X0-7.5 meq/l). During the infusion of 1.0 mM heptanol, uniform ventricular tachycardia was induced in four of eight animals. Infusion of heptanol, but not potassium, increased the susceptibility to presumably reentrant ventricular tachycardia in normal myocardium. This suggests that agents that affect cellular coupling may have markedly different arrhythmogenic consequences than agents that primarily alter active membrane properties. cellular coupling; ventricular tachycardia; myocardial conduction
forslowingcardiacconduction velocity and producing conduction block include depression of active membrane properties, i.e., a reduction in fast sodium conductance or decreased excitability, and alterations in passive cell-to-cell electrotonic coupling (13). Recently, cell coupling, modulated primarily via intracellular gap junctional conductance, has been recognized as playing an important role in arrhythmogenesis (10, 26, 27). Abnormal, dissociated conduction is an important constituent of the electrophysiological substrate responsible for reentrant arrhythmias in the setting of healed myocardial infarction (10, 12, 22, 25, 27, 28). In addition, there is increasing evidence that myocardial tissue anisotropy may also play a role in arrhythmogenesis by contributing to slow conduction and influencing the location of conduction block (9, 10, 26). Heterogeneously distributed abnormalities in cell-to-cell electrical coupling have been implicated as a primary mechanism for the slow dissociated conduction associated with healed myocardial infarction (27). Even under normal conditions, the architecture of the myocardial cell determines the distribution of gap junctions such that effective axial resistance to current flow is greatest transverse to myocardial fiber orientation (7, 36). This anatomic anisotropy in turn influences conduction (24, 25). Systemic hyperkalemia can profoundly depress cardiac conduction. However, the cardiac arrhythmias as-
THEPOSSIBLEMECHANISMS
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$2.00
Copyright
sociated with hyperkalemia are relatively benign up to the point of “sine wave” ventricular tachycardia (VT) or fibrillation (VF) (8, 35). The difference between the arrhythmias associated with hyperkalemia and chronic myocardial infarction may relate to the global depression in conduction associated with hyperkalemia versus the more discrete nature of infarction. It is also possible that the nature of the conduction abnormality caused by cellular uncoupling is intrinsically more arrhythmogenic. To test this hypothesis we designed a system to deliver agents at precise, constant concentrations to localized regions of the canine left ventricle in vivo. This permitted testing for susceptibility to arrhythmias during the infusion of agents, which cause conduction slowing and block by different mechanisms. An added advantage of this system is that infused agents are diluted in the venous circulation and produce minimal systemic effects. Although there are no known agents that solely affect gap junctional conductance, heptanol has been reported to slow conduction by decreasing gap junctional conductance and to have minimal effects on action potential rate of depolarization at low concentrations (3, 9). In isolated ventricular myocytes and myocyte pairs the concentration of heptanol required to produce half block of fast sodium conductance is an order of magnitude greater than that which produces half block of gap junctional conductance (20, 23). Our results show that in animals infused with heptanol, programmed ventricular stimulation was more likely to result in the induction of uniform VT than in animals infused with potassium even though the degree of myocardial slowing was comparable. METHODS General Experiments were performed on 11 adult mongrel dogs of either sex, weighing 9-15 kg. The dogs were anesthetized with pentobarbitol sodium (30 mg/kg iv), intubated, and ventilated with room air using a volume-cycled, positive pressure ventilator. A 2-mm catheter was placed into each carotid artery; the right carotid was used for arterial pressure monitoring, the left carotid was used as the blood supply for the coronary bypass system. A 1.5-mm catheter was placed in the right internal jugular vein for administration of saline and additional anesthesia as necessary. Body temperature was monitored and maintained by use of a heated table. Exhaled carbon dioxide concentration, arterial blood pressure, and a lead II surface electrocardiogram (ECG) were monitored continuously. The heart was approached through a left lateral thoracotomy and suspended in a pericardial sling. Animals were killed at the conclusion of the experiments with an overdose of pentobarbital sodium. All experiments were performed in accordance with the
0 1992 the American
Physiological
Society
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EFFECTS
OF HEPTANOL
AND
K ON
NIH “The Guiding Principles in the Care and Use of Laboratory Animals” [DHEW Publication No. (NIH) 80-23, Revised 1978, Office of Science and Health Reports, DRR/NIH, Bethesda, MD 202051 and approved by the Council of the American Physiological Society. Coronary
Perfusion
System
A coronary bypasssystemwasconstructed using a peristaltic pump (Masterflex) to supply arterial blood to a reservoir so that pulsatile pressurewas dampedand reservoir output wasat constant flow. The details of this system are schematizedin Fig. 1. The blood supply for the perfusion system originated from the left carotid arterial cannula and perfused the left anterior descendingcoronary artery (LAD). Potassiumor heptanol could be introduced into the perfusate at known, constant concentrations through a syringe pump attached to a sidearm of the system; the volume infused through the sidearm at maximum flow rates contributed insignificantly (~5%) to total coronary flow. The bypass system contained a stainlesssteel coil positioned inside the chest cavity, which allowed passivereheating of the perfusate to body temperature. A l.O-mm OD stainless steel cannula formed the distal end of the perfusion system. The LAD coronary artery was dissectedfree proximal to the first diagonal branch. After systemic anticoagulation (heparin sodium,200 U/kg iv followed by 100U *kg-‘. h-i), the proximal LAD wasoccluded,and the cannula from the bypasssystemwas introduced through an arteriotomy and securedby a snare so that coronary flow wastotally determined by the perfusion system. The total time of LAD flow interruption was 150 min. More specifically, changesin epicardial activation times were~5% over this time period, and no arrhythmias were induced in any animal with repeatedprogrammed stimulation every 30 min (5).
ARRHYTHMIA
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INDUCTION
Electrophysiological
Recordings
A 2 x 1 cm plaque consisting of 50 silver electrodesarranged as40 bipolar pairs (10 x 4 array, l-mm interbipole distance) was sutured to the epicardium in the LAD perfusion field. The plaque waspositioned with its long axis parallel to the longitudinal axis of the musclefibers that form the superficial epicardial layer, perpendicular to the LAD. Bipolar electrodeswere positioned at the borders of the plaque for pacing to establish conduction in either the longitudinal or transverse directions relative to fiber axis. Bipolar stainlesssteel plunge electrodes wereplacedin the left atrium for atria1pacing, the anterior right ventricle (outside the perfusion field) to serve as a reference recording for mapping, and at the left ventricular apex for programmedstimulation. Electrograms were filtered at 40-150 Hz band pass,displayedon a memory oscilloscope,and stored on an electrostatic strip-chart recorder at a paper speedof 250 mm/s and on magnetic tape. The meanepicardial activation time, that is, the averagetime required from the onset of ventricular activation to the intrinsic deflection at each of the 40 bipolar recording sites, was measured during sinus rhythm. The time of intrinsic activation at each site was defined as the peak of a monophasicrecording or the baselinecrossingof a biphasic recording. Activation times were referenced relative to the activation time recorded from the anterior right ventricular plungeelectrode (i.e., a site outside the perfusion field), which ultimately wasreferencedto the QRS onset of the surface ECG lead II. Activation time reflected conduction through the Purkinje system,Purkinje-musclejunction, and endocardium to the epicardium where the recording was made. A typical sinus activation map exhibiting nearly simultaneousactivation acrossthe entire area is shown in Fig. 2. Measurementswere made using a manual digitizing tablet (GTCO) interfaced with a computer (Hewlett-Packard 9836). The time resolution of this system is +O.l ms. In a similar manner, isochronalmapsof activation times were constructed during conduction longitudinal or transverse to fiber axis. Typical longitudinal and transverse conduction maps are shown in Fig. 3, A and B. Local activation times were referenced from the pacing artifact. The isochronal activation mapswere analyzed to estimatethe axis of fiber orientation. For longitudinal conduction (Fig. 3A) the major axis of the isothrones was visually determined (solid line). This was confirmed by inspecting the isochronesthat were distant from the pacing site during transverse conduction (Fig. 3B), which parallel the fiber axis and correspond to the axis determined in Fig. 3A. Using a linear regressionanalysis, conduction velocity during longitudinal and transverse propagation was calculated
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Fig. 1. Schematicview of coronaryperfusionsystem.Arterial bloodis suppliedfrom left carotidartery to a roller pumpwith a reservoirsystem. Heptanolor potassiumcan be introducedusinga syringepump thorougha sidearmin the system.Bloodisdeliveredat constantflowto the fieldof the left anteriordescending coronaryartery, whichhasbeen occludedproximally.Seetext for further discussion.
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Fig. 3. Example of activation maps during epicardial pacing longitudinal (A) and transverse (B) to fiber axis. Numbers refer to local activation times at each bipolar recording site, referenced to pacing artifact. Computer-generated isochronal lines are displayed at 2-ms intervals. Solid lines in A and B represent axis of longitudinal and transverse conduction, respectively. Activation time measurements and distances from pacing site at points along these lines were used to compute conduction velocity. Points used for determination of electrogram duration are those within dotted lines, which radiate at angles t10” from longitudinal axis and k30” from transverse axis.
from the slope of a plot of activation times versus distance from the pacing stimulus taken from sites on the longitudinal or transverse axis (solid lines in Fig. 3, A and B). The duration and morphology of the local bipolar electrogram were also characterized at each site. The duration of the electrogram was defined as the time between the first component with an amplitude >O.l mV to the last deflection above 0.1 mV. All 40 sites were included in the calculation of electrogram duration during sinus rhythm. Recording sites activated by propagation in the longitudinal direction, defined as those located within t 10” of parallel to fiber orientation (dashed lines in Fig. 3A), were included for determination of electrogram duration during longitudinal conduction. Similarly, sites within t30" of perpendicular to fiber axis (dashed lines in Fig. 3B) were included for the determination of electrogram duration during transverse conduction. Experimental Protocols Control experiments. In a separate series of experiments in which the LAD bypass was performed in six animals, repeated measurements of activation time during sinus rhythm, epicardial conduction velocity, and response to programmed ventricular stimulation remained constant over a 15O-min period (5). Infusion experiments. The effects of infusion of known constant concentrations of potassium and/or heptanol were studied in 11 animals. The serum potassium concentration was mea-
K ON
ARRHYTHMIA
INDUCTION
sured at the beginning of each experiment and averaged 3.8 t 0.2 meq/l. After baseline conduction measurements and programmed stimulation were performed in the control state, isotonic potassium chloride was infused through the sidearm of the perfusion system to bring the total serum potassium concentration inside the system to 5.0-5.5 meq/l. This group is referred to as the K, or 5.5 meq/l group. Conduction measurements and programmed stimulation were performed after a 5-min period of infusion, based on pilot experiments that indicated a steadystate effect at this time period. The potassium infusion was then discontinued, and a 15-min washout period followed the conclusion of the stimulation protocol. When conduction properties returned to baseline, the experiment continued, and similar determinations were made during the infusion of isotonic KC1 to reach a concentration of 7.0-7.5 meq/l (K, or 7.5 meq/l group) within the perfusion system. Conduction properties were again measured after the washout phase of the final potassium infusion; this determination served as the control for the heptanol infusion experiments. Heptanol (60 mM solution in 50% ethanol) was infused to attain serum concentrations of 0.5 and 1.0 mM sequentially (H,, HZ), and measurements of conduction and programmed stimulation were performed in a similar manner at each concentration, but after a IO-min infusion period. Separate pilot experiments documented the absence of an effect of the ethanol vehicle on conduction or susceptibility to inducible arrhythmias. This order of infusion experiments was used because of concern over the lack of complete reversibility of heptanol’s effect on conduction. The entire protocol, with infusion of all substances at each concentration, was not performed in all animals. There were six animals in the K, group, six in the K2 group, six in the H, group, and eight in the H2 group. Programmed Stimulation Constant current pulses were generated by a custom-made programmable stimulator and delivered as 2.0-ms square wave pulses at twice diastolic threshold. The pacing protocol included single, double, and triple ventricular extrastimuli, scanning diastole, delivered after eight beat-paced drive trains at a cycle length just below the sinus cycle length. The end point of stimulation was either the induction of a ventricular arrhythmia or completion of the protocol. Any repetitive activity lasting more than three beats was characterized according to the following schema: 1) uniform VT, defined by the presence of a regular repetitive electrogram morphology on the epicardial and surface ECG recordings with a cycle length ~100 ms; 2) polymorphic VT (PMVT), defined by the presence of continuously changing epicardial electrogram morphology, surface ECG lead axis, and cycle length; or 3) VF, defined by chaotic appearing electrograms on both the surface and epicardial leads with irregular R-R intervals 400 ms. Statistical Analysis Results are presented as means * SD where appropriate. The effects of potassium and heptanol infusion on conduction and electrogram characteristics are presented as percent change from control values. Differences in measured variables between control and high-dose potassium and heptanol groups were compared by two-tailed Student’s t test for paired data. A P value < 0.05 was considered statistically significant. RESULTS
Effect of Potassium and Heptanol Infusion on Electrogram Duration and Morphology
The mean electrogram durations in the control groups were 16.2 t 2.7 ms during sinus rhythm, 22.0 t 3.1 ms during epicardial conduction longitudinal to fiber axis,
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EFFECTS
OF HEPTANOL
AND
and 30.8 t 2.8 ms during conduction transverse to fiber axis. Infusion of 5.5 meq/l potassium caused no change in electrogram duration. During infusion of 7.5 meq/l potassium, significant increases in electrogram duration of 69.1 t 39.4% (P = 0.01) during sinus rhythm and 31.1 t 16.7% (P = 0.02) during longitudinal conduction were observed. This increase was not associated with fractionation of the waveform but with a decrease in the amplitude and attenuation of the rapid components. A nonsignificant increase in electrogram duration of 9.7 t 19.2% was seen in the K2 group during transverse conduction. The electrogram duration during longitudinal and transverse conduction could not be determined in experiment 9 because of conduction block during the K2 infusion. Heptanol infusion did not cause significant changes in electrogram duration at either concentration. In addition, heptanol infusion did not produce fractionated electrograms or cause the disappearance of individual electrogram components. There was no significant change in the QRS duration in surface lead II with either potassium or heptanol infusion. Changes in the S-T segment and T wave of the ECG consistent with systemic hyperkalemia were not observed during potassium infusions. Effect of Potassium and Heptanol Infusion on Epicardial Activation in Sinus Rhythm
The mean epicardial activation time during sinus rhythm in the control state was 26.2 t 5.4 and 26.8 t 4.5 ms for the potassium and heptanol control groups, respectively. Infusion of 5.5 meq/l potassium caused a decrease of 5.2 t 2.7% in the mean activation time during sinus rhythm (Table 1). Mean activation time increased markedly with infusion of 7.5 meq/l potassium, with a change from control of 33.2 t 23.8% (P = 0.03). Heptanol infusion caused a concentration-dependent increase in activation time of 3.2 t 5.0% at 0.5 mM and 15.3 t 7.9% at 1.0 mM (P = 0.003; Table 2). Effect of Potassium and Heptanol on Conduction Velocity During Epicardial Pacing
The control values for conduction velocity longitudinal and transverse to fiber orientation in the potassium exTable 1. Effects of potassium Sinus Expt
No.
1 2 3 4 5 6 7 8 9 10 Mean &SD P value
Control, ms
30.7 19.1 17.8 27.1 33.0 24.4 21.5 30.7 31.8 25.8 26.2 5.4
on epicardial
Activation 5.5 meq/l (%change)
-5.2 -9.9 -6.4 -3.9 -6.0 ND ND ND -2.7 -2.1 -5.2 2.7
CV (L), longitudinal conduction velocity; block. Negative changes in sinus activation velocity.
activation
K ( 3N ARRHYTHMIA
H1385
INDUCTION
periments were 0.456 t 0.045 and 0.153 t 0.030 m/s, respectively. Infusion of 5.5 meq/l potassium caused a slight increase in epicardial conduction velocity, resulting in a change of 2.9 t 6.1% during longitudinal and 3.5 t 9.7% during transverse conduction (Table 1). A similar potassium-induced acceleration in conduction velocity has been reported in vitro (11, 30). Conduction velocity decreased substantially during infusion of 7.5 meq/l potassium; longitudinal conduction velocity slowed by 42.2 t 30.8% (P = 0.04) and transverse conduction velocity slowed by 17.8 t 31.1% (P = NS). Epicardial conduction block preventing calculation of longitudinal and transverse conduction velocity occurred in one experiment in the K2 group. The control longitudinal and transverse conduction velocities in the heptanol experiments were 0.432 t 0.031 and 0.135 t 0.041 m/s. Infusion of 0.5 mM heptanol had little effect on epicardial conduction velocity, causing a slowing of 4.4 t 7.3% longitudinal and 5.0 t 4.2% transverse to fiber orientation (Table 2). In the 1.0 mM heptanol group, longitudinal and transverse conduction velocity decreased 12.7 t 11.3 and 16.4 t 10.5% compared with control (P < 0.05 for both comparisons). Conduction block during epicardial pacing was not observed with heptanol infusion. Although heptanol caused conduction slowing during impulse propagation in both directions relative to fiber axis, a preferential effect on transverse conduction was not observed. Effects of Potassium and Heptanol Infusion on Induction of Ventricular Arrhythmias
There was no significant change in the ventricular effective refractory period during any of the infusions. Programmed stimulation in the control state resulted in the initiation of VF in 1 of 11 animals with no inducible arrhythmias in the remaining 10. There were no changes in ventricular effective refractory period or in the sinus cycle length with either potassium or heptanol infusion. Repeated programmed stimulation during potassium infusion initiated VF in two of six K1 experiments and in five of six K2 experiments (Table 3). In the other four K1 experiments there were no inducible arrhythmias, and in
time and conduction cv UJ
Time 7.5 meq/l (%change)
ND 22.8 ND ND ND 26.6 56.8 43.8 55.1 ND 33.2 23.8 0.03
Control, m/s
5.5 meq/l (%change)
0.470 0.475 0.526 0.451 0.444 0.484 0.460 0.485 0.372 0.396 0.456 0.045
ND, not determined; time denote improved
0.4 6.3 -3.2 -3.8 2.5 ND ND ND 5.6 -2.9 2.9 6.1
CV CT) 7.5 meq/l (%change)
Control, 4s
5.5 mey/l (%change)
7.5 mey/l (%change)
ND -23.8 ND ND ND -56.2 -61.3 -72.1 NC ND -42.2 30.8 0.04
0.177 0.144 0.145 0.129 0.134 0.225 0.145 0.169 0.160 0.122 0.155 0.030
16.9 6.9 2.8 7.0 ND ND ND ND -12.5 11.9 3.5 9.7
ND -7.6 ND ND 13.4 0.4 66.9 27.2 NC ND -17.8 31.1 0.25
CV (T), transverse conduction conduction; negative changes
velocity; NC, not calculated because of conduction in CV (L) or CV (T) denote decreased conduction
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H1386
EFFECTS
OF HEPTANOL
Table 2. Effects of heptanol on epicardial Sinus Expt
No.
Control, ms
1 2 3 4 5 6 7 8 Mean +SD P value See Table
25.0 23.6 32.9 24.7 20.5 30.4 32.4 25.1 26.8 4.5 1 for abbreviations
Activation
No.
Group
Coupling
activation
0.5 mM (Y&change)
K ON
ARRHYTHMIA
cv 1.0 mM (S&change)
ND ND 6.5 2.3 -2.0 11.0 3.5 -2.1 3.2 5.0
5.1 7.2 15.4 8.3 17.7 23.4 27.1 18.4 15.3 7.9 0.003
INDUCTION
time and conduction
Time Control, m/s
0.42 1 0.432 0.49 1 0.444 0.438 0.440 0.381 0.413 0.432 0.03 1
CV (T)
(11)
0.5 mM (%change)
1.0 mM (Y&change)
Control, m/s
0.5 mM (%change)
1.0 mM (Ghange)
ND ND -4.9 -2.9 4.3 -17.0 -6.6 0.5 -4.4 7.3
-3.3 -23.8 -19.3 -23.1 -2.1 -26.6 -20.5 1.0 -12.7 11.3 0.02
0.171 0.127 0.134 0.251 0.146 0.175 0.138 0.134 0.153 0.041
ND ND -3.7 -6.8 -2.7 -10.9 -7.2 1.5 -5.0 4.2
-15.8 -3.2 -10.4 -15.1 -10.3 -30.3 -34.1 -11.9 -16.4 10.5 0.004
and description.
Table 3. Results of programmed Expt
AND
Intervals,
stimulation ms
Arrhyt,hmia
Control 300/140/100/100 None 280/140/130 VT (CL 130, 21 beats) H2 Control 350/190/140/120 None 350/170/150/120 None Kr Control 350/170/120/100 None 350/170/120/100 None Kr 330/170 VF K2 330/170 PMVT (CL 150, 4 beats) Hz Control 290/140/110/160 VF 350/160/170 VF Kr Control 350/170/130/100 None 350/150/140 VF Kr Control 350/180/150/130 None 350/l 70/l 70 VF K2 310/170/150/120 None K H, 320/200/170/140 None 7 Control 320/170/130/120 None 320/190 VF & 320/180/140/120 None HI 320/190/150/140 VF Hz 8 Control 330/200/170/140 None 300/150/140 VF K2 300/190/180/170 None HI 300/190/200/170 None H2 9 Control 250/170/140/120 None 280/170/140 PMVT (CL 130, 6 beats) K2 280/170/130/120 None HI 280/170/130 VT (CL 135, 12 beats) Hz 10 Control 250/150/130/110 None 250/140/130/110 None K, K, 250 (drive) VF 250/170/130 VT (CL 140, 11 beats) HI 250/160/140 VT (CL 135, 6 beats) 5 11 Control 350/180/140/150 None 330/180/150/130 None K* 350/190/170/150 None HI H, 320/190/140/120 VT (CL 130, 27 beats) K, and K%, potassium infusion, 5.0-5.5 and 7.0-7.5 meq; H, and Hg, heptanol infusion, 0.5 mM and 1.0 mM; VT, ventricular tachycardia; PMVT, polymorphic ventricular tachycardia; CL, cycle length. * Data on coupling interval refer to the stimulation that resulted in arrhythmia induction or the closest coupling intervals attained if no arrhythmias were induced.
the remaining Kz experiment PMVT (mean cycle length, 130 ms), which lasted six beats before degenerating to VF, was induced. A typical example of arrhythmia induction during infusion of 7.5 meq/l potassium is shown in Fig. 4. VF was initiated with single or double ventricular extra-
stimuli in four of five cases in the K2 group and during the drive train in the remaining experiment. During infusion of heptanol at a concentration of 0.5 mM, no arrhythmias were induced in five of six experiments; an episode of uniform, nonsustained VT with a cycle length 140 ms, lasting 11 beats before converting spontaneously to sinus rhythm, was initiated in experiment 10 (Table 3). Programmed stimulation during the infusion of 1.0 mM heptanol resulted in the induction of uniform VT in four of eight animals, with a mean cycle length of 133 ms (range 130-135 ms). VT episodes lasted 6-27 beats before degenerating to VF (Table 3). An example of the induction of uniform VT during 1.0 mM heptanol infusion is shown in Fig. 5. For the remaining experiments, no arrhythmia was induced in two cases, PMVT degenerating to VF after four beats, and VF were initiated in one experiment each. DISCUSSION
The major finding of this study is the differential response to programmed stimulation during the infusion of heptanol and potassium. Uniform VT (6-27 beats) was induced in four of eight animals in the H2 group. Only polymorphic VT and VF, which are generally considered nonspecific arrhythmias (4), were induced during the infusion of potassium. The type of arrhythmia induced was not related to the degree of conduction slowing produced by the various infusions because heptanol caused an intermediate amount of slowing compared with the two doses of potassium. Therefore heptanol produced a qualitatively different effect on arrhythmia inducibility than potassium. Importantly, the induction of ventricular arrhythmias cannot be explained as a “side effect” of prolonged perfusion by the LAD bypass system. In a separate series of experiments (5), over a period of 2 h (which was greater than the amount of time required for the completion of the drug infusion experiments), control animals consistently had no inducible arrhythmias with serial programmed stimulation. In addition, activation times during sinus rhythm and conduction velocity during epicardial pacing remained constant over a similar time period within the perfusion field. Thus this model
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EFFECTS
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