Factors related to the induction of ventricular fibrillation in the normal canine heart by programmed electrical stimulation

Effect of Shenfu Injection on Porcine Renal Function after Cardiopulmonary Resuscitation

Objective

To comprehensively evaluate the protective effect of Shenfu injection (SFI) on renal ischaemia/reperfusion injury (IRI) after cardiopulmonary resuscitation (CPR) through neutrophil gelatinase-associated lipocalin (NGAL) and to explore effective monitoring of early renal injuries after CPR.

Methods

Thirty healthy minipigs were randomly divided into 3 groups: sham operation (SO) (n = 6), control (n = 12), and SFI (n = 12). The SO group underwent only catheterization, whereas the control and SFI groups were subjected to program-controlled electrical stimulation to establish a cardiac arrest (CA) model due to ventricular fibrillation. After CPR, the return of spontaneous circulation was achieved. Each animal in the SFI group was intravenously injected with SFI after resuscitation. Haemodynamic parameters were monitored at baseline and 2, 6, 12, and 24 hr after CPR. At each time point, venous blood samples were collected for NGAL, creatinine, and ATPase screening.

Results

After CA, the MAP, CPP, and CO of the animals in the control and SFI groups decreased significantly. However, at 6 hr after CPR, the MAP, CPP, and CO of the animals in the SFI group began to recover gradually; the differences between the control and SFI groups were significant (P < 0.005). The renal damage immediately after CPR appeared to be significant in the pathological examinations. However, the degree of renal injury in the SFI group improved significantly, and the apoptosis index was also notably reduced. The blood and urine NGAL levels were clearly elevated after CPR. The greatest increase in NGAL was found in the control group, which was significantly different from that of the SFI group (P < 0.001). SFI can significantly increase the ATPase activity of kidney tissues after CPR and improve abnormal caspase-3 protein expression.

Conclusion

SFI can effectively prevent acute kidney injuries caused by CPR through improving energy metabolism and inhibiting apoptosis.

Role of abnormal repolarization in the mechanism of cardiac arrhythmia

In cardiac patients, life-threatening tachyarrhythmia is often precipitated by abnormal changes in ventricular repolarization and refractoriness. Repolarization abnormalities typically evolve as a consequence of impaired function of outward K⁺ currents in cardiac myocytes, which may be caused by genetic defects or result from various acquired pathophysiological conditions, including electrical remodelling in cardiac disease, ion channel modulation by clinically used pharmacological agents, and systemic electrolyte disorders seen in heart failure, such as hypokalaemia. Cardiac electrical instability attributed to abnormal repolarization relies on the complex interplay between a provocative arrhythmic trigger and vulnerable arrhythmic substrate, with a central role played by the excessive prolongation of ventricular action potential duration, impaired intracellular Ca²⁺ handling, and slowed impulse conduction. This review outlines the electrical activity of ventricular myocytes in normal conditions and cardiac disease, describes classical electrophysiological mechanisms of cardiac arrhythmia, and provides an update on repolarization-related surrogates currently used to assess arrhythmic propensity, including spatial dispersion of repolarization, activation-repolarization coupling, electrical restitution, TRIaD (triangulation, reverse use dependence, instability, and dispersion), and the electromechanical window. This is followed by a discussion of the mechanisms that account for the dependence of arrhythmic vulnerability on the location of the ventricular pacing site. Finally, the review clarifies the electrophysiological basis for cardiac arrhythmia produced by hypokalaemia, and gives insight into the clinical importance and pathophysiology of drug-induced arrhythmia, with particular focus on class Ia (quinidine, procainamide) and Ic (flecainide) Na⁺ channel blockers, and class III antiarrhythmic agents that block the delayed rectifier K⁺ channel (dofetilide).

Ventricular repolarization time, location of pacing stimulus and current pulse amplitude conspire to determine arrhythmogenicity in mice

AIM

In this study, we investigate the impact of altered action potential durations (APD) on ventricular repolarization time and proarrhythmia in mice with and without genetic deletion of the K⁺ -channel-interacting protein 2 (KChIP2^-/- and WT respectively). Moreover, we examine the interrelationship between the dispersion of repolarization time and current pulse amplitude in provoking ventricular arrhythmia.

METHODS

Intracardiac pacing in anesthetized mice determined refractory periods and proarrhythmia susceptibility. Regional activation time (AT), APD and repolarization time (=AT + APD) were measured in isolated hearts using floating microelectrodes.

RESULTS

Proarrhythmia in WT and KChIP2^-/- was not sensitive to changes in refractory periods. Action potentials were longer in KChIP2^-/- hearts compared to WT hearts. Isolated WT hearts had large apico-basal dispersion of repolarization time, whereas hearts from KChIP2^-/- mice had large left-to-right ventricular dispersion of repolarization time. Pacing from the right ventricle in KChIP2^-/- mice in vivo revealed significant lower current pulse amplitudes needed to induce arrhythmias in these mice.

CONCLUSION

Large heterogeneity of repolarization time is proarrhythmic when pacing is delivered from the location of earlier repolarization time. Ventricular repolarization time, location of the pacing stimulus and the amplitude of the stimulating current pulse are critical parameters underlying arrhythmia vulnerability.

Effect of anisotropy on ventricular vulnerability to unidirectional block and reentry by single premature stimulation during normal sinus rhythm in rat heart

Single high-intensity premature stimuli when applied to the ventricles during ventricular drive of an ectopic site, as in Winfree's "pinwheel experiment," usually induce reentry arrhythmias in the normal heart, while single low-intensity stimuli barely do. Yet ventricular arrhythmia vulnerability during normal sinus rhythm remains largely unexplored. With a view to define the role of anisotropy on ventricular vulnerability to unidirectional conduction block and reentry, we revisited the pinwheel experiment with reduced constraints in the in situ rat heart. New features included single premature stimulation during normal sinus rhythm, stimulation and unipolar potential mapping from the same high-resolution epicardial electrode array, and progressive increase in stimulation strength and prematurity from diastolic threshold until arrhythmia induction. Measurements were performed with 1-ms cathodal stimuli at multiple test sites (n = 26) in seven rats. Stimulus-induced virtual electrode polarization during sinus beat recovery phase influenced premature ventricular responses. Specifically, gradual increase in stimulus strength and prematurity progressively induced make, break, and graded-response stimulation mechanisms. Hence unidirectional conduction block occurred as follows: 1) along fiber direction, on right and left ventricular free walls (n = 23), initiating figure-eight reentry (n = 17) and tachycardia (n = 12), and 2) across fiber direction, on lower interventricular septum (n = 3), initiating spiral wave reentry (n = 2) and tachycardia (n = 1). Critical time window (55.1 ± 4.7 ms, 68.2 ± 6.0 ms) and stimulus strength lower limit (4.9 ± 0.6 mA) defined vulnerability to reentry. A novel finding of this study was that ventricular tachycardia evolves and is maintained by episodes of scroll-like wave and focal activation couplets. We also found that single low-intensity premature stimuli can induce repetitive ventricular response (n = 13) characterized by focal activations.NEW & NOTEWORTHY We performed ventricular cathodal point stimulation during sinus rhythm by progressively increasing stimulus strength and prematurity. Virtual electrode polarization and recovery gradient progressively induced make, break, and graded-response stimulation mechanisms. Unidirectional conduction block occurred along or across fiber direction, initiating figure-eight or spiral wave reentry, respectively, and tachycardia sustained by scroll wave and focal activations.

Effect of continuous compression and 30:2 cardiopulmonary resuscitation on cerebral microcirculation in a porcine model of cardiac arrest

BACKGROUND

The effect of rescue breathing on neurologic prognosis after cardiopulmonary resuscitation (CPR) is controversial. Therefore, we investigated the cerebral microcirculatory and oxygen metabolism during continuous compression (CC) and 30:2 CPR (VC) in a porcine model of cardiac arrest to determine which is better for neurologic prognosis after CPR.

METHODS

After 4 min of ventricular fibrillation, 20 pigs were randomised into two groups (n=10/group) receiving CC-CPR or VC-CPR. Cerebral oxygen metabolism and blood flow were measured continuously using laser Doppler flowmetry. Haemodynamic data were recorded at baseline and 5 min, 30 min, 2 h and 4 h after restoration of spontaneous circulation (ROSC).

RESULTS

Compared with the VC group, the mean cortical cerebral blood flow was significantly higher at 5 min ROSC in the CC group (P<0.05), but the difference disappeared after that time point. Brain percutaneous oxygen partial pressures were higher, and brain percutaneous carbon dioxide partial pressures were lower, in the VC group from 30 min to 4 h after ROSC; significant differences were found between the two groups (P<0.05). However, no significant difference of the cerebral oxygen extraction fraction existed between the two groups.

CONCLUSIONS

Inconsistency of systemic circulation and cerebral microcirculation with regard to blood perfusion and oxygen metabolism is common after CPR. No significant differences in cortical blood flow and oxygen metabolism were found between the CC-CPR and VC-CPR groups after ROSC.

Effect of rescue breathing during cardiopulmonary resuscitation on lung function after restoration of spontaneous circulation in a porcine model of prolonged cardiac arrest

OBJECTIVE

The destruction of the pulmonary structure after cardiopulmonary resuscitation may lead to lung function breakdown. The aim of this study was to investigate lung function after cardiopulmonary resuscitation and the influence of rescue breathing on lung function.

DESIGN

Prospective, randomized animal study.

SETTING

A university animal research laboratory.

SUBJECTS

Twenty-eight male domestic pigs weighing 30 ± 2 kg.

INTERVENTIONS

The animals were randomized into three groups: continuous compressions (n = 12), 30:2 compression/rescue ventilation cardiopulmonary resuscitation (n = 12), and sham cardiopulmonary resuscitation (n = 4). Ventricular fibrillation was induced in the continuous compressions and compression/rescue ventilation groups.

MEASUREMENTS AND MAIN RESULTS

Cardiac output, extravascular lung water, and airway resistance were measured at baseline and 1, 2, and 4 hrs after restoration of spontaneous circulation. Thoracopulmonary compliance, lower inflection point, and dead space were calculated. Lung ventilation/perfusion scans with Tc were performed 48 hrs before the experiment and 24 hrs after restoration of spontaneous circulation. Conventional histopathology evaluation was performed. Dead space, airway resistance, lower inflection point, and extravascular lung water significantly increased and compliance decreased after restoration of spontaneous circulation in the continuous compressions and compression/rescue ventilation groups. Lung injury was more severe in the continuous compressions group. Significant differences were found between the two groups in the three time points after restoration of spontaneous circulation (p < 0.05). Variables of the sham cardiopulmonary resuscitation group remained stable during the whole protocol. Poor ventilation/perfusion and mismatch were found after restoration of spontaneous circulation, but the injury was mitigated in the compression/rescue ventilation group. Histopathology injury in the compression/rescue ventilation group was also improved.

CONCLUSIONS

Appropriate rescue breathing during cardiopulmonary resuscitation does not influence the prognosis of cardiac arrest or the hemodynamics after restoration of spontaneous circulation but can improve lung function and alleviate lung injury.

Load-distributing band improves ventilation and hemodynamics during resuscitation in a porcine model of prolonged cardiac arrest

Background

The use of mechanical cardiopulmonary resuscitation (CPR) has great potential for the clinical setting. The purpose of present study is to compare the hemodynamics and ventilation during and after the load-distributing band CPR, versus the manual CPR in a porcine model of prolonged cardiac arrest, and to investigate the influence of rescue breathing in different CPR protocols.

Methods

Sixty-four male pigs (n = 16/group), weighing 30 ± 2 kg, were induced ventricular fibrillation and randomized into four resuscitation groups: continuous load-distributing band CPR without rescue ventilation (C-CPR), load-distributing band 30:2 CPR (A-CPR), load-distributing band CPR with continuous rescue breathing (10/min) (V-CPR) or manual 30:2 CPR (M-CPR). Respiratory variables and hemodynamics were recorded continuously; blood gas was analyzed.

Results

Tidal volume produced by compressions in the A-, C- and V-CPR groups were significantly higher compared with the M-CPR group (all p < 0.05). Coronary perfusion pressure of the V-CPR group was significantly lower than the C-CPR group (p < 0.01), but higher than the M-CPR group. The increasing of lung dead space after restoration of spontaneous circulation was significantly greater in the M-CPR group compared with the A-, C- and V-CPR groups (p < 0.01). Blood pH gradually decreased and was lower in the M-CPR group than that in the A-, C- and V-CPR groups (p < 0.01). PaO₂ of the A-, C- and V-CPR groups were significantly higher and PaCO₂ were significantly lower compared with the M-CPR (both p < 0.05). Cerebral performance categories were better in the A-, C- and V-CPR groups compared with the M-CPR group (p < 0.0001).

Conclusions

The load-distributing band CPR significantly improved respiratory parameters during resuscitation by augmenting passive ventilation, and significantly improved coronary perfusion pressure. The volume of ventilation produced by the load-distributing band CPR was adequate to maintain sufficient gas exchange independent of rescue breathing.

The effects of 3 different compression methods on intrathoracic pressure in a swine model of ventricular fibrillation

OBJECTIVE

The aim of this study was to provide a realistic comparison of 3 different extracorporeal compression methods during cardiopulmonary resuscitation on intrathoracic pressure (ITP), hemodynamics, and oxygen metabolism in a swine model of ventricular fibrillation (VF).

METHODS

Eight minutes after the development of VF, pigs were subjected to 3 different extracorporeal compression methods: traditional artificial manual compression, mechanical compression using an AutoPulse apparatus, or manual sucker. Heart rhythm was assessed by electrocardiography after 5 cycles of extracorporeal compression. If VF still occurred, electrical defibrillation was performed. After defibrillation, an additional 5 cycles of extracorporeal compression were performed. Resuscitation was considered to have failed if the above procedure was continued for 30 minutes without return of spontaneous circulation. Hemodynamics and ITP waveforms were monitored continuously. Oxygen metabolism indices were measured, and success rates were compared among the groups.

RESULTS

Manual compression showed advantages over both of the other methods in terms of maximal ITP and fluctuation amplitude, hemodynamic and oxygen dynamic changes, convenience of administration, and duration of treatment. Survival rates and cerebral performance category scores for the manual compression group were significantly higher than that for the other groups.

CONCLUSIONS

Mechanical compression cannot replace traditional artificial manual compression, which remains the preferred method for cardiopulmonary resuscitation.

Infusion of 4°C normal saline can improve the neurological outcome in a porcine model of cardiac arrest

BACKGROUND

This study sought to investigate induction of therapeutic hypothermia using ice-cold intravenous fluid after cardiopulmonary resuscitation (CPR). The effects on temperature, hemodynamics, cognitive performance and the accompanying neurohistopathological changes, and apoptosis were assessed.

METHODS

Fourteen piglets had 4 minutes of untreated ventricular fibrillation, followed by CPR. The animals in which spontaneous circulation was restored were randomly assigned to two groups: the hypothermia group (n = 7) was given an infusion of 4°C cold normal saline solution 30 mL/kg at an infusion rate of 1.33 mL/kg/min, followed by 10 mL/kg/h to 4 hours after restoration of spontaneous circulation; the control group (n = 7) was given the same infusion at room temperature. Variables were measured repeatedly until 4 hours after restoration of spontaneous circulation. Neurocognitive performance was evaluated 24 hours after CPR. Then animals were killed and the brains were removed for histopathology at 24 hours after restoration of spontaneous circulation. Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling method was used for apoptosis evaluation.

RESULTS

Compared with the control group, the core temperature of the hypothermia group was significantly decreased (p < 0.01). The cerebral performance categories at 24 hours after restoration of spontaneous circulation in the hypothermia group were better than that in the control group (p < 0.05). The percentage of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling-positive cells in the cortex and dentate gyrus of the hippocampus were significantly reduced in the hypothermia group compared with the control group at 24 hours after restoration of spontaneous circulation. By observation of transmission electron microscopy, the neuron damages were significantly reduced in hypothermia group.

CONCLUSION

4°C normal saline solution is a safe and effective method to reduce brain damages and prevent apoptotic cell death after cardiac arrest.

Effect of continuous compressions and 30:2 cardiopulmonary resuscitation on global ventilation/perfusion values during resuscitation in a porcine model

OBJECTIVE

Rescue ventilations during bystander resuscitation, although previously considered essential, interrupt the continuity of chest compressions and might have deleterious effects in basic life support. This study was undertaken to analyze the global ventilation/perfusion values of continuous compressions and 30:2 cardiopulmonary resuscitation to determine the effectiveness for each approach in a porcine model of prolonged bystander cardiopulmonary resuscitation for ventricular fibrillation.

DESIGN

Prospective, randomized animal study.

SETTING

A university animal research laboratory.

SUBJECTS

Twenty-four male domestic pigs (n = 12/group) weighing 30 ± 2 kg.

INTERVENTIONS

All animals had ventricular fibrillation induced by programmed electrical stimulation instruments and were randomized into two groups. Continuous compressions or 30:2 compression/rescue ventilation cardiopulmonary resuscitation was performed in each group.

MEASUREMENTS AND MAIN RESULTS

Continuous respiratory variables, hemodynamic parameters, and blood gas analysis outcomes were recorded, and global ventilation/perfusion values were calculated. Alveolar minute volume and global ventilation/perfusion values decreased progressively after ventricular fibrillation, but cardiac output was stable. The global ventilation/perfusion value was higher in the ventilation cardiopulmonary resuscitation group than that in the continuous compression group (p < .0001) and was higher than normal. Coronary perfusion pressure was progressively decreased after 6 mins of cardiopulmonary resuscitation and greatly fluctuated in the ventilation cardiopulmonary resuscitation group. Coronary perfusion pressure was higher in the continuous compression group than that in the ventilation cardiopulmonary resuscitation group after 9 mins of cardiopulmonary resuscitation (p < .05). Values for pH and Pao2 progressively decreased, but there were no significant differences between the two groups, except for pH at 12 mins of cardiopulmonary resuscitation and Paco2 after 3 mins of cardiopulmonary resuscitation.

CONCLUSIONS

In the first 12 mins of cardiopulmonary resuscitation, continuous compressions could maintain relatively better coronary perfusion pressure, Pao2, and global ventilation/perfusion values than 30:2 cardiopulmonary resuscitation. Therefore, rescue ventilation during 12 mins of simulated bystander cardiopulmonary resuscitation did not improve hemodynamics or outcomes compared with compression-only cardiopulmonary resuscitation.

Wave emission from heterogeneities opens a way to controlling chaos in the heart

The effectiveness of chaos control in large systems increases with the number of control sites. We find that electric field induced wave emission from heterogeneities (WEH) in the heart gives a unique opportunity to have as many control sites as needed. The number of pacing sites grows with the amplitude of the electric field. We demonstrate that WEH has important advantages over methods used in clinics, and opens a new way to manipulate vortices in experiments, and potentially to radically improve the clinical methods of chaos control in the heart.

The pinwheel experiment revisited: effects of cellular electrophysiological properties on vulnerability to cardiac reentry

In normal heart, ventricular fibrillation can be induced by a single properly timed strong electrical or mechanical stimulus. A mechanism first proposed by Winfree and coined the “pinwheel experiment” emphasizes the timing and strength of the stimulus in inducing figure-of-eight reentry. However, the effects of cellular electrophysiological properties on vulnerability to reentry in the pinwheel scenario have not been investigated. In this study, we extend Winfree's pinwheel experiment to show how the vulnerability to reentry is affected by the graded action potential responses induced by a strong premature stimulus, action potential duration (APD), and APD restitution in simulated monodomain homogeneous two-dimensional tissue. We find that a larger graded response, longer APD, or steeper APD restitution slope reduces the vulnerable window of reentry. Strong graded responses and long APD promote tip-tip interactions at long coupling intervals, causing the two initiated spiral wave tips to annihilate. Steep APD restitution promotes wave front-wave back interaction, causing conduction block in the central common pathway of figure-of-eight reentry. We derive an analytical treatment that shows good agreement with numerical simulation results.

Pacing after shocks stronger than the upper limit of vulnerability: impact on fibrillation induction

BACKGROUND

After upper-limit-of-vulnerability (ULV) shocks of the same strength and coupling interval (CI) during the T wave, (1) the epicardial activation pattern (EAP) for the first postshock cycle is indistinguishable between shocks that do (VF) and do not (NoVF) induce ventricular fibrillation (VF) and (2) >/=3 cycles in rapid succession always occur during VF but not during NoVF episodes. To study the role of these rapid cycles, rapid pacing was performed after a shock stronger than the ULV that by itself did not induce rapid cycles and VF.

METHODS AND RESULTS

A 504-electrode sock was sutured to the heart in 6 pigs to map EAPs. The S2 shock strength and S1-S2 CI at the ULV were determined by T-wave scanning with an up/down protocol. Ten shocks 50 to 100 V above the ULV (aULV) were delivered at the same S1-S2 CI to confirm that VF was not induced. Then, the postshock interval after aULV shocks was scanned with an S3 pacing stimulus from the LV apex until the shortest S2-S3 CI that captured was reached. This was repeated for S4, S5, etc, until VF was induced. To induce VF, 3 pacing stimuli (S3-S5) with progressively shorter CIs were required; S3 or S3, S4 never induced VF. After cycle S5, which induced VF, 2 EAP types occurred: focal (74%) and reentrant (26%).

CONCLUSIONS

At least 3 cycles with short CIs are necessary for VF induction after aULV shocks. Cycles S3-S4 may create the substrate for cycle S5 to initiate VF.

Modulated dispersion explains changes in arrhythmia vulnerability during premature stimulation of the heart

BACKGROUND

Previously, we have shown that a premature stimulus can significantly modulate spatial gradients of ventricular repolarization (ie, modulated dispersion), which result from heterogeneous electrophysiological properties between cells. The role modulated dispersion may play in determining electrical instability in the heart is unknown.

METHODS AND RESULTS

To determine if premature stimulus-induced changes in repolarization are a mechanism that governs susceptibility to cardiac arrhythmias, optical action potentials were recorded simultaneously from 128 ventricular sites (1 cm2) in 8 Langendorff-perfused guinea pig hearts. After baseline pacing (S1), a single premature stimulus (S2) was introduced over a range of S1S2 coupling intervals. Arrhythmia vulnerability after each premature stimulus was determined by measurement of a modified ventricular fibrillation threshold (VFT) during the T wave of each S2 beat (ie, S2-VFT). As the S1S2 interval was shortened to an intermediate value, spatial gradients of repolarization and vulnerability to fibrillation decreased by 51+/-9% (mean+/-SEM) and 73+/-45%, respectively, compared with baseline levels. As the S1S2 interval was further shortened, repolarization gradients increased above baseline levels by 54+/-30%, which was paralleled by a corresponding increase (37+/-8%) in vulnerability.

CONCLUSIONS

These data demonstrate that modulation of repolarization gradients by a single premature stimulus significantly influences vulnerability to ventricular fibrillation. This may represent a novel mechanism for the formation of arrhythmogenic substrates during premature stimulation of the heart.

Vulnerability to ventricular fibrillation

One of the factors that favors the development of ventricular fibrillation is an increase in the dispersion of refractoriness. Experiments will be described in which an increase in dispersion in the recovery of excitability was determined during brief episodes of enhanced sympathetic nerve activity, known to increase the risk of fibrillation. Whereas in the normal heart ventricular fibrillation can be induced by a strong electrical shock, a premature stimulus of moderate intensity only induces fibrillation in the presence of regional ischemia, which greatly increases the dispersion of refractoriness. One factor that is of importance for the transition of reentrant ventricular tachycardia to ventricular fibrillation during acute regional ischemia is the subendocardial Purkinje system. After selective destruction of the Purkinje network by lugol, reentrant tachycardias still develop in the ischemic region, but they do not degenerate into fibrillation. Finally, attempts were made to determine the minimal mass of thin ventricular myocardium required to sustain fibrillation induced by burst pacing. This was done by freezing of subendocardial and midmural layers. The rim of surviving epicardial muscle had to be larger than 20 g. Extracellular electrograms during fibrillation in both the intact and the "frozen" left ventricle were indistinguishable, but activation patterns were markedly different. In the intact ventricle epicardial activation was compatible with multiple wavelet reentry, in the "frozen" heart a single, or at most two wandering reentrant waves were seen. (c) 1998 American Institute of Physics.

Comparison of left ventricular cryolesions created by liquid nitrogen and nitrous oxide

OBJECTIVES

This study was designed to compare the cryosurgical lesions produced by liquid nitrogen (-196 degrees C) and nitrous oxide (-76 degrees C).

BACKGROUND

Cryosurgical ablation is a useful method of arrhythmia surgery, but information on the dimensions of cardiac lesions produced by modifying cryoprobe temperature is limited.

METHODS

We compared the dimensions, volumes and electrophysiologic effects of cryolesions created by a liquid nitrogen cryoprobe (Group I) and a nitrous oxide cryoprobe (Group II) on the left ventricular myocardium in the beating canine heart. Exposure time was compared at 1, 2, 3 and 4 min. In each of 18 dogs, two to four lesions were created on the left ventricle and analyzed: 35 lesions created with use of the nitrous oxide cryoprobe and 30 lesions created with the liquid nitrogen cryoprobe. Lesions were measured at the time of induced death 6 weeks postoperatively and assessed by tissue staining with the Masson trichrome technique.

RESULTS

The volumes (mm3) of the cryolesions created by the liquid nitrogen cryoprobe were significantly larger (p < 0.05) than those of lesions created by nitrous oxide: 826 +/- 163 versus 493 +/- 197 at 1 min; 1,101 +/- 327 versus 666 +/- 185 at 2 min; 1,356 +/- 318 versus 787 +/- 258 at 3 min and 1,735 +/- 534 versus 923 +/- 376 at 4 min.

CONCLUSIONS

Decreasing the temperature of the cryoprobe by using liquid nitrogen increases the volume of the lesions. Programmed electrical stimulation before and 6 weeks after cryoablation indicated no arrhythmogenicity.

Effect of high-current stimulation in patients with sustained ventricular tachycardia rendered noninducible by antiarrhythmic drugs

Successful antiarrhythmic drug therapy for sustained ventricular tachycardia (VT) is presumed to be related to effects on myocardium within the re-entrant circuit. To test the hypothesis that prevention of VT induction may be related to effects on myocardium other than that directly involved in the tachycardia circuit, high-current stimulation was used to achieve shorter coupling intervals in 22 patients with sustained uniform VT that was rendered noninducible by antiarrhythmic agents during stimulation at twice threshold. Sustained uniform VT was induced in 10 patients in response to high-current stimulation (group 1), including 4 tachycardias with the same morphology observed in the baseline study. There were no inducible arrhythmias in 12 patients (group 2). Patients were receiving several different antiarrhythmic regimens, but there was no particular drug associated with the induction of VT using high-current stimulation. There was no statistically significant difference between groups 1 and 2 in baseline VT cycle length (247 +/- 41 vs 253 +/- 44 ms), drug-induced increase in effective refractory period (20 +/- 15 vs 16 +/- 7%), QRS duration (25 +/- 10 vs 20 +/- 17%) or maximal current strength delivered (10.9 +/- 5.3 vs 9.3 +/- 4.0 mA). There was no significant difference in local activation with high-current stimulation between groups 1 and 2. In conclusion, sustained uniform VT was induced in 45% (10 of 22) of patients whose arrhythmias were rendered noninducible by antiarrhythmic agents during programmed stimulation at twice threshold.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of ventricular fibrillation versus monomorphic ventricular tachycardia during programmed stimulation. Role of premature beat conduction delay

BACKGROUND

Premature stimuli can cause ventricular fibrillation (VF) during electrophysiological testing. The electrophysiological correlations associated with the onset of VF were evaluated in 40 patients who had this rhythm induced during programmed ventricular stimulation. These parameters were compared with those observed in 51 patients who had inducible sustained monomorphic ventricular tachycardia (VT) and 45 patients who had no inducible sustained ventricular tachyarrhythmias.

METHODS AND RESULTS

Shortest premature coupling intervals for S2, S3, and S4 at induction of tachycardia or before achieving refractoriness, corresponding conduction latencies (defined as the time from the premature stimulus to the upstroke of the depolarization wave front recorded 35 mm away from the stimulation site), and ventricular activation times (defined as the time from the premature stimulus to the end of the depolarization wave) were compared. The mean coupling intervals were longest in the inducible VT patients: 300 +/- 30, 254 +/- 57, and 228 +/- 32 msec for S2, S3, and S4, respectively. In the inducible VF group, the coupling intervals were 260 +/- 37, 208 +/- 20, and 213 +/- 30 msec. In the group with no inducible VT or VF, these coupling intervals were 251 +/- 24 (p less than 0.01 versus inducible VT group), 209 +/- 27 (p less than 0.001 versus inducible VT group), and 194 +/- 21 msec (p less than 0.05 versus inducible VT and VF groups). The coupling interval of the last premature extrastimulus was above 200 msec in 70% of the patients in whom VF was induced. The largest increases in latency and activation times were recorded in patients in whom VF was induced. The cumulative increase in latency, defined as increased conduction time from baseline, summed for all the premature stimuli was also the greatest at initiation of VF. In contrast, the smallest increases in these parameters were noted in the patients with no inducible VT or VF. Measurements of total activation time yielded similar results as those recorded for latencies. The most important parameters distinguishing the VT patient population from the other two groups were the low ejection fractions and the longer coupling intervals at which VT was induced, whereas in the VF group, the most important discriminating factor was cumulative activation time. Sixty-three percent of the inducible VF patients presented with abnormal hearts (myocardial infarction or cardiomyopathy), whereas 88% of the inducible VT patients had abnormal hearts. In contrast, only 25% of the patients in whom no arrhythmia was induced presented with abnormal hearts. Mean ejection fraction was 32 +/- 15% for the inducible VT group, 45 +/- 13%* for the inducible VF group, and 51 +/- 17%* for patients with no inducible VT/VF (*p less than 0.001 versus VT).

CONCLUSIONS

The results suggest that 1) initiation of ventricular tachycardia during programmed ventricular stimulation occurs with minimal conduction latency; 2) because of the large overlap in coupling intervals where VF or VT were induced, a single coupling interval cannot be recommended to adequately separate these groups; and 3) induction of VF was preceded by increased latency and prolongation of the local activation time. These parameters should not be allowed to prolong if VF is to be avoided during programmed stimulation. In addition, 4) the initiation of VF during electrophysiological studies is often associated with the presence of structural heart disease; such structural disease may promote conduction latency and the development of VF.

Effect of pacing current strength on indexes of myocardial activation in humans: influence of chronic infarction and polarity of pacing

The effects of current strength (threshold to 20 mA) and pacing polarity (bipolar versus unipolar) on indexes of ventricular activation during endocardial pacing (cycle length 400 to 500 ms) from 10 normal and 17 abnormal left ventricular sites were assessed in 19 patients. Abnormal sites were infarcted and demonstrated an electrogram duration greater than 70 ms and amplitude less than 3 mV during sinus rhythm. Bipolar pacing was performed from poles 1 (cathode) and 3 (1 cm interelectrode distance) of a quadripolar catheter. Unipolar cathodal pacing was performed from the tip electrode (pole 1). Local activation was indexed by the interval from the pacing stimulus to 1) the onset of the QRS complex, 2) the largest rapid deflection of the local electrogram, and 3) the end (total duration) of the local electrogram recorded from poles 2 and 4 of the quadripolar catheter used for left ventricular pacing. Distant activation was indexed by the interval from pacing stimulus to electrograms recorded at the right ventricular apex and outflow tract. Bipolar and unipolar pacing of normal sites produced a modest homogeneous reduction of all activation times by 3 to 11 ms (median) with increments in current strength from threshold (0.8 mA) to 20 mA. Bipolar pacing of abnormal sites showed marked (up to 110 ms) and heterogeneous changes in local (median 22 to 30 ms) as well as distant (median 14 to 23 ms) activation times with increases in current strength from threshold (2.7 mA) to 20 mA.(ABSTRACT TRUNCATED AT 250 WORDS)

Limited value of programmed electrical stimulation from multiple right ventricular pacing sites in clinically sustained ventricular fibrillation or ventricular tachycardia associated with coronary artery disease

One-hundred and fifty patients with coronary artery disease and a documented history of sustained ventricular tachyarrhythmias were studied to determine if programmed electrical stimulation (PES) from a second right ventricular (RV) pacing site optimizes the induction of such sustained arrhythmias. The first PES test was performed from 2 RV pacing sites (apex and outflow tract or septum) using the apex first in each patient. All patients underwent a second PES within 6 to 24 hours of the first; both studies used up to 4 ventricular extrastimuli, in the absence of antiarrhythmic treatment. The second PES was performed from a single RV apical site using a pacing catheter retained from the first study. During the first day's study, 74 patients (49%) had sustained ventricular tachycardia induced from the RV apex. Only 11 of the remaining 76 patients (7% of the total group) were inducible exclusively from a second RV pacing location during the first day's testing. Seven of these 11 patients, as well as 15 additional patients who did not have ventricular tachycardia induced from either site on the first day's study, were inducible from the RV apex during the second drug-free study. Among patients with sustained ventricular tachyarrhythmias, limiting PES to a single RV site, with the option of performing a second study in those who are initially noninducible is more effective in inducing sustained ventricular tachyarrhythmias than is PES performed from 2 RV pacing sites.

Strength-interval relation for ventricular functional refractoriness

Successful initiation and termination of presumed reentrant ventricular tachycardia frequently depends on the ability to deliver closely coupled impulses to the region of the tachycardia origin. To evaluate systematically the relative influence of local latency and large-scale conduction delay in limiting the delivery of closely coupled impulses, the strength-interval relation of the effective refractory period (RP), and the local and remote functional RP in 35 patients at paced cycle length of 500 ms were measured. The pacing threshold was less than or equal to 0.25 mA in all patients. The drive-train (S1) and the extrastimulus (S2) were applied from the same site, the right ventricular (RV) apex, in 25 patients, and from separate sites (RV apex and RV outflow tract) in 10 patients. The effect of procainamide (plasma concentration 10.1 +/- 2.3 micrograms/ml) on the strength-interval relations in 10 patients was also assessed. Although effective RP decreased significantly with each successive increase in current strength (p less than 0.001), local functional RP decreased only up to current strength of 4 mA, and remote functional RP decreased only up to 2 mA. Procainamide shifted the effective RP and local and remote functional RP strength-interval curves uniformly to the right without altering their relation. These data indicate that large-scale conduction delay provides the principal limitation for using increasing current strengths of a single extrastimulus to initiate or terminate ventricular tachycardia.

Induction of ventricular arrhythmias by programmed ventricular stimulation: a prospective study on the effects of stimulation current on arrhythmia induction

A protocol for programmed ventricular stimulation is described in which the effect of increasing stimulation current on ventricular refractoriness and arrhythmia induction was specifically examined. The protocol was evaluated prospectively in 70 patients undergoing electrophysiological study for documented or suspected ventricular arrhythmias. Programmed electrical stimulation was performed at the right ventricular apex and outflow tract using single and double extrastimuli and burst pacing. Stimulation currents of 2, 5, 10, and 20 mA were used in ascending order. The initial (lowest) current was never less than twice diastolic threshold and was maintained during each stimulation run until refractoriness was reached. The current was then increased to the next level to facilitate premature capture until refractoriness was encountered at 20 mA or a sustained arrhythmia occurred. Ventricular arrhythmias were induced in 34 patients, 31 of whom had presented with a sustained ventricular arrhythmia. The incidence of induced arrhythmias was low in those patients who had presented with symptoms alone, a non-sustained arrhythmia, or a sustained arrhythmia in association with a predisposing clinical event. Only one patient with a negative result had further ventricular arrhythmias during the mean follow up period of 15 months. Although each increase in stimulation current caused a decrease in measured ventricular refractoriness, this resulted in only four arrhythmias. Only one arrhythmia was induced above 5 mA. These results suggest that this simple protocol using two extrastimuli and a single stimulation current of 5 mA will reliably identify most patients who have symptomatic ventricular arrhythmias.

Mechanism of prevention of sudden death by nadolol: differential actions on arrhythmia triggers and substrate after myocardial infarction in the dog

Electrocardiographic monitoring and provocative ventricular pacing were used to evaluate control and nadolol treatment groups 6 to 24 hours after left anterior descending coronary artery ligation in the dog. During the 6 to 24 hour period, the control group (n = 20) developed ventricular triplets at rates exceeding 270/min. Seven dogs spontaneously developed sustained monomorphic ventricular tachycardia (421 +/- 12 beats/min) at 13 +/- 2 hours. Sustained monomorphic ventricular tachycardia was present for 38 +/- 8 seconds before ventricular fibrillation developed. One dog developed recurrent monomorphic ventricular tachycardia, with six episodes lasting from 8 to 72 seconds (375 to 425 beats/min). At 24 hours, ventricular pacing produced sustained monomorphic ventricular tachycardia (378 +/- 12 beats/min) in 9 of 13 surviving animals. Nadolol administration 6 hours after coronary artery ligation (n = 19) lowered both the rate (241 +/- 8 versus 328 +/- 8 beats/min; p = 0.001) and the incidence (8 +/- 6 versus 198 +/- 61 per hour; p = 0.004) of rapid ventricular triplets and prevented sudden arrhythmic death (0%; p = 0.005). Nadolol failed to prevent sustained monomorphic ventricular tachycardia (88%; 365 +/- 12 beats/min) produced by ventricular pacing. The data suggest that nadolol prevents spontaneous sustained monomorphic ventricular tachycardia by selectively suppressing the arrhythmia trigger (rapid ventricular triplets) without altering the underlying arrhythmia substrate.

Mechanisms and relevance of arrhythmias induced by high-current programmed ventricular stimulation

Programmed ventricular stimulation was performed at 10 mA with up to 3 extrastimuli in 15 patients studied for indications other than sustained ventricular tachycardia and with no sustained arrhythmias induced at twice diastolic threshold. Stimulation with 10 mA produced 6 new instances of ventricular fibrillation (VF), 1 of which may have been clinically relevant. No sustained ventricular tachycardia was induced. VF was induced with triple extrastimuli in 5 of 6 cases. The increased arrhythmogenicity of 10-mA stimulation was related to shortened ventricular refractory periods (S2 267 +/- 21 vs 231 +/- 22 ms, p less than 0.0001; S3 217 +/- 15 vs 178 +/- 15 ms, p less than 0.0005) and did not occur without at least 2 extrastimulus coupling intervals being less than was possible at twice diastolic threshold. Stimulation with 10 mA also resulted in greater increments in extrastimulus local conduction time (27 +/- 19 vs 54 +/- 15 ms, p less than 0.001) and intraventricular conduction time (27 +/- 17 vs 45 +/- 18 ms, p less than 0.005) as coupling intervals were shortened from 360 ms to just beyond ventricular refractoriness. VF was induced more frequently in patients with cardiomyopathy (p less than 0.05). Thus, the increase in arrhythmogenicity with 10-mA stimulation with triple extrastimuli is predominantly manifest as VF, which occurs with considerable frequency and is of uncertain clinical significance. This technique should be used with great caution, and only after other stimulation modalities have been attempted.

Arrhythmia inducibility and ventricular vulnerability in a chronic feline infarction model

Ventricular tachyarrhythmias are the cause of sudden cardiac death in ischemic heart disease. Reliable animal models are necessary to study techniques for identifying individuals at risk and to develop effective modes of therapy. The purpose of the present study was to evaluate the inducibility of ventricular tachyarrhythmias and vulnerability to ventricular fibrillation and to correlate these findings with changes in ventricular refractoriness in a chronic feline model. Twelve conditioned cats were randomly divided into two groups: group A, sham-operated controls (n = 5); or group B, permanent occlusion of the left anterior descending coronary artery (n = 7). Two weeks later, the following measurements were made: (1) assessment of refractory periods at several ventricular sites; (2) inducibility to ventricular tachyarrhythmias; and (3) determination of ventricular fibrillation threshold. After electrophysiologic testing, the animals were killed and the hearts were studied histologically. Ventricular fibrillation thresholds were significantly lower in group B compared with group A (13 +/- 3 vs 46 +/- 9 mA; p less than 0.01). One of the sham-operated controls had induction of nonsustained ventricular tachycardia, while six of the group B animals had reproducible, inducible ventricular tachyarrhythmias (p less than 0.01). There was a significant dispersion in effective refractory periods between normal and infarcted sites in group B (46 +/- 6 msec) not seen in group A (12 +/- 2 msec, p less than 0.01). The group A cats demonstrated minimal damage to the myocardium or cardiac architecture. Group B cats demonstrated extensive, transmural, homogeneous infarcts of approximately 30% of the anterior wall of the left ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)

Mode of stimulation versus response: validation of a protocol for induction of ventricular tachycardia

Electrophysiologic studies were prospectively performed in 91 consecutive patients referred for evaluation of sustained ventricular tachycardia or sudden cardiac death. Fifty-two patients had a history of sustained ventricular tachycardia and 39 patients had a history of sudden cardiac death. The identical stimulation protocol was used in all patients. The stepwise protocol involved atrial pacing, burst ventricular pacing, single, double, and triple extrastimuli during ventricular pacing. Stimulation was performed at the right ventricular apex at two and five times diastolic threshold. Using this protocol, ventricular tachycardia was inducible in 48 (92%) of the 52 patients with a history of sustained ventricular tachycardia and in 28 (72%) of 39 patients with a history of sudden cardiac death (p less than 0.02). The use of a third extrastimulus increased the yield of inducible ventricular tachycardia by 37% in patients with a history of sustained ventricular tachycardia and by 25% in patients with a history of sudden cardiac death. Stimulation at five times diastolic threshold and stimulation from the right ventricular outflow tract added a 15% increment in overall yield of inducible ventricular tachycardia in patients with a history of sustained ventricular tachycardia, and a 26% increment in yield in patients with a history of sudden cardiac death. Forty-four (92%) of the 48 inducible patients in the sustained ventricular tachycardia group had inducible monomorphic ventricular tachycardia as compared to 19 (68%) of 28 patients in the sudden cardiac death group (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Dispersion of ventricular repolarization and arrhythmia: study of two consecutive ventricular premature complexes

The effect of two consecutive ventricular premature stimuli (S1S2) during atrial pacing on dispersion of repolarization and inducibility of ventricular arrhythmias was studied in 16 dogs under control conditions and in four dogs in the presence of an increased dispersion of repolarization during atrial pacing induced by general hypothermia and regional warm blood perfusion via selective cannulation of the distal branch of left anterior decending coronary artery. Dispersion of repolarization was measured as the maximal difference between the ends of six simultaneously recorded monophasic action potentials (MAPs) from anterior ventricular surface, and consisted of MAP duration difference and activation time difference. Dispersion of repolarization during atrial pacing at control was 29 +/- 7 msec (activation time difference 4 +/- 6 msec, MAP duration difference 25 +/- 8 msec), that after S1 at paraseptal the site was 81 +/- 8 msec (activation time difference 73 +/- 12 msec, MAP duration difference 8 +/- 5 msec), and that after S1S2 was 148 +/- 27 msec (activation time difference 103 +/- 21, MAP duration difference 44 +/- 26 msec). Neither S1 nor S1S2 induced ventricular arrhythmia. Hypothermia and regional warm blood reperfusion increased dispersion of repolarization during atrial pacing to 70 +/- 22 msec (activation time difference 9 +/- 3 msec, MAP duration difference 61 +/- 19 msec). During hypothermia and regional warm blood reperfusion, S1 produced a dispersion of repolarization of 149 +/- 29 msec (activation time difference 85 +/- 8 msec, MAP duration difference 64 +/- 23 msec) and did not induce ventricular arrhythmia.(ABSTRACT TRUNCATED AT 250 WORDS)

Day-to-day variations in inducibility of ventricular tachyarrhythmias during the late postmyocardial infarction phase in conscious dogs

The inducibility of ventricular tachyarrhythmias was studied daily on days 3 through 8 after experimental myocardial infarction in 15 conscious dogs. Although a sustained ventricular tachyarrhythmia was produced on one or more occasions in 11 of 15 dogs (73%), there was marked daily variation in the results of programmed stimulation. Ventricular fibrillation or sustained ventricular tachycardia was elicited in six dogs on day 3 after infarction. In two of these dogs, no sustained tachycardia could be induced by day 7. In nine dogs sustained ventricular arrhythmias were not inducible on day 3. By day 6 to 7, sustained ventricular tachycardia was inducible in five of these dogs. In clinical practice similar variation in the inducibility of sustained ventricular arrhythmias may conceivably complicate the use of results of programmed ventricular stimulation as determinants of risk of sudden death after myocardial infarction.

Effects of high stimulation current on the induction of ventricular tachycardia

Programmed stimulation at 2 right ventricular sites with 1 to 3 extrastimuli was performed at current strengths of twice diastolic threshold (1.0 +/- 0.2 mA, mean +/- standard deviation) and 10 mA in 41 patients undergoing an electrophysiologic study because of sustained ventricular tachycardia (VT) (11 patients), nonsustained VT (19 patients) or unexplained syncope (11 patients). In 26 patients, VT was not induced by programmed stimulation at twice diastolic threshold. Programmed stimulation at 10 mA induced VT or ventricular fibrillation in 16 of these 26 patients (62%). In 4 of 16 patients, the coupling intervals of the extrastimuli that induced VT/ventricular fibrillation at 10 mA were all equal to or longer than the shortest coupling intervals resulting in ventricular capture at twice diastolic threshold. Fifteen patients had inducible VT at twice diastolic threshold. Programmed stimulation at 10 mA induced a similar VT in 12 of these patients, but resulted in no VT induction in 3 of 15 patients (20%), despite ventricular capture at the same coupling intervals that had induced VT at twice diastolic threshold. This study shows that programmed stimulation at a high current strength may either facilitate or prevent induction of VT. Facilitation of VT induction usually is attributable to a shortening of ventricular refractoriness and the ability of extrastimuli at 10 mA to capture the ventricle at shorter coupling intervals than possible at twice diastolic threshold. However, in 25% of cases, the facilitation of VT induction by 10-mA stimuli is not explained by a shortening of ventricular refractoriness.(ABSTRACT TRUNCATED AT 250 WORDS)

Sinus rhythm mapping in healed experimental myocardial infarction: contrasting activation patterns for inducing ventricular tachycardia versus fibrillation

The inducibility of ventricular tachycardia (VT) and fibrillation (VF) is variable in healed myocardial infarction (MI) in the dog. To better understand the electrophysiologic basis for these arrhythmias, MI was produced in dogs by ligating the left anterior descending artery. One week later, epicardial mapping was performed with the dog in sinus rhythm using a hand-held bipolar electrode. Transmural mapping was performed with the dog in sinus rhythm with 4 pairs of bipolar electrodes mounted on a #14 needle. Ventricular arrhythmias were induced by the S1S2S3 technique or 3- to 5-beat burst pacing at twice diastolic threshold. Only VF could be induced in 11 dogs, while sustained VT was induced in 6 dogs. Significantly more marked and more extensive delay in activation was seen both in the epicardium and transmurally in dogs with VT than in dogs with VF. In addition, dogs with VT had morphologic evidence of a large transmural MI, whereas dogs with VF had only a subendocardial MI. It is concluded that inducible sustained VT in the dog is usually associated with a large transmural MI and an activation sequence in sinus rhythm characterized by an extensive area of marked delay in activation. This activation pattern in sinus rhythm presumably is necessary to provide the underlying electrophysiologic milieu for sustained reentry.

Clinical significance of ventricular fibrillation-flutter induced by ventricular programmed stimulation

Two hundred twenty-four patients underwent ventricular programmed stimulation (VPS) without prior documentation of the clinical occurrence of sustained ventricular tachycardia (VT) or ventricular fibrillation-flutter (VF). Indications for VPS were: palpitations or nonsustained VT during ambulatory monitoring (85 patients), syncope or presyncope (137 patients), and a family history of sudden death (two patients). Sustained VF requiring transthoracic defibrillation was initiated by VPS in 18 patients (8.0%). Four patients were treated for inducible VF with antiarrhythmic agents directed by electropharmacologic testing; five patients were treated empirically; nine patients received no therapy. No patient has had a cardiac arrest or sudden death during a follow-up period 25.2 +/- 13.8 months (mean +/- standard deviation). VF was initiated by two ventricular extrastimuli in three patients and by three extrastimuli in 15 patients. The incidence of VF was similar in patients with and without previous symptoms (8.8% vs 6.9%) or heart disease (7.1% vs 9.6%). It was significantly higher when VPS at three ventricular sites with a current of 5 mA (pulse width 2 msec) was compared to programmed stimulation at two ventricular sites with a current twice diastolic threshold (pulse width 2 msec) (15.2% vs 3.0%, p less than 0.05). VF initiated by VPS in patients without prior VT or VF appears to be a nonspecific finding. Antiarrhythmic therapy for VF may not be necessary in these patients.

Serial analysis of electrically induced ventricular arrhythmias in a canine model of myocardial infarction

To determine the rate of induction, specificity and evolution of electrically induced postmyocardial infarction ventricular arrhythmias, 10 dogs that underwent a sham operation and 20 dogs with experimental transmural apical myocardial infarction underwent serial closed chest electrophysiologic studies with programmed ventricular stimulation under light anesthesia 1, 2, 4 and 6 weeks after the operation. The reproducibility of the electrically induced ventricular arrhythmias was at a maximum when three extrastimuli were used during ventricular pacing for induction. The reproducibility of the arrhythmias was also a function of the age of the infarct. Electrically induced sustained monomorphic ventricular tachycardia, observed in 45 to 50% of the animals, was a highly specific postinfarction finding (0% specificity in control animals, regardless of the mode or timing of programmed cardiac stimulation), whereas nonsustained polymorphic ventricular tachycardia was not. The specificity of induced ventricular fibrillation was a function of the mode and timing of programmed stimulation. The rate of induction of the electrically induced ventricular arrhythmias did not change significantly during the 6 week period after myocardial infarction. A large infarct size (determined by postmortem examination) and a low left ventricular ejection fraction (determined during premortem cardiac catheterization) were the only variables identified that predisposed the animals to electrically induced sustained monomorphic ventricular tachycardia. These factors, however, did not correlate with the presence of electrically induced ventricular fibrillation or nonsustained ventricular tachycardia.