The effects of coronary artery disease on the ventricular fibrillation threshold in man

Humidity and Ventricular Fibrillation: When Wet Welding can be Fatal

INTRODUCTION

Arc welding is generally considered very safe electrically. There have been electrocution cases with welders in high humidity environments. When dry, the flux coatings tend to have sufficient electrical resistance to limit the current below that required for the induction of VF (ventricular fibrillation).

METHODS

We tested 4 welding electrodes for resistance in both dry and wet conditions. To estimate the cardiac current density - in a worst-case scenario - we used a 20k element finite-element bioimpedance model with 1 cm of skin and fat along with 1 cm of muscle before the heart of 5 cm dimensions. Between the heart and a metal plate we assumed 5 cm of lung and 1 cm of skin and fat.

RESULTS

Welding electrode flux is highly resistive when dry. However, when saturated with moisture the resistance is almost negligible as far as dangerous currents in a human. The FEM model calculated a current density of > 7 mA/cm² on the ventricular epicardium with a source of 80 V at the welding rod.

CONCLUSION

In conditions of high humidity, a supine operator, in contact with a coated welding electrode to the precordial region of the body can be fibrillated with the AC open-circuit voltage. Most reported DC fatalities were probably due to pseudo-DC outputs that were merely rectified AC without smoothing.

Comparing the incidence of ventricular arrhythmias during epicardial ablation in swine versus canine models

BACKGROUND

Choosing the appropriate animal model for development of novel technologies requires an understanding of anatomy and physiology of these different models. There are little data about the characteristics of different animal models for the study of technologies used for epicardial ablation. We aimed to compare the incidence of ventricular arrhythmias during epicardial radiofrequency ablation between swine and canine models using novel epicardial ablation catheters.

METHODS

We conducted a retrospective study using data obtained from epicardial ablation experiments performed on swine (Sus Scrofa) and canine (Canis familiaris) models. We compared the incidence of ventricular arrhythmias during ablation between swine and canine using multivariate regression analysis. Six swine and six canine animals underwent successful epicardial radiofrequency ablation. A total of 103 ablation applications were recorded.

RESULTS

Ventricular arrhythmias requiring cardioversion occurred in 13.11% of radiofrequency ablation applications in swine and 9.75% in canine (relative risk: 117.6%, 95% confidence interval [CI]: 83.97-164.69, animal-based odds ratio [OR]: .55, 95% CI: .23-61.33; P = .184). When adjusting for application position, duration of ablation and power, the odds of developing potentially lethal ventricular arrhythmia in swine increased significantly compared to canine (OR: 3.60, 95% CI: 1.35-9.55; P = .010).

CONCLUSIONS

The swine myocardium is more susceptible to developing ventricular arrhythmias compared to canine model during epicardial ablation. This issue should be carefully considered in future studies.

Transthoracic cardiac stimulation thresholds for short pulses

INTRODUCTION

The most common cause of death due to electric shock is ventricular fibrillation (VF). This work reviews applicable results from the literature and provides an estimation model for the risk of VF with short-duration pulses.

METHODS AND RESULTS

For 1 ms pulses, the predicted current and charge thresholds required for successful transthoracic cardiac stimulation were 1.12 A and 1.12 mC, respectively. For pulses of 0.1 ms durations, the transthoracic current and charge thresholds predicted by the model are 10.9 A and 1.09 mC, respectively.

CONCLUSION

In humans, the charge required for single-response cardiac capture using transthoracic electrodes and 0.1 ms pulses is at least 0.5 mC. The transthoracic charge required to trigger repetitive ventricular responses in humans is at least several times higher than that for single responses. Hence, in adult humans, the transthoracic charge threshold required to induce repetitive ventricular responses, tachycardia, or fibrillation, with 0.1 ms pulses is expected to be significantly greater than 1 mC.

Electrical safety of conducted electrical weapons relative to requirements of relevant electrical standards

INTRODUCTION

TASER(®) conducted electrical weapons (CEW) deliver electrical pulses that can inhibit a person's neuromuscular control or temporarily incapacitate. TASER X26, X26P, and X2 are among CEW models most frequently deployed by law enforcement agencies. The X2 CEW uses two cartridge bays while the X26 and X26P CEWs have only one. The TASER X26P CEW electronic output circuit design is equivalent to that of any one of the two TASER X2 outputs. The goal of this paper was to analyze the nominal electrical outputs of TASER X26, X26P, and X2 CEWs in reference to provisions of several international standards that specify safety requirements for electrical medical devices and electrical fences. Although these standards do not specifically mention CEWs, they are the closest electrical safety standards and hence give very relevant guidance.

METHODS

The outputs of two TASER X26 and two TASER X2 CEWs were measured and confirmed against manufacturer and other published specifications. The TASER X26, X26P, and X2 CEWs electrical output parameters were reviewed against relevant safety requirements of UL 69, IEC 60335-2-76 Ed 2.1, IEC 60479-1, IEC 60479-2, AS/NZS 60479.1, AS/NZS 60479.2 and IEC 60601-1. Prior reports on similar topics were reviewed as well.

RESULTS AND CONCLUSION

Our measurements and analyses confirmed that the nominal electrical outputs of TASER X26, X26P and X2 CEWs lie within safety bounds specified by relevant requirements of the above standards.

Contemporary analysis of predictors and etiology of ventricular fibrillation during diagnostic coronary angiography

OBJECTIVES

To assess the incidence, investigate the predictors and analyze the causes of ventricular fibrillation (VF) during coronary angiography (CA) on the condition of current techniques.

METHODS

From April 2004 to January 2007, a total 22,254 patients (27,798 procedures) received CA procedures in our center; 27 patients developed VF during CA. This report was to retrospectively analyze the clinical basic characteristics, coronary angiographic characteristics and CA procedure records of these patients.

RESULTS

The incidence of VF during CA was 0.097%. The incidence of VF in radial approaches and femoral approaches was 0.076% and 0.147% (p = 0.085). The VF patients had higher coronary artery bypass grafting (CABG) rates (11.1% vs 2.3%, p = 0.024) and were more likely to have a three-vessel disease (59.3% vs 31.2%, p = 0.002) and a total occlusion lesion (25.9% vs 11.1%, p = 0.014) than non-VF patients. On logistic regression analysis, three-vessel disease (OR: 2.582, 95% CI: 1.165-5.720, p = 0.019) and the history of CABG (OR: 3.959, 95% CI: 1.160-13.513, p = 0.028) were the two independent predictors of VF occurrences. Among 27 episodes of VF, 13 were ischemia-related; 11 were manipulation-related; two were contrast-related; one was hypokalemia-related; and the causes remain unclear in five episodes.

CONCLUSIONS

The incidence of VF during CA is low on the condition of current techniques. The severity of coronary artery disease (CAD) is an independent predictor of VF occurrence during CA. Acute ischemia and inappropriate manipulation may be the two main causes in VF development.

Coronary artery disease, coronary revascularization, and outcomes in chronic advanced systolic heart failure

BACKGROUND

Associations between coronary artery disease (CAD) and outcomes in systolic heart failure (HF) and that between coronary artery bypass graft (CABG) surgery and outcomes in patients with HF and CAD have not been examined using propensity-matched designs.

METHODS

Of the 2707 patients with advanced chronic systolic HF in the Beta-Blocker Evaluation of Survival Trial (BEST), 1593 had a history of CAD, of whom 782 had prior CABG. Using propensity scores for CAD we assembled a cohort of 458 pairs of CAD and no-CAD patients. Propensity scores for prior CABG in those with CAD were used to assemble 500 pairs of patients with and without CABG. Matched patients were balanced on 68 baseline characteristics.

RESULTS

All-cause mortality occurred in 33% and 24% of matched patients with and without CAD respectively, during 26 months of median follow-up (hazard ratio {HR} when CAD was compared with no-CAD, 1.41; 95% confidence interval {CI}, 1.11-1.81; P=0.006). HR's (95% CIs) for CAD-associated cardiovascular mortality, HF mortality, and sudden cardiac death (SCD) were 1.53 (1.17-2.00; P=0.002), 1.44 (0.92-2.25; P=0.114) and 1.76 (1.21-2.57; P=0.003) respectively. CAD had no association with hospitalization. Among matched patients with HF and CAD, all-cause mortality occurred in 32% and 39% of those with and without prior CABG respectively (HR for CABG, 0.77; 95% CI, 0.62-0.95; P=0.015).

CONCLUSIONS

In patients with advanced chronic systolic HF, CAD is associated with increased mortality, and in those with CAD, prior CABG seems to be associated with reduced all-cause mortality but not SCD.

Mechanisms of ventricular fibrillation during coronary angioplasty: increased incidence for the small orifice caliber of the right coronary artery

BACKGROUND

Ventricular fibrillation (VF) is not an infrequent complication of percutaneous transluminal coronary angioplasty (PTCA). However, it is not clear why there is a marked discrepancy in the higher incidence of VF during right coronary artery (RCA) approach.

METHODS AND RESULTS

We analyzed in detail every case of VF occurring in 905 consecutive PTCA procedures to investigate possible mechanisms. Sixteen patients (M/F=15/1, mean age: 71 +/- 8 years) with VF during PTCA for the RCA as Group I. Those 51 patients (M/F=48/3, mean age: 70 +/- 9 years) without VF during PTCA for the RCA engagement were designated as Group II. Patients were equipped with cardiac event recorder (CardioCall, Reynolds Medical, UK) before the PTCA, and we set the time period 1 min before and after the event. The lead II was selected to check the QRS width, QTc interval, ST segment change and RR interval before and after event. A total of 905 PTCA procedures were included. There were 561 procedures for the left coronary artery and three events (0.5%) with spontaneous VF. However, there were 16 events (4.6%) of VF during 344 PTCA procedures for the right coronary artery. The incidence of VF for the right side PTCA was significantly higher than for the left side. The orifice of RCA in Group I was smaller than Group II (orifice of RCA in Group I vs. Group II - 2.7+/-0.8 vs. 4.1+/-1.2 mm, P<0.001). Most cases (68.7%) presented with ST segment depression before the onset of VF.

CONCLUSION

A small caliber of RCA and associated ST segment changes played important roles in the patients with VF during the PTCA.

Evaluation of cardiac vulnerability and antifibrillatory properties of anti-arrhythmic drugs

A method of evaluating the antifibrillatory properties of drugs by their effect on the acceleration of the cardiac rhythm by electric pulses was developed. It permitted measurement of fibrillation thresholds and the maximal driving frequency of stimulation. The heart was accelerated in closed chest dogs, and this increased the fibrillation thresholds after the application of lidocaine (1 mg/kg), quinidine (5 mg/kg), and novocainamide (15 mg/kg body wt.). The development of an original programmed stimulator increased the accuracy of the method by means of establishing the initial and terminal stimulation rates and observing the constant steps of change of pulse intervals. Four methods of causing fibrillation were compared: (1) a single pulse during the vulnerable phase of the cardiac cycle; (2) a train of pulses overlapping the vulnerable phase; (3) sequential R on T pacing; (4) simple acceleration of the cardiac rhythm. In addition to the other methods, the method of accelerating the heart rate differs in that only a small amplitude of stimulating pulses is needed. The present method may be used in the case of an unstable initial cardiac rhythm.

The effect of beta-adrenergic blockade on vulnerability to ventricular fibrillation and inducibility of ventricular arrhythmia in short- and long-term feline infarction models

Previous investigation, predominantly in the short-term canine model, has documented a potent antifibrillatory effect of beta-adrenergic blockade. To determine whether the protection afforded by beta blockade is species- and model-specific, we studied 23 chloralose-anesthetized cats. Eight animals were studied over a short term and underwent serial determinations of the ventricular fibrillation (VF) threshold prior to and 1 minute after occlusion of the left anterior descending coronary artery (LAD) and immediately following reperfusion of a 10-minute occlusion. Beta-blocking doses of intravenous propranolol (P) (0.5 mg/kg) attenuated the fall in VF threshold during acute ischemia. Increasing the dose of P to 1 mg/kg did not provide further protection, nor did P protect against reperfusion VF. The other 15 animals underwent a preliminary surgical procedure during which the LAD was completely and irreversibly occluded (nine animals) or in which a sham procedure was performed (six animals). Two weeks later, we measured ventricular refractoriness at several left ventricular sites, ventricular inducibility using programmed electrical stimulation, and VF thresholds both before and after administration of intravenous P (1 mg/kg). Ventricular refractory periods in the infarcted zones were significantly increased compared with normal sites and with values obtained in sham-operated animals. In addition, VF thresholds in the infarcted animals were lower than those obtained in the sham-operated group. Before treatment, a reproducible sustained ventricular tachyarrhythmia was induced by means of programmed stimulation in seven of the nine chronically infarcted animals but in none of the sham-operated animals (p less the 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular fibrillation during coronary angiography: the precatheterization QT interval

Ventricular fibrillation during coronary angiography is associated with contrast-induced changes in repolarization and thus pre-catheterization abnormalities could predispose to this event. We retrospectively examined angiograms, pre-catheterization electrocardiograms and records of 26 consecutive patients who had ventricular fibrillation during coronary angiography, and compared these patients to controls matched for age, sex, and left ventricular function. Diatrizoate meglumine was used as the angiographic contrast agent in all instances. Catheterization findings and the prevalence of prior myocardial infarction were similar in both groups. However, pre-catheterization QT intervals in the ventricular fibrillation group (0.43 +/- 0.05 sec) were significantly longer than in control patients (0.39 +/- 0.04 sec, P less than 0.005) as were their QT intervals corrected for heart rate (QTc) (0.47 +/- 0.04 vs 0.42 +/- 0.03 sec; P less than 0.001). Only seven of the 16 patients (44%) with ventricular fibrillation who had a precatheterization QTc greater than 0.44 sec had the arrhythmia during angiography of a critically stenosed (greater than 75%) coronary artery, whereas VF followed injection of critically stenosed vessels in eight of 10 (80%) of those with a normal QTc (p NS). After a follow-up period of 24 to 54 months (mean 39), two ventricular fibrillation patients have died (one suddenly), as compared to five in the control group (two suddenly) (p NS). Therefore, pre-catheterization QT prolongation was associated with ventricular fibrillation during coronary angiography, but ventricular fibrillation did not necessarily portend a worse long-term prognosis.

The role of lipid peroxidation in pathogenesis of arrhythmias and prevention of cardiac fibrillation with antioxidants

The present paper shows the arrhythmogenic effect of a direct induction of lipid peroxidation (LP) on isolated auricles; it is demonstrated that preendured stress potentiates this effect, while antioxidants prevent it. Subsequently, in studying the mechanism of the LP arrhythmogenic effect it was established that stress, like the LP induction, disorders the activity of Na, K-ATPase and accelerates thermodenaturation of this enzyme which plays a key role in maintaining the transmembrane potential and the electrical stability of the heart. Antioxidants prevent the enumerated shifts. Based on these data, the antioxidant BHT was successfully applied for prevention of the fall in cardiac fibrillation threshold in stress and experimental myocardial infarction, and also for prevention of cardiac fibrillation itself under acute ischemia and reoxygenation of the heart.

Incidence and clinical significance of ventricular fibrillation induced with single and double ventricular extrastimuli

Of 718 patients evaluated for suspected or documented ventricular tachyarrhythmias, ventricular fibrillation (VF) was induced in 28 (incidence 3.9%) by single and double extrastimuli. Nine of the 28 patients had suspected but no clinically documented ventricular tachycardia (VT) or VF (group 1), 11 had documented VT (group 2) and 8 had out of hospital VF (group 3). In group 1, electropharmacologic control was achieved in 8 patients with the initial agent tested; however, symptoms recurred in 6 patients. In 4 patients the drug was discontinued. After a follow-up of 26 +/- 11 months in group 1, no patient had died. In only 2 of 19 patients in groups 2 and 3 were arrhythmias controlled with the initial agent; 15 patients had VT and 2 VF. Control with class I agents was achieved in 9 of 19 patients and none died until the drug regimen was changed empirically in 3 of these 9. Ten patients, all from groups 2 and 3, were treated empirically with amiodarone; 3 died. All patients died either suddenly or in VT. The mortality rate in groups 2 and 3 after a mean follow-up of 24 +/- 9 months was 32% (p less than 0.05). Continued symptoms and no deaths in group 1 suggests a nonclinical nature of induced VF. Control of induced VF on serial drug testing in group 2 and 3 also indicates a false-negative drug efficacy response, as pharmacologic control of emergent VT on subsequent studies appeared essential to their survival despite control of induced VF. Thus, even with single or double premature stimuli, induction of VF can be a nonclinical response, especially in patients without clinical VF.

Can potentially significant polymorphic ventricular arrhythmias initiated by programmed stimulation be distinguished from those that are nonspecific?

Polymorphic ventricular arrhythmias (PVAs) initiated by programmed electrical stimulation may be a nonspecific response or evidence of ventricular electrical instability. To determine if PVAs initiated in patients with spontaneous sustained ventricular tachycardia or fibrillation differ from those which are clearly a nonspecific response in structurally normal hearts, the initiation, characteristics, and relationship to ventricular repolarization of PVAs greater than five beats in duration were evaluated in 32 patients without structural heart disease and in 36 patients with spontaneous sustained ventricular arrhythmias more than 9 days after myocardial infarction. Patients received one to four extrastimuli during sinus rhythm and right ventricular pacing. In a comparison with patients who completed the same steps (defined by the basic drive cycle length and number of extrastimuli) in the stimulation protocol, there was no difference in the cumulative risk of initiation of a PVA between the patients with and those without heart disease at any step. This risk was 51% vs 38% for patients who received two or fewer extrastimuli at four basic cycle lengths (p = NS). PVAs were initiated by the same mean number of extrastimuli (2.3 +/- 0.5 vs 2.6 +/- 0.9 p NS) with the same degree of prematurity in both groups. Forty-four percent of the PVAs in the myocardial infarction group had a cycle length greater than 250 msec or a coupling interval of the first tachycardia beat to its initiating stimulus greater than 320 msec as opposed to only one (6%) in the group without heart disease (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Ventricular fibrillation

Ventricular fibrillation is the most common mechanism of sudden unexpected cardiac death in persons with asymptomatic or symptomatic coronary artery disease. The electrophysiologic mechanisms reviewed in this article include: automaticity of pacemaker fibers, transformation of nonpacemaker into pacemaker fibers, "injury" currents and reentry. Some of the conditions facilitating ventricular fibrillation include bradycardia, long QT syndrome, electrocution, electrolyte imbalance, drugs, sympathetic stimulation and myocardial ischemia. Electrophysiologic studies during acute myocardial ischemia suggest that the earliest activity at the onset of arrhythmia may originate at the sites of the surviving Purkinje fibers or at the epicardial rim. Reentrant arrhythmias arising in ischemic myocardium are attributed to nonhomogeneous distribution of local hyperkalemia and acidosis.

Vulnerability to ventricular fibrillation in patients with clinically manifest ventricular tachycardia

Ventricular vulnerability may be assessed by measuring the threshold current for the induction of ventricular fibrillation (VF). This technique has been widely utilized in animal experimentation and has been safely applied in a small number of clinical studies. We measured the VF threshold (VFT), using the single stimulus technique in 10 patients with coronary artery disease just prior to the institution of cardiopulmonary bypass. There were no adverse effects of VFT measurement. Three patients had nonsustained ventricular tachycardia (VT) on 24-hour ambulatory monitoring and had VFTs of 10, 14, and 16 mA. In this group VF was induced without any preceding repetitive ventricular responses. Seven patients had no repetitive forms on ambulatory monitoring. Their VFTs ranged from 30 to greater than 40 mA (mean greater than 37). Repetitive extrasystoles were regularly observed in this group at current intensities which ranged from 53% to 80% of the VFT. Thus patients with manifest VT appear to have an enhanced vulnerability to VF. Single or multiple responses were not observed in these patients but appeared to be present in patients with coronary disease and no demonstrable rhythm disorder.

The repetitive extrasystole as an index of vulnerability to ventricular fibrillation during myocardial ischemia in the canine heart

The repetitive extrasystole threshold (RET) is a reliable measure of vulnerability to ventricular fibrillation (VF) during diverse interventions in the normal heart. Whether this relationship also holds during varying degrees of myocardial ischemia has not been adequately explored. In 15 chloralose-anesthetized dogs, circumflex coronary blood flow (CBF) was decreased progressively with the use of an externally applied balloon occluder. There was a statistically significant correlation between the RET and ventricular fibrillation threshold (VFT) until left circumflex coronary artery flow was reduced by 90% of control values (r = 0.92). During reductions of CBF of 90% or greater, the VFT fell more than the RET and the RET/VFT ratio was disrupted. Total coronary occlusion, whether performed abruptly or gradually (5 minutes), likewise resulted in a disproportionate decline in VFT. During sustained total coronary occlusion, the VFT recovered to control values within 15 minutes, and the relationship between the RE and VF thresholds was restored. We conclude that the vulnerable period threshold for provoking repetitive extrasystole is a reliable index of vulnerability to VF during myocardial ischemia and remains so until nearly total occlusion of a major coronary vessel.

Increased risk of ventricular fibrillation associated with temporary pacemaker use during coronary arteriography

Each injection of angiographic contrast dye during coronary arteriography represents a "natural experiment" in which the human ventricular fibrillation threshold is transiently reduced. Few factors, however, have been identified which favor the actual occurrence of ventricular fibrillation in this setting. Of 3906 consecutive patients undergoing selective coronary arteriography with sodium meglumine diatrizoate, 66 (1.7 percent) experienced dye-induced ventricular fibrillation, from which all were successfully defibrillated. Analysis of these cases revealed, unexpectedly, that patients in whom temporary right ventricular pacemakers were employed had an incidence of ventricular fibrillation nearly six times that found in the entire group undergoing arteriography (10 percent vs. 1.7 percent, respectively; P less than .001). Those individuals receiving pacemakers were distinguished from other studied patients only by a higher prevalence of conduction abnormalities. Although there is normally a low probability that mechanical stimulation by a pacing catheter can induce ventricular fibrillation, it is postulated that such an occurrence may be more likely after ventricular vulnerability has been increased by contrast dye.

Pharmacologic treatment of cardiac arrhythmias: 25 years of progress

This review of practical and theoretical advances in antiarrhythmic drug therapy consists of four parts. Part 1, on clinical applications, compares the approaches to treatment 25 years ago with those of today, examines the current status of antiarrhythmic drugs used 25 years ago, reports on drugs approved for clinical use during the past 25 years, reviews new experimental drugs and suggests an approach to classification of antiarrhythmic drugs. Part 2 summarizes the contributions of cellular electrophysiology to the understanding of drug action, with emphasis on the drug-induced block of the voltage- and time-dependent properties of the rapid sodium channel. The subsequent section contains a brief discussion of the impact made by the new knowledge and the new diagnostic technology on the contemporary practices. The main conclusions are 1) that the more rational approach to treatment has benefited proportionately more patients with supraventricular than with ventricular arrhythmias, and 2) that new advances have made it possible to design successful treatments for certain patients with problems that could not be resolved in the past.

Reduced human ventricular fibrillation threshold associated with contrast-induced Q-T prolongation

Injection of contrast dye during selective coronary arteriography prolongs the Q-T interval well beyond that expected for the accompanying bradycardia. A case is presented in which a 5 mA stimulus delivered prematurely by an improperly sensing ventricular demand pacemaker initiated ventricular fibrillation during coronary arteriography, although no evidence of coronary or other cardiac disease was found. An analysis is made of the progressive Q-T and T changes following contrast dye injection, and their relation to the onset of ventricular fibrillation. The implications of these observations are discussed in light of existing data on human ventricular vulnerability.

A model of chronic ischemic arrhythmias: the relation between electrically inducible ventricular tachycardia, ventricular fibrillation threshold and myocardial infarct size

To study the relation between inducible ventricular tachycardia and ventricular vulnerability, myocardial infarction was created in 22 closed chest mongrel dogs by inflating a balloon catheter in the left anterior descending coronary artery for 2 hours. The presence of inducible ventricular tachycardia was determined by programmed electrical stimulation of the right ventricle in each dog before and 4 days after infarction, using a transvenous electrode catheter and a "clinical" stimulation protocol. In each dog the repetitive ventricular response threshold and the ventricular fibrillation threshold were measured before and 4 days after infarction. Ventricular tachycardia was not inducible in any dog before infarction. After infarction, sustained ventricular tachycardia was inducible in 10 (45 percent) of 22 dogs and nonsustained tachycardia in an additional 4 dogs (18 percent). Ventricular fibrillation threshold was greatly reduced 4 days after infarction in dogs with inducible sustained tachycardia (mean +/- standard deviation 29 +/- 11 to 10 +/- 5 mA, p less than 0.001); the mean threshold did not change significantly in dogs without inducible sustained tachycardia. Both the ventricular fibrillation threshold and mean ventricular repetitive response threshold were reduced in the dogs with sustained ventricular tachycardia; neither was significantly altered in the dogs without sustained tachycardia. The magnitude of change in the two thresholds frequently differed; hence, a correlation was weak between the control and postinfarction repetitive response/fibrillation threshold ratio (r = 0.41). Postmortem measurement of infarct size demonstrated an association between this measurement and the presence of inducible ventricular tachycardia. Sustained ventricular tachycardia was not inducible in the presence of a small infarct. It is concluded that: (1) inducible ventricular tachycardia on the 4th day after myocardial infarction is associated with a considerable decrease in the ventricular fibrillation threshold; (2) changes in the repetitive response and fibrillation thresholds after myocardial infarction may not be parallel, complicating the use of the repetitive ventricular response threshold as a substitute for the ventricular fibrillation threshold in the postinfarction state; (3) a large infarct predisposes the heart to electrically inducible sustained ventricular tachycardia.

Relation of the endocardial and epicardial ventricular fibrillation thresholds of the right and left ventricle

Ventricular fibrillation thresholds were measured on the endocardium and epicardium of the right or left ventricle, or both, in 13 dogs. The electrodes, introduced through a right or left atriotomy to avoid injury to the ventricles, were aligned opposite and parallel to each other on the endocardium and epicardium. The ventricular fibrillation threshold was measured during atrial pacing by delivering a train of impulses to the ventricle during the vulnerable period after every 12th paced complex in 1 milliampere (mA) increments of current until fibrillation ensued. The mean (+/- standard deviation) right ventricular epicardial and endocardial fibrillation thresholds were 18.3 +/- 5.3 and 17.6 +/- 5.3 mA, respectively, (values not significantly different). However, the fibrillation threshold of 36.1 +/- 9.5 mA in the left ventricular epicardium was significantly higher than the value of 20.7 +/- 9.4 mA on the left ventricular endocardium. These data suggest that the proximity of the fibrillating electrodes and Purkinje network may be a factor in the measurement of ventricular vulnerability.