Electroide cather recording during malignant ventricular arrythmia following experimental acute myocardial ischemia. Evidence for re-entry due to conduction delay and block in ischemic myocardium

Attenuating loss of cardiac conduction during no-flow ischemia through changes in perfusate sodium and calcium

Myocardial ischemia leads to conduction slowing, cell-to-cell uncoupling, and arrhythmias. We previously demonstrated that varying perfusate sodium (Na⁺) and calcium (Ca²⁺) attenuates conduction slowing and arrhythmias during simulated ischemia with continuous perfusion. Cardioprotection was selectively associated with widening of the perinexus, a gap junction adjacent nanodomain important to ephaptic coupling. It is unknown whether perfusate composition affects the perinexus or ischemic conduction during nonsimulated ischemia, when coronary flow is reduced or halted. We hypothesized that altering preischemic perfusate composition could facilitate perinexal expansion and attenuate conduction slowing during global ischemia. To test this hypothesis, ex vivo guinea pig hearts (n = 49) were Langendorff perfused with 145 or 153 mM Na⁺ and 1.25 or 2.0 mM Ca²⁺ and optically mapped during 30 min of no-flow ischemia. Altering Na⁺ and Ca²⁺ did not substantially affect baseline conduction. Increasing Na⁺ and decreasing Ca²⁺ both lowered pacing thresholds, whereas increasing Ca²⁺ narrowed perinexal width (W_p). A least squares mean estimate revealed that reduced perfusate Na⁺ and Ca²⁺ resulted in the most severe conduction slowing during ischemia. Increasing Na⁺ alone modestly attenuated conduction slowing, yet significantly delayed the median time to conduction block (10 to 16 min). Increasing both Na⁺ and Ca²⁺ selectively widened W_p during ischemia (22.7 vs. 15.7 nm) and attenuated conduction slowing to the greatest extent. Neither repolarization nor levels of total or phosphorylated connexin43 correlated with conduction slowing or block. Thus, perfusate-dependent widening of the perinexus preserved ischemic conduction and may be an adaptive response to ischemic stress.NEW & NOTEWORTHY Conduction slowing during acute ischemia creates an arrhythmogenic substrate. We have shown that extracellular ionic concentrations can alter conduction by modulating ephaptic coupling. Here, we demonstrate increased extracellular sodium and calcium significantly attenuate conduction slowing during no-flow ischemia. This effect was associated with selective widening of the perinexus, an intercalated disc nanodomain and putative cardiac ephapse. These findings suggest that acute changes in ephaptic coupling may serve as an adaptive response to ischemic stress.

Is external defibrillation an electric threat for bystanders?

BACKGROUND

Safety precautions during defibrillation and cardioversion are generally taken very seriously. The actual hazard for bystanders and rescuers, however, has rarely been investigated. Recently, continuing chest compressions during defibrillation has been suggested to improve outcome from cardiac arrest. This article is to review reports on electric shocks to persons other than patients and to discuss the pertinent biomedical principles.

METHODS

Systematic search in medical literature databases and consecutive hand-search of reference lists.

RESULTS

A total of 29 adverse events are reported in the medical literature; seven due to accidental or intentional defibrillator misuse, three due to device malfunction, four during training/maintenance procedures, and 15 during regular resuscitation efforts. Tingling sensations and minor burns are frequently reported consequences of inadvertent shocks. There are no accounts on immediate life-threatening conditions or long-term disability in rescuers/bystanders inflicted by defibrillation/cardioversion of a patient. Discharging a defibrillator directly to a healthy person's chest can be lethal.

CONCLUSIONS

External electric therapy is likely to be safer than traditionally assumed, especially with self-adhesive thoracic electrodes. Sound clinical experiments are urgently needed before safety measures are revised.

Acute subendocardial ischaemia leads to homogenous prolongation in ventricular repolarization

Acute transmural ischaemia often shortens ventricular repolarization and increases repolarization dispersion, leading to life threatening ventricular arrhythmias in animal models and human subjects. Experimental studies and clinical observations have shown that acute subendocardial ischaemia rarely causes serious ventricular arrhythmia. We hypothesized that the different arrhythmia outcomes between transmural and subendocardial ischaemia are largely due to the homogenous prolongation in ventricular repolarization after acute subendocardial ischaemia. Further experimental studies on a subendocardial model are required to assess the changes in ventricular repolarization and its spatial dispersion, and to investigate the role of these changes in the pathogenesis of ventricular arrhythmias. These studies will facilitate our understanding on the mechanisms of life-threatening ventricular arrhythmias during acute myocardial ischaemia.

Alternans and higher-order rhythms in an ionic model of a sheet of ischemic ventricular muscle

Life-threatening arrhythmias such as ventricular tachycardia and fibrillation often occur during acute myocardial ischemia. During the first few minutes following coronary occlusion, there is a gradual rise in the extracellular concentration of potassium ions ([K(+)](0)) within ischemic tissue. This elevation of [K(+)](0) is one of the main causes of the electrophysiological changes produced by ischemia, and has been implicated in inducing arrhythmias. We investigate an ionic model of a 3 cmx3 cm sheet of normal ventricular myocardium containing an ischemic zone, simulated by elevating [K(+)](0) within a centrally-placed 1 cmx1 cm area of the sheet. As [K(+)](0) is gradually raised within the ischemic zone from the normal value of 5.4 mM, conduction first slows within the ischemic zone and then, at higher [K(+)](0), an arc of block develops within that area. The area distal to the arc of block is activated in a delayed fashion by a retrogradely moving wavefront originating from the distal edge of the ischemic zone. With a further increase in [K(+)](0), the point eventually comes where a very small increase in [K(+)](0) (0.01 mM) results in the abrupt transition from a global period-1 rhythm to a global period-2 rhythm in the sheet. In the peripheral part of the ischemic zone and in the normal area surrounding it, there is an alternation of action potential duration, producing a 2:2 response. Within the core of the ischemic zone, there is an alternation between an action potential and a maintained small-amplitude response ( approximately 30 mV in height). With a further increase of [K(+)](0), the maintained small-amplitude response turns into a decrementing subthreshold response, so that there is 2:1 block in the central part of the ischemic zone. A still further increase of [K(+)](0) leads to a transition in the sheet from a global period-2 to a period-4 rhythm, and then to period-6 and period-8 rhythms, and finally to a complete block of propagation within the ischemic core. When the size of the sheet is increased to 4 cmx4 cm (with a 2 cmx2 cm ischemic area), one observes essentially the same sequence of rhythms, except that the period-6 rhythm is not seen. Very similar sequences of rhythms are seen as [K(+)](0) is increased in the central region (1 or 2 cm long) of a thin strand of tissue (3 or 4 cm long) in which propagation is essentially one-dimensional and in which retrograde propagation does not occur. While reentrant rhythms resembling tachycardia and fibrillation were not encountered in the above simulations, well-known precursors to such rhythms (e.g., delayed activation, arcs of block, two-component upstrokes, retrograde activation, nascent spiral tips, alternans) were seen. We outline how additional modifications to the ischemic model might result in the emergence of reentrant rhythms following alternans. (c) 2000 American Institute of Physics.

Significance of R-on-T phenomenon in early ventricular tachyarrhythmia susceptibility after acute myocardial infarction in the thrombolytic era

We investigated the clinical significance and mechanism of the R-on-T phenomenon in the current thrombolytic era as potential precipitant of R-on-T-induced early ventricular tachyarrhythmias in patients with a thrombolysed acute myocardial infarction. We also examined the role of QT dispersion on ventricular vulnerability and its association with R-on-T-initiated ventricular tachyarrhythmias. A total of 93 patients underwent 24-hour Holter monitoring starting at hospital admission before thrombolysis. Patients were classified into 2 groups: those with (n = 76) and those without (n = 17) reperfusion according to electrocardiographic criteria. All R-on-T ventricular premature complexes (VPCs) and R-on-T-initiated arrhythmic events (ventricular tachycardia [VT], ventricular fibrillation) were counted to estimate arrhythmia density and severity in 2 time periods during and after completion of thrombolysis. Measurements of QT and QTc intervals and dispersion parameters were obtained on the 12-lead electrocardiogram before thrombolysis and at 24 hours in patients with and without R-on-T VTs. Overall, R-on-T VPCs were rarely observed (1.8% of total VPCs over 24 hours), occurring more frequently during than after thrombolysis (at a rate of 8 vs 0.6 VPCs/hour, p = NS) and at a higher rate during thrombolysis in nonreperfused than in perfused patients (15 vs 8/hour, p = NS). Three VF episodes were observed in 1 reperfused patient, and all were R-on-T initiated. Episodes of nonsustained R-on-T VTs (3.3% of total VTs over 24 hours) appeared more frequent during than after thrombolysis (at a rate of 0.8 vs 0.05 VPCs/ hour, p = NS), and compared with non-R-on-T VTs they were significantly faster (374 +/- 56 ms vs 411 +/- 69 ms; p < 0.05), with a trend toward longer duration. Our findings indicate that R-on-T VPCs and R-on-T VTs are early rare features in acute myocardial infarction, and do not serve as triggers of severe ventricular tachyarrhythmia. The study of ventricular repolarization did not elicit an identifiable risk factor of R-on-T VT susceptibility.

Ischemia-induced changes of the signal-averaged electrocardiogram: experimental investigation during percutaneous transluminal coronary angioplasty balloon-occluded coronary artery

INTRODUCTION

The influence of myocardial ischemia on the detection of an arrhythmogenic substrate with the signal-averaged ECG is unclear.

METHODS AND RESULTS

In 80 patients with single vessel coronary artery disease and a critical stenosis of the left anterior descending vessel selected after coronary angiography in whom percutaneous transluminal coronary angioplasty (PTCA) was planned, we retrospectively investigated the signal-averaged ECGs in the time domain before, during, and after occlusion of the coronary artery by the PTCA balloon. Forty patients were resuscitated from ventricular fibrillation (VF group), and 40 patients had no ventricular arrhythmia (non-VF group). Late potentials were seen at rest in 26 of 40 patients in the VF group. During ischemia, the duration of the filtered QRS complex and the duration of low-amplitude signals < 40 microV increased significantly. In another 14 patients in the VF group, late potentials were observed only during ischemia. In 4 of 26 patients in the VF group without prior infarction but with severe ischemia present at rest, successful PTCA eliminated preexistent late potentials. In the non-VF group, one patient had late potentials present at rest. In two patients with prior infarction, late potentials were provokable only during transmural ischemia.

CONCLUSION

Myocardial ischemia is able to modify detection of an arrhythmogenic substrate with the signal-averaged ECG.

Relation between infarct artery patency at late angiography after acute myocardial infarction and signal-averaged electrocardiography

The angiograms of 89 patients were reviewed from the LATE Ancillary Study (randomized trial of recombinant tissue plasminogen activator vs placebo in patients with symptom onset after 6 hours of myocardial infarction) to determine patency of the infarct-related artery (IRA). In the occluded IRA group (n = 35), the incidence of signal-averaged electrocardiographic abnormality (fQRS > 120 ms) was significantly higher (p = 0.04), the filtered QRS duration was significantly longer (p = 0.007), and the V40 was significantly shorter (p = 0.02), compared with the patent IRA group (n = 54).

Ablation of ventricular tachycardia in the setting of coronary artery disease

Ablation of reentrant ventricular tachycardia (VT) is an accepted therapy for certain patients with VT caused by coronary artery disease (CAD). Its use is currently limited to patients with sustained, monomorphic, hemodynamically tolerated VT. The use of entrainment in mapping reentrant VT has made possible increasingly accurate localization of critical sites on the reentrant pathway that are amenable to ablation. Recent work has examined the accuracy with which various mapping criteria are able to predict successful ablation of reentrant VT in patients with CAD. Other recent studies have investigated attempted ablation of all inducible VTs in patients with multiple VT morphologies. In the future, substrate mapping may make possible ablation of VT in patients with nonsustained or fast, hemodynamically unstable VTs, thus allowing VT ablation to become a first-line therapy for many patients with VT in the setting of CAD.

Evolution of late potential parameters in thrombolyzed acute myocardial infarction might predict patency of the infarct-related artery

The objective was to predict the patency grade of an infarct-related artery by identifying the time course of the changes of the late potential parameters before, during, and shortly after thrombolysis. The study population consisted of 51 patients with acute myocardial infarction (AMI) who received thrombolytic therapy within 3.2 +/- 1.3 hours from the onset of symptoms. Multiple signal-averaged electrocardiograms (SAECGs) were recorded before, during, and shortly after thrombolysis. A total of 489 single-averaged electrocardiographic tracings were evaluated. Late potentials were defined as: QRS duration > 114 ms, low amplitude signals (LASs) > 38 ms, and root mean square (RMS) < 20 microV. Late potentials were found in 37% of patients (21 before and 16 during the first 2 hours of thrombolysis), disappeared in all of patients within 89 +/- 75 minutes (range 25 to 350) but reappeared and persisted in 12% of patients, all with an occluded artery (grade 0). The late potential parameters (QRS, LAS, RMS) showed a gradual improvement which occurred earlier (2 vs 4 hours) and was more marked (0.01 vs 0.05) in cases with a patent artery. This improvement expressed by the late potential parameter index (LnQRS + LnLAS - LnRMS) predicts the patent artery with a sensitivity of 0.94 and specificity of 0.79. The improvement of late potential parameters jointly with close to normal initial values or the late potential parameter index and its changes constituted a satisfactory prediction of the patency grade. Thus, the signal-averaged electrocardiographic technique is capable of predicting the early success or failure of thrombolytic therapy.

Anterior septal coronary artery infarction in the canine: a model of ventricular tachycardia with a subendocardial origin. Ablation and activation sequence mapping

BACKGROUND

In humans, chronic ventricular tachycardia (VT) is usually associated with myocardial infarcts that involve the interventricular septum. In an effort to more closely mimic the anatomic substrate that gives rise to chronic VT in humans, we developed a canine model of VT in which the anterior septal coronary artery was ligated. The site of earliest activation, the subsequent activation sequence, and the mechanism of VT associated with the resultant ventricular septal infarct was then evaluated to determine if this model accurately reflected the characteristics of human VT.

METHODS AND RESULTS

Seventeen dogs underwent occlusion-reperfusion ventricular septal infarcts. Four to 7 days later, electrophysiological studies were performed. VT was initiated by programmed electrical stimulation and terminated by pacing at a cycle length of 50% to 75% of the VT cycle length. Electrophysiological studies were performed using a 256-channel mapping system. A total of 15 VT morphologies were mapped in 9 animals. Fourteen of 15 morphologies had septal subendocardial sites of earliest activation and 1 had a septal midwall site of earliest activation. VT ablation was performed using a nitrous oxide cryoprobe and confirmed the site of earliest activation by subsequently rendering VT noninducible. Electrophysiological studies demonstrated four distinct VT activation sequences: (1) circular reentrant (n = 7), (2) concentric spread (n = 5), (3) figure-of-eight (n = 2), and (4) septal midwall (n = 1).

CONCLUSIONS

This canine model of ventricular septal infarction produces VTs with sites of earliest activation and activation sequences similar to those in humans. A reentrant mechanism as the basis of these arrhythmias is supported by the following observations: (1) all VT was initiated and terminated with programmed electrical stimulation; (2) VT activation sequences were consistent with reentry; and (3) precise interruption of the sequence terminated the VT and rendered it noninducible.

Detection and localization of prolonged epicardial electrograms with 64-lead body surface signal-averaged electrocardiography

BACKGROUND

Prolonged, fractionated ventricular electrograms often are detectable after myocardial infarction and are a marker for an arrhythmia-prone state. QRS late potentials detected on the body surface with signal-averaged electrocardiography (SAECG) are thought to arise from the diseased tissue that generates prolonged ventricular electrograms and as such are also a marker for arrhythmias. A limitation of the current SAECG technique is that recordings are obtained from only three bipolar lead pairs. Because late potentials probably arise from multiple small sources in the heart, more extensive sampling of the body surface may contribute additional information to the SAECG: The present study investigates the additional sensitivity of SAECG using 64 body surface leads in detecting prolonged epicardial electrograms and examines its use in determining the epicardial location of prolonged electrograms.

METHODS AND RESULTS

Dogs were studied before and 5-10 days after either lateral left ventricular (n = 13) or right ventricular (n = 8) myocardial infarction. Greater prolongation of signal-averaged QRS duration was detected with 64-lead SAECG (postinfarction QRS duration, 100.3 +/- 16.3 msec) than with three-lead SAECG (postinfarction QRS duration, 89.4 +/- 10.1, p = 0.0005). Nineteen of the 21 dogs (90%) had prolonged epicardial electrograms detected over the infarct. The correlation between epicardial electrogram duration and signal-averaged QRS duration calculated from individual leads was much better for 64-lead SAECG (r = 0.88, p less than 0.0001) than for three-lead SAECG (r = 0.53, p = 0.01), and the difference was most marked in cases with longer electrogram durations (more than 100 msec). Local late potential maxima on the thorax after lateral left ventricular infarction were located to the left and inferior compared with those after right ventricular infarction (p = 0.006).

CONCLUSIONS

SAECG with more extensive recording from the body surface using 64 leads detects greater QRS prolongation than three-lead SAECG, and the longer QRS durations detected correspond to the duration of prolonged epicardial electrograms. Body surface location of late potentials corresponds to the epicardial location of the prolonged electrograms. This application of body surface mapping techniques to SAECG may permit more sensitive detection of arrhythmia-prone states and may aid in identifying arrhythmia sources.

Study of the influence of left bundle branch block on the signal-averaged electrocardiogram: a qualitative and quantitative analysis

To study the influence of left bundle branch block (LBBB) on the signal-averaged electrocardiogram (SAECG), quantitative and qualitative analyses of SAECG parameters were undertaken in 48 patients with electrocardiographic evidence of intrinsic LBBB and in 39 patients with a "normal" surface QRS duration (less than 120 msec) who underwent right ventricular pacing-induced LBBB. We assumed pacing of the right ventricular apex to be a suitable model of this conduction defect. Sustained monomorphic ventricular tachycardia (SMVT) was inducible in 16 of 48 patients with intrinsic LBBB and in 23 of 39 patients with pacing-induced LBBB. Utilizing a filter setting of 25 to 250 Hz, late potentials were defined as a total filtered QRS duration greater than or equal to 120 msec, a root mean square voltage in the terminal 40 msec (RMS 40) of less than or equal to 25 microV, and the duration of signals less than 40 microV (LAS 40) of greater than or equal to 38 msec. Only RMS 40 and LAS 40 criteria were used in patients with LBBB. Prolongation of LAS 40 and fragmentation of signals in the terminal portion of the filtered QRS were characteristic of all patients with LBBB aberration. Of those patients with intrinsic LBBB, the mean total filtered QRS duration, RMS 40, and LAS 40 for inducible and noninducible patients were significantly different (170 +/- 28, 16 +/- 10, 55 +/- 24, and 153 +/- 18 msec, 25 +/- 10 microV, 33 +/- 16.9 msec; p = 0.04, 0.009, and 0.007, respectively). Noninducible patients with a normal QRS duration demonstrated a 60% decrement in the mean RMS 40 value during pacing-induced LBBB. These changes resulted in a 59% false positive incidence of late potentials during pacing-induced LBBB. This correlated with a similarly low mean RMS 40 value in patients with intrinsic LBBB and no inducible SMVT, hence giving rise to a false positive incidence of late potentials of 63%. Since "standard" RMS 40 and LAS 40 criteria resulted in low specificity and positive predictive value, new parameters were selected and analyzed. The combination of RMS 40 less than or equal to 17 microV plus LAS 40 greater than or equal to 55 msec yielded the best overall statistical result, with a sensitivity, specificity, and total predictive accuracy of 69%, 81%, and 77%, respectively.

IN CONCLUSION

(1) A reduction of RMS 40, prolongation of LAS 40, and fragmentation of signals in the terminal portion of the filtered QRS are characteristics of LBBB.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of bepridil and lidocaine on the intraventricular conduction in acutely ischaemic and infarcted canine myocardium

Effects of bepridil, an antiarrhythmic and antianginal drug, on intraventricular conduction in acutely ischaemic and infarcted myocardium were examined in anaesthetized dogs, and compared with those of lidocaine. Bepridil at doses of 2 and 5 mg/kg markedly prolonged the conduction time of a premature excitation induced by a ventricular stimulation in the infarcted zone. The effect of bepridil was dependent on a coupling time of the stimulation. Bepridil showed a marked effect at a coupling time of 150 ms, while it showed no significant effect at a prolonged coupling time of 1 s. In other words, the effect of bepridil was interval-dependent. Lidocaine showed a similar interval-dependent effect, but the effect of lidocaine at a longer coupling time was less than that of bepridil. The premature stimulation produced severely delayed conduction which resulted in reentrant beats. Bepridil blocked these conductions, thereby preventing reentrant beats. In contrast to the depressant effect of bepridil in the infarcted myocardium, bepridil prevented the prolongation of conduction time during acute ischaemia. The alternation of the ST-T complex during acute ischaemia which is also an important arrhythmogenic factor was also attenuated by bepridil. Contrary to bepridil, lidocaine significantly enhanced the conduction delay and the alternation in the ST-T complex. In conclusion, bepridil as well as lidocaine showed an interval-dependent depression of the conduction in the infarcted zone of the heart, whereas during acute ischaemia bepridil in contrast to lidocaine attenuated the conduction delay and ST-T alternans.

Electrophysiological basis of ventricular late potentials

The presence of late potentials on the body surface recording was correlated with ventricular activation maps of reentrant circuits in the postinfarction canine model of reentrant excitation. Late potentials were found to correlate with delayed myocardial activation. However, during a reentrant rhythm complete diastolic activity on the body surface could not be detected if the mass of electrically active cells was too small and/or if very slow conduction in part of the reentrant circuit generated low amplitude extracellular potentials. Myocardial zones responsible for late potentials during a basic rhythm (e.g., sinus rhythm) may not necessarily be part of the critical zone of slow conduction during reentrant activation. Dynamic changes in late potentials are not amenable to temporal signal averaging techniques but could be detected by a high resolution beat-to-beat recording. A thorough understanding of the electrophysiological limitations of late potentials in the signal-averaged ECG could result in better utilization of the technique in clinical practice as well as in the development of new approaches for the detection of the arrhythmogenic substrate.

Late potentials on the signal-averaged electrocardiogram after canine myocardial infarction: correlation with induced ventricular arrhythmias during the healing phase

Signal-averaged electrocardiograms (ECGs) and programmed ventricular stimulation were serially performed in 12 dogs (3 weeks of age) after experimental anteroapical myocardial infarction. At electrophysiologic study, sustained ventricular tachyarrhythmia was induced in seven dogs on at least one occasion. Of a total of 39 electrophysiologic studies, sustained monomorphic ventricular tachycardia was induced in seven studies and ventricular fibrillation in eight studies. In the remaining studies, no ventricular arrhythmia could be induced with triple ventricular extrastimuli. There was considerable day to day variability in the response to programmed stimulation and the results of the signal-averaged ECG. The signal-averaged QRS complex was significantly longer in dogs with inducible ventricular tachycardia or fibrillation (61 +/- 5 versus 57 +/- 3 ms, p = 0.02), had a lower terminal QRS amplitude (24 +/- 20 versus 46 +/- 33 microV, p = 0.04) and a longer late potential duration (19 +/- 4 versus 15 +/- 3 ms, p = 0.003) compared with that in animals with no inducible ventricular arrhythmia. Late potentials were defined as a total QRS duration greater than 58 ms, a terminal QRS amplitude less than 20 microV and a late potential duration greater than 18 ms. Using this definition, late potentials were seen in two distinct phases--immediately after coronary ligation and then beyond the first 72 h after infarction. The appearance of late potentials coincided with a change in arrhythmia inducibility from no ventricular arrhythmia to initiation of sustained monomorphic ventricular tachycardia. There is a close relation between inducibility of ventricular tachycardia in experimental canine myocardial infarction and the appearance of late potentials on the surface ECG.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of anterograde flow in the infarct artery on the incidence of late potentials after acute myocardial infarction

In patients after myocardial infarction, survival is influenced by the presence or absence of anterograde flow in the infarct artery, and late potentials on signal-averaged electrocardiography identify those at risk for tachyarrhythmias and sudden death. To assess the frequency of late potentials in survivors of first infarction, coronary arteriography and signal-averaged electrocardiography were performed in 109 subjects (64 men, 45 women, aged 30 to 77 years), 49 with (group I) and 60 without (group II) anterograde flow in the infarct artery. The groups were similar in age, sex, infarct artery, severity of coronary artery disease and left ventricular function. However, only 4 (8%) of group I had late potentials, whereas 24 (40%) of group II had late potentials (p less than 0.001). Thus, anterograde flow in the infarct artery after myocardial infarction is associated with a low incidence of late potentials on signal-averaged electrocardiography, whereas the absence of anterograde flow is more often associated with late potentials.

Improving the predictive ability of the signal-averaged electrocardiogram with a linear logistic model incorporating clinical variables

To improve the predictive accuracy of the signal-averaged electrocardiogram, we created a linear logistic model for predicting ventricular tachycardia during electrophysiologic testing. This signal-averaged electrocardiographic model was created from data obtained from 214 patients undergoing electrophysiologic testing (70 had ventricular tachycardia during electrophysiologic testing) by using stepwise logistic regression to rank eight clinical and nine signal-averaged electrocardiographic variables. The best predictors were ejection fraction, history of infarction, ventricular ectopic pairs or nonsustained ventricular tachycardia on Holter monitoring, QRS duration after 25-Hz filtering, and root mean square voltage of the terminal 40 msec of the QRS complex after 40- and 80-Hz filtering. Cross validation (a statistical technique that can be used to accurately evaluate how a predictive model will perform on a prospective patient population) was used to validate the model. After cross validation, the model's sensitivity was 91% and specificity was 59% for predicting ventricular tachycardia during electrophysiologic testing. This model compared favorably with established 25-Hz late-potential criteria (QRS duration of more than 110 msec and root mean square voltage of less than 25 microV of the terminal 40 msec of the QRS complex; sensitivity, 64%; specificity, 85%) and with established 40-Hz late-potential criteria (QRS duration of more than 114 msec or root mean square voltage of less than 20 microV of the terminal 40 msec of the QRS complex or duration of the low-amplitude signal less than 40 microV at the terminal QRS complex that is greater than 38 msec; sensitivity, 84%; specificity, 54%).(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical application of the signal-averaged electrocardiogram and "late potentials"

The signal-averaged ECG has proven to be a valuable tool for identifying patients at risk of ventricular arrhythmias. This computerized method of analyzing standard ECGs identifies microvolt-level late potentials that represent delayed conduction through diseased myocardium. This diseased myocardium is a potential substrate for reentrant ventricular arrhythmias. In select patient groups, the signal-averaged ECG predicts electrophysiologic testing results. Problems remain and continued development is needed to evaluate patients with conduction system disease, the patient without coronary artery disease but at risk of sudden death, and proper general application of the technique.

Influence of rate-dependent cellular uncoupling on conduction change during simulated ischemia in guinea pig papillary muscles: effect of verapamil

This study was performed to determine if the changes in cellular coupling induced by simulated ischemia were rate-dependent and if they contributed to the rate-dependent conduction slowing that occurs in this setting. We also sought to determine if the known ability of verapamil to prevent ischemia-induced conduction changes might be related to the preservation of cellular coupling. We studied the effects of increasing stimulation frequency from 0.5 to 2.0 Hz on the simultaneous changes in the maximum rate of rise (Vmax) of the action potential upstroke, conduction velocity, and internal longitudinal resistance (ri) determined by the voltage ratio method in superfused guinea pig papillary muscles under conditions of simulated ischemia (SI). When stimulation frequency was 0.5 Hz, 30 minutes of SI caused a 16.5% decrease in Vmax, a 16% increase in ri, and a 12.9% decrease in conduction velocity. When stimulation frequency was increased to 2.0 Hz, 30 minutes of SI caused a 30% decrease in Vmax, a 72.9% increase in ri, and a 21.4% decrease in conduction velocity. Thus, the changes were rate-dependent. Verapamil (1 X 10(-6) M) did not influence the changes in these parameters during SI at 0.5 Hz nor the decrease in Vmax during SI at 2.0 Hz, but it did prevent the rate-dependent increase in ri. Verapamil also prevented the rate-dependent decrease in conduction velocity induced by SI. Our results suggest that during simulated ischemia the rate-dependent component of the increase in Ri contributes to the rate-dependence of the conduction slowing.(ABSTRACT TRUNCATED AT 250 WORDS)

Nonuniform recovery of excitability in the left ventricle

The purpose of this study was to determine left ventricular activation, dispersion of refractoriness, and total recovery time in patients with coronary artery disease and ventricular tachycardia and in patients with the long QT syndrome and to compare these patients with a group of normal patients. Left ventricular endocardial catheter mapping and left ventricular refractory period determination were performed in 18 patients. Group 1 consisted of seven patients with no heart disease and no arrhythmia; group 2 consisted of six patients with previous infarction and sustained ventricular tachycardia; and group 3 consisted of five patients with prolonged QT interval and previous cardiac arrest. Total left ventricular endocardial activation was significantly longer in group 2 (75 +/- 23 msec, mean +/- SD) compared with group 1 (34 +/- 9 msec, p less than 0.01) and group 3 (42 +/- 5 msec, p less than 0.05). Dispersion of refractoriness was significantly greater in group 3 (87 +/- 27 msec) than in group 1 (40 +/- 14 msec, p less than 0.01) and group 2 (53 +/- 14 msec, p less than 0.05). Dispersion of total recovery time was significantly greater in group 2 (90 +/- 30 msec) than in group 1 (52 +/- 14 msec, p less than 0.05) as well as group 3 (114 +/- 43 msec) compared with group 1 (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous compared with induced onset of sustained ventricular tachycardia

Electrophysiological characteristics of the modes of initiation of 16 episodes of spontaneously occurring sustained ventricular tachycardia recorded in 16 patients by two-channel ambulatory electrocardiographic monitoring were compared with the characteristics of ventricular tachycardia induced by programmed electrical stimulation. Eleven episodes of spontaneous ventricular tachycardia began after a single ventricular premature depolarization (VPD), three episodes after two VPDs, and two episodes after five VPDs. By comparison, only four episodes of sustained ventricular tachycardia were induced with a single VPD. Each episode of spontaneous ventricular tachycardia was initiated by a late coupled VPD (RR':QT ratio greater than 1.0). The VPD was often morphologically similar to the ensuing ventricular tachycardia (eight of 11 episodes that began after a single VPD). No correlation was found between the modes of initiation of spontaneous and induced ventricular tachycardia. We hypothesize that concealed decremental slow conduction, reflected in the long coupling intervals of VPDs initiating ventricular tachycardia, is of critical importance in initiating ventricular tachycardia. We conclude that major differences exist in the timing and number of VPDs associated with the onsets of spontaneous and induced sustained ventricular tachycardia.

Natural history of late potentials in the first ten days after acute myocardial infarction and relation to early ventricular arrhythmias

Serial signal-averaged electrocardiograms (ECGs) were performed every 48 hours in 50 patients admitted to the coronary care unit with acute myocardial infarction. The prevalence of late potentials was 32% at presentation (mean time to recording 12.4 +/- 6.6 hours after onset of chest pain) and increased progressively throughout the hospital stay. New late potentials were recorded in patients with no prior acute myocardial infarction as early as 3 hours after the onset of chest pain and as late as 8 days. Late potentials appeared transiently in only 3 patients. The detection of late potentials in the initial signal-averaged ECG identified patients with clinically significant early ventricular arrhythmias with a sensitivity of 80% and specificity of 72%. The predictive accuracy was 38% for a positive test and 94% for a negative test. Patients with early ventricular arrhythmias had significantly lower voltage in the terminal 40 ms of the filtered QRS complex (16 +/- 8 vs 32 +/- 19 microV, p less than 0.01) than those without arrhythmias. The signal-averaged ECG may be useful in identifying patients at high risk of developing clinically significant early ventricular arrhythmias after acute myocardial infarction.

Mechanisms underlying the antiarrhythmic and arrhythmogenic actions of quinidine in a Purkinje fiber-ischemic gap preparation of reflected reentry

The effects of therapeutic levels of quinidine were studied in an ischemic gap preparation of reflected reentry. The preparation consisted of a Purkinje fiber mounted in a three-compartment chamber. A narrow central compartment was perfused with a solution prepared to mimic the extracellular milieu at a site of ischemia. Quinidine in concentrations that exert little effect on normal Purkinje tissue, 1 to 2 micrograms/ml, greatly impaired conduction and markedly prolonged refractoriness across the ischemic gap. The drug effected these changes by (1) extending the inexcitable zone within the depressed region, (2) decreasing the amplitude of the input signal entering this zone, and (3) decreasing the excitability of the tissue beyond the depressed zone (evaluated by current clamp techniques). These actions of the drug produced both antiarrhythmic and proarrhythmic effects. When the initial level of conduction impairment was high, quinidine totally suppressed reflected reentry at all frequencies by precipitating complete anterograde conduction block. At intermediate levels of block, the drug generally caused a prominent shift of the frequency dependence of reentrant activity to lower stimulation rates. Finally, when conduction was relatively less impaired, quinidine created the conditions for reflected reentry to occur. Our results suggest that the heart rate dependence of reentrant arrhythmias might be of prognostic value in the administration of antiarrhythmic drugs.

Alternation in refractoriness and in conduction delay in the ischemic myocardium associated with the alternation in the ST-T complex during acute coronary occlusion in anesthetized dogs

The effective refractory period (ERP) during the period of the alternation of the ST-T complex (STTA) in the ischemic myocardium was determined using a synchronized system of electrical stimulators and a bipolar epicardial electrode during acute coronary occlusion in anesthetized dogs. Epicardial unipolar electrograms (UP), epicardial bipolar electrograms (BP) and monophasic action potentials (MAP) were also recorded from ischemic areas. ERP in ischemic areas was prolonged, as ischemia progressed, and during the period of STTA ERP also alternately changed. A lower or negative deflection of the ST-T complex was accompanied by a longer ERP, and a higher or monophasic ST-T complex was accompanied by a shorter ERP. A good correlation was observed between the degrees of alternation in ERP and of STTA. MAP also showed an alternation in its duration and its amplitude. The alternation in duration preceded the alternation in amplitude. A lower or negative deflection of the ST-T complex corresponded to a MAP with a longer duration and a larger amplitude. During the period of STTA, a long conduction delay followed the negative deflection of the ST-T complex. A marked conduction delay appeared only during a period of STTA. Diltiazem and nifedipine inhibited STTA and an alternation in ERP. In conclusion, STTA is associated with an alternation in ERP and in conduction delay, and calcium antagonists attenuate both STTA and an alternation in ERP.

Late potentials and arrhythmogenesis

There are three current prognostic indicators of ventricular electrical instability: categorization and stratification of spontaneous ventricular arrhythmias from standard ECG recordings; programmed electrical stimulation; direct recording of delayed depolarization potentials, usually referred to as late potentials. Of the three, the latter offers a new and promising approach. Late potentials represent delayed activation potentials of diseased myocardial zones and may prove to be a strong independent marker of the propensity to develop reentrant ventricular arrhythmias and sudden cardiac electrical death. The problem in identifying late potentials on the body surface is that the signal is smaller than the electrical noise produced by various sources. Two different techniques have been utilized to improve the signal-to-noise ratio: first, signal averaging, which is applicable to regular repetitive electrocardiographic signals but cannot detect moment-to-moment dynamic changes in the signal; second, low-noise or high-resolution electrocardiography that utilizes spatial averaging techniques as well as other noise-reducing measures to record the late potentials on a beat-to-beat basis. This technique has the potential of directly identifying malignant "reentrant" versus benign "focal" ventricular rhythms. The present report discusses the electrophysiologic basis of late potentials and the clinical results of both signal-averaged and low-noise recordings for evaluation of ventricular electrical instability, particularly in patients with ischemic heart disease.

Relation of late potentials to site of origin of ventricular tachycardia associated with coronary heart disease

Signal-averaged electrocardiograms and endocardial catheter mapping were performed in 41 patients with coronary artery disease and sustained ventricular tachycardia (VT) to determine the relation between signal-averaged late potentials (SA-LPs) and catheter-mapped late activity (CM-LA) to the site of origin of VT. The 41 patients had 79 morphologically distinct VTs. Either CM-LA or SA-LP was present during sinus rhythm in 37 of 41 patients (90%). Twenty-two out of 30 patients (73%) had CM-LA corresponding to SA-LP during normal sinus rhythm. Patients with SA-LP had a significantly greater number of sites of CM-LA, which were later and longer in duration than patients without SA-LP present during sinus rhythm. In a select group of patients, those with both SA-LP and CM-LA, the site of origin of VT was located at or adjacent to a site of CM-LA during sinus rhythm in 38 of 44 (86%); however, 36 of 78 sites (46%) of CM-LA were clearly distant from the site of origin of VT. In conclusion, CM-LA corresponding to SA-LP in patients with VT is sensitive but not specific for the site of origin of VT.

Potentiating effects of a ventricular premature beat on the alternation of the ST-T complex of epicardial electrograms and the incidence of ventricular arrhythmias during acute coronary occlusion in dogs

A relation between ST-T alternans (STTA) and incidence of ventricular arrhythmias (VA) was examined, and the effects of a premature beat on these were tested during acute coronary occlusion in dogs. Epicardial unipolar electrograms, epicardial and endocardial bipolar electrograms and monophasic action potentials were recorded during coronary occlusion. During the period of STTA, a remarkably late conduction and its alternation were also observed in ischemic areas. The more serious delay appeared during the negative deflection of the ST-T complex. VA followed mainly the negative deflection of the ST-T complex. The first premature activation appeared in either ischemic areas or the border areas. A ventricular premature beat (VPB) which occurred spontaneously or was produced by an electrical stimulation potentiated STTA, and additional VA frequently followed the negative deflection of the ST-T complex of the second sinus beat after the VPB. A VPB pronounced the conduction delay during the second sinus beat. The discordant STTA became concordant after the VPB. The duration of a monophasic action potential of the second sinus beat after the VPB in mildly ischemic areas decreased. These changes and enhanced STTA may contribute to the occurrence of the additional VA.

Experimental studies into mechanisms of cardiac arrest

Experimental studies have revealed that a wide variety of different pathophysiological mechanisms may induce ventricular fibrillation (VF) and cardiac arrest during acute myocardial ischaemia or infarction. Distinct phases of enhanced vulnerability (the amount of current required to stimulate ectopic activity in the heart following application of an extra stimulus) to VF follow coronary occlusion and correspond to 'pre-hospital', 'in-hospital' and 'out-of-hospital' periods of arrhythmogenesis. Electrophysiological evidence suggests very early (phase 1a) VF results from multiple re-entrant excitation within the ischaemic zone. Slowed and fragmented conduction and inhomogeneities in refractoriness rapidly develop which mapping studies show to occur in association with development of spatial inhomogeneities in residual blood flow distribution and metabolism. Onset of VF may be triggered by adrenergic mechanisms or influenced by peripheral metabolic responses. Automatic mechanisms (spontaneous pacemaker activity) may induce later VF or VF on reperfusion or trigger re-entry. Findings indicate no single therapeutic approach to be likely to protect against all forms of cardiac arrest.

The effects of milrinone on conduction, reflection, and automaticity in canine Purkinje fibers

Milrinone is a newly developed analogue of amrinone possessing potent positive inotropic action. Electrophysiologic actions of the drug have not been reported. In this study microelectrode techniques were used to assess the electrophysiologic effects of milrinone in canine false tendons homogeneously superfused with either normal or high-K Tyrode's solution and in Purkinje fibers mounted in a three-compartment chamber in which the central segment was depressed with an "ischemic" solution. Milrinone (0.2 to 20 micrograms/ml) caused no major changes in the action potential characteristics, refractoriness, or conduction velocity in fibers exposed to normal Tyrode's solution, but markedly improved conduction and abbreviated or eliminated postrepolarization refractoriness in the ischemic gap preparations. The drug also exerted important effects on reflected reentry generated in these preparations. Depending on the initial level of block, milrinone (1) suppressed the arrhythmia, (2) shifted its frequency dependence, or (3) created the conditions that allowed reflection to occur. Similar results were obtained in homogeneously depressed fibers. At similar concentrations, milrinone caused a relatively small enhancement of automaticity. Thus, in addition to its inotropic actions, milrinone produces important electrophysiologic effects. By restoring or improving conduction through areas of depressed conductivity, the drug may exert either antiarrhythmic or arrhythmogenic effects.

Mechanisms of phase 1a and 1b early ventricular arrhythmias during acute myocardial ischemia in the dog

Two phases of ventricular arrhythmia occur within the first 30 minutes of experimental myocardial ischemia. Possible differences in their mechanisms of pathogenesis were investigated in anesthetized dogs by detailed mapping of patterns of epicardial activation and regional myocardial blood flow during phase 1a and phase 1b early ventricular arrhythmias induced by high ligation of the left anterior descending coronary artery. Data were derived from 80 sites in a 4 by 5 cm area of left ventricular anterior free wall and displayed using computer graphics. Regional myocardial blood flow and the relation of regional flow to epicardial delay did not differ significantly during the 2 phases of arrhythmia in central ischemic or nonischemic areas, although epicardial flow in border region segments was increased during phase 1b. Significantly greater mean epicardial delays and spatial heterogeneity of epicardial delay (assessed by intersite variance within the ischemic area) occurred during phase 1a arrhythmias. Serial studies show striking increases in spatial heterogeneity of delays during phase 1a, but not during phase 1b, relating to temporal dispersion of a phenomenon of transient prolongation of activation delay at individual electrode sites. These data are consistent with the concept that phase 1a and 1b arrhythmias arise through different electrophysiologic mechanisms independent of flow-dependent effects.

Electrophysiologic and myocardial metabolic changes in the acute phase of partial coronary occlusion

The acute effects of the partial reduction of coronary blood flow (CBF) on electrical and metabolic changes in myocardium were studied in 59 dogs. In seven dogs with a CBF reduction of 20% to 49%, the adenosine triphosphate (ATP) content (3.59 +/- 0.45 mumol/gm) and the width of the composite electrogram (54 +/- 5 msec) were not significantly different from those of 14 control dogs. In 14 dogs with a CBF reduction of 50% to 74%, ATP decreased significantly (3.09 +/- 0.30 mumol/gm, p less than 0.01); however, widening of the composite electrogram was not noted. Malignant ventricular arrhythmias developed in 5 of 10 dogs with a CBF reduction of 75% to 90% and 6 of 14 dogs with a 100% occlusion, but in none of the dogs with a CBF reduction of less than 75%. Marked widening of the composite electrogram (94 +/- 42 msec, p less than 0.05; and 115 +/- 54 msec, p less than 0.001) and further decrease in ATP (2.49 +/- 0.28 mumol/gm, p less than 0.001; and 2.66 +/- 0.45 mumol/gm, p less than 0.05) were noted in these two groups with a CBF reduction of greater than or equal to 75%. In conclusion, more advanced ischemia was mandatory for electrical derangements than for metabolic deterioration.

Electrophysiologic observations during the spontaneous initiation of ischemia-induced ventricular fibrillation

Electrophysiologic features of spontaneous, ischemia-induced ventricular fibrillation were studied in 17 dogs using multiple endocardial bipoles positioned in normal and ischemic zones and at the border of ischemic myocardium. All dogs showed ventricular tachyarrhythmias prior to the initiation of ventricular fibrillation. The heart rate prior to the fatal arrhythmia in the ventricular fibrillation dogs was significantly faster than that of nonventricular fibrillation dogs. There was no difference in the coupling intervals of the initial premature complex between episodic and sustained ventricular arrhythmia in most dogs. However, shorter coupling intervals initiated sustained arrhythmia in some dogs. Sites of initiation of arrhythmia were mostly in the ischemic zone. Furthermore, diastolic electrical activity was consistently observed in the ischemic zone during fatal arrhythmia in dogs showing diastolic activity. Cycle length during the fatal arrhythmia prior to ventricular fibrillation gradually shortened, whereas cycle length of episodic ventricular tachycardia remained approximately 200 msec followed by lengthening prior to restoration of sinus rhythm. The disparity of local activation (time differences between the earliest and latest onset of the activation in the five recording sites) increased during the fatal arrhythmia. Examples of progressive intraventricular block (Wenckebach-like) between the border and the center of ischemic myocardium leading to ventricular fibrillation are resynchronization of this disparity leading to the termination of ventricular tachycardia are shown. The recording of continuous electrical activity using bipolar electrodes with an interelectrode distance of 1 mm suggests a smaller reentrant pathway during fatal arrhythmia. Our observations confirm the importance of endocardial recordings within ischemic myocardium, and adds new insight into the events leading to both episodic and sustained ventricular tachycardia.

Ventricular arrhythmias in patients with rest angina: correlation with ST segment changes and extent of coronary atherosclerosis

Major ventricular arrhythmias occurring concurrently with myocardial ischemia are presumed to be the most frequent mechanism for sudden cardiac death. Two hundred eighteen catheterized patients with angina pectoris at rest were reviewed to identify clinical, ECG, and arteriographic features that might correlate with the presence of serious ventricular arrhythmias occurring during episodes of rest pain. Ventricular arrhythmias during episodes of rest pain were significantly more common in patients who manifested transient ST segment elevation in the anterior leads and in patients with marked transient ST segment shifts (greater than 5 mm). Ventricular arrhythmias during episodes of rest pain were not more common in patients with extensive coronary artery disease.

Relation between late potentials on the body surface and directly recorded fragmented electrograms in patients with ventricular tachycardia

The relation between low-amplitude, late potentials on the body surface and directly recorded electrograms in 8 patients with and 11 patients without ventricular tachycardia (VT) was studied. Bipolar X,Y,Z leads were signal-averaged and filtered with a digital technique. All patients had catheter endocardial left ventricular maps. The VT group had medically intractable VT and an endocardial excision was performed for control of VT. Before bypass, epicardial maps were obtained in the operating room. All studies were performed during normal sinus rhythm. Four patients without VT, each with a previous myocardial infarction, had fragmented endocardial electrograms recorded at 2.0 +/- 1.2 sites. The latest electrogram for each patient ended 87 +/- 8 ms after QRS onset, within the high-amplitude portion of the filtered QRS complex. All patients with VT had fragmented electrograms recorded at 6.1 +/- 3.1 sites/patient. Eighty-eight percent of the fragmented electrograms were endocardial. The latest fragmented electrogram for each patient ended 161 +/- 43 ms after QRS onset, significantly later than the fragmented electrograms from the patients without VT (p = 0.002). Six VT patients had low-amplitude, late potentials at the end of the filtered QRS complex. In these patients, the last 40 ms of the filtered QRS complex contained a higher proportion of fragmented electrograms compared with earlier segments of the QRS complex (68% versus 27%, p less than 0.001). Two patients with VT did not have late potentials. One patient with left bundle branch block had delayed left ventricular epicardial activation which masked the fragmented electrograms. The other had fragmented electrograms of brief duration which ended 80 +/- 12 ms after QRS onset, during the time of normal ventricular activation. It is concluded that the late potential corresponds to delayed, fragmented electrographic activity. Failure to record a late potential may arise from delayed ventricular activation at other sites from bundle branch block or fragmented electrograms of a brief duration.

Effects of antiarrhythmic surgery on late ventricular potentials recorded by precordial signal averaging in patients with ventricular tachycardia

In seven patients with documented ventricular tachycardia (VT) and prior myocardial infarction, late potentials (LP) were recorded at the end of or after the QRS complex from the body surface using high-gain amplification and the signal averaging technique (RC filter settings 100 to 300 Hz). In 6 to 7 patients VT could be initiated by programmed right ventricular stimulation; in one case, VT was inducible only from the left ventricle during surgery. Surgery was guided by epi- and endocardial mapping. In most cases besides resection of scar tissue, a partial or complete subendocardial encircling ventriculotomy was performed. Postoperatively, LPs were abolished in five cases, VT being no longer inducible. In the remaining two patients, LPs were still present. VT was still inducible in one of these two cases whereas in the other case, no programmed testing was done postoperatively. These data suggest that the abolition of LPs by surgery is closely related to the disappearance of the propensity to stimulus-induced VT. Thus, the averaging technique represents a new approach to the noninvasive control of the efficacy of surgery in patients with VT and prior myocardial infarction.

Effects of premature depolarization on refractoriness of ischemic canine myocardium

In 25 pentobarbital anesthetized dogs we measured refractory periods (RPs) of regularly driven complexes and premature ventricular depolarizations (PVDs) with a range of coupling intervals or of regularly driven complexes and the complex following the PVD, i.e. the postextrasystolic depolarization (PED). Measurements were made during control periods and during occlusion of a branch of the left anterior descending coronary artery. The difference in control and occlusion RPs was less following some PVDs with short coupling intervals than following other PVDs with longer coupling intervals. Variations in the coupling interval of PVDs had less effect on RPs of the PVDs in ischemic than in nonischemic tissue. RPs of PEDs were prolonged with respect to RPs of regularly driven complexes in both ischemic and nonischemic tissue, but the prolongation in ischemic tissue was significantly greater than that in nonischemic tissue, 8 +/- 4 msec and 2 +/- 2 msec respectively, p less than .001. The difference in effect of PVDs on RPs of ischemic and nonischemic tissue results in greater disparity of refractoriness between ischemic and nonischemic tissue following some long coupling interval PVDs than following some PVDs with shorter coupling intervals. In addition the greater prolongation of RPs of PEDs in ischemic than in nonischemic tissue can result in increased disparity in RPs than the disparity between ischemic and nonischemic tissue present during regular drive.

Ventricular fibrillation after intravenous atropine in a patient with atrioventricular block

A 69-year-old black woman with complete AV block developed ventricular fibrillation following an IV injection of 1 mg of atropine sulphate. After a successful DC countershock, the ECG showed a polymorphous ventricular tachycardia which subsided spontaneously. Cardiac catheterization revealed a small left ventricular diverticulum and normal coronary arteries. This seems to be the first reported case of atropine-induced ventricular fibrillation in a patient with complete AV block. The fact that this occurred without previous change of the ventricular rate suggests that the adverse action of atropine was mediated through a mere vagolytic effect at the ventricular level.

Arrhythmogenic effects of carbon monoxide in experimental acute myocardial ischemia: lack of slowed conduction and ventricular tachycardia

To investigate the possibility that carbon monoxide (CO) at high clinically encountered levels may have arrhythmogenic effect in initial minutes of acute myocardial ischemia, 14 dogs underwent thoracotomy and recording of bipolar epicardial electrograms during brief (6-minute) left anterior descending coronary artery (LAD) occlusions. Eight of the 14 dogs were studied both before and after CO pretreatment in which carboxyhemoglobin levels of 6.8% to 14.6% (mean 10.4 +/- 2.8%) were achieved. Epicardial electrograms showed no change in degree of ischemic myocardial conduction slowing after CO, and there was no significant difference between CO and no-CO LAD occlusions in incidence of spontaneous ventricular tachycardia. Since the degree of myocardial conduction slowing is thought to be importantly related to susceptibility to ventricular arrhythmias in acute ischemia, this study suggests that CO may not be arrhythmogenic during early minutes of acute myocardial ischemia.