Endocardial monophasic action potential recordings for the detection of myocardial ischemia in man: a study using atrial pacing stress and myocardial perfusion scintigraphy

Beta blockers as anti-arrhythmic agents

Although beta-adrenergic blocking agents are not always considered anti-arrhythmic drugs, the results of several recent trials have suggested an anti-arrhythmic mechanism for at least part of their mortality benefit in the treatment of chronic congestive heart failure. We review background experimental and clinical evidence for the anti-arrhythmic actions of beta-blockers and then review the results of published beta-blocker heart failure trials. A majority of trials showed improvement in overall survival as well as reduction in sudden death and ventricular arrhythmias with beta-blocker treatment. Although different effects were seen with different specific agents, these trials overall support a clinically significant anti-arrhythmic effect of several beta-blockers.

The Electrical Restitution Curve Revisited:

The electrical restitution curve (ERC) traditionally describes the recovery of action potential duration (APD) as a function of the interbeat interval or, more correctly, the diastolic interval (DI). Often overlooked in modeling studies, the normal ventricular ERC is triphasic, starting with a steep initial recovery at the shortest DIs, a transient decline, and a final asymptotic rise to a plateau phase reached at long DIs. Recent studies have proposed that it would be advantageous to lower the slope of the ERC by drug intervention, as this might reduce the potential for electrical alternans and ventricular fibrillation. This review discusses the pros and cons of a flat versus steep slope of the ERC and draws attention to mechanisms thatjustify the (physiologically) steep slope, rather than a flat slope, as a better design against arrhythmias. Five potential mechanisms are discussed, which allows for a different interpretation of the effect of the slope on arrhythmogenicity. The most important appears to be the physiologic rate adaptive shortening of APD that, by reciprocal lengthening of the DI, allows the subsequent APD to move more quickly from the steep initial ERC phase onto the flat phase. A less steep initial ERC phase would protract the transition toward more fully recovered APD and, in fact, may perpetuate electrical alternans. The triphasic ERC time course in normal myocardium cannot be explained by or fitted to single exponentials or single ion channel recovery kinetics. A simple tri-ionic model is suggested that may help explain the shape of the ERC at various repolarization levels and place APD recovery into perspective with intracellular calcium recycling and recovery of contractile force.

Effects of Valsalva maneuver on QT dispersion in patients with ischemic heart diseases

Previous studies showed that increased QT dispersion (QTd) has been observed during episodes of myocardial ischemia or infarction and identify the patients at risk of arrhythmia or sudden death. The objective of this study is to investigate the relationship between coronary artery disease and QTd during the Valsalva maneuver. The study population included 85 subjects (21 with normal coronary arteries, 35 with stable angina pectoris, and 29 with unstable angina pectoris). Twelve-lead surface ECGs were recorded at 50-mm/sec paper speeds and were obtained before the Valsalva maneuver and during the strain phase. The results indicate a significant difference in mean time increase between the control group and the group with stable angina pectoris (mean difference = 16.10 milliseconds, p<0.000), and between the control group and the group with unstable angina pectoris (mean difference = 35.26 milliseconds, p<0.000). The mean difference in time between these groups was also compared (mean difference = 19.17 milliseconds), and was statistically significant (p<0.000). There are some conditions like constipation, severe coughing spells, nausea, vomiting, and carrying or lifting heavy objects that increase intrathoracic pressure and may increase QT dispersion. Therefore, all these conditions should be treated appropriately and carrying or lifting heavy objects is forbidden, especially in patients with coronary artery disease.

Effects of atrial pacing on QT dispersion in patients with coronary artery disease without angina pectoris and ST segment depression

The aim of this study was to investigate QT dispersion during atrial pacing in patients with coronary artery disease (CAD) without clinical ischemia, such as angina pectoris and ST segment depression. Thirteen patients with normal coronary arteries and 42 patients with CAD (12 with single-vessel, 16 with two-vessel and 14 with three-vessel disease) having no angina pectoris or ST segment depression during atrial pacing with maximum rate of 120/minute were enrolled in the study. Twelve-lead surface ECGs were recorded at 100 mm/second paper speed before pacing, at maximum pacing rate, and during the recovery period for measurement of QT interval parameters. Corrected QTd (QTcd) increased from 43.4 +/- 8.1 to 49.3 +/- 9.5 ms (p < 0.05) in the control group, from 46.1 +/- 8.1 to 74.3 +/- 7.7 ms (p < 0.0001) in the single-vessel disease group, from 48.5 +/- 10.4 to 93.8 +/- 22.1 ms in the two-vessel disease group (p < 0.0001), and from 49.7 +/- 13.6 to 128.5 +/- 31 ms (p < 0.0001) in the three-vessel disease group at peak atrial pacing period. A positive correlation was found between the severity of CAD and QTcd (r = 0.49, p < 0.0001). It was found that pacing-induced QTc dispersion identifies coronary disease extent, even when there is no ST depression or T wave inversion during pacing.

Investigating the dual nature of endothelin-1: ischemia or direct arrhythmogenic effect?

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide, which may also elicit severe ventricular arrhythmias. The aims of our study were to compare the effects of total left anterior descending coronary artery (LAD) occlusion to intracoronary (ic.) ET-1 administration and to investigate the pathomechanism of ET-1 induced arrhythmias in 3 groups of anesthetized, open-chest mongrel dogs. In group A (n=10) a total LAD occlusion was carried out for 30 min, followed by a 60 min reperfusion period. In groups B and C ET-1 was administered into LAD for 30 min at a rate of 30 pmol/min (n=6) and 60 pmol/min (n=8). Epi- and endocardial monophasic action potential (MAP) recordings were performed to detect electrophysiologic changes and ischemia Blood samples for lactate measurements were collected from the coronary sinus (CS) and from the femoral artery. Infrared imaging was applied to follow epimyocardial heat emission changes. At the end of the ET-1 infusion period coronary blood flow (CBF) was reduced significantly in groups B and C (deltaCBF30MIN B: 21+/-2%, p<0.05; C: 35+/-2%, p<0.05), paralleled by a significant epimyocardial temperature decrease in group C (deltaT30MIN: -0.65+/-0.29 degrees C, p<0.05). Two dogs died of ventricular fibrillation (VF) in the reperfusion period in group A. Ventricular premature contractions and non-sustained ventricular tachycardic episodes appeared in group B, whereas six dogs died of VF in group C. Significant CS lactate level elevation indicating ischemia was observed only in group A from the 30th min occlusion throughout the reperfusion period (control vs. 30 min: 1.3+/-0.29 vs. 2.2+/-0.37 mmol/l, p<0.05). Epi- and endocardial MAP durations (MAPD90) and left ventricular epicardial (LV(EPI)) upstroke velocity decreased significantly in group A in the occlusion period. ET-1 infusion significantly increased LV(EPI) MAPD90 in group B and both MAPD90-s in group C. In conclusion, ischemic MAP and CS lactate changes were observed only in group A. Although ET-1 reduced CBF significantly in groups B and C, neither MAP nor lactate indicated ischemic alterations. ET-1 induced major ventricular arrhythmias appeared before signs of myocardial ischemia developed, though reduced CBF presumably contributed to sustaining the arrhythmias.

Prediction of the onset of atrial fibrillation after cardiac surgery using the monophasic action potential

OBJECTIVE

To show that the monophasic action potential (MAP) recorded continuously from human epicardium may be used to predict the imminent onset of atrial fibrillation or flutter (AF) following surgery, thus allowing prophylactic treatment to be started.

PATIENTS

22 patients (14 male, 8 female; mean (SD) age 64 (12) years) undergoing aortic valve replacement.

SETTING

Tertiary referral centre.

METHODS

Over a mean observation period of 8 (2.7) days (range 4 to 14), nine episodes of AF were seen in six patients. Before AF, specific and significant alterations of the MAP morphology were observed. In seven of nine episodes the MAP shortened (25 (4)% 60 minutes before AF), developed a triangular shape, and the plateau amplitude decreased from 5.3 (1.2) to 2 (0.2) mV. In the two remaining episodes the beat to beat variability of cycle length and MAP duration at 90% repolarisation (MAPd90) increased significantly from 24 (7) ms and 12 (8) ms (24 hours before AF) to 137 (27) ms and 56 (11) ms (30 minutes before AF) respectively. AF was successfully treated by the administration of sotalol in three cases and by a combination of verapamil and digoxin in a further four. Previously observed changes of MAPd90 and MAP morphology regressed after conversion to sinus rhythm.

CONCLUSIONS

The continuous and intermediate term recording of the MAP from atrial epicardium appears to be a valid tool for detecting imminent AF after cardiac surgery with a high sensitivity (99%) and specificity (88%). Optimised antiarrhythmic treatment may thus be given selectively for prophylaxis.

Acute ischaemia: a dynamic influence on QT dispersion

BACKGROUND

The aim of this study was to test the hypothesis that acute myocardial ischaemia increases QT dispersion measured from the 12-lead electrocardiogram.

METHODS

Incremental atrial pacing was used to induce myocardial ischaemia in 18 patients with coronary artery disease and QT dispersion was measured. Six patients with normal coronary arteries served as the control group.

FINDINGS

All the patients with coronary artery disease developed angina and/or ST depression accompanied by marked increases in QT dispersion (mean increase 38 ms, 95% CI 30 to 45 ms, p < 0.001). In contrast, in the six patients with normal coronary arteries who remained without symptoms and without ST changes, there was no significant change in QT dispersion in response to pacing. Baseline QT dispersion did not distinguish those patients with coronary artery disease from those with normal coronary arteries (44 ms [95% Cl 39-49 ms] vs 40 ms [25-55 ms]), respectively.

INTERPRETATION

These results demonstrate that myocardial ischaemia induced by incremental atrial pacing in patients with coronary artery disease causes an acute increase in QT dispersion. Such "inducible" QT dispersion may prove more useful than resting QT dispersion in assessing the individual risk of arrhythmic events in patients with coronary artery disease.

Human ventricular action potential duration during short and long cycles. Rapid modulation by ischemia

BACKGROUND

Mechanisms underlying the initiation of ventricular arrhythmias in ischemia by a premature beat or after a pause remain unclear. The kinetics of electrical restitution, which is the modulation of action potential duration (APD) by an abrupt alteration in cycle length, may be important.

METHODS AND RESULTS

We recorded one or two simultaneous monophasic action potentials (MAPs) from the right ventricular septum during balloon occlusion of the left anterior descending coronary artery (LAD) (14 patients), which is expected to induce ischemia at the recording site, and during occlusion of the right coronary artery (RCA) (7 patients), which is not expected to induce ischemia at the recording area. The latter acted as a control. A test pulse sequence was incorporated whereby during steady-state pacing, test beats of altered cycle length were interpose. During LAD occlusion, APD for basic beats shortened from 260 +/- 4 to 236 +/- 4 ms (P < .0001), whereas the control group (RCA occlusion) showed no significant change (251 +/- 7 to 249 +/- 9 ms; P = NS). LAD occlusion resulted in flattening of the slope relating APD of test beats to diastolic interval (P = .001), whereas in the control group (RCA occlusion) the slope remained unchanged. Similar results were obtained during a second occlusion.

CONCLUSIONS

LAD occlusion in patients during balloon angioplasty shortened MAP duration of basic beats and minimized, abolished, or reversed the normal APD/diastolic-interval relation of test beats of altered cycle length at sites served by the occluded vessel. The results suggest that ischemia flattens the electrical restitution curve in the human endocardium. These findings may have important implications in arrhythmogenesis.

Ventricular pacing threshold and refractoriness after defibrillation shocks in patients with implantable cardioverter-defibrillators

The aim of this study was to examine the effect of ventricular fibrillation and a subsequent defibrillation shock on ventricular excitability and refractoriness in human beings. We studied 16 consecutive patients with implantable cardioverter-defibrillators undergoing follow-up studies. The pre- and post-shock pacing threshold, ventricular effective refractory period, monophasic action potential duration, and serum catecholamine levels were measured. Compared with the baseline state, immediately after ventricular fibrillation, and a successful defibrillation shock: (1) the ventricular effective refractory period decreased from 251 +/- 24 ms to 222 +/- 30 ms (p < 0.01), (2) the monophasic action potential duration decreased from 210 +/- 16 ms to 179 +/- 23 ms (P < 0.01) at 50% repolarization and from 274 +/- 24 ms to 240 +/- 26 ms (P< 0.01) at 90% repolarization, (3) the pacing threshold was not significantly altered and, (4) serum levels of epinephrine and norepinephrine were elevated. These results show that although ventricular fibrillation and subsequent defibrillation had no effect on the ventricular pacing threshold in human beings, it was associated with a decrease in post-shock monophasic action potential duration and ventricular effective refractory period, contrary to some previously reported findings.

Repolarization gradients derived by subtraction of monophasic action potential recordings in the human heart. Studies incorporating altered mechanical loading and ischemia

Information derived from the analysis of the electrocardiographic waveform remains one of the most valuable diagnostic aids in modern cardiology. Paradoxically, although changes in the ST-T segment probably have the widest clinical application, it is the analysis of this repolarization phase that has been surrounded by the greatest difficulties in interpretation.

Monophasic action potentials: concepts to practical applications

Monophasic action potential (MAP) recordings reproduce the repolarization time course of intracellular action potentials with high accuracy and provide precise information on the local activation time. With the advantage of in vivo application and the development of the safer and simpler contact catheter technique, MAP recording has become the method of choice for evaluating myocardial repolarization changes. This review aims to provide information on practical application of MAP recording in the clinical setting. MAPs can easily be recorded from the endocardium with the contact catheter technique in the electrophysiology laboratory and from the epicardium with electrode probes during open heart surgery. The technical aspects are described in detail. The rate dependence of myocardial excitability and repolarization and the effect of antiarrhythmic drugs on MAP duration and effective refractory period are thoroughly reviewed. The use of MAPs in detecting myocardial ischemia, in studying early afterdepolarization and triggered arrhythmias, in measuring dispersion of repolarization, in identifying intracardiac conduction and the development of the T wave, and in verifying the arrhythmogenic effect of mechanoelectric feedback are presented. Computerized automatic analysis of MAPs and the limitations of the MAP technique are also discussed.

Direct effect of dobutamine on action potential duration in ischemic compared with normal areas in the human ventricle

BACKGROUND AND OBJECTIVES

The arrhythmogenic effect of beta-adrenoceptor stimulation is complex and may differ in ischemic and normal myocardium. In this study we examined the differential effect of beta-adrenergic stimulation on ventricular action potential duration and, hence, dispersion of repolarization in potentially ischemic versus nonischemic human ventricular myocardium.

METHODS

Simultaneous biventricular monophasic action potentials were recorded in 14 patients (28 recording sites) during infusion of dobutamine in incremental doses (low dose 5 micrograms/kg per min, high dose 10 to 15 micrograms/kg per min) during atrial pacing. Perfusion at the action potential recording site was assessed by incorporating myocardial perfusion scintigraphy with injection of technetium-99m hexakis-2-methoxy-2-methylpropyl-isonitrile during the recording at peak doses of dobutamine. Action potential duration during dobutamine infusion was compared with that during atrial pacing to identical rates in the absence of dobutamine.

RESULTS

In 21 normal zone recordings, dobutamine produced a variable effect over that produced by atrial pacing to identical heart rates, either lengthening or shortening the action potential duration. The mean (+/- SEM) value for the additional effect of dobutamine was 0.9 +/- 2.5 ms with low doses and -4 +/- 2.6 ms with high doses (p = NS). In seven recordings from potentially ischemic zones, low dose dobutamine had a similar effect (mean change -3.4 +/- 6.5 ms; p = NS vs. normal zone values). However, the high dose dobutamine invariably shortened the action potential duration by a mean of -22.9 +/- 2.9 ms. (p less than 0.05 vs. low dose in ischemic areas, p less than 0.01 vs. normal zone recordings). Pacing alone or the addition of dobutamine had no significant effect on the normal dispersion of action potential duration between two nonischemic recording sites. In recordings in a normal and an abnormally perfused site, high dose dobutamine significantly altered the dispersion of action potential duration.

CONCLUSIONS

These results suggest a different effect of beta adrenergic stimulation in potentially ischemic compared with nonischemic human ventricular myocardium. The abnormal dispersion of repolarization thus created may well be important in beta-receptor-mediated arrhythmogenesis during myocardial ischemia.