Mechanisms underlying ventricular tachycardia and fibrillation in the ischemic heart: relation to nonlinear dynamics

Changes in ventricular repolarization during percutaneous transluminal coronary angioplasty in humans assessed by QT interval, QT dispersion and T vector loop morphology

OBJECTIVES

Based on clinical, epidemiological, and experimental data, transient cardiac ischaemia is one of the major triggering factors of malignant ventricular arrhythmia. According to animal studies, increased dispersion of repolarization is of pathophysiological relevance in this context. Therefore we explored the impact of myocardial ischaemia during single vessel coronary angioplasty on the change in ventricular repolarization, measured by QT and JT intervals and their dispersion in the 12-lead electrocardiogram. We also assessed a novel method, the 3-dimensional T vector loop, to find out whether it was sensitive to changes in ventricular repolarization during ischaemia, and whether there was any correlation with changes in the dispersion of the QT and/or JT intervals.

DESIGN

This study was prospective with consecutive patients. Only patients in sinus rhythm and without bundle branch block were included.

SETTING

All coronary angioplasties were performed at Norrlands University Hospital, Umeå. The analysis of the material was performed at the Karolinska Hospital, Stockholm.

SUBJECTS

Twenty-nine consecutive patients went through 30 elective one-vessel percutaneous transluminal coronary angioplasty (PTCA) procedures. PTCA was performed in 10 stenoses of the left anterior descending, 10 of the left circumflex, and 10 of the right coronary artery.

INTERVENTIONS

A 12-lead electrocardiogram was recorded continuously as part of routine monitoring of the patient during PTCA and the T vector loop was calculated from the simultaneously recorded. X, Y, Z leads.

MAIN OUTCOME MEASURES

Repolarization was assessed by the QRS, QT and JT intervals as well as by the T vector loop parameters (Tarea, Tavplan, and Teigenv) before and at the end of the first occlusion during PTCA.

RESULTS

PTCA, with an average occlusion time of 171 +/- 60 s (mean +/- SD), induced ischaemia on the 12-lead electrocardiogram in 73% of cases. The overall response for the 30 procedures was a significantly increased dispersion of ventricular repolarization, both corrected and uncorrected for heart rate. QT dispersion increased by, on average, 19% from 74 +/- 35 to 88 +/- 36 ms, QTc dispersion by 27% from 71 +/- 39 to 90 +/- 42 ms, and JTc dispersion by 19% from 78 +/- 32 to 94 +/- 43 ms (P < 0.05). The T vector loop became more circular and bulgy during occlusion (all three parameters changed by between 33% and 59%). There was a significant correlation between changes in one of the T vector loop parameters (Teigenv), and changes in JT and QT dispersion in the left anterior descending group.

CONCLUSIONS

Transient ischaemia during PTCA induced significant changes in ventricular repolarization, especially during occlusion of the left anterior descending artery and resulted in a significant increase in both QT and QTc dispersion. The degree of QT dispersion was such that several patients were at risk of ventricular arrhythmia, if a proper triggering extrasystole had occurred. In addition, and as an original observation, the 3-dimensional T vector loop morphology seemed even more sensitive to coronary occlusion than QT dispersion.

Estimation of 3-D conduction velocity vector fields from cardiac mapping data

A method to estimate three-dimensional (3-D) conduction velocity vector fields in cardiac tissue is presented. The speed and direction of propagation are found from polynomial "surfaces" fitted to space-time (x, y, z, t) coordinates of cardiac activity. The technique is applied to sinus rhythm and paced rhythm mapped with plunge needles at 396-466 sites in the canine myocardium. The method was validated on simulated 3-D plane and spherical waves. For simulated data, conduction velocities were estimated with an accuracy of 1%-2%. In experimental data, estimates of conduction speeds during paced rhythm were slower than those found during normal sinus rhythm. Vector directions were also found to differ between different types of beats. The technique was able to distinguish between premature ventricular contractions and sinus beats and between sinus and paced beats. The proposed approach to computing velocity vector fields provides an automated, physiological, and quantitative description of local electrical activity in 3-D tissue. This method may provide insight into abnormal conduction associated with fatal ventricular arrhythmias.

Detection of life-threatening cardiac arrhythmias using the wavelet transformation

Time-frequency wavelet theory is used for the detection of life threatening electrocardiography (ECG) arrhythmias. This is achieved through the use of the raised cosine wavelet transform (RCWT). The RCWT is found to be useful in differentiating between ventricular fibrillation, ventricular tachycardia and atrial fibrillation. Ventricular fibrillation is characterised by continuous bands in the range of 2-10 Hz; ventricular tachycardia is characterised by two distinct bands: the first band in the range of 2-5 Hz and the second in the range of 6-8 Hz; and atrial fibrillation is determined by a low frequency band in the range of 0-5 Hz. A classification algorithm is developed to classify ECG records on the basis of the computation of three parameters defined in the time-frequency plane of the wavelet transform. Furthermore, the advantage of localising and separating ECG signals from high as well as intermediate frequencies is demonstrated. The above capabilities of the wavelet technique are supported by results obtained from ECG signals obtained from normal and abnormal subjects.

Effects of blockade of fast and slow inward current channels on ventricular fibrillation in the pig heart

OBJECTIVE

To determine the contribution of fast and slow inward channels to the electrocardiogram (ECG) of ventricular fibrillation.

METHODS

Ventricular fibrillation was induced by endocardial electrical stimulation in pigs anaesthetised with pentobarbitone sodium (30 mg/kg intravenously). ECGs simultaneously recorded from the body surface (lead II) and from the endocardium were studied by power spectrum analysis (0-40 Hz).

RESULTS

The mean (SEM) dominant frequency of fibrillation (9.0 (1.1) Hz in lead II at 0-40 s) did not change significantly with time in pigs given intravenous saline. However, the dominant frequency was significantly reduced by intravenous pretreatment with the class I antiarrhythmic drugs, lignocaine (3 mg/kg, 6.5 (0.5) Hz; 10 mg/kg, 4.2 (0.6) Hz), mexiletine (3 mg/kg, 6.2 (0.4) Hz; 10 mg/kg, 5.5 (0.4) Hz), and disopyramide (2.5 mg/kg, 5.4 (0.6) Hz). After flecainide (3 mg/kg, 6.9 (0.5) Hz) the reduction in frequency was not significant. Similar data were obtained with endocardial recordings. In contrast pre-treatment with verapamil (0.2 mg/kg, 11.7 (0.8) Hz; and 1.0 mg/kg, 12.9 (1.6) Hz) produced a significantly higher dominant frequency of fibrillation than saline and widened the bandwidth of frequencies around the dominant frequency.

CONCLUSIONS

These results indicate that voltage-dependent sodium channel currents contribute to the rapid frequencies of ventricular fibrillation. Blockade of L-type inward calcium channel activity increases the fibrillation frequency and fractionates the frequencies of the fibrillation wavefronts.

Ventricular fibrillation is not always due to multiple wavelet reentry

INTRODUCTION

It is not known whether ventricular fibrillation (VF) is always caused by multiple wavelet reentry, or if it could also be caused by a single wandering reentrant wavefront. Activation mapping of the entire ventricles during VF is practically impossible.

METHODS AND RESULTS

We studied VF in a two-dimensional sheet of left ventricular subepicardial tissue of isolated, Langendorff-perfused pig hearts. Left and right endocardial cryoablation via probes filled with liquid nitrogen caused coagulation necrosis of the right ventricle, interventricular septum, and most of the left ventricular wall, leaving a thin subepicardial layer intact. Left ventricular epicardial activation patterns were constructed based on simultaneous recording of 128 unipolar extracellular electrograms. Regular pacing through a central electrode before and after freezing revealed that, following cryoablation, the activation pattern no longer showed evidence of involvement of the Purkinje system, and that it was compatible with propagation through a two-dimensional anisotropic tissue sheet. VF was induced by burst pacing. When the mass of surviving subepicardium was < 10 g, no VF could be induced; when it was between 10 and 20 g, VF was nonsustained; when it was > 20 g, VF was sustained. Unipolar extracellular electrograms during VF before and after cryablation could not be distinguished from each other; however, epicardial activation patterns were markedly different. In the intact left ventricle, up to six different wavefronts were simultaneously present during a 100-msec time window. In the "frozen heart," during a period of about 0.5 seconds, at most two wandering reentrant waves were simultaneously present; sometimes only one reentrant wave was seen in a 100-msec time window.

CONCLUSION

The extracellular waveform during VF can be caused by different forms of reentry: multiple wavelet reentry (on the order of six different wavefronts), two independent wandering reentrant waves, and a single wandering reentrant wave.

Comparison of four techniques for recognition of ventricular fibrillation from the surface ECG

Four ventricular fibrillation (VF) detection techniques were assessed using recordings of VF to evaluate sensitivity and VF-like recordings to evaluate specificity. The recordings were obtained from Coronary Care Unit patients. The techniques were: threshold crossing intervals (TCI); peaks in the autocorrelation function (ACF); signal content outside the mean frequency (VF-filter); and signal spectrum shape (spectrum). Using 70 extracts, each 4 s long, from VF recordings, the VF filter achieved a sensitivity of 77 per cent; the ACF, TCI and spectrum algorithms had sensitivities of 67, 53 and 46 per cent, respectively. Susceptibility to false alarms was assessed using 40 extracts from VF-like recordings. The TCI algorithm was the most specific (93 per cent), while the spectrum, VF filter and ACF algorithms had specificities of 72, 55 and 38 per cent, respectively. The TCI algorithm achieved overall sensitivity of 93 per cent and specificity of 60 per cent. The spectrum, VF filter and ACF algorithms had overall sensitivities of 80, 93 and 87 per cent, and overall specificities of 60, 20 and 0 per cent, respectively.

The use of statistical methods in the analysis of clinical studies

It is "standard" to analyze data from a clinical trial using a narrowly defined probabilistic mathematical model. This paper examines the ways in which mathematical models, in general, can be used in clinical research, the meaning of probability in the examination of clinical trials, and the philosophical flaws in the current "standard" method. An alternative formulation is proposed which is more flexible and which comes closer to meeting the needs of medical science. In this alternative formulation, significance tests are applied to the data from a study only as a first step to determine whether the data are worth further examination. After that, clinically relevant questions are answered with 50 and 95% confidence bounds. The initial significance test is tailored to be directed at a narrow class of hypotheses that, in turn, are dictated by clinical expectations.