Showing papers on "QRS complex published in 1989"

Decreased incidence of ventricular late potentials after successful thrombolytic therapy for acute myocardial infarction.

Abstract: In some patients with acute myocardial infarction, low-amplitude potentials that prolong the QRS complex, termed "late potentials," can be recorded on a signal-averaged electrocardiogram. The presence of these late potentials is known to be associated with an increase in the risk of ventricular tachycardia and sudden death. Because patients with acute myocardial infarction who receive thrombolytic therapy have a reduced incidence of ventricular tachyarrhythmia and sudden death, we sought to determine whether such patients also have a decreased incidence of late potentials. We studied 106 patients less than 75 years of age who were admitted with a first myocardial infarction and in whom a signal-averaged electrocardiogram was recorded within 48 hours of admission. Within four hours of the onset of chest pain, tissue plasminogen activator (t-PA) was given to 44 patients, and 62 were treated conventionally. In the t-PA group, late potentials were recorded in 2 of 44 patients (5 percent), as compared...

Estimating cardiac transmembrane activation and recovery times from unipolar and bipolar extracellular electrograms: a simulation study.

Abstract: A model of one-dimensional action potential propagation was used to compare activation times and recovery times measured from simulated unipolar and bipolar electrograms with the activation and recovery times measured from simulated transmembrane action potentials. Theory predicts that the intrinsic deflection--the time of the maximum negative slope of the unipolar electrogram QRS complex--corresponds to the time of maximum positive slope of action potential depolarization. Similarly, the time of the maximum positive slope of the unipolar electrogram T wave corresponds to the time of maximum negative slope of action potential repolarization. This study showed that the difference between the unipolar electrogram activation time and the action potential activation time and the difference between the unipolar electrogram recovery time and the action potential recovery time were small during ideal conditions of uniform propagation in a long cable. Nonideal conditions, however, were associated with activation time differences in excess of 1.8 msec and recovery time differences in excess of 30 msec (243 msec in certain conditions). Nonideal conditions that had a major influence were changes in activation sequence, propagation in a short cable, and propagation through regions of nonuniform coupling resistance and/or nonuniform membrane properties. Nonideal conditions that had a smaller influence were variations in distance from the measurement site to the simulated tissue surface, nonzero reference potentials, and the addition of distant events. Recovery time differences were more sensitive to the nonideal conditions than were activation time differences, and both depended on the action potential shape. When distant events significantly contributed to the unipolar electrogram waveform, the time differences when bipolar electrograms were used were less than those when unipolar electrograms were used; however, under other conditions, the time differences were comparable. Results showed that activation times and especially recovery times measured from electrograms can be greatly affected by conditions independent of changes in the underlying action potential waveforms.

Spectral mapping of the electrocardiogram with Fourier transform for identification of patients with sustained ventricular tachycardia and coronary artery disease

Abstract: In time domain analysis, detection of late potentials is limited by high pass filtering, noise interference and the necessity to exclude patients with bundle branch block. We therefore used frequency analysis with Fourier transform of multiple segments of the surface electrocardiogram (25 segments, size 80 ms, time shift 3 ms) during sinus rhythm after signal averaging. Thirty-two post-myocardial infarction patients with sustained ventricular tachycardia (VT), 19 post-myocardial infarction patients without VT and 17 healthy subjects were studied. A total of 18 patients had bundle branch block. In 24 out of 32 patients with VT, three-dimensional spectral plots were characterized by spectral peaks greater than 10 dB in the range of 40-200 Hz in segments only at the end of QRS and the early ST wave, but not far outside the QRS. In only 2 out of 19 patients without VT and in 1 out of 17 healthy subjects could such peaks be observed. Noise caused spectral peaks throughout all segments. Sixteen out of 18 patients with bundle branch block were correctly classified with spectral mapping. With the Simson method, patients with bundle branch block had to be excluded, abnormal results were found in 10 out of 19 patients with VT, but also in 5 out of 15 patients without VT and in 3 out of 16 healthy subjects. Thus, spectral mapping of the electrocardiogram offers promise for better identification of patients prone to sustained VT in the presence of coronary artery disease.

QRS morphology representation and noise estimation using the Karhunen-Loeve transform

Abstract: The authors describe robust methods for deriving Karhunen-Loeve (KL) basis functions, which can be used to represent the QRS complex. Using a five-term KL expansion of a 200-ms interval, which includes the QRS complex and part of the ST segment, one can represent morphology on two simultaneous ECG (electrocardiographic) leads with sufficient fidelity for beat classification. The residual error of the representation is an ideal estimate of the instantaneous noise content of the signal and permits identification of events for which the morphologic information is unreliable. The authors have compared the performance of the current KL-based arrhythmia analysis program with its predecessor (which uses a set of time-domain features for morphology representation but is otherwise identical to the newer program). In evaluations using the MIT-BIH and AHA (American Heart Association) databases, and a newly developed database containing approximately 2.5 million annotated beats (including over 80000 premature ventricular contractions) from 27 long-term ECG recordings, it was found that beat classification errors using the KL transform were as little as one-fourth of those for the older program. >

Heart data monitoring method and apparatus

Abstract: Improved ambulatory heart data monitoring and recording apparatus and method are described. The apparatus provides a power partitioning and management circuit that enables the extended, continuous ECG monitoring of an ambulatory patient's cardiovascular performance and selective recording of arrhythmic events that transpire during such extended monitoring, wherein the circuit supplies reduced, unregulated DC power to potentially volatile data storage circuit elements in the event of the failure of the primary DC power source. An improved method validates QRS complexes by determining, during a learning period, characteristic criteria of the individual patient's QRS waveform most representative of a valid QRS complex as opposed to motion or other artifacts, and thereafter uses such learned criteria in heart rate monitoring. In another improved method, the occurrence of an arrhythmic event is determined by monitoring not only the patient's average heart rate relative to predetermined physiological norms but also the rate of change of the average heart rate, thereby reducing the likelihood that the apparatus gives false positive indications of abnormality.

Amplification of flecainide-induced ventricular conduction slowing by exercise. A potentially significant clinical consequence of use-dependent sodium channel blockade.

Abstract: Proarrhythmic effects of flecainide acetate have been reported during exercise, but the mechanism for the arrhythmogenic interaction between flecainide and exercise is unknown. We hypothesized that the sinus tachycardia of exercise may enhance flecainide-induced conduction slowing by increasing use-dependent sodium channel blockade, thereby facilitating the occurrence of ventricular reentry. To evaluate the modulation of flecainide's effects by exercise, we studied 19 patients who were receiving therapeutic doses of flecainide for the treatment of cardiac arrhythmias. Sixteen patients underwent treadmill exercise testing by a modified Bruce protocol. During exercise, QRS duration increased progressively from 94 +/- 22 msec (mean +/- SD) at rest to 116 +/- 25 msec (p less than 0.001) at a mean heart rate increase of 84 +/- 32 beats/min. The patient with the greatest QRS increase developed a monomorphic ventricular tachycardia at peak exercise. At rest, the QRS duration after treatment with flecainide increased 12.1 +/- 10.0% compared with the pretreatment value, and with exercise, the QRS duration increased by a further 28.1 +/- 17.0% compared with the predrug value. We found that the best predictor of further exercise-induced QRS slowing was the change in QRS duration produced by flecainide at rest (r = 0.76, p = 0.001). In an age- and disease-matched control group, the QRS duration did not change during exercise that caused a similar heart rate increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrocardiographic left ventricular hypertrophy and effects of antihypertensive drug therapy in hypertensive participants in the Multiple Risk Factor Intervention Trial.

Abstract: Data are reported on electrocardiographic left ventricular hypertrophy (ECG LVH) among 8,012 men classified as hypertensive at baseline in the Multiple Risk Factor Intervention Trial. Compared with those allocated to the usual care (UC) control group, men allocated to the special intervention (SI) group experienced a mean reduction of 4 mm Hg in diastolic blood pressure and 7 mm Hg in systolic blood pressure, over 6 years of follow-up. There were 378 new cases of ECG LVH during follow-up; the incidence in the SI group was about 23% less than that in the UC group (4.2 vs 5.4% 2P 1 , R wave in aVL and S wave in V 3 ). In SI and UC groups combined, the presence of ECG LVH either at baseline or at follow-up was associated with several-fold increases in death from cardiovascular diseases in general, and death from coronary artery disease in particular. While mortality from cardiovascular disease, including coronary artery disease, was not significantly different in SI and UC groups during follow-up, moderate though still potentially worthwhile benefits that might accompany the observed ECG changes cannot be excluded.

Importance of the endpoint of noise reduction in analysis of the signal-averaged electrocardiogram.

Abstract: Reduction of random noise by signal averaging is required to uncover ventricular late potentials (LPs). Noise reduction is dependent upon the ambient noise before the study and the number of signal-averaged QRS complexes. Prior studies have used a fixed number of QRS complexes (e.g., 150 to 200) for performing signal-averaged electrocardiograms (SAECGs). Because of variable background noise levels, it was hypothesized that variable noise levels after processing could interfere with detection of LP. Accordingly, SAECGs were performed for each patient to 2 prespecified noise endpoints (expressed as the standard deviation/square root of number of beats): 1.0 microV, which has been used previously as a minimal residual noise level, and 0.3 microV, a low level that generally can be attained in less than 450 beats. Root mean square-voltage noises in the 40-Hz high pass filtered vector magnitude for these studies were 1.36 +/- 0.57 and 0.58 +/- 0.28 microV, respectively. The relative prevalence of LP was evaluated in 3 groups. Group I was comprised of 26 patients with sustained ventricular tachyarrhythmias, group II included 59 patients after myocardial infarction and group III had 14 normal volunteers. The prevalence of LP was greater in group I (69 vs 46%, p less than 0.001) and group II (34 vs 24%, p less than 0.01) with the 0.3-microV studies. In group III, the prevalence did not change (7 vs 7%, difference not significant). The greater detection of LP was due to improved resolution of the terminal low-amplitude QRS segment. Therefore, using the 0.3 microV level instead of 1.0 microV increased sensitivity of LP without loss of specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics, prognosis and treatment of the ventricular arrhythmias of right ventricular dysplasia.

Abstract: We studied 58 cases of arrhythmogenic right ventricular dysplasia (ARVD). Sustained monomorphic ventricular tachycardia (VT) was present in 50 patients, ventricular fibrillation (VF) in three (two also having VT), and non-sustained VT in the remaining seven. Different morphologies of VT were documented in 24 of the 50 patients with sustained VT. They had a left bundle branch block pattern in 96% of cases, without extreme deviation of the QRS axis, and a QRS relatively narrow and ample. These sustained VTs were triggered by provocative techniques. Holter recordings showed frequent ventricular extrasystoles in the great majority of cases. They were polymorphic in 78%, with runs of VT in 59% of patients. Spontaneous onset of VT occurred during exercise in 60% of cases, preceded by a sinus rate increase when recording was available. This is more frequent in angiographically localized forms of ARVD than in diffuse forms, and tends to disappear during follow-up. Only four cardiac deaths occurred after a follow-up of 8.8 +/- 7.2 years: three by acute heart failure, and only one by recurrent VF. Spontaneous disappearance of VT which became non-inducible was seen in four cases. Single antiarrhythmic drug therapy was judged satisfactory in 21 cases, and combined therapy in 19 other cases. Surgery of fulguration was performed in 17 cases, with 14 successes (nine of them with combined antiarrhythmic therapy). Despite a frequent lack of control of VT by antiarrhythmic drugs, the follow-up of ARVD seems good in patients with sustained VT. Some arguments favour the concept of a diffuse and progressive disease.

Invalidation of the resting electrocardiogram obtained via exercise electrode sites as a standard 12-lead recording.

Abstract: The difficulty in interpreting the standard 12-lead electrocardiogram (ECG) due to the interference from muscle potentials produced by arm and leg motion makes it unsuitable during the exercise treadmill test. Likewise, the exercise lead placement ECG cannot substitute for the standard ECG due to significant errors in the former's diagnostic interpretation. This study compares the ECGs recorded via standard and exercise sites regarding frontal and horizontal plane axes, diagnosis and location of myocardial infarction and estimation of infarct size using the complete 54-criteria and 32-point Selvester QRS scoring system. The altered limb lead locations on the exercise ECG caused the QRS vectors to artifactually appear to be directed more inferiorly, posteriorly and rightward, producing a marked rightward mean frontal plane axis shift of +48 ° (p

Clinical course of endomyocardial fibrosis.

Abstract: The survival pattern, morbidity, and clinical course of 145 patients with endomyocardial fibrosis who were followed up between November 1975 and June 1987 were studied. The diagnosis was confirmed in all cases by cardiac angiography, or echocardiography, or necropsy. Percentage survival at the end of one and 9.5 years was 76.11 and 26.35 respectively. History, physical examination, electrocardiography, and cardiac catheterisation were studied at the first presentation. The determinants of early mortality were studied by univariate Kaplan-Meier estimates compared by the log rank test and Cox proportional hazards multiple regression analysis. Significant univariate predictors of early mortality were QRS axis above +90 degrees, intraventricular conduction delay (QRS duration greater than 0.12 s), duration of symptoms before presentation, New York Heart Association functional classes III and IV, presence of embolic episodes, right atrial mean pressures greater than 20 mm Hg, right ventricular end diastolic pressure greater than 20 mm Hg, and aortic oxygen saturation less than 85%. The significant multivariate predictors of mortality were cyanosis, New York Heart Association functional class at first presentation, and right atrial mean pressure greater than 20 mm Hg. The bleak prognosis of endomyocardial fibrosis did not substantially improve despite advances in the medical management of congestive cardiac failure during the period of the study.

Signal-averaged electrocardiography in the time and frequency domains.

Abstract: The noninvasive signal-averaged electrocardiographic detection of late potentials correlates with the spontaneous occurrence of sustained ventricular tachycardia (VT). Frequency analysis of the electrocardiographic signal from the terminal QRS and ST segment also correlates with sustained VT. This study was designed to compare these 2 methods by analysis of signals recorded from the same hardware system. Signals were recorded from 234 patients with prior myocardial infarctions with a commercially available signal-averaging system. Patients were classified into 2 groups: group 1 consisted of 84 patients with VT and group 2 consisted of 150 patients without VT. In the frequency domain, magnitude and energy area ratios and peak ratios of the spectral plot from 20 to 50 Hz over 0 to 20 Hz were calculated for a 140-ms interval starting 60 ms after the beginning of the QRS. In the time domain, the duration of the filtered QRS was 121 ± 29 ms for group 1 and 110 ± 25 ms for group 2 (p < 0.002). The duration of the terminal QRS < 40 μV was 45 ± 21 ms in group 1 and 36 ± 18 ms in group 2 (p < 0.001). The root-mean-square amplitude of the terminal 40 ms of the QRS was 25 ± 24 μV in group 1 and 36 ± 33 μV in group 2 (p < 0.004). In the frequency domain, the log peak magnitude ratio was 3.14 ± 0.97 in group 1 and 2.66 ± 1.23 in group 2 (p < 0.004) and the log peak energy ratio was −0.32 ± 1.99 in group 1 and −1.38 ± 2.55 in group 2 (p < 0.002). The log magnitude energy ratio was 3.97 ± 0.72 in group 1 and 3.72 ± 0.95 in group 2 (p < 0.04) and the energy area ratio was 1.01 ± 1.4 in group 1 and 0.57 ± 1.91 in group 2 (p = 0.06). Stepwise multiple logistic regression showed that (in order of importance) the duration of the filtered QRS, magnitude peak ratio, energy area ratio, conduction defect score and myocardial infarction location were useful in identifying the patient with VT. Analysis of the signal-averaged electrocardiogram in both the time and frequency domains revealed that both methods have value for the identification of patients with sustained VT.

The electrocardiographic features of hypothermia.

Abstract: Hypothermia results in the development of several characteristic electrocardiographic changes. As the core body temperature decreases, several changes in cardiac rhythm occur. Prolongation of the PR, QRS, and QT intervals are also seen. Muscle tremor artifact may be present, even in the absence of clinical shivering. A characteristic secondary deflection on the terminal portion of the QRS complex (Osborn wave) is usually found. All of these features are reversible with rewarming.

Wall motion asynchrony prolongs time constant of left ventricular relaxation.

Abstract: To evaluate the effects of wall motion asynchrony on left ventricular (LV) relaxation, we performed atrioventricular sequential pacing with the second stimulation at six epicardial sites in open-chest anesthetized dogs. Myocardial segment lengths in the basal, mid, and apical LV free wall were measured by ultrasonic crystals. The standard deviation of interval from the onset of the QRS complex to that of elongation in each segment length was used as a quantitative index for asynchrony (asynchrony index, AI). The AI increased significantly in all sequential pacing modes compared with the control right atrial pacing. The time constant (T) of LV relaxation derived from exponential fit with zero-asymptote was prolonged significantly in all sequential pacing modes except for pacing at the LV base. In each dog there was a good correlation between changes in AI and T [r = 0.61 - 0.98 (mean = 0.84)]. Since the regional inactivation process of the myocardium is considered to be unchanged during these interventions, we concluded that asynchronous wall motion plays an important role in the impairment of LV relaxation.

Prognostic significance and evolution of late ventricular potentials in the first year after myocardial infarction: a prospective study.

Abstract: We performed a prospective study of the high-frequency components of the terminal portion of the QRS complex in 220 patients who survived acute myocardial infarction. Signal-averaged electrocardiograms (SA-ECGs) were performed before hospital discharge (16 +/- 6 days) and then serially at regular intervals over the following year. SA-ECGs were processed using a 40 Hz high-pass bidirectional filter. Duration of "filtered" QRS (D-normal value less than 120 ms), duration of the low-amplitude signals (D40 - n.v. less than 39 ms) and last 40 ms voltage of the QRS complex (V40 - n.v. greater than 20 microV) were measured. Late potentials (LPs) were defined as the presence of two or more abnormal values. In addition, 24-hour Holter monitoring was performed in 208 patients and left ventricular ejection fraction (LVEF) was determined by scintigraphy in 111. Sixty-two patients (group 1) had LPs, 158 had normal SA-ECGs (group 2). Spontaneous normalization of SA-ECGs occurred in 20% of patients after 6 months, although the mean values of D, D40 and V40 did not change significantly and the reproducibility was very good for all the indexes during all the follow-up controls. Three patients had sudden death and three presented again with spontaneous, sustained ventricular tachycardia. Five of 62 (8%) group 1 patients had an arrhythmic event compared with one of 158 patients (0.6%) in group 2. The sensitivity of SA-ECGs as a predictor of arrhythmic events was 83% with a specificity of 73%. Patients with subsequent arrhythmic events had longer filtered QRS (133 +/- 19 vs 104 +/- 16 ms; P less than 0.001), longer duration of the low-amplitude signals (54 +/- 15 vs 33 +/- 14 ms; P less than 0.01), and lower voltages in the last 40 ms of the filtered QRS (11 +/- 3 vs 36 +/- 25 microV; P less than 0.02) and, moreover, higher peak CK values and lower LVEF than those without such events. In conclusion, SA-ECGs provide important prognostic information in identifying patients at risk of arrhythmic events after myocardial infarction although dynamic changes of LPs are observed during the first year after myocardial infarction.

Programmed electrical stimulation at potential ventricular reentry circuit sites. Comparison of observations in humans with predictions from computer simulations.

Abstract: The purpose of this study was to define specific types of resetting responses to programmed electrical stimulation during human ventricular tachycardia and to use computer simulations of reentry circuits to assess the possible mechanisms and pacing site location relative to the reentry circuit for each type of response. The effects of scanning single stimuli at 35 left ventricular endocardial sites during sustained monomorphic ventricular tachycardia in 12 patients were studied. In considering alterations in QRS configuration and the delay between the stimulus and the advanced QRS, we identified three types of resetting responses to scanning stimuli consistent with stimulation at sites in or near the reentry circuit at 12 abnormal endocardial sites in eight patients. Type 1: all capturing stimuli were followed after a delay by early QRS complexes that had the same configuration as the tachycardia complexes. Type 2: late stimuli reset tachycardia as in type 1 but early stimuli reset the tachycardia after altering the QRS configuration. Type 3: late stimuli reset tachycardia as in type 1, but early stimuli advanced tachycardia with a short stimulus to QRS delay without altering the QRS configuration. In the simulations, premature depolarization of sites in the circuit produced orthodromic and antidromic wavefronts. The orthodromic wavefront propagated through the circuit and exited the circuit at the same site as did the previous tachycardia wavefronts and advanced the tachycardia without altering the configuration of the advanced QRS. The antidromic wavefront of relatively late stimuli was confined within or near the circuit by collision with the orthodromic wavefront of the preceding tachycardia beat and failed to alter ventricular activation distant from the circuit. Therefore, the QRS configuration after the stimulus was unchanged. A type 1 response occurred when all capturing stimuli produced this effect. However, with increasing stimulus prematurity, the antidromic wavefront propagated farther before colliding with an orthodromic wavefront, and under some conditions, it exited the circuit from a site other than the original circuit "exit," and altered the ventricular activation sequence distant from the circuit and, therefore, the QRS configuration, producing a type 2 pattern. The type 3 pattern occurred when the antidromic wavefront of early premature beats captured the original circuit exit. The effect of a stimulus was dependent on the stimulus prematurity, the relative conduction times from the stimulation site to the potential sites of "exit" from the circuit, and the timing of the excitable gap at the stimulation site.(ABSTRACT TRUNCATED AT 400 WORDS)

Signal-averaged electrocardiography of the terminal QRS in healthy young adults.

Abstract: DANFORD, D.A., et al.: Signal-Averaged Electrocardiography of the Terminal QRS in Healthy Young Adults Interpretation of signal-averaged electrocardiograms (SAECG) in the young could he of value in detecting those at risk for episodic ventricular tachycardia, but suffers from a lack of data in normal young people. The purpose of this study is to determine normal values for QRS duration and the duration and amplitude of terminal potentials on the SAECG in young adults. Thirty-two normal medical students were examined. With high pass futering at 25 Hz, normal total QRS duration (QRS) varied as a function of sex and hody size whereas low amplitude signal duration (LAS) did not. Ninety-five percent confidence limits are: QRS (male) 85–117 msec. QRS (female) 76–102 msec, and LAS 6–35 msec. Root mean square voltage of the terminal QRS showed a broad scatter, however none was < 20 microvolts. High pass filtering at 40 Hz did not change the QRS duration, but resulted in significantly longer LAS duration and diminished RMS voltage. Because of the longer QRS and shorter LAS previously reported in the presence of right bundle branch hlock, the normal values reported here should not be applied in the presence of intraventricular conduction delay following surgical repair of congenital heart disease. They will, however, provide a basis for interpretation of SAECG in young adults with normal QRS duration.

Potentiation by mild hypothermia of ventricular conduction disturbances and reentrant arrhythmias induced by bupivacaine in dogs

Abstract: High concentrations of bupivacaine and profound hypothermia individually cause intraventricular conduction disturbances and reentrant arrhythmias. The effects of the combination of relatively low concentrations of bupivacaine and mild hypothermia are unknown and are the subject of this study. Three groups (n = 10–12) of dogs anesthetized with thiopental-chloralose were treated as follows: group 1, bupivacaine + hypothermia; group 2, bupivacaine alone; group 3, hypothermia alone. Bupivacaine was administered as a 4 mg/kg iv bolus followed by an iv infusion of 0.1 mg.kg-1.min-1. Hypothermia, i.e., a 4° C reduction in core temperature, was produced by cooling the blood with an extracorporeal circuit. The peripheral ECG was recorded to determine the duration of QRS complexes and the QT interval. Conduction time and effective refractory period (ERP) of ventricular contractile tissue were measured with right ventricular endocavitary electrodes. Measurements were made with the heart paced at 180 beats/min and without pacing. In group 1 dogs, bupivacaine (plasma level, 2.8 ± 0.3 μg/ml) initially caused a prolongation of conduction time and QRS duration, which were further lengthened (approximately doubled) by a temperature decrease of 4° C from baseline. The QT interval and ERP also were increased but to a lesser degree. In dogs in which the effects were most pronounced, rhythm disorders, such as wave burst arrhythmias (most common), premature systoles, ventricular tachycardia, and even ventricular fibrillation, occurred either spontaneously or during pacing. Bupivacaine alone (group 2) increased QRS duration and conduction time significantly, whereas hypothermia alone (Group 3) did not cause changes in any conduction variables. In neither group were dysrhythmias observed. Thus, the combination of moderate bupivacaine concentrations and 4° C hypothermia cause significant cardiac arrhythmias. These results may explain the occasional cardiac disorders that have occurred during apparently uncomplicated regional anesthesia with this agent.

Predicting ventricular tachycardia cycle length after procainamide by assessing cycle length-dependent changes in paced QRS duration.

Abstract: To determine if paced cycle length-dependent changes in the QRS duration correlate with the change in ventricular tachycardia (VT) cycle length after procainamide, we measured the QRS duration during sinus rhythm and during right ventricular pacing before and after procainamide (mean concentration, 9.9 micrograms/ml) in 18 patients with morphologically identical VT induced at both study periods. Pacing was performed at 600 msec or the longest cycle length that allowed for uninterrupted capture and at a cycle length that was within 50 msec of the VT cycle length observed during the control study (mean, 313 +/- 51 msec). After procainamide, the VT cycle length increased from 285 +/- 62 to 368 +/- 70 msec (percent change, 30 +/- 13%). The QRS duration during sinus rhythm increased from 125 +/- 25 to 145 +/- 29 msec (percent change, 16%). The QRS duration at both paced cycle lengths was the same in the baseline state (191 +/- 26 msec). However, the change in QRS duration after procainamide at the shorter paced cycle length compared to a 39 +/- 13 msec (18%) increase at the longer paced cycle, p less than 0.001. There was a significant correlation between the percent change in QRS duration at the shorter paced cycle length and the percent change in VT cycle length (r = 0.84, p less than 0.001) with the relation expressed by the regression equation: percent change in VT cycle length = -2.8 + 1.16 x percent change in QRS duration.(ABSTRACT TRUNCATED AT 250 WORDS)