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Biomedical subjects

M Malik

Publications and source records attributed to M Malik.

At least 235 records · Page 13Linked to original sources
PubMed1991

Risk stratification for arrhythmic events in postinfarction patients based on heart rate variability, ambulatory electrocardiographic variables and the signal-averaged electrocardiogram.

The value of heart rate variability, ambulatory electrocardiographic (ECG) variables and the signal-averaged ECG in the prediction of arrhythmic events (sudden death or life-threatening ventricular arrhythmias) was assessed before hospital discharge in 416 consecutive survivors of acute myocardial infarction. During the follow-up period (range 1 to 1,112 days), there were 24 arrhythmic events and 47 deaths. The initial relation between several prognostic factors and arrhythmic events was explored with use of the Kaplan-Meier product limit estimates of survival function. Impaired heart rate variability less than 20 ms (p less than 0.0000), late potentials (p less than 0.0000), ventricular ectopic beat frequency (p less than 0.0000), repetitive ventricular forms (p less than 0.0000), left ventricular ejection fraction less than 40% (p less than 0.02) and Killip class (p less than 0.02) were identified as significant univariate predictors of arrhythmic events. When these variables were analyzed by using a stepwise Cox regression model, only impaired heart rate variability, followed by late potentials and repetitive ventricular forms remained independent predictors of arrhythmic events. The combination of impaired heart rate variability and late potentials had a sensitivity of 58%, a positive predictive accuracy of 33% and a relative risk of 18.5 for arrhythmic events and was superior to other combinations including those incorporating left ventricular function, exercise ECG, ventricular ectopic beat frequency and repetitive ventricular forms. These results suggest that a simple method of assessment based on heart rate variability and the signal-averaged ECG can select a small subgroup of survivors of myocardial infarction at high risk of future life-threatening arrhythmias and sudden death.

Arrhythmias, Cardiac↗
PubMed1991

Cardiac electrophysiological experiments in numero, Part II: Models of electrophysiological processes.

This article is the second part of a three article series reviewing computer simulation models of the heart, in particular of cardiac electrophysiology. The previous section of the review discussed the methodological principles of the construction and application of computer models. This article overviews the development of mathematical and computer modeling studies applied to cardiology. The models are classified according to the physiological processes that were simulated; this article distinguishes models oriented to cardiac mechanics, hemodynamics, and electrophysiology. The electrophysiology models are discussed in more detail and the review classifies them into four main categories: models of cellular processes, models of tissue behavior, models of the ventricular electric field, and models of macroconduction disturbances. In each category, the historical development of the models and their key achievements are described.

Animals↗
PubMed1991

Multifactorial prediction of arrhythmic events after myocardial infarction. Combination of heart rate variability and left ventricular ejection fraction with other variables.

Autonomic dysfunction has recently been shown to identify postinfarction patients at a high risk of arrhythmic events. Therefore, the predictive characteristics of heart rate variability and the left ventricular ejection fraction in combination with other prognostic variables--mean heart rate, late potentials, and ventricular ectopic beat frequency greater than 10/hour (VE10)--were examined in 417 postinfarction patients. The heart rate variability index was the most important factor for the stratification of patients at high risk of arrhythmic events after myocardial infarction and optimum stratification was based on the combination of the heart rate variability index with late potentials or with frequent ventricular ectopic beats.

Death, Sudden, Cardiac↗
PubMed1991

The effect of age on the electrophysiological and autonomic correlates of sudden death after acute myocardial infarction.

To examine the influence of age on the autonomic and electrophysiological correlates of sudden death after myocardial infarction, 223 patients aged less than 60 and 195 patients aged greater than or equal to 60 were followed up for a mena of 790 days. The patients had Holter monitoring and a signal-averaged ECG 5-11 days after infarction. A mean ventricular ectopic beat frequency greater than 10 beats/hour (VE10) was present in 17.0% of young versus 28.2% of old patients (P less than 0.01); a low heart heart variability index in 17.9% of young but in 32.3% of old patients (P less than 0.001) and late potentials in 17.5% but 32% of young and old patients, respectively (P less than 0.01). There was no difference in the incidence of sudden death between young and old patients (3.6% vs 3.1%). However, sudden death accounted for 50%, compared with 24% of all deaths in the young and old groups, respectively (P less than 0.01). Sudden death was more closely associated with low heart rate variability and VE10 in the young than in the older group. The predictive values of a heart rate variability index less than 20 units with VE10 in younger patients were a sensitivity of 50%, a positive predictive accuracy (PPA) of 33% and risk ratio (RR) of 18 (P less than 0.001); these values did not reach significance in older patients (16.7%, 4.3% and 1.4%, respectively.)(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMed1991

Cardiac electrophysiological experiments in numero, Part I: Concepts and strategies of mathematical and computer models.

This article is the first of three articles that review mathematical and computer models of the heart and describe their construction, development, research potential, and clinical utility. This article explains the methodological principles of mathematical and computer simulation of biomedical systems. The strategies of model construction, testing, and application are presented; the advantages and limitations of computer simulation studies are explained, and the basic value of computer simulation for cardiological research and practice is discussed.

Cardiac Pacing, Artificial↗
PubMed1991

Cardiac electrophysiological experiments in numero, Part III: Simulation of arrhythmias and pacing.

This paper is the third and final part of a series of articles reviewing mathematical and computer models of the electrophysiological processes. This section reviews the arrhythmia simulation and discusses models of arrhythmogenic processes, fibrillation and defibrillation, and of heart-pacemaker interaction. The models of arrhythmogenesis are classified into three main sections: models of reentry and vortex reentry, models of myocardial electrotonic interactions, and models of macroreentrant supraventricular tachycardias. This final part of the review discusses the future potential of mathematical and computer models of different cardiac processes.

Arrhythmias, Cardiac↗
PubMed1991

Prognostic value of reduced heart rate variability after myocardial infarction: clinical evaluation of a new analysis method.

The relation between heart rate variability, measured from standard 24 hour electrocardiogram recordings in patients convalescent after a myocardial infarction, and the occurrence of sudden death and spontaneous, symptomatic, sustained ventricular tachycardia were assessed in a consecutive series of 177 patients admitted with acute myocardial infarction and surviving to 7 days. In addition to the analysis of heart rate variability, the occurrence of non-sustained arrhythmias on 24 hour electrocardiographic monitoring, and the results of clinical assessment, signal averaged electrocardiography and ejection fraction were analysed and were related to outcome. During a median of 16 months of follow up (range 10-30 months) there were 17 end point events (11 (6.2%) sudden deaths) and six (3.4%) episodes of sustained ventricular tachycardia. An index of the width of the frequency distribution curve for the duration of individual RR intervals was used to measure heart rate variability. This mean (SD) index was significantly smaller in those with end point events (16.8 (8.0)) than in those without events (29.0 (11.2)). The relative risk of the occurrence of an end point event in those with a heart rate variability index less than 25 was 7.0. Multivariate analysis showed that of all the variables examined a reduced heart rate variability index was the single most powerful predictor of end point events. Measurement of heart rate variability by this simple, automated, operator-independent method provided useful information on the arrhythmic propensity in patients convalescent after myocardial infarction.

Aged↗
PubMed1991

Baroreflex sensitivity and electrophysiological correlates in patients after acute myocardial infarction.

BACKGROUND: Several studies have identified transient disturbances of autonomic function during the acute and recovery phases of myocardial infarction, and it has recently been suggested that survivors of acute myocardial infarction with depressed vagal tone may be at increased risk of sudden or arrhythmic death. METHODS AND RESULTS: To investigate this hypothesis, parasympathetic function was assessed by arterial baroreflex sensitivity (BRS) testing (using the phenylephrine method) and by heart rate variability (HRV) analysis from 24-hour Holter recording in 68 patients at day 7-10 after infarction. The relation between autonomic tone and markers of arrhythmic propensity, including programmed ventricular stimulation (PVS) and late potentials in addition to other clinical variables, was examined. BRS for the whole group was 7.0 +/- 4.7 msec/mm Hg and was inversely correlated with age (r = 0.53, p less than 0.001) but not with left ventricular ejection fraction (r = 0.035, p = NS). In those patients in whom sustained monomorphic ventricular tachycardia (SMVT) was induced, BRS was significantly reduced (p = 0.001) as was HRV (p = 0.007) and left ventricular ejection fraction (p = 0.022). The strongest association between any variable (including HRV, BRS, late potentials, left ventricular ejection fraction, exercise testing, Q waves, and infarct site) and the induction of sustained monomorphic ventricular tachycardia was depressed BRS with a relative risk of 36.28 (95% confidence interval, 5-266). CONCLUSIONS: This study confirms that depressed BRS identifies a subgroup at high risk for arrhythmic events after myocardial infarction and that programmed ventricular stimulation may be safely limited to this group without any loss of predictive accuracy.

Arrhythmias, Cardiac↗
PubMed1991

Ultrasound in choledocholithiasis.

Twenty-five patients of obstructive jaundice due to choledocholithiasis, were prospectively evaluated by ultrasonography and cholangiography. Ultrasound could demonstrate choledocholithiasis in 10 patients (40%). Choledocholithiasis in non-dilated ducts could be demonstrated only in one patient. All cases were diagnosed by cholangiography. Ultrasound, though an accepted modality of choice for diagnosing cholelithiasis, has a limited role in the diagnosis of choledocholithiasis. Ultrasonography (USG) is the screening modality of choice to distinguish obstructive from non-obstructive jaundice. It is highly accurate in the diagnosis of cholelithiasis but its role in detection of choledocholithiasis is less clear. Choledocholithiasis complicates 10% of all cases of cholecystitis and occurs in 2-4% of postcholecystectomy patients. The present study deals with the diagnostic value of USG in cases of choledocholithiasis subjected to USG prior to cholangiography and surgery.

Adult↗
PubMed1990

Circadian rhythm of heart rate variability after acute myocardial infarction and its influence on the prognostic value of heart rate variability.

This study examined heart rate (HR) variability in patients surviving acute myocardial infarction (AMI) to find the optimum time and duration of recording of the ambulatory electrocardiogram for the prediction of the risk of sudden cardiac death, or serious arrhythmic events, or both. Twenty patients (group I) who initially survived an AMI but later experienced serious events (death or symptomatic sustained ventricular tachycardia) during a 6-month follow-up were compared with 20 patients (group II) who remained free of complications for greater than 6 months after discharge. Groups I and II were matched with regard to age, gender, infarct site, ejection fraction, and beta-blocker treatment. HR variability was assessed in the 24-hour electrocardiograms recorded during the first 2 weeks after an AMI and in various portions of the complete 24-hour recording, with both the beginning and the length of the analyzed portion varied by 20 minutes (a total of 5,113 possibilities). The maximum reduction of HR variability in group I patients was systematically found when assessing HR variability in recordings starting approximately at 6 A.M. and lasting for approximately 8 hours. In the low-risk patient, the diurnal rhythm of HR variability is more marked than in the high-risk patient and the long-term components of HR variability due to the diurnal variation must be included in the measurement of HR variability when using it as a long-term predictor of risk from arrhythmic events after an AMI.

Circadian Rhythm↗
PubMed1990

Heart rate variability.

Reduced heart rate variability carries an adverse prognosis in patients who have survived an acute myocardial infarction. This article reviews the physiology, technical problems of assessment, and clinical relevance of heart rate variability. The sympathovagal influence and the clinical assessment of heart rate variability are discussed. Methods measuring heart rate variability are classified into four groups, and the advantages and disadvantages of each group are described. Concentration is on risk stratification of postmyocardial infarction patients. The evidence suggests that heart rate variability is the single most important predictor of those patients who are at high risk of sudden death or serious ventricular arrhythmias.

Arrhythmias, Cardiac↗
PubMed1990

Changes in the distribution of ventricular ectopic beats in long-term electrocardiograms.

Only simple methods have been used to assess antiarrhythmic and proarrhythmic effects when comparing baseline and on-therapy Holter recordings taken from the same patient. The paper suggests a new method for definition of these effects based upon comparisons of statistical distributions of ectopic beats. The method is based on multiple random sampling of the recordings and on application of the Smirnov test to compare the samplings. The results of these multiple comparisons are subsequently evaluated using the chi-squared test. An analysis is reported of Holter recordings made on seven patients suffering from ventricular arrhythmia but with anatomically normal heart. In each patient, one baseline recording and one recording on each of three different drugs were made. The results show that the definition of proarrhythmic effects can be partly addressed in a precise mathematical way. The method can also detect a significant change in the character of arrhythmia which can neither be classified as antiarrhythmic nor as proarrhythmic.

Arrhythmias, Cardiac↗
PubMed1990

Significance of long term components of heart rate variability for the further prognosis after acute myocardial infarction.

STUDY OBJECTIVE: The study examined heart rate variability to find out whether shorter ECG records can predict long term mortality following acute myocardial infarction as efficiently as 24 h recordings. DESIGN: Heart rate variability was assessed in 24 h electrocardiograms recorded during the first 2 weeks following acute myocardial infarction and in separate 1 h portions of the complete recording. The spectral analysis of complete 24 h records was performed and different short and long term components of heart rate variability were used to distinguish between patients with and without later complications. SUBJECTS: 20 patients who initially survived acute myocardial infarction but later experienced serious events (death or symptomatic sustained ventricular tachycardia) during a 6 month follow up (group I) were compared with 20 patients (group II) who remained free of complications for more than 6 months after discharge and who were matched with group I for age, gender, infarct site, ejection fraction, and beta blocker treatment. MEASUREMENTS AND MAIN RESULTS: The distinction based on components limited to changes of heart rate within periods less than or equal to 1 h was as significant (p less than 0.001, paired t test) as when using the components limited to changes of periods less than or equal to 10 h. However, heart rate variability of separate 1 h portions of the complete 24 h records differed between the groups significantly only for certain 1 h intervals of the day (the p values varied from 0.2 to 0.0005). CONCLUSIONS: Whilst the maximum value of short term heart rate variability is sufficient for stratification of the high risk post-myocardial infarction patients, an arbitrarily selected short term ECG recording is unlikely to register the maximum heart rate variability. It is concluded that the heart rate variability assessed from arbitrary 1 h electrocardiographic records is not as prognostically important as the variability estimated from 24 h recordings.

Animals↗
PubMed1990

Effect of activation sequence on ventricular refractoriness as determined by extrastimuli.

STUDY OBJECTIVE: The aim was to determine by the extrastimulus method the effect on the right and left ventricular effective refractory periods of moving the site of the pacing train away from the site of the extrastimulus. DESIGN: The ventricular effective refractory period was measured at the right and left ventricular apices using pacing trains at two heart rates, delivered to the ipsilateral ventricle, the contralateral ventricle, and the right atrium. SUBJECTS: Seven patients (six male), mean age 52 years (range 26-75 years), with either documented (six) or suspected (one) ventricular tachycardia were studied. Four had ischaemic heart disease and the remaining three had morphologically normal hearts. MEASUREMENTS AND MAIN RESULTS: The pacing train and extrastimulus delivered in the right ventricle produced the shortest effective refractory period at both heart rates: 220.8(SD 19) ms and 207.9(16) ms respectively. As the pacing train was moved to the right atrium, the effective refractory period lengthened to 246.4(22) ms and 219.3(20) ms at the two heart rates. There was further lengthening as the site of the pacing train was moved to the left ventricle, to 269.2(20) ms and 240.7(35) ms respectively. The same pattern was observed in the left ventricular effective refractory periods as the pacing train was moved from left ventricle to right ventricle and to right atrium. CONCLUSIONS: The ventricular effective refractory period lengthens as the site of the pacing train is moved away from the site of the extrastimulus. This may be explained by the effects of the distribution of the pacing energy within the myocardium and by intercellular electrotonic interactions. This has important clinical implications for the arrhythmogenic mechanisms of ventricular tachyarrhythmias.

Adult↗
PubMed1990

Termination of macro-reentrant tachycardia by a single extrastimulus delivered during the 'effective' refractory period: a computer modeled 'case report'.

A computer model of cardiac excitation sequences was used to reproduce atrioventricular (AV) reentrant tachycardia (AVRT) and its termination by a single 'on-circuit' extrastimulus. The model simulated activation waves revolving along a one-dimensional circular pathway, the portions of which represented the atrial, AV nodal, His-Purkinje, ventricular, and accessory pathway sections of the tachycardia circuit. The modeled pathway was composed of 289 elements. The model distinguished only the depolarised and resting states of constituent elements, but introduced differential refractoriness and conduction velocity for each element. These values approximated the natural situation established in a patient suffering from AVRT associated with the right bundle branch block. The results of the study suggest that: (A) the usual impression of a regular recovery wave and of a regular excitable window moving uniformly along the macro-reentrant circular path is incorrect; (B) during the tachycardia, islands of repolarized cells appear which are surrounded by tissue that is still refractory; (C) an extrastimulus which captures the island of early repolarized tissue may cause an excitation restricted to a small part of the myocardium but the local refractoriness following such an extrastimulus may be sufficient to terminate the tachycardia.

Atrioventricular Node↗