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.

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.

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.

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)

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.

Computer simulation of overdrive pacing during atrioventricular reentrant tachycardia

The study used a computer model of cardiac excitation to reproduce atrioventricular (AV) reentrant tachycardia and to evaluate the possibility of its termination by overdrive burst pacing. The model simulated activation waves radiating along a one-dimensional circular pathway, the portions of which represented the atrial, AV nodal, His-Purkinje, ventricular, and bypass parts of the tachycardia circuit. The pathway consisted of 289 elements. Only depolarised and resting states of elements were modelled. Differential refractoriness and conduction velocity for each element and the cycle length dependence of AV nodal decremental conduction were introduced. The experiments with the model examined the ability of overdrive 'on-circuit' pacing to terminate the tachycardia in order to determine the relevance of: (a) the coupling interval of the first beat in the burst; (b) the cycle length of the burst; (c) the number of stimuli in the burst; (d) His-Purkinje refractoriness; and (e) the degree of AV nodal decremental conduction. The results suggested that: (A) the general impression of a regular recovery wave and of a regular excitable window moving uniformly along the macro-reentrant circular path is incorrect; (B) the use of overdrive bursts of several stimuli with a short coupling interval has unpredictable effects; (C) the use of faster bursts with a cycle length only slightly shorter than the tachycardia cycle length is more safe (with respect to tachycardia reinitiation) and for certain combinations of the coupling interval and cycle length, prolonged bursts do not reinitiate the tachycardia; (D) the likelihood of tachycardia termination is increased by prolonging the refractoriness of the tachycardia circuit and by reducing AV nodal decremental conduction.

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.

Age-related normal values of signal-averaged electrocardiographic variables after acute myocardial infarction

The study examined standard time domain variables of a signal-averaged electrocardiogram (SAECG) in 328 survivors of acute myocardial infarction. The correlation of these variables with age and the influence of age on the prediction of postinfarction arrhythmic complication (sudden death [n = 12] or sustained ventricular tachycardia, or both [n = 14]) from the SAECG were investigated. Statistically highly significant correlations (p less than or equal to 0.00002) between age and SAECG variables were found. Compared with patients aged less than 60 years, the SAECG-based stratification of arrhythmic complications after myocardial infarction in patients greater than 60 years had lower sensitivity for the same values of specificity and lower specificity for the same values of sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the predictive characteristics of heart rate variability index and left ventricular ejection fraction for all-cause mortality, arrhythmic events and sudden death after acute myocardial infarction

Heart rate (HR) variability index and left ventricular ejection fraction (EF) were compared for the prediction of all-cause mortality, arrhythmic events and sudden death in 385 survivors of acute myocardial infarction. For arrhythmic events, where, for a sensitivity of 75%, HR variability index had a specificity of 76%, EF had a specificity of only 45%. An EF of less than or equal to 40% had a sensitivity of 42% and a specificity of 75% for arrhythmic events; for the same sensitivity an HR variability index of 20 U had a specificity of 92%. An EF less than or equal to 40% had a sensitivity of 40% and a specificity of 73% for sudden death; HR variability index had a specificity of 83% for the same sensitivity. For all cause mortality, where, for a sensitivity of 75%, HR variability index had a specificity of 52%, EF had a specificity of 40%. It is concluded that HR variability index appears a better predictor of important postinfarction arrhythmic complications than left ventricular EF, but both indexes perform equally well in predicting all-cause mortality.

Computer simulation of electronic interactions during excitation and repolarisation of myocardial tissue

A three-dimensional model of myocardial tissue has been developed which incorporates electrotonic interactions between neighbouring myocytes. The algorithms of the model are based on asynchronous planning of discrete events. Each cellular element is described in a logical way and traces a predefined action potential which is dynamically modified depending on the electrotonic interactions. The model has a low computational complexity and has been implemented on personal workstations even for experiments investigating arrhythmogenic processes and simulating several tens of cycles in blocks composed of several thousand elements. The paper describes the algorithmic implementation of the model and presents three series of experiments examining the dependence of the accuracy of the model on the size of the modelled tissue, changes of the shape of simulated T-waves due to electrotonic interactions and arrhythmogenic processes caused by lowering the threshold of the electric flow which excites individual cells.

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.

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.

Evaluation of receiver operator characteristics--optimum time of day for the assessment of heart rate variability after acute myocardial infarction

The study reports a mathematical method for deciding which clinical data are of importance for treatment decisions in a given clinical setting. The method comprises the following steps: (A) the receiver operator characteristic (ROC) functions of the compared sets of data are computed; (B) the design and aim of the clinical study is expressed as an integral measure on the space of sensitivity values (this reflects the preference of low or high sensitivity values dependent on the clinical targets); (C) the sets of data being compared are characterised by the non-linear integrals of their ROC functions. The approach has been used to compare mean heart rate (HR) and heart rate variability (HRV) data calculated in 5113 different portions of 24-h ECG recordings and assessed in 365 patients surviving acute myocardial infarction, in order to evaluate the utility of Holter recording of varying lengths and starting times for the prediction of sudden cardiac death and/or serious arrhythmic events. The results of the study show that this approach is capable of evaluating and comparing the sets of medical data used for identification of patients who are at increased risk. The experimental part of the study showed that the optimum recording interval for the assessment of HR and HRV data in patients who survived acute myocardial infarction depends on the aim of the identification of increased risk patients. The optimum interval of recording is different for an identification which requires a low number of false negative cases and permits a higher number of false positive cases, than for the situation where a low number of false positive cases are required and a higher number of false negative cases are permissible.

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.

Variability of the intracardiac electrogram: effect on specificity of tachycardia detection

Correlation has been described as a method of high sensitivity to distinguish ventricular arrhythmias from sinus rhythm but the specificity of this algorithm has not been assessed. Ten patients with a history of chronic ventricular tachycardia were studied. The ventricular endocardial electrogram was recorded during sinus rhythm at rest immediately following exercise and during their clinical ventricular tachycardia. Each complex recorded during these sample periods was correlated with a template constructed during sinus rhythm at rest. Although for each patient the range of correlation values obtained at rest were clearly separated from those obtained during ventricular tachycardia, in 69% of cases there was overlap of the range in sinus tachycardia and ventricular tachycardia.

Temporal electrogram analysis: algorithm development

The automatic discrimination of physiological from pathological tachycardias by rate criteria alone lacks adequate specificity. Tachycardia detection algorithms based upon morphological analysis of the endocardial electrogram have been attributed high specificity although their specificity has not been proven. A previous study had shown temporal electrogram analysis (TEA) to be an algorithm of high sensitivity in the detection of ventricular arrhythmias despite low computational demands. In this study, the specificity and potential for automatic implementation have been assessed. Manual adjustment of thresholds for individual patients gave a maximum potential sensitivity of 97% (26/27 arrhythmias correctly recognized as non-sinus). The use of automatic setting of thresholds reduced sensitivity to 81%. The specificity of the algorithm, as assessed by exercise testing, was only 60%.

Hemodynamic status during famotidine infusion

Histamine H2 antagonists, which reduce gastric acid secretion, are often used in the intensive care setting for the prophylaxis of stress ulcers. This double-blind, placebo-controlled study evaluated hemodynamic parameters in 11 stable, critically ill patients receiving famotidine. Repeated-measures ANOVA demonstrated that famotidine had no significant effect on baseline hemodynamic measurements and that there was no significant difference in hemodynamic values following the famotidine infusion as compared with NaCl 0.9% placebo (p greater than 0.05).

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.

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.

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.

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.

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.

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.

Heart rate variability in relation to prognosis after myocardial infarction: selection of optimal processing techniques

Automatic analysis of heart rate variability from Holter recordings may be invalidated by beat recognition errors and recording artefact, necessitating filtering and editing of the computer-recognized RR interval sequence. Two new methods for heart rate variability analysis have been developed, based on an estimation of the width of the main peak of the frequency distribution curve of the computer-recognized normal-to-normal beat sequence. These methods are independent of a low level of recognition error and artefact, thus removing the need for operator-dependent, time-consuming editing. The value of the new methods (heart variability indices 1 and 2) in identifying patients with serious events (death and symptomatic, sustained documented ventricular tachycardia) during a 6-month follow-up after acute myocardial infarction was assessed in a case-control study comparing 20 patients who had experienced such events (Group I) with 20 patients who, following admission with acute myocardial infarction, had remained free of complications for greater than 6 months after discharge (Group II). Group II was selected to match Group I with regard to age, sex, infarct site, ejection fraction, and beta-blocker treatment. Analysis of the unfiltered computer-recognized normal-to-normal interval sequence showed that heart rate variability indices 1 and 2 were significantly lower (P less than 0.005, P less than 0.002) in those who had experienced events compared with those free from complications. Two other methods of expressing heart rate variability, including the standard deviation method, in combination with four different data-filtering techniques, gave less significant distinction between those with and without events during follow-up. It is concluded that using the methods described, reduced heart rate variability in patients at risk from death or sustained ventricular tachycardia after acute myocardial infarction can be detected automatically from unfiltered Holter tape recordings even in the presence of a low level of beat recognition error and recording artefact.

Mechanism of Wenckebach periods: hypothesis based on computer modeling experiments

Wenckebach periodicity is characterized by progressive lengthening of conduction intervals and by progressive shortening of the intervals between conducted excitations. Although different hypotheses have been suggested to explain the mechanisms of Wenckebach periods, no serious proposition explaining both components of the phenomenon has yet been reported. A computer model simulating detailed mechanisms of excitation transmission and electrotonic interactions between neighboring cardiac cells has been employed to investigate the conduction properties of a one-dimensional cable composed of simulated cells. When introducing gradual prolongation of the recovery phase for the elements in the center of the cable and when incorporating physiologically realistic shapes of premature action potential curves into the simulation experiments, the model was able to reproduce all aspects of Wenckebach periodicity. Systematic evaluation with simulation experiments showed that a shorter duration of premature action potentials (i.e., of action potentials resulting from excitation of a cell before it has been fully repolarized) produced shortening of intervals between conducted excitations during a Wenckebach period.

Long-term spectral analysis of heart rate variability--an algorithm based on segmental frequency distributions of beat-to-beat intervals

Reduced heart rate variability has been reported as a predictor of long-term mortality in recent myocardial infarction patients. However, it has not been systematically investigated whether the reduction in heart rate variability in those post myocardial infarction patients who later suffer death or severe arrhythmias is caused by a reduction of short-term variability of heart rate (such as respiratory arrhythmia) or whether the differences in long term variability (such as diurnal rhythm) are involved. In order to perform such an evaluation, a new algorithm has been developed which permits different wavelength components (including the long-term components due to diurnal rhythm) of heart rate variability to be approximated. In general, the method uses segmental frequency distributions of durations of intervals between successive normal cardiac beats. To assess the spectral components of heart rate variability, a scale of wavelength limits is used and for each limit of this scale, the algorithm excludes the rate changes of wavelength longer than the given bound. The method was applied to the analysis of electrocardiograms recorded in 14 post myocardial infarction patients who later suffered death or ventricular tachycardia, and in 14 other randomly selected patients with an uncomplicated course following acute myocardial infarction. The rate variability spectra obtained for both groups of patients were compared statistically and the results showed that the groups of positive and negative cases were most significantly distinguished when including both short- and long-term components of heart rate variability. Separate evaluation of different wavelength components showed that the very long-term components of heart rate variability were more powerful in distinguishing between positive and negative cases than the short term components.

Computer model of cardiac repolarization processes and of the recovery sequence

A computer model simulating both excitation and recovery processes within a block of heart muscle tissue has been developed and implemented on different IBM PC AT compatible computers. The model incorporates blocks of tissue consisting of several thousand elements and introduces phenomena which are completely or partly omitted in other existing cardiac electrophysiology models. These phenomena include the electric anisotropy of the tissue, different durations of repolarization in different layers of tissue, and the different shapes of action potential which correspond to cells excited when not fully recovered. Implementation of the model on small personal computers requires the use of a special data structure management and an effective algorithmic background. The program of the model is written in PASCAL and uses dynamically allocated data structures and the asynchronous simulation technique of event planing. These techniques are described in detail. The model has been used in various experiments. Results of simulation studies are presented in the form of modeled three-lead electrocardiographic records. The experimental series which are described include basic patterns of regular activation sequences, modeling of premature beats, simulation of effects due to fast pacing, models of ischemia and infarction, simulation of reentry mechanisms with a special reference to the initiation of ventricular fibrillation, and models of late potentials. The future development of more realistic models of the cardiac recovery process is also discussed.

Computer simulation of myocardial fibrillation using a one dimensional model of excitation and recovery processes

A computer model of cardiac excitation sequences and recovery processes has been employed to reproduce chaotic behaviour of the simulated tissue and to investigate how different variables of the model influence the degree of disorganisation in modelled episodes. The model emphasises the electrophysiological features of excitation transmission and repolarisation processes and introduces phenomena which are omitted or seriously simplified in most of the existing models of cardiac tissue electrophysiology. These phenomena include abnormal shapes of action potential curves corresponding to premature excitation of cells which have not fully recovered, excitation transmission based on transmembrane voltages, and the electrotonic interactions between neighbouring cells during their repolarisation phases. The model has been used to examine a one dimensional cable of simulated cells in which a central area with shortened refractoriness was used to enable the "reflection" processes to initiate conduction and repolarisation disturbances. In some cases, the chaotic nature of the reproduced episodes resembled fibrillation myocardium. The degree of the simulated chaos depended on different variables of the model. This study included a systematic evaluation of the influence of the shapes of action potential curves, of the threshold of transmembrane voltages initiating an excitation wave, of the degree of the electrotonic interactions of neighbouring cells, and of various combinations of these variables. The results showed that in this model, the maximum disorganisation was achieved when combining the negative influences of all variables, and that changing the shape of the action potential curves prevented the modelled chaos more fully than changes in the other variables.

Compensating conduction times as a mechanism of alternating reentry tachycardia: computer modelling experiments

This article concerns a currently reported hypothesis explaining the alternations of conduction time during intra atrioventricular (AV) nodal reentrant tachycardia (AVNRT). This hypothesis supposes simple mutual influence of sequences of intranodal conduction during reentry tachycardia that use the same circuit in different tachycardia cycles. It has been suggested that delayed conduction prolongs the recovery time of the circuit, thus making it possible to transmit the excitation wave with a faster speed in the next tachycardia loop; this less delayed loop shortens the recovery interval of the pathway and results in subsequently delayed conduction of the following tachycardia cycle. A computer model simulating intra-AV nodal reentrant conduction was used to imitate and prove this hypotheses. The results are negative, showing that the suggested mechanism is too simplistic and that either the pathophysiologic background of the AVNRT cycle length oscillations must be different or additional phenomena must be considered to improve the hypothesis.

Diagnosis of paced electrocardiograms by inverse computer modeling of pacemaker actions

The explanation and interpretation of ECGs recorded from paced patients may be very difficult because of pacemaker specification, programmed parameters, and idiosyncrasy and also because of hidden interactions between the device and its environment. Moreover, the recent development of very sophisticated pacemakers makes the heart-pacemaker interaction (HPI) difficult to understand even when the recorded ECG is accompanied by an event marker. A computer system providing an automatic analysis of the HPI based on ECG data has been developed and implemented on an IBM PC AT computer. The system evaluates all possible combinations of HPI events and establishes whether they correspond to the pacemaker. The system inputs interactively the description of the pacemaker mode and programming, and the description of the analyzed ECG in the form of timing of "definite" and "possible" sensing events and generator pulses. The computer performs the analysis and establishes whether the device operates correctly within its permitted error. The system has been tested by evaluating different examples of paced ECGs. The presented examples confirm the ability of the system and show the potential for its future clinical use. Future development of the system and the provision of computer support for the understanding of the HPI of complex DDDR pacemakers are discussed.

Computer modeling of cardiac rhythm disturbances and heart-pacemaker interaction

Different computer models have been developed in order to study various aspects of cardiac electrophysiological processes. These models can be classified according to many parameters and also in respect to their application areas. One group of the models is devoted to computer simulation of cardiac rhythm disturbances and to reproduction of interactions between the heart and an artificial pulse generator. This report overviews the recent models of arrhythmias and heart-pacemaker interaction. Special attention is paid to (1) functioning of fundamental model elements, (2) structure of the heart model, (3) pacemaker models, and (4) forms of results offered by simulation experiments. Existing models are classified and compared. To illustrate the medical capability of rhythm and pacemaker models, three computational experiments are presented with model atrioventricular reentry tachycardia and reentry tachycardia mediated by a DDD pacemaker. Future development and utilization of arrhythmia and pacemaker models are briefly discussed.

Modification of the DDD pacing mode to prevent junctional reentry tachycardia: computer modelling experiments

This paper examines the possibility of using short atrioventricular (AV) delay dual chamber pacing to prevent junctional reentry tachycardia mediated by an accessory pathway or by an intra-AV nodal circuit. For this purpose, a clinically realistic computer simulation model of cardiac rhythm and heart-pacemaker interactions has been used. The computational experiments compared the actions of two pacemaker models: (A) a clinically realistic DDD mode operating with quasi-Wenckebach prolongation of the AV delay; and (B) a new modification of the DDD mode introducing independent counters for the atrial and ventricular refractory periods of the heart, and the possibility of instantaneous or shortly delayed atrial pacing triggered by a sensed or paced ventricular event. The pathological phenomena modelled in the experiments simulate different possibilities of tachycardia initiation. These disorders include: (1) single atrial premature beats (APBs), (2) salvos of APBs, (3) closely coupled pairs of APBs, (4) ventricular premature beats initiating an antidromic reentry tachycardia, and (5) ventricular ectopic beats initiating an AV nodal reentry tachycardia. The computational results prove that many possible mechanisms of initiation of junctional reentry tachycardia are beyond the prophylactic capabilities of current sophisticated DDD pacemakers (A). The results also show that the suggested pacing mode (B) improves anti-tachycardia prophylaxis even when responding to complex pathological episodes of the natural cardiac activity. Future development of the suggested mode (B) is discussed.

Theoretical evaluation of the Rosenblueth hypothesis

Rosenblueth's hypothesis states that atrioventricular (AV) nodal conduction delay and Wenckebach periodicity of AV transmission are not due to overall decremental conduction within the AV node but are due to a single step delay which is caused by a special element or layer of the AV nodal tissue. This paper discusses some theoretical considerations which allow detailed evaluation of the original hypothesis. Two artificial conduction structures which incorporate the Rosenblueth phenomenon are presented and tested by theoretical experiments that consider the potential of these structures to produce (a) basic pattern of Wenckebach periods, (b) decremental shortening of RR intervals during Wenckebach periods. These experiments are also employed to test whether or not the Rosenblueth concept can be used to explain (c) appropriate dependence of AV conduction changes on the prematurity of atrial depolarizations, and of (d) alternating cycle lengths such as may be seen with atrioventricular reentrant tachycardia. The results of the theoretical considerations show that the original concept of the Rosenblueth hypothesis is sufficient to explain (a) but it cannot be used for realization of (b), (c) and (d). A modification of the original concept complying with both (a) and (b) is proposed. This modified structure can also reproduce (c), but not simultaneously with (b). The experiments show that anisotropy of intra AV nodal conduction may create an electrophysiological mechanism of single-step delay. Different anisotropic conduction structures have to be considered to reproduce phenomenon (d).

The pacemaker inverse problem--computer diagnosis of paced electrocardiograms

Like the electrocardiographic inverse problem, the object of which is the algorithmical analysis and diagnosis of the electrocardiogram (ECG), the pacemaker inverse problem is the analysis of the ECG in order to establish details of heart-pacemaker interaction (HPI) with special reference to the diagnosis of pacemaker failure. The solution to this problem is of practical importance, since it is often difficult to evaluate such records clinically. The ECG patterns of natural cardiac depolarizations and paced events may not be distinguishable and many combinations of potential pacemaker response must be taken into account. A computer system providing automatic analysis of the HPI, based on ECG data, has been developed and implemented on an IBM PC AT computer. The system uses a complex algorithm which enables the evaluation of all possible combinations of HPI events, and establishes for each of these combinations its correspondence to the specified pacemaker algorithm. The system is written in Pascal and its source text has approximately 11,000 lines. The first version of the system has been tested with algorithms of the dual chamber cardiac pulse generator AUTIMA-II (Telectronics). The interactive input of the system allows the pacemaker algorithm and the ECG, in the form of timing of "definite" and "possible" sensing events and pacemaker pulses, to be specified. The analysis establishes whether the device operates correctly within permitted error. Should one or more correct explanations of the specified case be found, the system prints simplified patterns of the ECG accompanied by a simulated marker channel tracing the possible HPIs.

Complexity of AV nodal function: complex nodal structure or complex behavior of nodal elements?

This paper discusses the hypothesis that the complex behavior of the atrioventricular (AV) node, with particular reference to Wenckebach periods, may be explained by the complexity of nodal structure and that complicated function of individual AV nodal fibers need not necessarily be considered. To prove this hypothesis, a computer model of cardiac excitation has been employed to simulate AV nodal function for two different anisotropic AV nodal model images. The experimental computer results show that the pattern of Wenckebach periods, and decremental and concealed AV nodal conduction can be explained without considering decremental conduction properties of individual AV nodal cells.

Hysteresis of the ventricular paced QT interval in response to abrupt changes in pacing rate

The rate of QT adaptation to abrupt changes in pacing rate was studied in seven patients with newly diagnosed complete heart block with a ventricular escape rate of less than 40 beats.min-1. Their median age was 70 (range 36-84) years, and none was taking any cardioactive medication known to affect the QT interval. From a baseline pacing rate of 50 or 110 beats.min-1 the ventricular rate was increased or decreased to a new level. The time taken for the ventricular paced QT interval to complete 90% of the change secondary to the change in rate was found to be 136(16) s (mean(SEM] when the rate was increasing and 189(25) s when the rate was decreasing (p less than 0.01). This time interval was independent of the magnitude of the rate change and the baseline heart rate from which the change occurred. Furthermore, the time course of QT adaptation was found to be exponential and was characterised by a time constant of 49.1(2.2) s when the rate was increasing and 60.4(2.0) s when the rate was decreasing (p less than 0.01). It is concluded that QT measurements in response to a change in pacing rate should take into account the time dependent nature of QT changes.

Computer modeling of DDD pacemakers for use in prophylaxis of junctional reentry tachycardia

A computer simulation model of cardiac rhythm and heart-pacemaker interactions has been used to examine the response of short AV delay DDD pacing to an atrial premature depolarization in patients with a potential junctional reentry circuit. Special attention was given to the possibility of using the pacemaker to prevent atrioventricular reentry tachycardia mediated by a bidirectional accessory pathway. The computational experiments examined: (a) the differences between pacing modes with constant and quasi-Wenckebach prolongation of AV delay; (b) the effects of imposing the restrictions of a pacemaker atrial refractory period and upper rate limit; and (c) the effects of varying the ventricular refractory period so that it was either the same or longer than that of the atria. The computational results prove that some possible modes of initiation of junctional reentry tachycardia are beyond the prophylactic capabilities of current DDD pacemakers. Future possibilities involving the use of computer modeling to develop more sophisticated pacemaker modes are briefly discussed.

A one-dimensional model of atrioventricular nodal conduction

A computer model of the cardiac conduction process and an integral equation mathematical model have been employed to study the functions of the atrioventricular (AV) node. Special attention has been paid to the dependence of conduction delay on cycle length. The models have been used to evaluate the question as to whether simple cycle length dependences of AV nodal conduction could cause the oscillations of cycle length which are sometimes observed in AV reentry tachycardia. The models consider the AV node as a linear structure in which the depolarisation wavefront radiates in one dimension only. A model representing the node by two parallel linear structures has also been examined. Some results obtained do not conform with those of natural circumstance and clinical experiments, suggesting that the above hypotheses of AV nodal function are too simple and restricted to explain the natural processes.

A study on a vibratory model of a human body

This paper is concerned with the modeling of the human body as a spring mass system. Based on certain assumptions, an analysis for evaluating the mass and stiffness values of the model is developed. As an illustration of the modeling procedure, a 15-degree-of-freedom model of a male body is considered. The computed natural frequencies of the model are found to be within the range of available experimental values.

Umbilical artery catheterization: a potential cause of refractory hypoglycemia

Two infants with persistent, symptomatic hypoglycemia are reported. Both were thought to have iatrogenic hyperinsulinism due to a malpositioned umbilical artery catheter. Repositioning of the catheter to avoid direct infusion into the arterial blood supply to the pancreas resulted in prompt cessation of hypoglycemia.