Forecasting of life threatening arrhythmias using the compression entropy of heart rate

OBJECTIVES

Ventricular tachycardia (VT) provoking sudden cardiac death (SCD) are a major cause of mortality in the developed countries. The most efficient therapy for SCD prevention are implantable cardioverter defibrillators (ICD). In this study heart rate variability (HRV) measures were analyzed for short-term forecasting of VT in order to improve VT sensing and to enable a patient warning of forth-coming shocks.

METHODS

The last 1000 normal beat-to-beat intervals before 50 VT episodes stored by the ICD were analyzed and compared to individually acquired control time series (CON). HRV analysis was performed with standard parameters of time and frequency domain as suggested by the HRV Task Force and furthermore with a newly developed and optimized nonlinear parameter that assesses the compression entropy of heart rate (Hc).

RESULTS

Except of meanNN (p = 0.02) we found no significant differences in standard HRV parameters. In contrast, Hc revealed highly significant (p = 0.007) alterations in VT compared with CON suggesting a decreased complexity before the onset of VT.

CONCLUSION

Compression entropy might be a suitable parameter for short-term forecasting of life-threatening tachycardia in ICD.

[Short-term heart rate turbulence analysis versus variability and baroreceptor sensitivity in patients with dilated cardiomyopathy]

New methods for the analysis of arrhythmias and their hemodynamic consequences have been applied in risk stratification, particularly to patients after myocardial infarction. This study investigates the suitability of shortterm heart rate turbulence (HRT) in comparison to heart rate and blood pressure variability as well as baroreceptor sensitivity analyses to characterize the regulatory differences in patients with dilated cardiomyopathy (DCM) and healthy controls. In this study, 30 minutes data from noninvasive continuous blood pressure and ECG of 37 DCM patients and 167 controls under standard resting conditions were analyzed. The results showed highly significant differences between DCM patients and controls in heart rate and blood pressure variability as well as baroreceptor sensitivity parameters. Applying a combined heart rate-blood pressure trigger, in 24.3% (9) of the DCM patients and in 11.3% (19) of the controls ventricular premature beats were detected. This fact demonstrates the constricted applicability of short-term HRT analyses. However, the HRT parameters showed significant differences in this subgroup with ventricular premature beats (Turbulence Onset: DCM: 1.80+/-2.72, CONTROLS: -4.34+/-3.10, p<0.001; Turbulence Slope: DCM: 6.75+/-5.50, CONTROLS: 21.30+/-17.72, p = 0.021). Considering all (including HRT) parameters in the subgroup with ventricular beats, a discrimination rate between DCM patients and controls of 88.0% was obtained (max. 6 parameters). In comparison, in the total group this rate was 86.3% (without HRT parameters). The comparable classification rates and the high correlations between heart rate turbulence and variability and baroreflex parameters point to a more universal applicability of the last-mentioned methods.

Model based analysis of the heart rate and blood pressure relationship

The main intention of this contribution is to measure the coupling between bivariate time series using the dual sequence method to estimate the baroreflex as well as a nonlinear regression approach, namely the maximal correlation method, to get a better understanding of the underlying processes. The data we analyse are heart rate and blood pressure variability time series from 27 patients with dilated cardiomyopathy as well as from a control group of 27 age- and sex-matched healthy subjects. The results strongly indicate and confirm the mechanisms of respiratory sinus arrhythmia in heart rate. Revealing the coupling direction and the strength of coupling between heart rate and blood pressure via optimal transformations in addition to the baroreflex estimation seems to be a very promising approach.

[Analysis of cardiovascular regulation after heart operation]

Baroreflex sensitivity, heart rate and blood pressure variability have been proven to predict fatal outcome in patients after acute myocardial infarction. This study aims at investigating the time dependent alterations in cardiovascular control to find new predictive parameters for arrhythmic events after surgery. 25 male patients with coronary heart disease following an aortocoronary bypass surgery were examined. The results show significant alterations in sympathetic and vagal mediated regulation. The extubation after 6 hours seems to influence primarily the sympathetic activation. Obviously, there is a vagal suppression 20 h after surgery, while the sympathetic tonus works in a normal range. This unbalanced interaction of the autonomous system seems to be a reason for the high incidence of atrial tachycardias in the early period after cardiac surgery.

Potential of feature selection methods in heart rate variability analysis for the classification of different cardiovascular diseases

In this study heart rate variability (HRV) analysis was applied to characterize patients suffering from coronary heart disease (CHD), dilated cardiomyopathy (DCM) and patients who had survived an acute myocardial infarction (MI). On the basis of several HRV parameters, an optimal discrimination between the different kinds of cardiovascular diseases and between the diseases and healthy controls (HC) was derived by feature selection and linear classification. For each task a small favourable subset of a set of 33 potentially interesting HRV measures was selected with the intention of improving the diagnostic value and facilitating the physiological interpretation of HRV analysis. Time- and frequency-domain parameters as well as parameters from non-linear dynamics were included in the analysis. With the expectation that different diseases are characterized by different phenomena, feature selection was applied for each task separately. Using the features optimal for one task to another task can reveal a loss in performance, but it turned out that one specific parameter set (set1: normalized low frequency LF/P and a non-linear variability measure WPSUM13) was applicable for all tasks, where diseased and healthy subjects have to be distinguished, without significant reduction in performance. This set seems to be a general marker for pathologic changes in HRV and might be used for early detection of heart diseases. The classification between different heart diseases requires another parameter set (set2: meanNN and sdaNN, reflecting the steady state behaviour of the heart rate and long and short term SEAR describing the spectral composition). However, the use of set1 for the separation of different kinds of diseases, where set2 is appropriate, led to significant reduction in performance and vice versa. This observation may be important for future developments of HRV measures especially suitable for the assessment of disease severity.

Recurrence-plot-based measures of complexity and their application to heart-rate-variability data

The knowledge of transitions between regular, laminar or chaotic behaviors is essential to understand the underlying mechanisms behind complex systems. While several linear approaches are often insufficient to describe such processes, there are several nonlinear methods that, however, require rather long time observations. To overcome these difficulties, we propose measures of complexity based on vertical structures in recurrence plots and apply them to the logistic map as well as to heart-rate-variability data. For the logistic map these measures enable us not only to detect transitions between chaotic and periodic states, but also to identify laminar states, i.e., chaos-chaos transitions. The traditional recurrence quantification analysis fails to detect the latter transitions. Applying our measures to the heart-rate-variability data, we are able to detect and quantify the laminar phases before a life-threatening cardiac arrhythmia occurs thereby facilitating a prediction of such an event. Our findings could be of importance for the therapy of malignant cardiac arrhythmias.

Heart rate variability before the onset of ventricular tachycardia: differences between slow and fast arrhythmias

BACKGROUND

We tested whether or not heart rate variability (HRV) changes can serve as early signs of ventricular tachycardia (VT) and predict slow and fast VT in patients with an implantable cardioverter defibrillator (ICD).

METHODS AND RESULTS

We studied the ICD stored 1000 beat-to-beat intervals before the onset of VT (131 episodes) and during a control time without VT (74 series) in 63 chronic heart failure ICD patients. Standard HRV parameters as well as two nonlinear parameters, namely 'Polvar10' from symbolic dynamics and the finite time growth rates 'Fitgra9' were calculated. Comparing the control and the VT series, no linear HRV parameter showed a significant difference. The nonlinear parameters detected a significant increase in short phases with low variability before the onset of VT (for time series with less than 10% ectopy, P<0.05). Subdividing VT into fast (cycle length <or=270 ms) and slow (>270 ms) events, we found that the onset of slow VT was characterized by a significant increase in heart rate, whereas fast VT was triggered during decreased heart rates, compared to the control series.

CONCLUSIONS

Our data may permit the development of automatic ICD algorithms based on nonlinear dynamic HRV parameters to predict VT before it starts. Furthermore, they may facilitate improved prevention strategies.

Advanced analysis of spontaneous baroreflex sensitivity, blood pressure and heart rate variability in patients with dilated cardiomyopathy

Baroreflex sensitivity (BRS) is an important parameter in the classification of patients with reduced left ventricular function. This study aimed at investigating BRS in patients with dilated cardiomyopathy (DCM) and in healthy subjects (controls), as well as comparing the values of BRS parameters with parameters of heart rate variability (HRV) and blood pressure variability (BPV). ECG, continuous blood pressure and respiration curves were recorded for 30 min in 27 DCM patients and 27 control subjects. The Dual Sequence Method (DSM) includes the analysis of spontaneous fluctuations in systolic blood pressure and the corresponding beat-to-beat intervals of heart rate to estimate bradycardic, opposite tachycardic and delayed baroreflex fluctuations. The number of systolic blood pressure/beat-to-beat interval fluctuations in DCM patients was reduced in comparison with controls (DCM patients: male, 154.4+/-93.9 ms/mmHg; female, 93.7+/-40.5 ms/mmHg; controls: male, 245.5+/-112.9 ms/mmHg; female, 150.6+/-55.8 ms/mmHg, P<0.05). The average slope in DCM patients was lower than in controls (DCM, 5.3+/-1.9 ms/mmHg; controls, 8.0+/-5.4 ms/mmHg; P<0.05). Discriminant function analysis showed that, in the synchronous range of the standard sequence method, the DCM and control groups could be discriminated to only 76% accuracy, whereas the DSM gave an improved accuracy of 84%. The combination of six parameters of HRV, BPV and DSM gives an accuracy of classification of 96%, whereas six parameters of HRV and BPV could separate the two groups to only 88% accuracy. Thus the DSM leads to an improved characterization of autonomous regulation in order to differentiate between DCM patients and healthy subjects. BRS in DCM patients is significantly reduced and apparently less effective.

Test-retest variation of pressure flow parameters in men with bladder outlet obstruction

PURPOSE

We assess short-term (5 to 10 minutes) and long-term (24 weeks) test-retest changes of repeated pressure flow examinations.

MATERIALS AND METHODS

The pressure flow charts of 84 patients with benign prostatic enlargement and bladder outlet obstruction who had received either androgen suppressive therapy or placebo were reviewed retrospectively. Pressure flow examinations were performed at baseline, and at weeks 24 and 48. Each pressure flow session included 3 sequential voids.

RESULTS

Median detrusor opening pressure, maximum detrusor pressure, detrusor pressure at maximum flow rate and minimum voiding pressure decreased statistically significantly from void 1 to 2, ranging from 9.5% to 15.8%. From void 2 to 3 during the same pressure flow session there was a further reduction in obstruction parameters. Median Abrams/Griffiths number was 10.7% lower at void 2 compared to void 1 (p <0.0001) and the urethral resistance algorithm was 3.2% lower (p <0.0001). Long-term test-retest changes from baseline to week 24 and from week 24 to week 48 for the pressure flow parameters studied were negligible.

CONCLUSIONS

Changes in pressure flow parameters at short-term test-retesting are considerable and probably of clinical significance. The standard pressure flow nomograms, which are based on single void pressure flow studies, might need modification when applied to repeat void studies.

Evaluation of renormalised entropy for risk stratification using heart rate variability data

Standard time and frequency parameters of heart rate variability (HRV) describe only linear and periodic behaviour, whereas more complex relationships cannot be recognised. A method that may be capable of assessing more complex properties is the non-linear measure of 'renormalised entropy.' A new concept of the method, RE(AR), has been developed, based on a non-linear renormalisation of autoregressive spectral distributions. To test the hypothesis that renormalised entropy may improve the result of high-risk stratification after myocardial infarction, it is applied to a clinical pilot study (41 subjects) and to prospective data of the St George's Hospital post-infarction database (572 patients). The study shows that the new RE(AR) method is more reproducible and more stable in time than a previously introduced method (p<0.001). Moreover, the results of the study confirm the hypothesis that on average, the survivors have negative values of RE(AR) (-0.11+/-0.18), whereas the non-survivors have positive values (0.03+/-0.22, p<0.01). Further, the study shows that the combination of an HRV triangular index and RE(AR) leads to a better prediction of sudden arrhythmic death than standard measurements of HRV. In summary, the new RE(AR) method is an independent measure in HRV analysis that may be suitable for risk stratification in patients after myocardial infarction.

Baroreflex sensitivity, heart rate, and blood pressure variability in normal pregnancy

Heart rate variability is a relevant predictor of cardiovascular risk in humans. However, to use heart and blood pressure (BP) variability or baroreflex sensitivity as markers for hypertensive pregnancy disorders, it is first necessary to describe these parameters in normal pregnancy. To accommodate the complexities of autonomic cardiovascular control we added parameter domains of nonlinear dynamics to conventional linear methods of time and frequency domains. The BP of 27 women with normal pregnancy and 14 nonpregnant women were monitored at a high resolution (200 Hz sampling frequency) using a Portapres for 30 min. The pregnant women were divided into groups of 32 or less or greater than 32 weeks of gestation. Pregnant and nonpregnant women were classified into subclasses of maternal age of less than 28 or 28 or more years. Except for two single parameter domains, we found no significant differences in heart rate and BP variability for pregnant women with different gestational age or different maternal age. Moreover, no significant differences in spontaneous baroreflex sensitivity could be found between pregnant women regardless of either their age or gestational age. In contrast, all measures of nonlinear dynamics of heart rate variability as well as all parameter domains of spontaneous baroreflex sensitivity showed significant changes between pregnant and nonpregnant women, whereas BP variability did not differ between those groups. This complex assessment of autonomic cardiovascular regulation has shown that the parameters tested are stable in the second half of normal pregnancy, and might have the potential to be excellent indicators of pathophysiologic conditions.

Short-term forecasting of life-threatening cardiac arrhythmias based on symbolic dynamics and finite-time growth rates

Ventricular tachycardia or fibrillation (VT-VF) as fatal cardiac arrhythmias are the main factors triggering sudden cardiac death. The objective of this study is to find early signs of sustained VT-VF in patients with an implanted cardioverter-defibrillator (ICD). These devices are able to safeguard patients by returning their hearts to a normal rhythm via strong defibrillatory shocks; additionally, they store the 1000 beat-to-beat intervals immediately before the onset of a life-threatening arrhythmia. We study these 1000 beat-to-beat intervals of 17 chronic heart failure ICD patients before the onset of a life-threatening arrhythmia and at a control time, i.e., without a VT-VF event. To characterize these rather short data sets, we calculate heart rate variability parameters from the time and frequency domain, from symbolic dynamics as well as the finite-time growth rates. We find that neither the time nor the frequency domain parameters show significant differences between the VT-VF and the control time series. However, two parameters from symbolic dynamics as well as the finite-time growth rates discriminate significantly both groups. These findings could be of importance in algorithms for next generation ICD's to improve the diagnostics and therapy of VT-VF.

Altered heart rate and blood pressure variability in mice lacking the Mas protooncogene

Heart rate variability is a relevant predictor of cardiovascular risk in humans. A significant genetic influence on heart rate variability is suggested, although the genes involved are ill-defined. The Mas-protooncogene encodes a G-protein-coupled receptor with seven transmembrane domains highly expressed in testis and brain. Since this receptor is supposed to interact with the signaling of angiotensin II, which is an important regulator of cardiovascular homeostasis, heart rate and blood pressure were analyzed in Mas-deficient mice. Using a femoral catheter the blood pressure of mice was measured for a period of 30 min and 250 data values per second were recorded. The mean values and range of heart rate and blood pressure were then calculated. Neither heart rate nor blood pressure were significantly different between knockout mice and controls. However, high resolution recording of these parameters and analysis of the data by non-linear dynamics revealed significant alterations in cardiovascular variability in Mas-deficient animals. In particular, females showed a strong reduction of heart rate variability. Furthermore, the data showed an increased sympathetic tone in knockout animals of both genders. The marked alterations detected in Mas-deficient mice of both genders suggest that the Mas-protooncogene is an important determinant of heart rate and blood pressure variability.

[Dual sequence method for analysis of spontaneous baroreceptor reflex sensitivity in patients with dilated cardiomyopathy]

The analysis of heart rate variability (HRV) and blood pressure variability (BPV) improves the characterization of patients with dilated cardiomyopathy (DCM). In this study we tested the hypothesis that patients with DCM and controls show a different behavior in the baroreflex (BR) regulation. In contrast to other methods, the new dual sequence method (DSM) analyzes the baroreflex sensitivity (BRS) as a response of the heart rate (interbeat interval, IBI) on dual spontaneous fluctuations of blood pressure (BP). The DSM includes the analysis of bradycardiac fluctuations (an increase of BP causes an increase of IBI) and tachycardiac fluctuations (decrease of BP causes a decrease of IBI) to obtain enhanced information about the sympathetic-vagal regulation. DCM patients show a 40-50% lower number of correlated blood pressure-heart rate fluctuations (DCM patients: male 154 +/- 93, female 93 +/- 40 vs. control group: m 245 +/- 112, f 150 +/- 55, p < 0.05). The BRS in DCM patients is significantly lower than in controls (5.2 +/- 1.9 vs. 8.0 +/- 5.4 (ms/mm Hg), p < 0.05). Using the DSM the discriminant function analysis (6 parameters) classifies correctly 84% of DCM patients and the control group. Using the classical sequence method, only 76% were correctly classified. The DSM is a useful method for analyzing the BRS based on the spontaneous BR to obtain an increased classification of patients with DCM. BRS in patients with DCM is significantly reduced and apparently more ineffective.