Alterations in heart rate variability and its circadian rhythm in hypertensive patients with left ventricular hypertrophy free of coronary artery disease

Effects of combination antihypertensive therapy on baroreflex sensitivity and heart rate variability in systemic hypertension

Earlier studies have shown that cardiovascular autonomic regulation is impaired in untreated or poorly controlled systemic hypertension. The purpose of this double-blind, randomized parallel trial was to evaluate whether improved blood pressure (BP) control can reverse this impairment. The study group consisted of 33 patients (age 45 to 63 years) with poor BP control who received randomized metoprolol or enalapril monotherapy. Baroreflex sensitivity (BRS) was assessed by phenylephrine test and time- and frequency-domain measurements of heart rate variability (HRV) were analyzed from 24-hour ambulatory electrocardiographic recordings during monotherapy and after 10 weeks of combination therapy with metoprolol + felodipine or enalaril + hydrochlorothiazide to lower casual BP to < 140/90 mm Hg. Intensified treatment decreased 24-hour systolic and diastolic BP from 139 +/- 12/86 +/- 8 mm Hg to 126 +/- 8/80 +/- 7 mm Hg (p <0.0001). BRS improved from 6.2 +/- 3.2 ms/mm Hg to 8.9 +/- 4.1 ms/mm Hg (p <0.0001) and measurements of HRV (e.g., SD of all RR intervals from 128 +/- 45 ms to 145 +/- 46 ms, p <0.001) improved significantly during the combination therapy. Changes in BRS and HRV were similar in magnitude in both treatment arms. Mean RR intervals were comparable before and after intensive antihypertensive therapy (850 +/- 124 ms vs 937 +/- 279 ms, p = NS). These data indicate that adequate BP control with modem antihypertensive combination therapy can improve cardiovascular autonomic function, which may partially explain the reduced cardiac mortality observed in patients with intensified antihypertensive therapy.

Power spectral analysis of the heart rate in hypertensive patients with and without left ventricular hypertrophy: the effect of a left ventricular mass reduction

OBJECTIVE

The aim of this study was to evaluate the spectral analysis of the heart rate in normotensive subjects and in hypertensive patients with and without left ventricular hypertrophy (LVH), under basal conditions and after a reduction in left ventricular mass.

SUBJECTS AND METHODS

In 12 normotensive subjects and 22 hypertensive patients (14 with and eight without LVH), we performed 24 h electrocardiogram Holter monitoring, ambulatory blood pressure monitoring and an echocardiographic study. Sequences of 512 R-R intervals, during daytime, afternoon and night-time periods, were taken for an evaluation of spectral analysis (Box-Jenkins method). We then calculated the absolute and percentage power spectral density of the peak centred at 0.10 Hz (low-frequency peak) and at 0.25 Hz (high-frequency peak).

RESULTS

At baseline, a daytime to night-time decrease in the low-frequency peak was detected in normotensives (P < 0.01) and in hypertensives without LVH (P < 0.01), while no change was observed in hypertensives with LVH. The power spectral density low-frequency peak during the daytime and night-time was significantly greater in hypertensives with LVH than in those without LVH (P < 0.001) and in normotensive subjects (P < 0.001). Fourteen of these patients with LVH were given effective long-term antihypertensive treatment and were studied again 20 days after the treatment had been withdrawn, when blood pressure had increased to pretreatment values. In eight patients showing a reduction in LVH, we found a significant decrease in the power spectral density low-frequency peak and an increase in the high-frequency peak during daytime and night-time in respect to basal conditions, and circadian variations in the spectral indices of heart rate variability were restored. In contrast, in six patients without reversal of LVH, the power spectral density low-frequency peak did not change in respect to basal conditions and remained significantly higher in comparison with the patients with LVH regression.

CONCLUSION

A reduction in LVH may be associated with restoration of daytime to night-time cardiac autonomic control, as evaluated by a power spectral analysis of the heart rate.

Reduced heart rate variability and new-onset hypertension: insights into pathogenesis of hypertension: the Framingham Heart Study

Heart rate variability (HRV) is a useful noninvasive tool to assess cardiac autonomic function. The purpose of this study was to (1) compare measures of HRV between hypertensive and normotensive subjects and (2) examine the role of HRV as a predictor of new-onset hypertension. The first 2 hours of ambulatory ECG recordings obtained from 931 men and 1111 women attending a routine examination at the Framingham Heart Study were processed for HRV. Three time-domain and 5 frequency-domain variables were studied: standard deviation of normal RR intervals (SDNN), percentage of differences between adjacent normal RR intervals exceeding 50 milliseconds, square root of the mean of squared differences between adjacent normal RR intervals, total power (0.01 to 0.40 Hz), high frequency power (HF, 0.15 to 0.40 Hz), low frequency power (LF, 0.04 to 0.15 Hz), very low frequency power (0.01 to 0.04 Hz), and LF/HF ratio. On cross-sectional analysis, HRV was significantly lower in hypertensive men and women. Among 633 men and 801 women who were normotensive at baseline (systolic blood pressure <140 mm Hg and diastolic blood pressure <90 mm Hg and not receiving antihypertensive treatment), 119 men and 125 women were newly hypertensive at follow-up 4 years later. After adjustment for factors associated with hypertension, multiple logistic regression analysis revealed that LF was associated with incident hypertension in men (odds ratio per SD decrement [OR], 1.38; 95% confidence interval [CI], 1.04 to 1.83) but not in women (OR, 1.12; 95% CI, 0.86 to 1.46). SDNN, HF, and LF/HF were not associated with hypertension in either sex. HRV is reduced in men and women with systemic hypertension. Among normotensive men, lower HRV was associated with greater risk for developing hypertension. These findings are consistent with the hypothesis that autonomic dysregulation is present in the early stage of hypertension.

Effect of clonidine on heart rate variability in congestive heart failure

In patients with congestive heart failure, abnormal heart rate variability is a predictor of total mortality and sudden cardiac death. Drugs that improve heart rate variability may have a potential role for improving the survival among these patients. The effects of clonidine were studied in 24 patients with congestive heart failure, sinus rhythm, a left ventricular ejection fraction <0.40, and systolic blood pressure > 115 mm Hg. A 6-minute corridor walk test and 24-hour Holter monitoring were performed before and 42+/-24 days after initiation of clonidine therapy (Catapres-TTS patch, mean dose: 0.33+/-0.21 mg). Changes in other medications used at baseline were not allowed. One patient died suddenly. Two patients did not complete the protocol due to worsening congestive heart failure, which required changes in medications, 1 patient discontinued due to hypotension, and 2 for personal reasons. Among the 18 patients who completed the protocol, the mean RR interval of sinus beats increased from 760+/-106 to 822+/-125 ms (p=0.001) and the distance covered during the 6-minute walk test increased from 1,148+/-277 to 1,255+/-359 feet (p=0.042). Systolic blood pressure decreased from 139+/-15 to 119+/-10 mm Hg (p <0.0001). The following increases were noted in the heart rate variability measurements: high-frequency power in 0.15 to 0.40 Hz: 4.58+/-1.07 to 4.94+/-1.17 In (ms), p=0.002; SD: 47.0+/-16.9 to 52.5+/-18.4 ms, p=0.034; SD of the mean of all RR intervals in 24 hours: 116+/-94 to 130+/-19 ms, p=0.033; SD of all 5-minute mean RR intervals: 106+/-44 to 124+/-66 ms, p=0.042; root-mean square of difference of successive RR intervals: 28.8+/-10.7 to 34.1+/-14.2 ms, p=0.017. Clonidine improves heart rate variability in the patients with congestive heart failure by increasing the parasympathetic tone. It is well tolerated by most patients with heart failure and may have a beneficial effect on exercise capacity.

Relationship Between Components of Heart Rate Variability and Doppler Echocardiographic Indices of Left Ventricular Systolic Performance in Patients with Coronary Artery Disease

Decreased heart rate variability (HRV) has been reported as prognostic predictor in coronary artery disease population. The aim of the study was to assess the relationship between cardiac autonomic tone disturbances and the degree of left ventricular dysfunction, estimated by 2-dimensional and Doppler echocardiography, in patients with coronary artery disease. Forty patients with angiographically proven coronary artery disease and 15 age-and sex-matched healthy subjects were included into the study (Group I and II, respectively). Routine clinical examinations, electrocardiography, coronary angiography, and 2-dimensional and Doppler echocardiography with assessment of LV systolic performance were done in all the patients and control subjects. Cardiac autonomic function was assessed by means of frequency-domain analysis of heart rate variability. The powers of LF and HF bands (P1 and P2), their ratio (P1/P2), and the logarithmic expression of powers were measured. The patients (Group I) had lower values of parasympathetic band of HRV and higher sympathetic activity than the healthy population (Group II) (p < 0.0002 for P2 and p < 0.04 for P1/P2 ratio). A positive significant correlation was seen between parasympathetic band of HRV and ejection fraction (r = 0.61, p < 0.0001), and mean acceleration of aortal flow (r = 0.56, p < 0.0001), whereas P1/P2 ratio inversely and significantly correlated with the same parameters (r = -0.46, p < 0.002 and r = -0.41, p < 0.008).

Depressed heart rate variability and arterial baroreflex in conscious transgenic mice with overexpression of cardiac Gsalpha

Recently, we developed a transgenic mouse with cardiac-specific Gsalpha overexpression (TG mouse), which exhibits enhanced postsynaptic beta-adrenergic receptor signaling, ultimately developing a cardiomyopathy. The goal of the present study was to determine whether cardiac Gsalpha overexpression alters autonomic cardiovascular control, which could shed light on the mechanism responsible for the later development of cardiomyopathy. Mean arterial pressure was increased (P<.05) in conscious, chronically instrumented TG mice (123+/-1 mm Hg) compared with age-matched wild-type (WT) control mice (103+/-1 mm Hg). Respiratory frequency was increased (P<.05) in TG mice (269+/-26/min) compared with WT mice (210+/-20/min). By use of telemetric techniques, baseline heart rate (HR) was elevated (P<.05) in conscious, untethered TG mice (696+/-13 bpm) compared with WT mice (568+/-28 bpm). Intrinsic HR, after propranolol and atropine or after ganglionic blockade with hexamethonium, was not different between TG and WT mice. Both the normal minute-to-minute and circadian variations of HR observed in WT mice were markedly blunted in TG mice. HR variability was assessed by the time-domain and frequency-domain methods. At baseline, time-domain analysis indices were reduced (P<.05) in TG mice compared with WT mice. Although the low frequency (LF) component was higher (P<.05) than the high frequency (HF) component in WT mice, the LF component was less (P<.05) than the HF component in TG mice. In addition, arterial baroreflex regulation of HR was markedly blunted in TG mice in response to both nitroglycerin-induced hypotension and phenylephrine-induced hypertension. The reduced LF/HF ratio in TG mice was surprising in view of enhanced beta-adrenergic signaling and may be due to reduced neural tone secondary to the elevated arterial pressure or alterations in arterial baroreflex control. Dobutamine infusion in WT mice also resulted in depressed HR variability. The combination of elevated baseline HR, arterial pressure, and respiratory frequency suggests that enhanced beta-adrenergic signaling in TG mice results in reduced HR variability, in terms of both minute-to-minute variability and the lack of circadian variations in HR. The lack of normal HR variability in general and the failure of HR to decline, even during sleep, may actually be critical mechanisms contributing to the ultimate development of cardiomyopathy in these animals.

Low-dose transdermal scopolamine decreases blood pressure in mild essential hypertension

BACKGROUND

Increasing cardiovascular parasympathetic nervous activity could have antihypertensive effects. Low-dose transdermal scopolamine increases vagal-cardiac modulation of sinus node and baroreflex sensitivity in healthy subjects and in cardiac patients.

OBJECTIVE

To study the short-term effects of transdermal scopolamine on blood pressure and cardiovascular autonomic control in patients with mild essential hypertension.

DESIGN

A randomized, double-blind, placebo-controlled crossover trial with 12 untreated middle-aged [aged 39+/-5 years (mean+/-SD)] patients with mild essential hypertension.

METHODS

We recorded the electrocardiogram, auscultatory sphygmomanometric and continuous photoplethysmographic finger arterial pressure, and spirometry signals with patients supine and 70 degrees tilted during controlled (0.25 Hz) breathing. Cardiovascular autonomic regulation was analyzed with power spectrum analysis of R-R interval and arterial pressure variability and a spontaneous sequence method for baroreflex sensitivity. In addition, a deep-breathing test was performed to assess maximal breathing-related sinus arrhythmia.

RESULTS

Transdermal scopolamine treatment significantly decreased blood pressure both when patients lay supine and when they were in the 70 degrees tilted position. Scopolamine also slowed heart rate and increased baroreflex sensitivity and R-R interval high-frequency variability for both body positionings. In addition, scopolamine accentuated respiratory sinus arrhythmia during deep breathing and blunted the tilt-induced increase in heart rate. Scopolamine did not affect blood pressure variability.

CONCLUSIONS

Transdermal scopolamine decreases arterial pressure, increases baroreflex sensitivity and accentuates vagal-cardiac modulation of sinus node in patients with mild hypertension. Our study supports the hypothesis that increasing cardiovascular parasympathetic activity could have antihypertensive effects in essential hypertension.

Perioperative myocardial ischemia is associated with a prolonged cardiac vagal dysfunction after non-cardiac surgery

BACKGROUND

Heart rate variability (HRV), a measure of cardiac autonomic balance seems to be linked to coronary artery disease (CAD). Impaired vagal input facilitates the generation of fatal arrhythmias and has a great impact on morbidity and mortality. The purpose of this study was to determine the relationship between perioperative HRV and the incidence of silent myocardial ischemia (SMI) and ventricular dysrhythmias in CAD-patients undergoing non-cardiac surgery.

METHODS

31 patients were studied by continuous Holter electrocardiography preoperatively and again on the evening before surgery until postoperative day 3. Three frequency and one time domain measures of HRV (TP, 0.01-1.00 Hz: total power of the amplitude spectral plot; LF, 0.04-0.15 Hz: low-frequency power; HF, 0.15-0.40 Hz: high-frequency power; MeanRR: mean of all coupling RR-intervals between normal beats) as well as ischemic events and ventricular couplets and runs were computed.

RESULTS

Depending on the presence of ischemic episodes, each patient was assigned to either the no SMI-group (13 pts) or the SMI-group (18 pts). MeanRR, TP and LF significantly declined in both groups over time. The parasympathetically dominated index HF, however, only decreased in the SMI-group. Normalized HF power (HF/TP) even increased in the no SMI-group resulting in a postoperative decrease in LF/HF ratio. TP, LF, and HF inversely correlated with ischemia parameters whereas HF/TP and HF/LF ratio correlated with the number of ventricular couplets. Incidence and severity of SMI significantly increased after surgery.

CONCLUSION

Postoperatively, a prolonged vagal withdrawal occurred in CAD-patients exhibiting perioperative SMI. Whether the increased incidence of SMI after surgery in conjunction with the observed parasympathetic derangement contributes to adverse cardiac outcome still has to be determined.

Circadian heart rate variability rhythm in shift workers

The objective of this study was to assess the influence of day-night cycle and sleep-awake period on the circadian pattern of heart rate variability (HRV). Twelve male oil refinery security shift workers, aged 39 +/- 7 years, were studied with 24-hour Holter monitor recordings during morning and night work periods. Hourly HRV parameters in the time and frequency domains were evaluated. For both shifts, all HRV parameters during awake or work periods were found not to be statistically different. In both day and night work shifts, the very low frequency and high-frequency components of HRV and the proportion of differences in successive R-R intervals greater than 50 ms increased during the sleep period, while the low frequency/high frequency ratio decreased. The low-frequency component in absolute units and the SD of the R-R interval did not show any variation in either shifts for the different periods. These results suggest that the circadian pattern of HRV seems to be predominantly related to sleep (supine) and wakefulness (standing) and remains independent of night-day cycle.

Dispersion of the QT interval and autonomic modulation of heart rate in hypertensive men with and without left ventricular hypertrophy

Left ventricular hypertrophy is an independent risk factor for sudden cardiac death in hypertension, but the mechanisms of electrical instability associated with hypertrophy are not well known. We studied dispersion of the QT interval, an index of inhomogeneity of repolarization, and heart rate variability, a measure of cardiac autonomic modulation, in a randomly selected population of 162 men with systemic hypertension and made comparisons between the patients with echocardiographic evidence of left ventricular hypertrophy (left ventricular mass index > or = 131 g/m2, n = 44) and those without hypertrophy (left ventricular mass index < 131 g/m2, n = 118). The heart rate-corrected QT dispersion (67 +/- 37 versus 53 +/- 21 milliseconds, P < .05) and QT apex dispersion (55 +/- 22 versus 44 +/- 16 milliseconds, P < .01) were significantly longer in the patients with left ventricular hypertrophy than in those without hypertrophy. Thirteen of the 44 patients (30%) with hypertrophy versus 7 of the 118 patients (6%) without hypertrophy had an abnormally long QT apex dispersion ( > 70 milliseconds) (P < .001). The time and frequency domain measures of heart rate variability did not differ significantly between the patient groups with and without left ventricular hypertrophy. The measures of heart rate variability were not related to QT dispersion or left ventricular mass index but had a negative correlation with blood pressure values (eg, r = -.30 between the low-frequency component of heart rate variability and systolic pressure, P < .001). Age, body mass index, antihypertensive medication, and the other demographic variables were similar between the groups, but the patients with left ventricular hypertrophy had higher systolic (P < .01) and diastolic (P < .01) pressures compared with the patients without hypertrophy. Left ventricular hypertrophy in hypertensive men is associated with inhomogeneity of the early phase of ventricular repolarization, favoring susceptibility to reentrant ventricular tachyarrhythmias. Abnormalities in cardiac autonomic function, which may trigger a spontaneous onset of arrhythmias, are related to elevated blood pressure but not specifically to left ventricular hypertrophy.

Heart rate variability in systemic hypertension

Low heart rate (HR) variability is a risk factor for cardiac mortality in various patient populations, but it has not been well established whether patients with long-standing hypertension have abnormalities in the autonomic modulation of HR. Time and frequency domain measures of HR variability were compared in randomly selected, age-matched populations of 188 normotensive and 168 hypertensive males (mean age 50 +/- 6 years for both). The standard deviation of the RR intervals was lower in the hypertensive subjects than in the normotensive ones (52 +/- 19 vs 59 +/- 20 mss; p <0.01), and the very low and low-frequency spectral components of HR variability analyzed as absolute units were reduced in the hypertensive patients relative to the normotensive controls (p <0.001 for both). Hypertensive subjects also had blunted changes of the normalized low- and high-frequency components in response to an upright (sitting) posture (NS) as compared with normotensive subjects (p <0.001 for both). Multiple regression analysis showed the standard deviation of the RR intervals to be predicted most strongly by systolic blood pressure, both in the patients with hypertension (beta--0.20, p=0.01) and in the normotensive subjects (beta--0.28, p=0.0002). After adjustment for the baseline differences in blood pressure and body mass index, none of the absolute measures of the HR variability or the responses of the normalized units of HR variability to a change in the body posture differed between the hypertensive subjects and normotensive controls. These data show that long-standing hypertension results in reduced overall HR variability and blunted autonomic responses to a change in body posture. Altered autonomic modulation of HR in hypertension is mainly due to elevated blood pressure and obesity in males with long-standing hypertension as compared with normotensive subjects.

Heart period variability in patients with variant angina

To clarify how cardiac autonomic control is affected in variant angina, we analyzed heart period variability in 35 patients with variant angina and in 19 control subjects. Patients with variant angina were divided into 1-vessel (group S, n = 17) and multivessel spasm groups (group M, n = 18) according to the site(s) of ST elevation on the electrocardiogram during attacks. The 24-hour Holter electrocardiogram recorded 6 +/- 3 days after the treatment with calcium antagonist was analyzed to avoid the possible influence of spontaneous attacks. In 5 group M patients, the electrocardiogram recorded 1 month after the treatment was also analyzed. There was no difference in the number of spontaneous attacks between groups S and M. The standard deviation of all normal RR intervals (SDNN) and the percentage of differences between adjacent normal RR intervals >50 (pNN50) in variant angina were slightly but significantly lower than those in controls. There were no differences in other indexes between variant angina and controls. When the data were analyzed separately in groups S and M, averaged RR intervals (MN), SDNN, pNN50, high-frequency power, and low-frequency power in group M were significantly lower than those in group S and controls, and the ratio of low- to high-frequency power in group M was significantly higher than that in group S and controls. There was no difference in any index between group S and controls. All abnormal indexes in group M recovered to levels similar to those in controls 1 month after the treatment. In conclusion, depressed cardiac vagal control and sympathetic-dominant sympathovagal interaction were present in patients with variant angina, especially in those with multivessel spasm.

Effect of 1 year of lisinopril treatment on cardiac autonomic control in hypertensive patients with left ventricular hypertrophy

In this study we evaluated in hypertensive patients the effects of drug-induced left ventricular hypertrophy regression on cardiac autonomic control, as assessed by means of heart period variability analysis. Power spectral analysis of 24-hour electrocardiographic monitoring was performed in 30 hypertensive patients with left ventricular hypertrophy at baseline, after 1 year of lisinopril treatment, and after 1 month of drug withdrawal. At the same times, patients underwent 24-hour blood pressure monitoring, echocardiographic study, and plasma renin activity assessment. Lisinopril treatment increased plasma renin activity and reduced 24-hour systolic and diastolic pressures (from 159 +/- 14 to 121 +/- 8 and from 103 +/- 7 to 80 +/- 3 mm Hg, respectively) and left ventricular mass index (from 159 +/- 33 to 134 +/- 26 g/m2); moreover, in 12 of 30 patients, left ventricular mass normalization was achieved. Drug withdrawal was followed by an increase in blood pressure without left ventricular mass modification. In the total study population, only high-frequency power was higher after lisinopril treatment. In the subgroup of patients with left ventricular mass normalization, daytime and nighttime high-frequency powers as well as nighttime total and very-low-frequency powers were higher after 1 year of treatment than at baseline. In the remaining 18 patients, power spectral measures after treatment were slightly lower than at baseline and were even lower after drug withdrawal. Thus, in hypertensive hypertrophic patients, lisinopril treatment improves sympathovagal imbalance when left ventricular mass normalization is achieved. In patients without left ventricular mass normalization, drug withdrawal is followed by a worsening of neural cardiac control.

Heart rate variability in pheochromocytoma

The results of our study show that parasympathetic tone was higher in patients with pheochromocytoma than in patients with primary hypertension. An unusual spectral form of vagal activity was seen during excessive beta-adrenergic stimulation, while persistent hypertension with an excessive alpha-adrenergic stimulation was responsible for low cardiac sympathetic tone, probably due to desensitization of beta-adrenergic receptors.

Circadian variation of heart rate variability in postinfarction patients with and without life-threatening ventricular tachyarrhythmias

INTRODUCTION

Determination of heart rate variability (HRV) is widely used for noninvasive assessment of cardiac autonomic tone. A decreased HRV is associated with an increased mortality in patients surviving an acute myocardial infarction. There are, however, only sparse data about the circadian variation of different components of HRV that may be linked to the well-known circadian fluctuations in the occurrence of sudden death. In addition, the potential prognostic impact of circadian variations of HRV has not been examined.

METHODS AND RESULTS

The present study compared the circadian variation of HRV from 14 postinfarction patients who had survived at least one episode of out-of-hospital cardiac arrest (cardiac arrest group) with that of 14 age- and sex-matched patients without a history of malignant arrhythmias after their index infarct (control group). Several time- and frequency-domain measures of HRV were assessed from 24-hour Holter recordings. Circadian variations of high- (HF), low- (LF), and total-frequency (TF) components were determined by calculating for each parameter the hourly difference from the day's mean. The average of these differences was calculated for every hour as well as for predefined day and night periods. There was no significant difference between the two groups with regard to HRV indices that predominantly reflect vagal tone, such as SDNN (78 +/- 25 vs 96 +/- 24 msec), pNN50 (2.7% +/- 4.6% vs 4.9% +/- 4.2%), or HF (6.3 +/- 3.0 vs 7.8 +/- 3.2 msec; cardiac arrest vs control group). There was also no significant difference in the circadian variation of LF or TF between the two groups during daytime and nighttime. However, a significant difference in circadian variation of HF was found during daytime (0.02 +/- 0.5 vs -0.6 +/- 0.5 msec; P = 0.006) and nighttime (0.19 +/- 0.64 vs 1.5 +/- 0.75 msec; P = 0.0002). In cardiac arrest survivors, there was no difference in the mean deviation of HF between the day- and the nighttime periods.

CONCLUSIONS

These results show an almost complete abolition in circadian variation of parasympathetic tone in postinfarction patients surviving an episode of out-of-hospital cardiac arrest, whereas circadian variation of sympathetic tone is comparable to that of postinfarction patients without arrhythmic episodes. These findings indicate that determination of diurnal variation of HRV may add to the prognostic value of HRV with respect to identifying patients at high risk of sudden death.