Circadian rhythmicity of heart rate and QTc interval in diabetic autonomic neuropathy: implications for the mechanism of sudden death

Circadian variation of the QT interval and heart rate variability and their interrelationship

BACKGROUND

Whether the QT interval displays circadian rhythm after heart rate correction is unresolved and the relationship of QT interval to heart rate variability (HRV) is uncertain.

OBJECTIVES

To test the hypothesis that there is a circadian rhythm to QT interval and HRV and determine the relationship between QTc and HRV.

METHODS

The hourly average ECG data from 24-h ECGs were examined in individuals (50 without medications and 9 on beta blockers only) with no evidence of coronary artery disease or structural heart disease. The QT duration of normal QRS complexes from a series of 30-s windows was measured. The presence of circadian rhythm was tested by the data analytic approach of goodness of fit to a cosine function.

RESULTS

QT interval with and without heart rate correction showed a circadian rhythm for five heart rate adjustment formulae except for the Bazett formula. HRV also showed circadian rhythm but with different acrophages and nadirs depending on the HRV component. There were significant (p < 0.05) positive correlations of QTc with pNN50 rms-SD and SDNN and significant (p < 0.05) negative correlations with SDANN and Tri. The beta blocker group did not generally show circadian rhythm for QT interval or HRV.

CONCLUSION

QT, after heart rate adjustment, and HRV have circadian rhythmicity. There are significant correlations between QT interval and HRV indices. Circadian rhythm was blunted with beta blockers. The data are consistent with the concept of a predominance of parasympathetic activity to increase QTc and sympathetic activity to shorten QTc, even after 'correction' of the QT interval for heart rate.

[Improving parasympathetic functioning to contain the inflammatory response of COVID-19 infection]

COVID-19 infection results in an unrestrained inflammatory reaction in serious cases. The autonomic nervous system (ANS), in particular the parasympathetic branch, helps to regulate the inflammatory response. A dysfunction of this branch, frequent in people at risk of developing COVID-19, favours a pro-inflammatory effect. Reinforcing and stimulating the parasympathetic ANS is possible and accessible to paramedical and medical professionals.

Evaluation of QT Liability for PF-05251749 in the Presence of Potential Circadian Rhythm Modification

PF-05251749 is a dual inhibitor of casein kinase 1 δ/ε, key regulators of circadian rhythm. As a result of its mechanism of action, PF-05251749 may also change the heart rate corrected QT (QTc) circadian rhythm, which may confound detection of drug-induced QTc prolongation. In this analysis, a nonlinear mixed effect model including a multioscillator function was developed in addition to fitting the prespecified linear mixed effect concentration-QTc model, to identify QTc liability of PF-05251749 in the presence of potential circadian rhythm change. The modeling results suggested lack of clinically meaningful QTc prolongation (upper bound of 90% confidence interval for ∆∆QTc < 10 milliseconds) and that the drug-induced QTc circadian rhythm change was not present. However, simulation results indicated that inference of drug-induced QTc prolongation could be misleading if the drug effect on QTc circadian rhythm is not properly addressed. The modeling and simulation results suggest that prespecification of the concentration-QTc model should be reconsidered for drugs with circadian rhythm modulation potential.

Prevalence of prolonged QT interval in patients with HCV-related chronic liver disease

Background

Hepatitis C virus (HCV) is a common disease in Egypt with a high socioeconomic burden and extra-hepatic manifestations as QT prolongation, but previous studies included mainly patients with advanced liver disease, so in this study, we aimed to delineate the prevalence of QT prolongation in early-stage HCV patients.

Results

The study included 874 HCV patients with early cirrhosis; in Child’s class A, 57 (6.5%) patients had prolonged QT interval corrected (QTc). There was significant higher proportion of cirrhotic patients in the prolonged QTc group (31.6%) vs. in the normal QTc group (11.5%). QTc was 424.39 ± 36.6 vs. 411.51 ± 32.89 ms in cirrhotic and non-cirrhotic patients, respectively (P, 0.001). There was significant higher proportion of Fibrosis 4 (FIB-4) ≥ 1.45 score in the prolonged QTc (77.2%) vs. in the normal QTc group (56.8%) (P, 0.003). QTc interval was 417.76 ± 34.12 ms in patients with FIB-4 score ≥ 1.45 vs. 406.78 ± 31.95 ms in those with FIB-4 < 1.45 (P, < 0.001). FIB-4 score value of 2.108 predicted prolonged QTc with a sensitivity of 63.2% and a specificity of 64.5% (P, < 0.001). Twenty-four patients of long QTc group sent ECGs after HCV eradication, and 19 patients (79%) showed QTc normalization.

Conclusions

HCV is associated with QTc prolongation even in patients with early chronic liver disease stages without significant fibrosis. Also, it is related to the degree of fibrosis and cirrhosis. At a cutoff value of 2.108, FIB-4 score can predict prolonged QTc. HCV eradication is associated with a high incidence of QTc normalization.

Diurnal Profile of the QTc Interval Following Moxifloxacin Administration

Understanding the physiological fluctuations in the corrected QT (QTc) interval is important to accurately interpret the variations in drug-induced prolongation. The present study aimed to define the time course of the effect of moxifloxacin on the QT interval to understand the duration of the responses to moxifloxacin. This retrospective analysis was performed on data taken from a thorough QT 4-way crossover study with 40 subjects. Each period consisted of a baseline electrocardiogram (ECG) day (day -1) and a treatment day (day 1). On both days, ECGs were recorded simultaneously using 2 different systems operating in parallel: a bedside ECG and a continuous Holter recording. The subjects were randomized to 1 of 4 treatments: 5 mg and 40 mg of intravenous amisulpride, a single oral dose of moxifloxacin (400 mg), or placebo. Standardized meals, identical in all 4 periods, with similar nutritional value were served. Bedside ECG results confirmed that the moxifloxacin peak effect was delayed in the fed state and showed that the Fridericia corrected QT prolongation induced by moxifloxacin persisted until the end of the 24-hour measurement period. The use of continuous Holter monitoring provided further insight, as it revealed that the moxifloxacin effect on QTc was influenced by diurnal and nocturnal environmental factors, and hysteresis effects were noticeable. The findings suggested that moxifloxacin prolongs QTc beyond its elimination from the blood circulation. This is of relevance to current concentration-effect modeling approaches, which presume the absence of hysteresis effects.

QT interval, corrected for heart rate, is associated with HbA1c concentration and autonomic function in diabetes

AIMS

To examine QT intervals corrected for heart rate (QTc) in adolescents with Type 1 diabetes compared with control subjects, and to determine associations with metabolic control and autonomic function.

METHODS

Resting electrocardiogram recordings of 142 adolescents with Type 1 diabetes [mean (sd) age 15.3 (2.0) years, diabetes duration 9.0 (3.5) years, HbA1c 71 (17) mmol/mol or 8.7 (1.6)%] and 125 control subjects [mean (sd) age 15.7 (2.5) years] were used to calculate QTc duration and derive mean heart rate and heart rate variability (HRV) values. Linear and logistic regression models were used to examine the associations between QTc, metabolic control and autonomic function (HRV and pupillary function).

RESULTS

QTc duration was not significantly different between subjects with Type 1 diabetes and control subjects (mean duration 392 vs 391 ms; P = 0.65). In the Type 1 diabetes group, QTc was positively associated with HbA1c [β = 4 (95% CI 2, 6); P < 0.001] and inversely associated with severe hypoglycaemic events [β = -10 (95% CI -20,-2); P = 0.01], less insulin/kg [β = -12 (95% CI -22, -2); P = 0.024] and less HRV. In the Type 1 diabetes group, QTc in the highest quintile (≥409 ms) vs quintiles 1-4 had more pupillary abnormalities (83 vs 56%; P = 0.03), lower pupillary maximum constriction velocity (4.8 vs 5.3 mm/s; P = 0.04), higher heart rate (78 vs 72 beats per min; P = 0.02) and lower HRV (standard deviation of mean NN intervals 4.0 vs 4.3 ms, P = 0.004 and root-mean-square difference of successive NN intervals 3.7 vs 4.1 ms; P = 0.004).

CONCLUSIONS

Although there are concerns about hypoglycaemia in general in people with Type 1 diabetes, chronic hyperglycaemia, rather than intermittent hypoglycaemia, appears to be more deleterious to autonomic cardiac function, even in adolescence. Longer QTc was associated with higher HbA1c concentration, lower risk of hypoglycaemia and autonomic dysfunction. Longitudinal studies are warranted.

Greater insulin resistance indicates decreased diurnal variation in the QT interval in patients with type 2 diabetes

Circadian variations in the QT interval (QT) and QT dispersion are decreased in patients with type 2 diabetes because of cardioneuropathy. Insulin resistance has been recently identified as an independent determinant of QT prolongation in normoglycemic women. However, the relationship between QT prolongation and the degree of insulin resistance as well as circadian variation remains unclear in diabetic patients. This study was designed to assess the relationship between insulin resistance and the circadian variation in QT measurements in patients with type 2 diabetes. In 14 patients with diabetes, QT, corrected QT (QTc), QT dispersion, QTc dispersion, and RR interval (RR) were analyzed using 12-lead Holter monitoring and commercial software. The degree of diurnal variation in each measurement was defined as the amplitude between the maximum and mean values on curves fitted using the mean cosinor method (A_QT, A_QTc, A_QT dispersion, A_QTc dispersion, and A_RR). The cosine curve was fitted to all measured values in each QT measurement and RR for 24 h. Insulin resistance (glucose infusion rate (GIR)) was measured using the euglycemic hyperinsulinemic glucose clamp method. The maximum QT, QTc, QT dispersion, and QTc dispersion were >450 ms. GIR was significantly correlated with A_QT only (r = 0.59, P < 0.05). GIR was not correlated with other variables, and was dependent only on the circadian variation in QT.

The ECG vertigo in diabetes and cardiac autonomic neuropathy

The importance of diabetes in the epidemiology of cardiovascular diseases cannot be overemphasized. About one third of acute myocardial infarction patients have diabetes, and its prevalence is steadily increasing. The decrease in cardiac mortality in people with diabetes is lagging behind that of the general population. Cardiovascular disease is a broad term which includes any condition causing pathological changes in blood vessels, cardiac muscle or valves, and cardiac rhythm. The ECG offers a quick, noninvasive clinical and research screen for the early detection of cardiovascular disease in diabetes. In this paper, the clinical and research value of the ECG is readdressed in diabetes and in the presence of cardiac autonomic neuropathy.

Diurnal heart rate patterns in inappropriate sinus tachycardia

BACKGROUND

Inappropriate sinus tachycardia (IAST) is a supraventricular tachycardia originating from the sinus node. Proposed etiologies for this symptom complex include autonomic dysfunction, abnormal automaticity, or hypersensitivity of the sinus node.

METHODS

Patients with IAST were identified by symptomatic tachycardia, with P-wave morphology consistent with origination from a sinus location. A matched set of control subjects was included. Hourly heart rate (HR) was measured as the average HR during each one hour period on an ambulatory 24-hour Holter monitor. Patients were further divided into two subgroups based on average daily HR < or = 80 and >80 bpm. Harmonic analysis was used to evaluate diurnal variation.

RESULTS

The mean HR was 86.0 +/- 12.8 beats per minute (bpm) in the IAST group and 73.9 +/- 8.6 bpm in the control group (P = 0.056). There was an increased overall heart rate for the IAST group, which appeared to be more prominent in the morning hours. In the IAST subgroup with average daily HR < or = 80, hourly HR appears similar to controls for the period 8 pm-8 am. However, in the late AM, the IAST group had an increase in HR not seen in the control subjects. In the IAST subgroup with average HR > 80, there appeared to be a fixed difference in HR compared to the control group, without hourly change.

CONCLUSIONS

Patients with IAST and elevated average daily HR exhibit normal diurnal variation around a higher mean HR. In contrast, patients with IAST and lower average daily HR had an exaggerated morning rise in HR. These diurnal patterns may be useful to classify the pathophysiology of IAST.

Assessment of QT interval and QT dispersion during electroconvulsive therapy using computerized measurements

BACKGROUND

Electroconvulsive therapy (ECT) used in the treatment of severe psychiatric disorders induces stimulation of the autonomic nervous system with initial parasympathetic outflow immediately followed by a sympathetic response. These responses induce an initial bradycardia, arrhythmias, and hypertension. QT dispersion (QTD), defined as maximal QT interval minus minimal QT interval on 12 leads of the surface electrocardiogram, reflects regional heterogeneity of ventricular repolarization. The effects of electrical stimulus due to ECT on QT interval and QTD are of considerable interest.

OBJECTIVE

: This study was designed to investigate the effects of electrical stimulation caused by ECT on RR interval, QT interval, the rate-corrected QT (QTc) interval, QTD, and the rate-corrected QTD (QTcD) under general anesthesia using computerized measurements.

METHODS

Thirty psychiatric patients scheduled for ECT were studied under propofol anesthesia. A 12-lead electrocardiogram was monitored to measure parameters. Muscle paralysis was achieved by administering succinylcholine 1 mg/kg intravenously, and the efficacy of ECT was determined by the tourniquet technique.

RESULTS

The RR interval and QT interval decreased significantly immediately after electrical stimulus, and returned to the baseline level 1 minute after electrical stimulus. In 25 out of 30 patients, the baseline value of QTc interval was higher than the normal limits, and the QTc interval decreased significantly for 2 minutes after electrical stimulus. In 27 out of 30 patients, the baseline values of QTD and QTcD were higher than the normal limits, and the QTD and QTcD increased significantly from immediately after electrical stimulus to 5 minutes after electrical stimulus.

CONCLUSIONS

The QTc interval, QTD, and QTcD, which were associated with increased risks of ventricular arrhythmias, increased significantly before anesthetic induction in patients with major depression. Electrical stimulus during ECT induced further increases of the QTD and QTcD.

Biological mechanisms of disease and death in Moscow: rationale and design of the survey on Stress Aging and Health in Russia (SAHR)

BACKGROUND

Prior research has revealed large differences in health and mortality across countries, socioeconomic groups, and individuals. Russia experiences one of the world's highest levels of all-cause and cardiovascular mortality, great mortality differences within the population, and a heavy burden of ill health. Psychological stress has been suggested as a likely explanation of health loss and premature death in Russia and Eastern Europe. However, physiological mechanisms connecting stress with health in Russia remain unclear since existing epidemiological data are scarce and limited to conventional risk factors.

METHOD AND DESIGN

The survey on Stress Aging and Health in Russia (SAHR) is addressing this knowledge gap by collecting an unusually rich database that includes a wide range of reported information, physical and cognitive health outcomes, and biomarkers in a sample of Muscovite men and women aged 55 and older. The total planned sample size is 2,000 individuals. The sample was randomly selected from epidemiological cohorts formed in Moscow between the mid-1970s and the 1990s and from medical population registers. The baseline data collection was carried out from December 2006 to June 2009. Interviews and medical tests were administered at hospital or at home according to standardized protocol. Questionnaire information includes health, socio-demographic characteristics, economic well-being, cognitive functioning, and batteries on stress and depression. Biomarkers include anthropometry, grip strength, resting ECG, conventional cardiovascular factors of risk such as lipid profile and blood pressure, and other biochemical parameters such as those related to inflammation, glucose and insulin resistance, coagulation, fibrinolysis, and stress hormones. In addition to these measurements, SAHR includes dynamic biomarkers provided by 24-hour ECG (Holter) monitoring. This method continuously registers the beat-to-beat heart rate in naturalistic conditions without restrictions on normal daily activities. It provides information about heart functioning, including heart rate variability and ischemic and arrhythmic events.Re-examination of the study subjects will be conducted in 2009-2011 and will focus on health, functional status, economic conditions, behaviors, and attitudes towards aging. The subjects are also followed up for mortality and non-fatal health events.

DISCUSSION

The SAHR will produce a valuable set of established and novel biomarkers combined with self-reported data for the international research community and will provide important insights into factors and biological mechanisms of mortality and health losses in Russia.

Cardiac repolarization interval in end-stage diabetic and nondiabetic renal disease

BACKGROUND AND HYPOTHESIS

QT interval length is influenced by autonomic nervous activity. In patients with diabetic autonomic neuropathy, both prolongation and shortening of ventricular repolarization has been reported. We studied diabetic and nondiabetic uremic patients to assess the effects of autonomic neuropathy on QT interval length.

METHODS

24-hour electrocardiogram recordings were performed in 12 diabetic and 11 nondiabetic renal transplantation patients, and in 12 control patients. Mean and corrected QT interval (QTc) during the 24-h period and intervals at predetermined heart rates at day and night periods were determined. The degree of autonomic neuropathy was assessed with cardiovascular autonomic function tests and measurement of heart rate variability.

RESULTS

In the diabetic group, severe autonomic neuropathy was present; in nondiabetic uremic patients, abnormalities were less severe. Mean QTc interval during 24 h was 444 +/- 24, 447 +/- 21, and 442 +/- 19 ms in the diabetic and nondiabetic uremic patients, and in the control groups, respectively, without any between-group difference. QT and QTc interval length did not differ among the groups when measured at heart rates of 70, 80, 90, or 100 beats/min.

CONCLUSIONS

In patients with autonomic failure caused by diabetes and/or uremia, QT interval length cannot be used as a diagnostic indicator of cardiac autonomic neuropathy.

Prolonged QT interval and reduced heart rate variability in patients with uncomplicated essential hypertension

A prolonged QT interval is a risk factor for ischemic heart disease in hypertensive subjects. Heart rate variability (HRV) is both an index of autonomic function and an important prognostic factor in several diseases. The aim of the present study was to evaluate the relation between a prolonged QT interval and autonomic nervous system function in patients with untreated uncomplicated essential hypertension. Two hundred and fifteen untreated patients with essential hypertension underwent a Holter ECG equipped with software dedicated to HRV and QT analyses. Nine percent of the patients showed a corrected QT (QTc) >or=440 ms. The HRV indexes in the time domain (SDNN, SDNN index, RMSSD, and pNN50) were significantly reduced in the patients with a prolonged QTc compared to those with a normal QTc (SDNN 24 h: 126.4+/-29.9 vs. 143.9+/-35.4 ms, p=0.02; SDNN index [nighttime]: 85.9+/-32.4 vs. 115.5+/-36.7 ms, p=0.0006; RMSSD 24 h: 22.2+/-7.7 vs. 31.2+/-13.0 ms, p=0.0007; pNN50 24 h: 4.4+/-4.9 vs. 9.7+/-8.4%, p=0.0006). The linear correlation analysis between QTc length and HRV parameters showed a significant negative correlation with all the time-domain indexes. Such a correlation was maintained for RMSSD 24 h, pNN50 24 h and SDNN index (nighttime) after correction for gender and age. The present study shows that, even prior to the development of cardiac hypertensive disease, a prolongation of the QTc and a reduced HRV, both markers of cardiovascular risk, coexist in a proportion of patients with untreated essential hypertension. Further studies are warranted to evaluate whether the combination of such markers can identify hypertensive patients at risk for life-threatening arrhythmias and sudden death. (Hypertens Res 2008; 31: 2003-2010).

Increased prevalence of ventricular fibrillation in patients with type 2 diabetes mellitus

Diabetes mellitus (DM) is an independent risk for cardiovascular disease. Furthermore, patients with DM have increased risk for ventricular arrhythmia that is thought to be secondary to coronary artery disease (CAD) or congestive heart failure (CHF). We hypothesized that DM may cause ventricular arrhythmias independent of CAD or CHF. Using a large database, we evaluated the occurrence of ventricular fibrillation in patients with DM adjusting for CAD and CHF. We used patient treatment files (PTF), documents of inpatients' admissions containing discharge diagnoses (ICD-9 codes) from all Veterans Health Administration Hospitals. The patients were stratified in two groups: ICD-9 code for DM (293 124) and a control group with ICD-code for hypertension (HTN) but no DM (552 623). ICD-9 codes for ventricular fibrillation were used for this study. We performed uni- and multivariant analysis adjusting for comorbid conditions. Ventricular fibrillation was present in 563 (0.2%) vs 781 (0.1%) in the control group. Using multivariate analysis, DM remained independently associated with ventricular fibrillation (odds ratio: 1.7; confidence interval: 1.5-1.9; P < 0.000). Patients with DM have significantly higher prevalence of ventricular fibrillation independent of CAD or CHF, which in part may explain the higher risk of sudden death in patients with DM.

Correlation between beat-to-beat QT interval variability and impaired left ventricular function in patients with previous myocardial infarction

BACKGROUND

Beat-to-beat QT interval variability (QTV) is associated with sudden cardiac death and New York Heat Association functional class severity. We sought to evaluate the relationship between QTV and left ventricular (LV) function in patients with previous myocardial infarction (MI).

METHODS

Fifty-nine patients with previous anterior MI were enrolled. LV ejection fraction (EF), LV end-systolic volume index (LVESVI), and LV end-diastolic volume index (LVEDVI) were measured by LV contrast angiography. QT interval was measured by automated analysis of 512-beat records of 12-lead electrocardiogram. The mean interval, standard deviation and variance in RR and QT intervals, and the QT variability index (QTVI) were calculated for each patient using two leads that corresponded with and without the infarction site. High-frequency power, low-frequency power, total-frequency power, and the ratio of low-frequency to high-frequency power in RR and QT intervals were calculated.

RESULTS

While measured indices of RR intervals and indices of QT intervals, which did not correspond with the infarction site, did not correlate with differences in LV function, measured indices of QT intervals, which corresponded with the infarction site, did correlate with differences in LV function. However, there were no correlations between the ratio of low-frequency to high-frequency power in QT intervals and EF or LVEDVI. Correlations between QTVI and LV function were observed, particularly between QTVI and LVESVI (r = 0.712, P < 0.0001).

CONCLUSION

In patients with previous anterior MI, there was variability in temporal dispersion of QT interval and a strong correlation between QTV corresponded with the infarcted site and LV function.

Evaluation of a 12-Lead Digital Holter System for 24-Hour QT Interval Assessment

BACKGROUND

Drug induced QT prolongation may precipitate life threatening cardiac arrhythmias. Evaluation of the QT prolonging effect of new pharmaceutical agents in a 'thorough QT/QTc study' is being mandated by FDA. The purpose of this study was to evaluate an automated 12-lead digital Holter system for a thorough QT/QTc study.

METHODS

Five healthy volunteers underwent 24-hour digital Holter monitoring. Each recording underwent a fully automated QT analysis (AQA) followed by an onscreen complete manual over read (MOR). Each recording was analyzed twice at least 2 weeks apart. The effect of data sampling (5-min segment/hour), the system sensitivity to detect 5-ms increase in QT, and the ability to assess circadian variation were evaluated.

RESULTS

The AQA resulted in identical QT for the first and second analyses, but with obvious errors in QT measurements. Compared to the complete onscreen MOR, the mean QT was longer with AQA (416 +/- 41 vs. 387 +/- 30 ms, p < 0.001), correlation; r = 0.3. The reproducibility of AQA with complete MOR was very good (QT: 387 +/- 30 vs. 387 +/- 30 ms, coefficient of variation: 0.2%, r = 0.986. The 5-min mean QT intervals correlated well with the hourly mean QT intervals (r = 0.994, p < 0.001, coefficient of variation = 1 ms) and both showed a similar circadian variation. The system was sensitive to detect a 5-ms change in QT intervals (5 +/- 2 ms, coefficient of variation = 0.6%, r = 0.998, p < 0.001).

CONCLUSIONS

The AQA is not an acceptable method, while the automatic analysis with complete MOR is a highly sensitive and reproducible method. Data sampling by analyzing 5-min segments per hour is sensitive and reproducible.

Modulation of ventricular repolarization and R-R interval is altered in patients with globally impaired cardiac 123I-MIBG uptake

BACKGROUND

Cardiac (123)I-metaiodobenzylguanidine (MIBG) imaging is widely used to assess cardiac sympathetic neuronal function. However, physiologic significance of impaired cardiac MIBG uptake is not fully elucidated. The purpose of the present study was to determine influences of abnormal cardiac sympathetic neuronal function on heart rate variability (HRV) and ventricular repolarization process.

METHODS

Twenty-nine patients with prior myocardial infarction were divided into two groups by a heart-to-mediastinum ratio (H/M) of MIBG scintigraphy. Ten patients with globally decreased MIBG uptake (group I: H/M < 1.5), 19 patients with partially decreased MIBG uptake (group II: H/M >or= 1.5), and 17 control subjects with normal MIBG uptake (group III) were studied. Holter recording and a standard 12-lead electrocardiography were used for evaluation of HRV, QT-RR relation, and QT dispersion.

RESULTS

Low, high, and total frequency components decreased in groups I and II, as compared to that of group III. The reduction of these frequency domain measures was more severe in group I than in group II, but the differences did not reach statistical significance. Circadian variation of frequency domain measures disappeared in group I. The slope of QT-RR relation was significantly greater in group I than in groups II and III. QT dispersion was also greater in group I (64 +/- 25 msec) than in group II (43 +/- 19 msec) and group III (28 +/- 9 msec).

CONCLUSION

These results suggest that patients with sympathetic neuronal dysfunction inferred from globally impaired cardiac MIBG uptake have an altered modulation of ventricular repolarization process as well as decreased HRV.

Drug-induced q-T prolongation

Drug therapy may induce Q-T prolongation by alteration of potassium ion currents in cardiac cells, resulting in abnormal repolarization. Q-T prolongation, whether congenital or acquired, has been associated with the development of the malignant dysrhythmia Torsade de Pointes (TdP), which may result in sudden death. Re-cent regulatory actions and drug withdrawals due to Q-T prolongation or TdP have focused attention on this issue. Although our understanding of the pathophysiology continues to evolve, both patient and medication factors contribute to the individual risk of drug-induced Q-T prolongation or TdP. The clinician should be aware of these issues when prescribing new drugs and should weigh the risks and benefits carefully when prescribing drugs known to prolong the Q-T interval.

Pharmacokinetic-pharmacodynamic modeling in the data analysis and interpretation of drug-induced QT/QTc prolongation

In this review, factors affecting the QT interval and the methods that are currently in use in the analysis of drug effects on the QT interval duration are overviewed with the emphasis on (population) pharmacokinetic-pharmacodynamic (PK-PD) modeling. Among which the heart rate (HR) and the circadian rhythm are most important since they may interfere with the drug effect and need to be taken into account in the data analysis. The HR effect or the RR interval (the distance between 2 consecutive R peaks) effect is commonly eliminated before any further analysis, and many formulae have been suggested to correct QT intervals for changes in RR intervals. The most often used are Bazett and Fridericia formulae introduced in 1920. They are both based on the power function and differ in the exponent parameter. However, both assume the same exponent for different individuals. More recent findings do not confirm this assumption, and individualized correction is necessary to avoid under- or overcorrection that may lead to artificial observations of drug-induced QT interval prolongation. Despite the fact that circadian rhythm in QT and QTc intervals is a well-documented phenomenon, it is usually overlooked when drug effects are evaluated. This may result in a false-positive outcome of the analysis as the QTc peak due to the circadian rhythm may coincide with the peak of the drug plasma concentration. In view of these effects interfering with a potential drug effect on the QTc interval and having in mind low precision of QT interval measurements, a preferable way to evaluate the drug effect is to apply a population PK-PD modeling. In the literature, however, there are only a few publications in which population PK-PD modeling is applied to QT interval prolongation data, and they all refer to antiarrhythmic agents. In this review, after the most important sources of variability are outlined, a comprehensive population PK-PD model is presented that incorporates an individualized QT interval correction, a circadian rhythm in the individually corrected QT intervals, and a drug effect. The model application is illustrated using real data obtained with 2 compounds differing in their QT interval prolongation potential. The usefulness of combining data of several studies is stressed. Finally, the standard approach based on the raw observations and formal statistics, as described in the Preliminary Concept paper of the International Conference on Harmonization, is briefly compared with the method based on population PK-PD modeling, and the advantages of the latter are outlined.

Blunted circadian variation in autonomic regulation of sinus node function in veterans with Gulf War syndrome

PURPOSE

To test the hypothesis that subtle abnormalities of the autonomic nervous system underlie the chronic symptoms reported by many Gulf War veterans, such as chronic diarrhea, dizziness, fatigue, and sexual dysfunction.

METHODS

Twenty-two ill Gulf War veterans and 19 age-, sex-, and education-matched control veterans underwent measurement of circadian rhythm of heart rate variability by 24-hour electrocardiography, ambulatory blood pressure recording, Valsalva ratio testing, sympathetic skin response evaluation, sweat imprint testing, and polysomnography. Investigators were blinded to case- or control-group status.

RESULTS

High-frequency spectral power of heart rate variability increased normally 2.2-fold during sleep in controls but only 1.2-fold in ill veterans (P <0.0001). In ill veterans as compared with controls, it was lower at night (P = 0.0006), higher during the morning (P = 0.007), but no different during the rest of the day (P = 0.8). The mean heart rate of ill veterans also declined less at night (P = 0.0002), and their corrected QT intervals tended to be longer over the full 24 hours (P = 0.07), particularly at night (P = 0.03). Blunting of the nocturnal heart rate dip in ill veterans was confirmed by 24-hour automatic ambulatory blood pressure monitoring (P = 0.05) and polysomnography (P = 0.03). These differences remained significant after adjusting for potential confounders. Cases and controls were similar on measures of sympathetic adrenergic and sudomotor function, sleep architecture, respiratory function, and circadian variation in blood pressure and body temperature.

CONCLUSION

Some symptoms of Gulf War syndrome may be due to subtle autonomic nervous system dysfunction.

Cardiac autonomic balance and QT dispersion during head-up tilt testing in diabetics with and without sensory neuropathy

BACKGROUND

Cardiovascular autonomic dysfunction is associated with higher mortality in diabetics. However, detection methods of early cardiac diabetic dysautonomia, and its correlation with severity of sensory neuropathy, have not been described.

METHODS

We analyzed the heart rate variability (HRV) by spectral analysis and QT dispersion in 23 diabetics with and without sensory neuropathy, and in 5 age-matched controls, in the supine position and during head-up tilt testing (HUT). Diabetics were divided into 3 groups according to the degree of sensory neuropathy.

RESULTS

In the spectral analysis of HRV, the high frequency components in the supine position decreased as a function of severity of the neuropathy. High frequency in diabetics was significantly decreased in the supine position, even in absence of sensory neuropathy. The low/high frequency ratio (L/H) in the supine position was similar among controls and the 3 patient groups. L/H in diabetics with moderate or severe neuropathy did not increase from the supine to the upright position. Baseline QT dispersion increased proportionally to the severity of sensory neuropathy and, in patients without apparent sensory neuropathy, QT dispersion increased significantly during HUT.

CONCLUSIONS

The spectral analysis of HRV and measurements of QT dispersion, before and during HUT, were reliable detection methods of early abnormalities in autonomic balance and may predict a risk of sudden cardiac death in diabetics.

Changes in cardiac repolarization during clinical episodes of nocturnal hypoglycaemia in adults with Type 1 diabetes

AIMS/HYPOTHESIS

Experimental hypoglycaemia leads to abnormal cardiac repolarization manifest by a lengthened QT interval and caused by adrenergic stimulation. However it is less clear whether spontaneous clinical episodes lead to similar changes. We have therefore measured cardiac ventricular repolarization and counterregulatory responses in patients with Type 1 diabetes during hypoglycaemic and euglycaemic nights.

METHODS

We studied 22 patients with Type 1 diabetes (mean age 40.4+/-17.2 years, duration of diabetes 17.2+/-9.3 years, HbA1c 8.2+/-1.2% overnight). Measurements were taken hourly of blood glucose, plasma potassium, catecholamines and high resolution electrocardiograms.

RESULTS

Hypoglycaemia (blood glucose level <2.5 mmol/l) occurred on 7 of the 22 nights. During overnight hypoglycaemia, QTc interval increased by 27 ms (+/-15) above baseline, compared with 9 ms (+/-19) during nights with no nocturnal hypoglycaemia (p=0.034, 95%CI 2, 35). Adrenaline increased by 0.33 nmol/l (+/-0.21) above baseline during hypoglycaemia, compared with -0.05 nmol/l (+/-0.08) during euglycaemia (p=0.001, 95%CI 0.19, 0.56 nmol/l). There was no significant difference between potassium, and noradrenaline concentrations between the two groups.

CONCLUSION/INTERPRETATION

QTc interval lengthens significantly during spontaneous nocturnal hypoglycaemia. Increases are generally less than those observed during experimental hypoglycaemia and could reflect attenuated sympathoadrenal responses during clinical episodes. The clinical relevance of these changes is uncertain but is consistent with the hypothesis that clinical hypoglycaemia can cause abnormal cardiac repolarization and an attendant risk of cardiac arrhythmia.

Beat-to-beat assessment of QT/RR interval ratio in severe heart failure and overt myocardial ischemia: a measure of electrical integrity in diseased hearts

The study was designed to assess the beat-to-beat variation of ventricular repolarization in patients with myocardial ischemia, hear failure, and in normal subjects. Autonomic tone may alter the dynamic QT/RR interval relation and thus may be involved in ventricular arrhythmia development, especially in the diseased heart. The study included 145 patients (age 16-86 years) with CHF (LVEF < or = 0.30) or unstable angina pectoris (LVEF > 0.60). The control group consisted of healthy volunteers giving physiological baseline measures for the evaluated parameters: cycle length, QT interval, and QT/RR interval ratio during three time periods. In patients with myocardial ischemia (LVEF > 0.60) and healthy subjects the QT/RR interval ratio did not reveal significant differences between both groups (QT/RR(CAD) = 0.36 +/- 0.77 vs QT/RR(controls) = 0.28 +/- 0.83; NS). In sharp contrast, in patients with severe heart failure, RR dependent instantaneous variation of the QT interval was almost missing and regression line analysis disclosed a QT/RR interval slope substantially enhanced by 196% (compared to normal subjects) and 131% (compared to CAD patients; P < 0.05) with a complete loss of circadian modulation (QT/RR(CHF) = 0.83 +/- 0.71 vs QT/RR(CAD) = 0.36 +/- 0.77 vs QT/RR(controls) = 0.28 +/- 0.83; P < 0.05). Beat-to-beat QT interval assessment provides a dynamic parameter of physiological and altered repolarization in defined study groups. Compared to other groups (preserved LVEF), patients with left ventricular impairment exhibited a significantly increased sensitivity of repolarization to cycle length (enhanced QT/RR interval ratio) and a blunted circadian modulation of the QT interval. This is consistent with concept that increased repolarization disparity may be deleterious being a potential pathophysiological basis for enhanced arrhythmic risk.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

Circadian rhythm of the corrected QT interval: impact of different heart rate correction models

A reduced circadian pattern in the QTc interval has been repeatedly reported to provide prognostic information in cardiac patients. However, the results of studies in healthy subjects in which different heart rate correction formulas were used are inconsistent regarding the presence and extent of diurnal variations in QTc. This study compared the diurnal variations in QTc obtained with four frequently used heart rate correction models with those based on individually optimized heart rate correction. In 53 subjects (25 men aged 27 +/- 7 years and 28 women aged 27 +/- 9 years) 12-lead digital ECGs were obtained every 30 seconds during 24 hours. The QT interval was measured automatically by six different algorithms provided by a commercially available device. The QT/RR relation was estimated by four common heart rate correction models and by an individually optimized correction model, QTc = QT/RR alpha. In each 24-hour recording, RR, QT, and WTc intervals of separate ECG samples were averaged over 10-minute intervals. Marked differences were found in the extent of the circadian pattern of QTc obtained with different formulas for heart rate correction. Under and overcorrection of the QT interval resulted in significant over- or underestimation of the circadian pattern. Thus, the extent of circadian variation in QTc depends highly on the heart rate correction formula used. To obtain proper insight regarding diurnal variation in QTc prolongation during pharmacologic therapy and/or to assess higher risk due to impaired autonomic regulation of ventricular repolarization, individualized heart rate correction is necessary.

Antipsychotic-related QTc prolongation, torsade de pointes and sudden death

Sudden unexpected deaths have been reported with antipsychotic use since the early 1960s. In some cases the antipsychotic may be unrelated to death, but in others it appears to be a causal factor. Antipsychotics can cause sudden death by several mechanisms, but particular interest has centred on torsade de pointes (TdP), a polymorphic ventricular arrhythmia that can progress to ventricular fibrillation and sudden death. The QTc interval is a heart rate-corrected value that represents the time between the onset of electrical depolarisation of the ventricles and the end of repolarisation. Prolongation of the QTc interval is a surrogate marker for the ability of a drug to cause TdP. In individual patients an absolute QTc interval of >500 msec or an increase of 60 msec from baseline is regarded as indicating an increased risk of TdP. However, TdP can occur with lower QTc values or changes. Concern about a relationship between QTc prolongation, TdP and sudden death applies to a wide range of drugs and has led to the withdrawal or restricted labelling of several. Among antipsychotics available in the UK, sertindole was voluntarily suspended, droperidol was withdrawn, and restricted labelling introduced for thioridazine and pimozide. The degree of QTc prolongation is dose dependent and varies between antipsychotics reflecting their different capacity to block cardiac ion channels. Significant prolongation is not a class effect. Among currently available agents, thioridazine and ziprasidone are associated with the greatest QTc prolongation. Virtually all drugs known to cause TdP block the rapidly activating component of the delayed rectifier potassium current (I(kr)). Arrhythmias are more likely to occur if drug-induced QTc prolongation coexists with other risk factors, such as individual susceptibility, presence of congenital long QT syndromes, heart failure, bradycardia, electrolyte imbalance, overdose of a QTc prolonging drug, female sex, restraint, old age, hepatic or renal impairment, and slow metaboliser status. Pharmacodynamic and pharmacokinetic interactions can also increase the risk of arrhythmias. Further research is needed to quantify the risk of sudden death with antipsychotics. The risk should be viewed in the context of the overall risks and benefits of antipsychotic treatment. It seems prudent, where possible, to select antipsychotics that are not associated with marked QTc prolongation. If use of a QTc-prolonging drug is warranted, then measures to reduce the risk should be adopted.

Abnormalities of rate-corrected QT intervals in Parkinson's disease-a comparison with multiple system atrophy and progressive supranuclear palsy

A number of patients with Parkinson's disease (PD) and multiple system atrophy (MSA), in whom sudden death does occur occasionally, have QT or rate-corrected QT (QTc) interval prolongation on electrocardiogram (ECG). Although these QT or QTc interval abnormalities are likely related to autonomic dysfunction, the pathophysiology remains unknown. The aim of this study was to compare the degree of QTc interval prolongation among akinetic-rigid syndromes, namely PD and related disorders, and to evaluate the relationship between QTc prolongation and severity of autonomic dysfunction. Thirty-four patients with PD, 22 with MSA, 11 with progressive supranuclear palsy (PSP) and 30 healthy controls underwent standard autonomic function tests, and electrocardiography variables (RR, QT and QTc intervals) were measured by an ECG recorder with an automated analyzer. The relationship between QTc interval and cardiovascular reflex tests were also analyzed. Orthostatic hypotension and decreased heart rate in response to respiratory stimuli were prominent in MSA, while these were relatively mild in PD. Unlike the RR and QT intervals, the QTc interval significantly differed among all groups (p<0.01). The QTc interval was significantly prolonged in PD (409+/-17 ms; p<0.001) and MSA (404+/-14 ms; p<0.05) compared with healthy controls (394+/-19 ms). Neither autonomic dysfunction nor QTc interval prolongation was evident in PSP. QTc intervals and cardiovascular reflexes did not correlate, except for Valsalva ratio. The QTc interval was obviously prolonged in PD patients to an extent that could not be accounted for simply by autonomic dysfunction levels. MSA patients showed slightly prolonged QTc intervals in spite of marked cardiovascular autonomic dysfunction. Abnormalities of the QTc may reflect the degeneration of cardioselective sympathetic and parasympathetic neurons that cannot be fully captured by cardiovascular autonomic function tests.

Influence of cardiac autonomic neuropathy on heart rate dependence of ventricular repolarization in diabetic patients

OBJECTIVE

Prolongation of the QT interval and increased QT dispersion are associated with a poor cardiac prognosis. The goal of this study was to assess the long-term influence of the autonomic nervous system on the heart rate dependence of ventricular repolarization in patients with diabetic autonomic neuropathy (DAN).

RESEARCH DESIGN AND METHODS

We studied 27 subjects (mean age 51.8 years) divided into three age- and sex-matched groups: nine control subjects, nine diabetic subjects with DAN (mostly at a mild stage; DAN+), and nine diabetic subjects without DAN (DAN-). DAN was assessed on heart rate variations during standard maneuvers (Valsalva, deep-breathing, and lying-to-standing maneuvers). No subject had coronary artery disease or left ventricular dysfunction or hypertrophy, and no subject was taking any drugs known to prolong the QT interval. All subjects underwent electrocardiogram and 24-h Holter recordings for heart rate variations (time and frequency domain) and QT analysis (selective beat averaging QT/RR relation, nocturnal QT lengthening).

RESULTS

Rate-corrected QT intervals (Bazett formula) did not differ significantly between the three groups. The diurnal and nocturnal levels of low frequency/high frequency, an index of sympathovagal balance, were significantly reduced in DAN+ subjects. Using the selective beat-averaging technique, a day-night modulation of the QT/RR relation was evidenced in control and DAN- subjects. This long-term modulation was significantly different in DAN+ subjects, with a reversed day-night pattern and an increased nocturnal QT rate dependence.

CONCLUSIONS

In diabetic patients with mild parasympathetic denervation, QT heart rate dependence was found to be impaired, as determined by noninvasive assessment using Holter data. Analysis of ventricular repolarization could represent a sensitive index of the progression of neuropathy. The potential prognostic impact of a reversed day-night pattern with steep nocturnal QT/RR relation still remains to be defined.

Diabetes, heart rate, and mortality

A large body of evidence indicates that a persistently high heart rate is associated with a significant risk for higher mortality and sudden death in individuals with a variety cardiovascular disorders, as well as in the general population. Heart rates elevated beyond a certain threshold have been found to be a risk factor for mortality in patients with hypertension, in survivors of myocardial infarction, and in patients with impaired cardiac function. Conversely, a naturally slow heart rate, or one that is slow by virtue of sympathetic blockade induced by pharmacologic agents, may result in longer survival. This is particularly evident in the case of beta-adrenergic blocking drugs, especially in patients after myocardial infarction and in those with acute as well as chronic cardiac failure, a syndrome in which there is a complex neurohormonal disturbance with elevated heart rate. Persistently elevated heart rate is also a feature of diabetes mellitus associated with autonomic neuropathy. Whether this also constitutes an independent risk factor for sudden and augmented mortality is not well defined. In this review, the data on the role of increased heart rate as a risk factor for mortality are examined in the context of other factors that may have therapeutic implications.

Does heart rate identify sudden death survivors? Assessment of heart rate, QT interval, and heart rate variability

The objective was to test whether the circadian variability of several electrocardiographic variables distinguishes sudden cardiac death survivors from heart disease patients without a history of cardiac arrest and from normal subjects. Heart rate, heart rate variability, and QT interval have been reported to identify survivors of sudden cardiac death. Computer-assisted continuous QT measurement and heart rate variability analysis were performed on 24-hour Holter records for three groups: (1) 14 sudden death survivors; (2) 14 control patients with diagnosis and therapy matched to survivors; and (3) 14 healthy subjects. There were no significant differences in 24-hour mean RR and QT intervals between groups. However, heart rate was significantly different between the three groups at night but not during the day because the expected nighttime decline was markedly blunted in survivors and somewhat blunted in control patients. The QT interval and frequency domain heart rate variability measures followed a similar circadian pattern. The mean QTc was significantly longer in control patients. The QTc had a wide range in all groups, but less in sudden death survivors. Of ten common time and frequency domain heart rate variability indices, only SDANN and SDNN were significantly lower in sudden death survivors. Reduced circadian variation of heart rate, with marked blunting of the nighttime heart rate decline, identifies sudden cardiac death survivors as well as does SDANN and SDNN, and, in contrast to heart rate variability measures, can easily be obtained from a Holter report without complex calculations.

Differences in beat-to-beat variability of the QT interval between day and night

The objective of this study was to evaluate the beat-to-beat variability of the QT interval during the day and night. A new algorithm was used to detect the onset of the QRS, the apex of the T wave, and end of the T in ambulatory electrocardiographic recordings. Beat-to-beat variability of QT, QaT, and QTc during the day and night was studied in the time, frequency, and chaotic domains. Participants were adults without clinical evidence of heart disease. Although the QT duration was higher (p = 0.0001) at night, the beat-to-beat variability of this interval was lower: in the time domain (decreased standard deviation, p = 0.0005), in the frequency domain (decreased low-frequency power of the spectra, p = 0.004), and the chaotic domain (tighter clustering of the points in the Poincaré plots). The high-frequency to low-frequency ratio of the power spectra of the QT (and the RR) was higher (p = 0.03) at night. Beat-to-beat QT variability in the time, frequency, and chaotic domains is decreased at night with shift of the QT modulation to higher frequencies corresponding to respiration and representing vagal preponderance. The techniques presented here and the findings in normal subjects may be useful in evaluating the risk for arrhythmic events in patients with heart disease.

QTc interval and scintigraphically assessed myocardial perfusion in newly diagnosed and long-term type 1 diabetes mellitus

In diabetes mellitus, heart rate corrected QT interval (QTc) has been suggested to be related to ischemic heart disease and increased risk of sudden cardiac death. The aim of the study was to analyze the length of QTc interval with regard to global and regional myocardial perfusion in type 1 diabetic patients. Myocardial perfusion was investigated in 20 newly diagnosed and 40 long-term type 1 diabetic patients without clinical evidence for coronary artery disease by means of Tc-99-methoxyisobutylisonitrile (Tc-99m-MIBI)-scintigraphy (myocardial uptake (MU) score: 1-6). Five consecutive RR and QT intervals of resting electrocardiogram (ECG) tracing were measured and corrected for the previous cycle length. ECG-based cardiac autonomic neuropathy (CAN) was assessed with five cardiac reflex tests. Length of QTc interval was 423+/-29 ms in newly diagnosed and 433+/-26 ms in long-term type 1 diabetic patients. Nine (45%) newly diagnosed and 18 (45%) long-term diabetic patients demonstrated a prolonged QTc interval (>440 ms). Both newly diagnosed and long-term diabetic patients did not display significant global or regional myocardial perfusion defects (mean MU scores<3). In newly diagnosed diabetic patients, the length of QTc interval was related to global, posterior and septal Tc-99m-MIBI uptake (p<0.05, respectively). In long-term diabetic patients, the length of QTc interval was associated with apical Tc-99m-MIBI uptake (p<0.05). Two (10%) newly diagnosed and 19 (48%) long-term type 1 diabetic patients demonstrated ECG-based CAN. In long-term type 1 diabetic patients, global myocardial Tc-99m-MIBI uptake did not differ significantly between patients with and without CAN. QTc interval was not significantly different between diabetic patients with and without ECG-based CAN (433+/-19 ms vs. 428+/-17 ms). Long-term diabetic patients, of whom 10 (25%) patients had microalbuminuria and seven (18%) patients had macroalbuminuria, demonstrated an association between QTc interval and albuminuria (p<0.05). The results somewhat suggest an association between QTc interval and vascular factors in type 1 diabetes mellitus. Future investigations are required to analyze the role of QTc interval in the pathogenesis of abnormalities of myocardial perfusion.

Understanding autonomic sympathovagal balance from short-term heart rate variations. Are we analyzing noise?

Heart rate variations reflect the output of the complex control of the heart mediated by the autonomic nervous system. Because of that, they also encode different types of information, namely the efferent outflow of reflex mechanisms involved in the beat-to-beat control of cardiac function, the efferent activity of neurohumoral elements involved in the control of other cardiovascular parameters and random noise resulting from the hysteresis of the different controllers. The degree to which power spectrum estimation methods will uncover the periodic component of heart rate variations is in direct relation with the status of the system under study. Although the utility of spectral methods is now established in mammalian research, very little is known on the utility of these techniques in non-mammalian cardiovascular research. This review covers this space by discussing the physiological significance of heart rate variations in non-mammalian vertebrates. A detailed account of the different steps of the technique, its limitations and the ways to overcome these problems are also presented. These are: the recording of the cardiac event signal, the detection and digital processing methods, the satisfaction of stationarity conditions, the problem of spectral leakage and the different methods to estimate the power spectrum.

Pharmacokinetics and pharmacodynamics following intravenous doses of azimilide dihydrochloride

Azimilide pharmacokinetics and pharmacodynamics were characterized in a safety and tolerance study of intravenously administered azimilide dihydrochloride. This was a parallel-group design (seven treatments), and 68 healthy volunteers received the drug. Single intravenous infusion doses (4.5 to 9 mg/kg) were administered over 60 minutes, and single 4.5 mg/kg intravenous infusion doses were also given over 15 or 30 minutes. Blood and urine specimens were collected and analyzed for azimilide and metabolites. QTc was measured as a marker of class III antiarrhythmic activity. Azimilide pharmacokinetics were dose proportional and did not differ among infusion rates. Azimilide pharmacodynamics did not differ among treatments. Mean Emax ranged from 23 to 28% delta QTc, with mean EC50 of 509 to 566 ng/mL. Peak circadian variation in QTc was equivalent to 14% of Emax. Rapid equilibration occurred between blood and the biophase. Unconfounded pharmacodynamic estimates required inclusion of circadian QTc variation in the pharmacodynamic model.

Evaluation of autonomic influences on QT dispersion using the head-up tilt test in healthy subjects

Our objective was to examine the autonomic influence on QT interval dispersion using the head-up tilt test in healthy subjects. RR and QT intervals, heart rate variability, and plasma norepinephrine concentration were measured in the supine position and tilting to 70 degrees for 20 minutes using a footboard support in 15 healthy male volunteers (mean age +/- SD: 28.0 +/- 4.5 years). The rate-corrected QT interval (QTc) was calculated using Bazett's formula, and QT and QTc dispersions were defined as the maximum minus minimum values for the QT and QTc, respectively, from the 12-lead ECG. Spectral analysis of the heart rate variability generated values for the low- and high-frequency powers (LF and HF) and their ratio (LF/HF). Compared with values obtained in the supine position, tilting significantly increased QT (P < 0.05) and QTc dispersion (P < 0.01), the LF/HF ratio (P < 0.0001), and plasma norepinephrine concentration (P < 0.0001), and significantly decreased HF (P < 0.0001). QTc dispersion was positively correlated with the LF/HF ratio and plasma norepinephrine concentration, and negatively correlated with HF. These results suggest that head-up tilt testing increases QT dispersion by increasing sympathetic tone and/or decreasing vagal tone in healthy subjects.

Autonomic modulation of QT intervals in post-myocardial infarction patients with and without ventricular fibrillation

The autonomic nervous system plays an important role in the genesis of sudden cardiac death. The aim of this study was to evaluate spatial autonomic QT interval modulation at the myocardial level. Circadian patterns of QT intervals and heart rate variability (HRV) components and their hourly linear correlations were determined by Holter recordings in 15 healthy subjects (controls), in 15 post-myocardial infarction (MI) patients resuscitated from ventricular fibrillation (VF) (VF group), and in 15 matched infarction patients without a history of arrhythmia events (MI group). QT intervals were measured in modified leads V1 and V5 individually at same stable heart rates during each hour and related to hourly measures of HRV. Controls had highly significant correlations between QT intervals and the high-frequency component of HRV (parasympathetic modulation), and between QT intervals and low- to high-frequency ratio (sympathetic modulation) uniformly in both leads (r from 0.62 to 0.81, p <0.001). The MI group had impaired sympathetic modulation in V5 (r = 0.34, p = NS), but had uniform and exaggerated sensitivity to parasympathetic modulation. In the VF group the QT difference between V1 and V5 leads correlated with parasympathetic modulation (r = 0.401, p <0.05) and sympathetic modulation (r = 0.446, p <0.05). Thus, normal subjects exhibit spatially uniform autonomic QT modulation. Myocardial damage can result in abolished, exaggerated, or regionally discordant QT modulation, and this may generate arrhythmic vulnerability.

Circadian variation of the QT interval in patients with sudden cardiac death after myocardial infarction

To evaluate the potential prognostic value of the circadian variation of QT intervals in predicting sudden cardiac death (SCD) in patients after myocardial infarction (MI), 15 pairs of post-MI patients (15 died suddenly within 1 year after MI [SCD victims] and 15 remained event-free [MI survivors]) were studied (mean age 60 +/- 8 years; 24 men and 6 women). The pairs were matched for age, gender, infarct site, presence of Q wave, left ventricular ejection fraction, thrombolytic and beta-blocker therapy. Fourteen normal subjects served as controls (mean age 55 +/- 9 years; 12 men). A 24-hour Holter electrocardiographic (ECG) recording was obtained from each subject. All recordings were analyzed using a Holter ECG analyser. QT, RR, and heart rate-corrected QT intervals (QTc) were automatically calculated by the analyzer, and hourly and 24-hour mean values of each measurement were derived from each recording. There was a pronounced circadian variation in the QT interval in parallel with the trend in the RR interval in normal subjects and in MI survivors. Circadian variation in both indexes was blunted in SCD victims. The QT interval was significantly longer at night than during the day in normal subjects (388 +/- 28 vs 355 +/- 21 ms, p = 0.001) and in MI survivors (358 +/- 25 vs 346 +/- 15 ms, p = 0.008), but not in SCD victims (357 +/- 32 vs 350 +/- 31 ms, p = 0.6). The 24-hour mean value of the QT interval in SCD victims did not differ significantly from that in normal subjects or MI survivors. The QT interval at night was significantly shorter in SCD victims than in normal subjects (357 +/- 32 vs 388 +/- 28 ms, p = 0.02), but daytime values were similar. The QT interval in SCD victims did not differ significantly from that of MI survivors at any time. The QTc interval exhibited a small circadian variation in normal subjects. This variation was abolished in SCD victims and MI survivors. The 24-hour mean value of QTc was significantly longer in SCD victims than in normal subjects (424 +/- 25 vs 402 +/- 21 ms, p = 0.02), and in MI survivors (424 +/- 25 vs 404 +/- 32 ms, p < 0.05). The QTc interval of SCD victims differed from that of normal subjects during both the day (421 +/- 25 vs 400 +/- 17 ms, p = 0.02) and night (424 +/- 26 vs 403 +/- 23 ms, p = 0.03). Thus, blunted circadian variation in QT intervals, abolished circadian variation in QTc intervals, and prolonged QTc intervals may suggest an increased risk of SCD in patients after MI.

Q-T interval prolongation in cirrhosis: prevalence, relationship with severity, and etiology of the disease and possible pathogenetic factors

Prolonged Q-T interval predicts severe arrhythmias and sudden death, and has been shown to occur in alcoholic liver disease and cirrhotic patients who are candidates for liver transplantation. This study first evaluated the prevalence of prolonged Q-T interval in a large population of unselected patients with cirrhosis, and assessed the relationship between abnormal Q-T, etiology, and severity of liver disease and mortality of patients. Possible causes of Q-T abnormality were also explored. Ninety-four patients with cirrhosis without overt heart disease and 37 control subjects with mild chronic active hepatitis were enrolled. Rate-corrected Q-T interval (Q-Tc) was assessed along with routine liver tests, Child-Pugh score, serum bile salts, electrolytes and creatinine, plasma renin activity, aldosterone, norepinephrine, atrial natriuretic factor and, gonadal hormones. Q-Tc was longer in patients with cirrhosis than in controls (440.3 +/- 3.2 vs. 393.6 +/- 3.7 ms; P < .001) and prolonged (> 440 ms) in 44 patients (46.8%) and 2 controls (5.4%; P < .001). Q-Tc length was not influenced by the etiology of cirrhosis and correlated with Child-Pugh score (r = .53; P < .001), liver tests such as prothrombin activity, and serum concentrations of albumin and bilirubin, plasma bile salts, and plasma norepinephrine. Multivariate analysis showed that only Child-Pugh score and plasma norepinephrine were independently correlated with Q-Tc duration. Over a median follow-up period of 19 months (range, 2-33 months), patients with Q-Tc longer than 440 ms had a significantly lower survival rate than those with normal Q-Tc. Q-T interval is frequently prolonged in patients with cirrhosis, regardless the etiology of the disease, worsens in parallel with the severity of the disease, and may have an important prognostic meaning. In addition to other undefined factors related to the severity of cirrhosis, sympathoadrenergic hyperactivity may play a pathogenetic role.

Changes in arrhythmia profile and heart rate variability during abrupt withdrawal of antiepileptic drugs. Implications for sudden death

Sudden unexpected death (SUD) has been associated with low or undetectable concentrations of antiepileptic drugs in patients with epilepsy suggesting that a sudden fall in plasma levels of these drugs might be a critical factor for the occurrence of SUD. We studied the changes in arrhythmia profile and heart-rate variability, during abrupt withdrawal of carbamazepine and phenytoin treatment in 10 patients with side effects on these drugs. Continuous ECG recording and daily measurements of drug plasma concentrations were performed from the last day of steady-state treatment and the following 4 days. Three patients had a 10-fold increase in ventricular premature beats. In addition, there was a significant reduction in heart-rate variability, assessed over 24 hours, in both the time (SDNN index, P = 0.03) and frequency domains from days 1-5. In the frequency domain analysis there was a significant reduction in total power (P = 0.01), very-low-frequency power (P = 0.004) and in low-frequency (LF) power (P = 0.01). Similar reductions in heart-rate variability and increases in ventricular automaticity have been associated with increased mortality in other patient groups. Two factors that might contribute to the increased rate of SUD in patients with epilepsy have thus been identified.

Changes in repolarization dynamicity and the assessment of the arrhythmic risk

At the present time, the assessment of the arrhythmic risk from surface ECG recordings is built on time-domain and frequent-domain analysis of high resolution ECG acquisition together with interlead variability of QT interval duration (QT dispersion). The corresponding raw ECG tracings are obtained in resting conditions. However, the dynamic aspects of the ECG signal is a rapidly evolving matter of interest. In addition to the beat-to-beat oscillations of the ventricular repolarization amplitude (QT alternans), there is growing evidence that the patterns of QT interval shortening with increasing heart rate are linked to susceptibility to ventricular arrhythmias. In this report, we will mainly address the association between QT dynamicity and the risk of developing torsades de pointes.

An abnormal relationship between blood pressure and pulse rate in amyotrophic lateral sclerosis

OBJECTIVES

To clarify the autonomic dysfunctions in amyotrophic lateral sclerosis (ALS) patients, we investigated the relationship between blood pressure (BP) and pulse rate (PR), QTc interval, and coefficient of variation in the R-R interval (CVR-R) in ALS patients.

MATERIALS AND METHODS

BP and PR were determined automatically every 30 min for 24 h in 6 patients with ALS and in 18 healthy age-matched volunteers. The QTc interval and CVR-R were also evaluated using an electrocardiogram.

RESULTS

The relationship between BP and PR was lost in the ALS group. There was no significant difference in the QTc interval or CVR-R between the ALS and control groups.

CONCLUSION

We speculate that the imbalance between BP and PR observed in the ALS group reflects a disorder in the balance between the sympathetic and parasympathetic nervous systems.

Can abnormalities of ventricular repolarisation identify insulin dependent diabetic patients at risk of sudden cardiac death?

OBJECTIVE

To study the possible association or QT dispersion and mean QTc intervals, as measured from standard 12 lead electrocardiograms, with baroreceptor-cardiac reflex sensitivity (BRS) in insulin dependent diabetic patients.

DESIGN

Comparative study of non-invasive assessment of BRS, QT interval, and QT dispersion.

SETTING

Large teaching hospital.

SUBJECTS

31 young asymptomatic, normotensive, insulin dependent diabetic patients, aged 20-55 years with normal clinical autonomic function.

METHODS

QT intervals and QT dispersion were measured by a single observer blinded to other data about the patients. BRS was measured after activating the baroreflex with a Valsalva manoeuvre, and the rate in change of R-R interval to increasing systolic pressure during phase 4 was measured; in addition sequence analysis of resting systolic blood pressure and heart rate was performed during standing. The alpha coefficient--an index of the overall gain of the baroreflex mechanisms--was estimated from spectral analysis data of systolic blood pressure and pulse interval variability.

RESULTS

Mean (SD) QTc interval was 406 (23) ms, QT dispersion was 44 (13) ms. There was no association between QT dispersion and any measurement of BRS. There was a negative correlation between mean QTc intervals and sequence analysis BRS (r = -0.355, P = 0.049), but no association with Valsalva BRS. The alpha coefficient, showed a significant negative correlation with mean QTc (r = -0.42, P = 0.008).

CONCLUSIONS

Abnormal BRS may be reflected in the heart by global prolongation of ventricular repolarisation, but not by dispersion of ventricular repolarisation. This may, in part, explain the increase in sudden cardiac death seen in IDDM patients.

Circadian variation of QT interval dispersion: correlation with heart rate variability

The circadian variation of QT interval dispersion and its correlation with heart rate variability (HRV) was examined in 17 normal subjects by using 24-hour recordings of three-lead electrocardiograms. Measurements of HRV, R-R intervals, and QT intervals were made for the first 6 minutes of each hour over a 24-hour period. Spectral analysis of HRV yielded low-frequency power (LF) (0.04-0.15 Hz), high-frequency power (HF) (0.15-0.40 Hz), and the ratio of LF to HF (LF/HF). A rate-corrected QT interval (QTc) was calculated by Bazett's formula, and QT and QTc dispersion was defined as the difference between the maximum and minimum values in any two leads. High-frequency power and QT interval were greater at night than during the day: conversely, LF/HF and dispersion of QT and QTc were greater during the day. The QTc interval remained virtually unchanged throughout the 24-hour period. The dispersion of QTc showed a significant negative correlation with HF and a significant positive correlation with LF/HF. The results suggest that an increased sympathetic tone or a decreased vagal tone increases QT dispersion in healthy subjects.

Increased QT dispersion and cardiac adrenergic dysinnervation in diabetic patients with autonomic neuropathy

Diabetic patients with autonomic neuropathy showed an increase in QTc dispersion correlated with cardiac adrenergic dysinnervation. A larger prospective study in a diabetic population is needed to assess whether QT dispersion increases the risk of arrhythmogenicity through autonomic dysfunction.