Sympathetic and vagal influences on rate-dependent changes of QT interval in healthy subjects

Analysis of the effect of cortisone on the QT interval

BACKGROUND

Cardiac death caused by malignant arrhythmias is very prevalent. Prolongation of the QT interval is a relevant aspect in arrhythmia mechanisms. Prior studies have revealed that the QTc interval could be shortened by cortisone. Moreover, in an animal model of long QT syndrome, cortisone treatment shortens the ventricular action potential duration. The present study investigated the effect of methylprednisolone (MPS) on the QTc interval in cardiovascularly healthy humans.

METHODS

Patients who had just been diagnosed with multiple sclerosis receiving MPS therapy were analysed prospectively. Demographic data, laboratory values, anti-arrhythmic medication and baseline and follow-up ECGs were extracted from the patients' medical records.

RESULTS

Seventy-eight patients were included. The mean ± standard deviation age was 47 ± 15 years. The values of the electrolytes were normal. All patients were treated with MPS for 3 or 5 days. The heart rate increased at the beginning of MPS therapy and decreased during the subsequent period. ECG measurements showed that the QTc interval was prolonged at the beginning of MPS therapy and shortened over the course of treatment. The longest QTc intervals were obtained by calculation with Bazett's formula.

CONCLUSIONS

In humans, cortisone shortens the QTc interval over time. The analysis indicates a cumulative effect of cortisone that lasts longer. The results of our pilot study reveal that cortisone might be added to therapeutic strategies in patients with long QT syndromes. Further clinical studies have to be carried out to analyze potential clinical options.

The effect of coronary slow flow on ventricular repolarization parameters

INTRODUCTION

Ischemia due to microvascular dysfunction may be responsible for the heterogeneity of ventricular repolarization in coronary slow flow. To our knowledge, there is no study in which QT interval, Tp-Te interval, index of cardiac-electrophysiological balance (iCEB), and frontal QRS-T angle were evaluated together in patients with CSF. In this study, we examined for the first time the relationship between all these myocardial repolarization parameters and CSF.

MATERIALS AND METHODS

The study group included 178 patients (99 female, mean age: 50.6 ± 8.6 years) with isolated CSF without stenotic lesions and with angiographically proven normal coronary arteries. The control group included 120 patients (71 female, mean age: 49.3 ± 9.4 years) with normal coronary angiography. QRS duration, QT interval, QTc interval, Tp-Te interval, Tp-Te/QT, Tp- Te/QTc, iCEB score, and frontal QRS-T angle were calculated from 12‑lead ECGs.

RESULTS

There was no significant difference in demographic parameters between the two groups. Compared with the control group, patients with CSF had significantly longer QTmax duration, QT dispersion, Tp-Te interval, and higher iCEB score, wider frontal QRS-T angle.

CONCLUSION

In our study, we found that many of the ventricular repolarization parameters were adversely affected in patients with CSF. Impaired parameters may be associated with the risk of malignant ventricular arrhythmias.

Baseline Corrected QT Interval Dispersion Is Useful to Predict Effectiveness of Metoprolol on Pediatric Postural Tachycardia Syndrome

OBJECTIVES

The study was designed to explore the role of baseline-corrected QT interval dispersion (QTcd) in predicting the effectiveness of metoprolol on pediatric postural tachycardia syndrome (POTS).

METHODS

There were two groups in the study, the discovery group and the validation group. The children with POTS in the discovery group were treated with oral metoprolol, with the completed necessary medical records, head-up tilt test (HUTT), blood chemistry, and 12-lead ECG before treatment at the pediatrics of Peking University First Hospital, China. According to whether the symptom score (SS) was reduced by more than 2 points after administration with oral metoprolol as compared with that before treatment, the children with POTS were separated into responders and non-responders. The demographic characteristics, hemodynamic indicators, and the QTcd of the two groups were compared, and the estimate of the baseline QTcd in predicting the treatment response to metoprolol was tested through a receiver operating characteristic (ROC) analysis. Other 24 children suffering from POTS who were, administrated with metoprolol at the pediatrics of Peking University First Hospital were included in the validation group. The sensitivity, specificity, and accuracy of the baseline QTcd in the prediction of the effectiveness of metoprolol on POTS were validated in children.

RESULTS

The pre-treatment baseline QTcd in responders treated with metoprolol was longer than that of the non-responders in the discovery group [(66.3 ± 20.3) ms vs. (45.7 ± 19.9) ms, p = 0.001]. The baseline QTcd was negatively correlated with SS after metoprolol treatment (r = -0.406, p = 0.003). The cut-off value of baseline QTcd for the prediction of the effectiveness of metoprolol on pediatric POTS was 47.9 ms, yielding a sensitivity of 78.9% and a specificity of 83.3%, respectively. The validation group showed that the sensitivity, specificity, and accuracy of the baseline QTcd ≥ 47.9 ms before treatment for estimating the effectiveness of metoprolol on POTS in children were 73.7, 80.0, and 75.0%, respectively.

CONCLUSION

Baseline QTcd is effective for predicting the effectiveness of metoprolol on pediatric POTS.

Gastroesophageal reflux disease is associated with abnormal ventricular repolarization indices

BACKGROUND/AIMS

Gastroesophageal Reflux Disease (GERD) is a clinical entity affecting the upper gastrointestinal tract. The pathophysiology of GERD has been associated with autonomic nervous system disorders. Autonomic nervous system disturbances in GERD patients have been shown to lead to fatal ventricular arrhythmias (VAs) that result in electrical and ventricular repolarization anomalies. The maximum to terminal electrocardiographic T wave (Tpe) has been associated with repolarization of transmural dispersion. In addition, a higher Tpe interval (TpeI) and Tpe/QT ratio correspond with VAs. The goal of this report was to assess ventricular repolarization, by TpeI and Tpe/QT ratio, in GERD patients.

MATERIALS AND METHODS

The study was comprised of 46 GERD subjects and 43 healthy volunteers. TpeI, cTpe, and Tpe/Q ratios were determined from electrocardiograms and associations of the groups were compared.

RESULTS

The clinical characteristics were similar between the two groups. TpeI, corrected Tpe (cTpe) interval, and Tpe/QT ratio were higher in subjects with GERD in comparison to the control group (P<0.001, P=0.018, and P<0.001, respectively).

CONCLUSION

TpeI and Tpe/QT ratio were higher in GERD patients. Patients with GERD may have an increased risk for VAs.

Effect of Isometric Exercise on QTc Interval

INTRODUCTION

The QTc interval is affected by heart rate, autonomic nervous system and diseases like diabetes. However, the affect of exercise which alters autonomic nervous system activity, on QTc is not clear. On the other hand, the incidence of sudden cardiac death increases many fold post exercise. These events may be better explained by studying the effect of exercise on QTc.

AIM

This study was designed with an aim to record the QTc interval changes in response to isometric exercise in a group of normal individuals with or without parental history of diabetes mellitus. Also the QTc duration was correlated with the LF-HF ratio.

MATERIALS AND METHODS

Twenty nine, healthy medical students were subjected to isometric hand grip test for 5min. ECG was recorded pre-exercise and at various time intervals post-exercise.

STATISTICAL ANALYSIS

All data are expressed in mean ± SD. Intra group comparison was done using paired t-test and unpaired t-test was used for comparison among group I and group II subjects, and among males and females.

RESULT

The difference in the pre and post exercise QTc values both within and between groups was statistically significant with group I subjects recording lower values. The post exercise LF: HF values were significantly increased when compared to pre exercise values in both the groups. There was no correlation between LF: HF and QTc.

CONCLUSION

A longer than normal QTc interval predisposes to arrhythmia. Exercise brings about detectable changes in the QTc interval after a period of isometric exercise in normal individuals which in high risk individuals may predispose to sudden cardiac death. In addition women may be more susceptible to post-exercise arrhythmia owing to a longer QTc even at rest.

Evaluation of Tpe interval and Tpe/QT ratio in patients with slow coronary flow

OBJECTIVE

Slow coronary flow (SCF) phenomenon is described as the delayed opacification of the distal vasculature and angiographically normal coronary arteries. Considerable studies have suggested that the interval from the peak to the end of the electrocardiographic T wave (Tpe) may correspond to the transmural dispersion of repolarization and that increased Tpe interval and Tpe/QT ratio are associated with malignant ventricular arrhythmias. In this study, we intended to evaluate ventricular repolarization in patients with SCF by using the Tpe interval and Tpe/QT ratio.

METHODS

The study population included 33 patients with angiographically proven SCF and 33 control patients with angiographically proven normal coronary arteries without associated SCF. Coronary flow rates of patients and the control group were documented by TIMI (Thrombolysis in Myocardial Infarction) frame count. From the electrocardiograms, Tpe interval and Tpe/QT ratio were calculated and compared between groups.

RESULTS

No statistically significant difference was found between the two groups in terms of basic characteristics. Mean Tpe interval, Tpe/QT ratio, and Tpe/QTc ratio were prolonged in the study group compared to the control group (p<0.001).

CONCLUSION

Tpe interval and Tpe/QT ratio were increased in SCF patients.

Alteration of the QT variability index in end-stage liver disease

Background

A prolonged QT interval can lead to malignant ventricular arrhythmias and sudden cardiac death, and has frequently been found in end-stage liver disease (ESLD). However, myocardial repolarization lability has not yet been fully investigated. We evaluated the QT variability index (QTVI), a marker of temporal inhomogeneity in ventricular repolarization and an abnormality associated with re-entrant malignant ventricular arrhythmias. We determined whether QTVI is affected by the head-up tilt test in ESLD.

Methods

We assessed 36 ESLD patients and 12 control subjects without overt heart disease before and after the 70-degree head-up tilt test. The electrocardiography signal (lead II) was recorded on a computer with an analog-to-digital converter. The RR interval (RRI) and QT interval were measured after recording 5 min of the digitized electrocardiography. Then, the QT intervals were corrected with Bazett's formula (QTc). QTVI was calculated through the following formula: QTVI = log10 [(QTv/QTm2)/(RRIv/RRIm2)], QTv/RRIv: variance of QTI/RRI, QTm/RRIm: mean of QT interval/RRI.

Results

Cirrhotic patients exhibited an elevated QTVI. In particular, Child class C patients had a significantly increased QTVI compared to Child class A patients and the control subjects in the supine position. However, the head-up tilt test did not cause a significant difference in QTVI in relation to the severity of ESLD.

Conclusions

Myocardial repolarization lability was significantly altered in end-stage liver disease. Our data suggest that the severity of ESLD is associated with the degree of the alteration in the QT variability index.

Cigarette Smoking Is Associated with Prolongation of the QTc Interval Duration in Patients with Type 2 Diabetes Mellitus

Aims. Aim of the study was to evaluate the effect of smoking on autonomic nervous system (ANS) activity and QTc interval duration in patients with Type 2 diabetes mellitus (T2DM). Methods. A total of 70 patients with T2DM (35 chronic smokers, 35 nonsmokers) treated with oral antidiabetic medications underwent continuous ECG Holter monitoring for 24 hours and analysis of time- and frequency-domain measures of heart rate variability (HRV). HRV over short time was also assessed using the deep breathing test. In addition, baroreflex sensitivity (BRS) was evaluated using the spontaneous sequence method. The mean QTc interval was measured from the 24-hour ECG recordings. Results. Smokers had lower body mass index (BMI) and exhibited higher 24-hour mean heart rate. There was no difference regarding all measures of ANS activity between the two groups. Smokers showed increased mean QTc duration during the 24 hours (439.25 ± 26.95 versus 425.05 ± 23.03 ms, P = 0.021) as well as in both day (439.14 ± 24.31 ms, P = 0.042) and night periods (440.91 ± 32.30 versus 425.51 ± 24.98 ms, P = 0.033). The association between smoking status and mean QTc interval persisted after adjusting for BMI. Conclusions. Cigarette smoking is associated with prolongation of the QTc interval in patients with T2DM by a mechanism independent of ANS dysfunction.

Usability of QTc dispersion for the prediction of orthostatic intolerance syndromes

BACKGROUND

Syncope is defined as transient loss of consciousness and muscle tone, usually of short duration. Noncardiac causes of syncope are classified as orthostatic intolerance syndromes (OIS). QT and QTc (corrected QT) dispersions are the measurements of myocardial instability and show predisposition to arrhythmias. In this study; clinical findings, QT and QTc dispersions of the patients who were diagnosed as OIS were evaluated retrospectively. Also, the aim of the study is to clarify the association of clinical characteristics of unexplained syncope with the outcome of the QT and QTc dispersions in children.

METHODS

We designed a retrospective study including 152 children and adolescents who had repeated unexplained syncope or presyncope between June 2002 and August 2010. Head-up Tilt table test (HUTT) were performed for all patients. Control group consisted of 67 healthy children. The QT and QTc dispersions were measured from the 12 ECG leads.

RESULTS

Eighty-four (55.2%) patients had positive and 68 (44.8%) had negative response to HUTT. QT and QTc dispersions were significantly higher in HUTT-positive group than in negative (p < 0.01, p < 0.001 respectively). Also, QTc dispersion was significantly higher in both vasovagal syncope and postural orthostatic tachycardia syndrome groups than in HUTT-negative group (p < 0.001, p < 0.05 respectively). Specifity and sensitivity of QTc dispersion for predicting positive HUTT are 76.5% and 59.5% respectively. The positive predictive value of the test calculated as 75.8%.

CONCLUSIONS

These results revealed that we can use QTc dispersion measurement as a noninvasive electrocardiographic test to evaluate OIS for predicting positive result before performing HUTT.

QT-RR hysteresis is caused by differential autonomic states during exercise and recovery

QT-RR hysteresis is characterized by longer QT intervals at a given RR interval while heart rates are increasing during exercise and shorter QT intervals at the same RR interval while heart rates are decreasing during recovery. It has been attributed to a lagging QT response to different directional changes in RR interval during exercise and recovery. Twenty control subjects (8 males, age 51 ± 6 yr), 16 subjects with type 2 diabetes (12 males, age 56 ± 8 yr), 71 subjects with coronary artery disease (CAD) and preserved left ventricular ejection fraction (LVEF) (≥50%) (51 males, age 59 ± 12 yr), and 17 CAD subjects with depressed LVEF (<50%) (13 males, age 57 ± 10 yr) underwent two 16-min exercise tests followed by recovery. In session 2, parasympathetic blockade with atropine (0.04 mg/kg) was achieved at end exercise. QT-RR hysteresis was quantified as: 1) the area bounded by the QT-RR relationships for exercise and recovery in the range of the minimum RR interval at peak exercise to the minimum RR interval + 100 ms and 2) the difference in QT interval duration between exercise and recovery at the minimum RR interval achieved during peak exercise plus 50 ms (ΔQT). The effect of parasympathetic blockade was assessed by substituting the QT-RR relationship after parasympathetic blockade. QT-RR hysteresis was positive in all groups at baseline and reversed by parasympathetic blockade (P < 0.01). We conclude that QT-RR hysteresis is not caused by different directional changes in RR interval during exercise and recovery. Instead, it is predominantly mediated by differential autonomic nervous system effects as the heart rate increases during exercise vs. as it decreases during recovery.

Effects of head-up tilt-table test on the QT interval

BACKGROUND

The QT interval shortens in response to sympathetic stimulation and its response to epinephrine infusion (in healthy individuals and patients with long QT syndrome) has been thoroughly studied. Head-up tilt-table (HUT) testing is an easy way to achieve brisk sympathetic stimulation. Yet, little is known about the response of the QT interval to HUT.

METHODS

We reviewed the electrocardiograms of HUT tests performed at our institution and compare the heart rate, QT, and QTc obtained immediately after HUT with the rest values.

RESULTS

The study group consisted of 41 patients (27 females and 14 males) aged 23.9 +/- 8.4 years. Head-up tilting led to a significant shortening of the RR interval (from 825 +/- 128 msec at rest phase to 712 +/- 130 msec in the upward tilt phase, P < 0.001) but only to a moderate shortening of the QT interval (from 363.7 +/- 27.9 msec during rest to 355 +/- 30.3 msec during upward tilt, P = 0.001). Since the RR interval shortened more than the QT interval, the QTc actually increased (from 403 +/- 21.5 msec during rest phase to 423.2 +/- 27.4 msec during upward tilt, P < 0.001). The QTc value measured for the upward tilt position was longer than the resting QTc value in 33 of 41 patients. Of those, 4 male patients and 2 female patients developed upward-tilt QTc values above what would be considered abnormal at rest.

CONCLUSIONS

During HUT the QT shortens less than the RR interval. Consequently, the QTc increases during head-up tilt.

RT variability unrelated to heart period and respiration progressively increases during graded head-up tilt

Open-loop linear parametric models were exploited to describe ventricular repolarization duration (VRD) variability during graded head-up tilt. Surface ECG and thoracic movements were recorded in 15 healthy humans (age: 24-54 yr, median: 28 yr; 6 women and 9 men). Tilt table inclinations ranged from 15 to 90 degrees and were varied in steps of 15 degrees . All subjects underwent recordings at every step in random order. Heart period was assessed as the time difference between two consecutive R-wave peaks (RR) and the respiratory signal (R) as the sampling of the thoracic movement signal at the R-wave peaks. VRD was measured automatically as the temporal difference between the R-wave peak and T-wave apex (RT(a)) or T-wave end (RT(e)). The best model decomposed RT variability as due to RR changes (RR-related RT variability) to direct respiratory-related inputs (R-related RT variability) and to unknown rhythmical sources unrelated to RR changes and R (RR-R-unrelated RT variability). Using this model, RT(e) variability was found to be less predictable than RT(a) variability and composed of a smaller fraction of RR-related RT variability and a larger fraction of RR-R-unrelated RT variability. Predictability progressively decreased with tilt table angles, suggesting increased complexity of RT regulation. RT variance progressively increased with tilt table inclination. This increase was characterized by a gradual rise of the amount of RR-R-unrelated RT variability, whereas the amount of RR-related RT variability remained unchanged. These results suggest that the amount of RT variability, complexity of RT dynamics, and amount of RR-R-unrelated RT variability increase with the magnitude of the sympathetic drive directly related to tilt table inclination. We propose the utilization of the amount of RR-R-unrelated RT variability instead of overall RT variability as an indirect measure of autonomic regulation directed to ventricles.

The correlation among QTc interval, hyperglycaemia and the impaired autonomic activity

BACKGROUND

The QT interval reflects the duration of the intracellular action potential. Little is known on the interval QT duration in non diabetic insulin-resistant subjects.

OBJECTIVE

The aims of the current study were to evaluate the QTc interval in three groups of non diabetic insulin-resistant subjects and the possible correlation between QTc and the autonomic nervous system's activity.

DESIGN

90 subjects were divided in subjects with impaired fasting glycaemia (IFG) and, by the results of OGTT, according to the criteria of ADA, in subjects with normal glucose regulation (NGR) and impaired glucose tolerance (IGT). Insulin resistance was evaluated by the homeostasis model assessment-index (HOMA-I). Heart rate variability and Qtc were calculated.

RESULTS

QTc interval is correlated with SDNN, LF n.u. and LF/HF.

CONCLUSION

We have observed that the QTc interval is prolonged in insulin-resistant subjects with associated impaired glucose metabolism, while no difference was reported between insulin-resistant and non insulin-resistant subjects with normal glucose regulation. We hypothesize that hyperglycaemia could play a major role than hyperinsulinemia on the QTc prolongation.

QT interval prolongation in future SIDS victims: a polysomnographic study

OBJECTIVE

Previous data have suggested that a prolonged QTc interval during the first days of life can be associated with some cases of sudden infant death syndrome (SIDS). Analysis of heart rate variability during sleep in future SIDS victims has shown findings compatible with an imbalance in autonomic tone. We hypothesized that some future SIDS infants could have longer QTc intervals during sleep, compared with healthy control infants, and that this difference would correlate with the autonomic imbalance already found in these infants.

METHODS

QTc intervals and a heart rate autoregressive power spectral analysis were calculated during the same periods in the polysomnographic sleep recordings of 18 infants who eventually died of SIDS and of 18 control infants. The control infants were matched for sex, gestational age, postnatal age, birth weight, and sleep position. The median postnatal age was 8 weeks.

RESULTS

Compared with control infants, future SIDS victims were characterized by having longer QTc intervals during total sleep (P = 0.019), rapid eye movement sleep (P = 0.045) and non-rapid eye movement sleep (P = 0.029). When the night was divided into 3 equal parts, this difference was always present but was most marked during the last part of the night. There was, respectively, a negative and a positive correlation between parasympathetic activity and sympathovagal balance and median and maximum QTc interval values.

CONCLUSION

Compared with QTc intervals in matched control infants, QTc intervals were increased in future SIDS victims. Such a prolongation could be related to the autonomic dysfunction already reported in these patients.

Open loop linear parametric modeling of the QT variability

Open loop linear parametric modeling approach was applied to describe the variability of the ventricular depolarization and repolarization duration (i.e. the QT interval from the ECG). Several model structures were compared. The model maximizing the goodness of fit describes the QT interval as a linear combination of its own past values plus two exogenous influences (i.e. heart period interval and respiration) and a colored noise. When this model was applied to a protocol imposing a progressive increase of the sympathetic activity and modulation (i.e. the graded head-up tilt), the goodness of fit gradually decreased, thus suggesting a progressive uncoupling between QT duration and heart period that cannot be the result of influences unrelated to heart period changes.

Strategic approach to fit-for-purpose biomarkers in drug development

The strategic, fit-for-purpose use of the combination of robust target engagement and well-qualified disease-related biomarkers enhances understanding of the mechanism of action, ties together preclinical and clinical data, enables the assessment of target engagement, facilitates early proof of concept and dose focusing, and increases the efficiency of early clinical development with improved quality of decision making. Significant progress in biomarker discovery, validation, and qualification has increased drug-development decision making and regulatory applications. Target engagement biomarkers are present early in a pathophysiologic cascade and inform on physical or biological interactions with the molecular target of the drug. Disease-related biomarkers are present late in the pathophysiologic cascade and are linked to clinical benefit; thus, they assess a drug's effect on a particular disease. Together, these concepts lay the groundwork for high-quality drug-development decision making and a framework for the acceptance and qualification of biomarkers for regulatory use.

Q-T interval (QT(C)) in patients with cirrhosis: relation to vasoactive peptides and heart rate

OBJECTIVE

Prolonged Q-T interval (QT) has been reported in patients with cirrhosis who also exhibit profound abnormalities in vasoactive peptides and often present with elevated heart rate (HR). The aim of this study was to relate QT to the circulating level of endothelins (ET-1 and ET-3) and calcitonin gene-related peptide (CGRP) in patients with cirrhosis. In addition, we studied problems with HR correction of QT.

MATERIAL AND METHODS

Forty-eight patients with cirrhosis and portal hypertension were studied during a haemodynamic investigation. Circulating levels of ETs and CGRP were determined by radioimmunoassays. Correction of QT for HR above 60 beats per min was performed using the methods described by Bazett (QT(C)) and Fridericia (QT(F)).

RESULTS

Prolonged QT(C) (above 440 ms), found in 56% of the patients, was related to the presence of significant portal hypertension and liver dysfunction (p < 0.05 to 0.001), but not to elevated ET-1, ET-3 or CGRP. When corrected according to Bazett, QT(C) showed no significant relation to differences in HR between patients (r = 0.07, ns). QTF showed some undercorrection of HR (r = -0.36; p < 0.02). During HR variation in the individual patient, QT(C) revealed a small but significant overcorrection (2.6 ms per heartbeat per min; p < 0.001). This value was significantly (p < 0.02) smaller with QTF (1.2 ms per heartbeat per min).

CONCLUSIONS

The prolonged QT(C) in cirrhosis is related to liver dysfunction and the presence of portal hypertension, but not to the elevated powerful vasoconstrictor (ET-1) or vasodilator (CGRP, ET-3) peptides. The problems with correction of the QT for elevated HR in cirrhosis are complex, and the lowest HR should be applied for determination of the QT.

Sympathoexcitation increases the QT/RR slope in healthy men: differential effects of hypoxia, dobutamine, and phenylephrine

INTRODUCTION

Dynamic ventricular repolarization assessed by QT/RR slopes studies the effects of modifications in cardiac repolarization independently of variations in RR interval (RR). The effects of changes in sympathetic and vagal activity on the QT/RR slope are controversial. We tested the hypothesis that sympathoexcitation is an important determinant of the QT/RR slope.

METHODS AND RESULTS

We compared the effects of a reflex sympathetic activation in response to hypoxia, to the direct effects of the infusion of the beta-adrenergic agent dobutamine, on the QTa (apex) and QTe (end)/RR slopes. Dobutamine was titrated to obtain similar increases in cardiac output than with hypoxia. Cardiac vagal activity was estimated by rMSSD and pNN50. In a second group of healthy subjects, we assessed the effect of a reflex cardiac vagal activation in response to phenylephrine infusion on the same variables. We observed a similar increase in QTa and QTe slopes during hypoxia and dobutamine (both P < 0.017 vs. normoxia), despite divergent changes in cardiac vagal activity, as rMSSD and pNN50 decreased with hypoxia compared to normoxia (P < 0.001) but increased during dobutamine infusion compared to hypoxia (P < 0.017). In contrast, these slopes did not change during the rises in rMSSD and pNN50 elicited by phenylephrine (P > 0.7).

CONCLUSION

Beta-adrenergic stimulation induces comparable increases in the QT/RR slopes than hypoxia, but in the presence of a larger cardiac vagal activity. Vagal cardiac activation by phenylephrine does not change the QT slopes. This reveals that the sympathetic system is an important determinant of QT/RR dynamicity in healthy men.

Autonomic effects on QT-RR interval dynamics after exercise

This study was designed to assess autonomic effects on the QT interval during recovery from exercise. Exercise is associated with an acute increased risk of sudden cardiac death. Evidence of impaired parasympathetic activity, such as low heart rate variability and heart rate recovery, and an increased QT interval are also associated with increased mortality. However, there is no clear pathophysiological link among these findings. Bicycle exercise testing was performed serially in 33 healthy volunteers (19 men; ages, 54 +/- 7 yr) under four conditions: 1) baseline, 2) beta-adrenergic blockade-intravenous propranolol (0.2 mg/kg) administered during exercise, 3) parasympathetic blockade-intravenous atropine (0.04 mg/kg) administered during exercise, and 4) double blockade with propranolol and atropine. ECGs were obtained every minute in recovery for 10 min and then at the 15th and 20th min, from which the QT and RR intervals were measured. Linear regression analyses were used to assess the individual QT-RR relationships for each subject for each condition. Relative to baseline, the QT-RR relationship with parasympathetic blockade was shifted to the left and had a steeper slope. In contrast, the QT-RR relationship with beta-adrenergic blockade was shifted to the right and had a less steep slope. The baseline and double-blockade QT-RR relationships were in the middle and essentially superimposable. There was a negative relationship between QT-RR slope and heart rate or RR interval recoveries, but it was significant only for the 1- and 2-min RR interval recoveries with low R(2) values of 0.124 and 0.114. The main parasympathetic effect in the postexercise recovery period is to counteract the sympathetically mediated QT prolongation. These data support the concept that parasympathetic tone may provide a natural antiarrhythmic effect during this time.

Effect of slow coronary flow on electrocardiographic parameters reflecting ventricular heterogeneity

QT interval dispersion reflects regional variations in ventricular repolarization and cardiac electrical instability. Previous studies have showed that QT interval dispersion changes during episodes of myocardial ischemia. Slow coronary flow (SCF) in epicardial coronary arteries is a rare and unique angiographic finding. Whether this pattern of flow is associated with electrocardiographic abnormalities is unknown. Therefore, this study was designed to investigate whether SCF results in electrocardiographic (ECG) changes compared to normal coronary flow. For this aim 24 patients with angiographically proven SCF who had no obstructive coronary lesion (group I) and 25 patients without coronary artery disease (group II) were included in the study. Both groups underwent a routine standard 12-lead surface electrocardiogram recorded at 50 mm/s during rest. QT dispersion (QTd), corrected QT (QTc), and corrected QT dispersion (QTcd) were calculated. Distributions of sex, age, body mass index (BMI), and cardiac risk factors were similar in the 2 groups. Mean heart rate was similar in the 2 groups (74 +/-8 vs 77 +/- 7 p > 0.05). Mean QRS interval durations were similar in the groups (92 +/-7 vs 90 +/-6 ms p > 0.005). In group I, QTd, QTcd, and QTc, were significantly higher than in group II (QTd: 73 +/-14 vs 40 +/-14; QTcd: 71 +/-15 vs 42 +/-9; QTc: 414 +/-14 vs 388 +/-13, respectively p <0.05). In conclusion, SCF was found to be associated with prolonged QT interval and increased QT dispersion. Ischemia in microvascular level and/or altered autonomic regulation of the heart may be responsible mechanisms.

The strength of QT-RR coupling decreases during graded head-up tilt

A cross-conditional entropy approach was applied to evaluate the degree of coupling between the beat-to-beat series of heart period (RR interval) and ventricular repolarization duration (QT interval). The strength of the QT-RR coupling was measured during graded head-up tilt test with table inclination randomly chosen in the set {0,15,30,45,60,75,90} in 17 healthy subjects. We found that RR and QT variabilities are significantly coupled during the entire experimental protocol and the strength of the QT-RR variability interactions progressively decreases as a function of the tilt angles. Results suggest that the fraction of QT interval variability independent of RR interval changes is not constant but depends on the level of sympathetic activity and/or amplitude of sympathetic modulation.

Reproducibility of QT parameters derived from 24-hour ambulatory ECG recordings in healthy subjects

AIM

To estimate the reproducibility of QT parameters derived from 24-hour ambulatory ECG recordings.

METHOD

Ten healthy volunteers aged 25 to 41 years participated. In two 24-hour ambulatory ECG recordings obtained 1 day apart, the QT interval was measured manually at stable heart rates in approximately 16 periods during daytime and 6 periods during nighttime. The association between the QT and RR interval was described by linear regression for day and nighttime separately and the following QT parameters were calculated: the QT interval at heart rate 60 beats/min during daytime (QT(60)day), slope(day), slope(night), and the difference in QT(60) between day and nighttime (DeltaQT(60)). The QT parameters were assessed four times for each participant to discriminate method inaccuracy from day to day variation. The reproducibility was estimated as the coefficient of repeatability, the relative error, and the ratio between within-subject variability and between-subject variability.

RESULTS

The coefficient of repeatability, the relative error and the ratio, respectively, were 19 ms, 1.8% and 0.5 for QT(60)day, 0.076, 21% and 0.68 for slope(day), 0.116, 43% and 1.37 for slope(night), and 37 ms, 325% and 1.19 for DeltaQT(60) when estimating the overall day to day reproducibility. Inaccuracy of QT measurement accounted for approximately 40% of this variation, whereas the error caused by selecting segments was small.

CONCLUSION

QT(60)day has a high reproducibility and may with advantage replace the conventional QT interval measured on a resting ECG. To assess QT dynamics, the slope of the regression line during daytime is suitable and the short term reproducibility acceptable for clinical trials. Regarding slope(night) and DeltaQT(60), the variation is high and the parameters should be used with caution.

Electrocardiogram, hemodynamics, and core body temperatures of the normal freely moving laboratory beagle dog by remote radiotelemetry

INTRODUCTION

The objectives of this study were to provide baseline normative values for circadian changes in the time-series data collected over the course of a normal day in laboratory-housed dogs and to assess the relative efficiency of standard correction formulas to correct for the variations in QT intervals and heart rate functions.

METHODS

One hundred and twenty-three beagle dogs (65 M, 58 F) were equipped with radiotelemetry transmitters and continuously monitored, while freely moving in their home cages. Electrocardiograms (ECGs), hemodynamic parameters (diastolic, systolic, and mean arterial pressures) as well as core body temperatures were recorded for 22 h.

RESULTS AND DISCUSSION

Blood pressures and core body temperatures demonstrated only very slight variations in their respective values over the 22-h monitoring period. ECGs were measured by a computerized waveform analysis program and quantitative elements reported as RR, PR, QRS, and QT intervals. Little circadian rhythmicity was demonstrated in the ECG intervals. Standard study-specific correction formulas appeared to satisfactorily normalize (i.e., compensate for) the relationship between heart rate and QT intervals in these beagle dogs but elevated the values of the QTc as compared to the uncorrected QT intervals. In sharp contrast, a subject-specific correction method based on analysis of covariance produced a more linear function between heart rates and QT intervals and, more importantly, provided QTc values within the normal range of actual, recorded QT interval data.

Investigation of the effects of physiological and vasodilation-induced autonomic activation on the QTc Interval in healthy male subjects

AIMS

Drug-induced prolongation of the QTc interval is an important marker for potential proarrhythmic action. Prolongation of the QTc interval results from alteration of the ionic currents that regulate cardiac repolarisation. Such effects may result from direct drug action or alternatively they could also occur indirectly by drug-induced modulation of autonomic tone, which is known to regulate cardiac repolarization. This study examined the effects of physiological and drug-induced autonomic activation on heart rate, QT and QTc intervals.

METHODS

We studied 29 healthy male subjects aged 18-30 years. Electrocardiographs were recorded before and during autonomic activation induced by mental activation, standing, exercise and glyceryl trinitrate (GTN) (0.5 mg sublingual)-induced vasodilation in the presence and absence of beta-blockade (atenolol 100 mg daily for 4 days). QT intervals were measured manually by electronic callipers and corrected using the Fridericia formula.

RESULTS

Heart rates were significantly increased during mental arithmetic, standing, exercise and GTN and this effect was significantly attenuated by atenolol, except for mental activation. QTc intervals were significantly reduced on standing and exercise and this was significantly attenuated by atenolol during exercise. In contrast, GTN increased QTc intervals (Delta = 5.7 ms, confidence interval +/- 3.2 ms, P < 0.005) and this was not attenuated by atenolol.

CONCLUSIONS

Alteration in QTc intervals may result from physiological manoeuvres and vasodilation, interventions known to induce autonomic activation. We suggest that QTc prolongation due to GTN is indirectly mediated and unlikely to carry any proarrhythmic effect. Understanding whether drug-induced QTc prolongation is directly or indirectly mediated may be important to determine any potential proarrhythmic risk.

Electrocardiogram, hemodynamics, and core body temperatures of the normal freely moving cynomolgus monkey by remote radiotelemetry

INTRODUCTION

The objectives of this study were to provide baseline normative values for circadian changes in the time-series data collected over the course of a normal day in laboratory-housed monkeys, and to assess the relative efficiency of standard correction formulas to correct for the variations in QT interval durations and heart rate functions.

METHODS

Ninety-nine cynomolgus monkeys (58 M, 41 F) were equipped with radiotelemetry transmitters and continuously monitored, while freely moving in their home cages. Electrocardiograms (ECGs), hemodynamic parameters (diastolic, systolic, and mean arterial pressures) as well as core body temperatures were recorded for 22 h from each of 99 monkeys. ECGs were measured by a computerized waveform analysis program and reported as RR, PR, QRS, and QT intervals.

RESULTS

Blood pressures and core body temperatures demonstrated a normal circadian variation in their respective values over the 22 h monitoring period. Standard study-specific correction formulas failed to satisfactorily normalize the relationship between heart rate and QT intervals in the cynomolgus monkeys. In contrast, a subject-specific correction method based on analysis of covariance produced a linear function between heart rates and QT intervals and provided QTc values within the normal range of actual, recorded data.

DISCUSSION

We believe these procedures represent the contemporary industry's preferred practice for measuring such parameters under the ICH guidelines, and are amenable to routine use in a variety of other relevant safety/efficacy studies.

Dynamic changes in the QT-R-R relationship during head-up tilt test in patients with vasovagal syncope

BACKGROUND

QT interval is influenced by preceding R-R intervals and autonomic nervous tone. Changes in QT intervals during vasovagal reflex might reflect autonomic modulation of ventricular repolarization; however, this issue has not been fully elucidated. This study aimed to evaluate dynamic response of QT interval to transient changes in R-R interval during vasovagal syncope (VVS) induced by head-up tilt test.

METHODS

Eighteen patients with VVS and 18 age-and sex-matched controls were studied. All patients with VVS had a positive mixed-type response to head-up tilt and all controls had a negative response. CM5-lead digital electrocardiogram (ECG) was recorded and QT intervals were analyzed using Holter ECG analyzer. Using scatter plots of consecutive QT and the preceding R-R intervals, QT-R-R relations during tilt-up and tilt-back or during vasovagal reflex were independently fitted to an exponential curve: QT (second) = A + B x exp[k x R-R (second)].

RESULTS

During the tilt-up, A, B, and k did not differ between patients with VVS and controls. During the tilt back, k showed equivalent positive value compared to the tilt-up (4.1 +/- 1.3 vs -4.6 +/- 0.9) in controls. However, k remained negative (-1.3 +/- 1.5) during vasovagal reflex in patients with VVS. In six patients, in whom metoprolol was effective in eliminating VVS, QT-R-R relationship during the tilt-back became similar to that in controls.

CONCLUSIONS

In patients with VVS, hysteresis of the QT-R-R relation is similarly shown during tilt-up as in controls, whereas this hysteresis is no longer evident and failure of QT prolongation is observed during VVS.

QT-Interval Turbulence Induced by Atrial and Ventricular Extrastimuli in Patients with Ventricular Tachycardia

Atrial and ventricular premature beats (APB, VPB) cause consistent changes in the sinus-cycle length known as heart rate (HR) turbulence. HR turbulence is described by turbulence onset (TO) and turbulence slope (TS). This article investigated into whether specific QT interval changes are associated with HR turbulence and whether these can be assessed using HR turbulence quantifiers. A total of 40 patients (54 +/- 16 years, 34 men) referred for electrophysiological evaluation for ventricular tachycardia (VT) underwent a study protocol prior to programmed electrical stimulation. The protocol consisted of three consecutive series of atrial and ventricular stimulation with single extrastimuli delivered from the high right atrium and the right ventricular apex at a coupling interval decremented from 750 to 400 ms at a 50-ms step. A 20-second period of sinus rhythm was allowed between ES. Turbulence onset and TS after an APB and a VPB were calculated separately using a dedicated computer algorithm. Consistent changes in the QT interval (QT turbulence) were observed immediately after premature beats and were determined using TO. QT TO was calculated as a relative difference between the QT interval of the first sinus cycle after the induced premature beat and the mean of the QT intervals of the two sinus cycles preceding the premature beat. QT turbulence was present following both APBs and VPBs but was less pronounced after an APB. In contrast to negative VPB-induced TO of HR turbulence reflecting early acceleration of the HR, VPB-induced TO of QT turbulence was positive showing lengthening of the QT interval following a VPB. The subgroup analysis has shown that QT turbulence was significantly attenuated in patients with ischemic heart disease and a left ventricular (LV) ejection fraction (EF) </=0.40 compared with those with nonischemic VT and left ventricular ejection fraction >0.40 (after an APB: 0.73 +/- 0.31% vs 2.58 +/- 0.31%, respectively, P = 0.0044; after a VPB: 1.18 +/- 0.29% vs 4.40 +/- 1.38%, respectively, P = 0.0026). Neither APB nor VPB produced late QT dynamics equivalent to TS of HR turbulence. QT-interval turbulence occurs in association with HR turbulence following induced VPB and APB but QT dynamics is opposite to that of HR. It can be expressed as TO but not TS. Patients with ischemic VT and LV dysfunction exhibited significantly lower QT TO values than those with nonischemic VT and normal ventricular function.

Comparison of QT interval duration and dispersion in elderly population versus healthy young subjects

We compared QT dispersion (QTd) in 75 elderly and 36 young subjects and found that those over the age of 75 years had higher QTd than those younger than 75. In addition those older than 75 years had higher QTd values compared to the younger subjects. We concluded that QTd increased especially over the age of 75.

Stability of Ventricular Repolarization in Conscious Dogs with Chronic Atrioventricular Dissociation and His-Bundle Pacing

Following AVN ablation, eight dogs were surgically instrumented for chronic (continuous) HIS-bundle pacing. For data collection, implanted pacemakers were transiently programmed to pace in stepwise ascending ramps at rates from 50 to 200 beats/min in 30-beat/min steps. Each rate was held for 60 seconds. At each rate, ECG signals were collected from conscious dogs for measurements of QT intervals during the last 10 seconds of each paced rate to construct a QT-HR ramp. This QT-HR ramp was repeated twice on each day of study 10 minutes apart. Dogs were randomly assigned to two groups and studied weekly for a minimum of 6 weeks. Group 1 dogs had pacemakers programmed to a rate of 80 beats/min for the duration of the study. Group 2 dogs were paced at 80 beats/min for weeks 1-3, then increased to 140 beats/min for weeks 4-6. The difference between paired QT-HR ramps within 1 day was <3 ms. QT-HR ramps were statistically indistinguishable over the 6-week study for group 1 dogs. Group 2 dogs experienced a slight flattening in the slope of the QT-HR ramps from week 3 to week 6 due to a reduction in QT interval at low HRs (50 and 80 beats/min) only. This conscious HIS-pacing model in dogs is a sensitive, stable, and reproducible method to define ventricular repolarization characteristics over a range of programmable HRs and experimental conditions.

Abnormalities of the repolarization characteristics of patients with heart failure progress with symptom severity

BACKGROUND

Congestive heart failure is a common condition with high mortality. Many of these deaths are sudden and unexpected. Ventricular action potential, surface repolarization (QT interval), and dispersion of repolarization are prolonged in the failing heart, contributing to arrhythmogenesis and sudden death. We studied the relationship between QT and heart rate (RR interval) from ambulatory recordings using a novel method in patients with ischemic heart disease and varying degrees of left-ventricular impairment (IHD) and compared them to healthy subjects (HS). We compare the degree of abnormality with the functional impairment and ejection fraction.

METHODS

Using a previously described automated method for continuous estimation of the QT/RR characteristic that incorporates a correction formula for compensation of QT adaptation lag (VERDA, Del Mar Reynolds Medical Ltd., Hertford, UK), we compared recordings from 41 IHD patients with age-matched HS.

RESULTS

IHD Patients have prolonged 24-hour mean QTo (461 ms vs 426 ms, P < 0.01), and abnormal rate dependence relative to controls (24-hour mean slope: 0.20 vs 0.14, P < 0.001; J: 0.38 vs 0.28, P < 0.001). There is increased temporal variation in J with respect to HS. These abnormalities of repolarization increase with worsening NYHA class, but do not correlate with ejection fraction.

CONCLUSIONS

The use of a universal correction formula to compare dynamic QT data in IHD patients is inappropriate. The observed progressive abnormalities may be responsible for the high incidence of sudden death through promotion of arrhythmias.

Acute non-selective beta-adrenergic blockade reduces prolonged frequency-adjusted Q-T interval (QTc) in patients with cirrhosis

BACKGROUND/AIMS

Earlier studies have shown a prolonged frequency-adjusted Q-T interval (QTc>0.440 s(1/2)) in a substantial fraction of patients with cirrhosis. The effect of beta-blockade on QTc is unknown, and its determination was the aim of the study.

METHODS

Seventeen patients with cirrhosis received 80 mg propranolol orally during a haemodynamic investigation with measurements at baseline and 90 min after propranolol ingestion.

RESULTS

Beta-blockade reduced cardiac output (-21%, P<0.001), heart rate (-20%, P<0.001), and the hepatic venous pressure gradient (HVPG, -17%, P<0.02). The mean QTc=0.460 s(1/2) was prolonged compared to 0.410 s(1/2) in age-matched controls (P<0.01). Whereas QTc decreased during beta-blockade in the cirrhotic patients (from 0.460 to 0.440 s(1/2), P<0.01), no effect was found in the subgroup with normal QTc (0.429 vs. 0.422 s(1/2), ns), and a reduction was seen in the patients with prolonged QTc (from 0.488 to 0.456 s(1/2), P<0.01). The percentage decrease in QTc was related to the reduction in HVPG (r=0.48, P=0.03) and cardiac output (r=0.56, P=0.02).

CONCLUSIONS

Acute non-selective beta-blockade reduces prolonged QTc towards normal values in patients with cirrhosis. The clinical significance of QTc reduction in arrhythmia is a topic for future research.

Does QTc interval predict the response to beta-blockers and calcium channel blockers in hypertensives?

The QT interval corrected for heart rate (QTc) is believed to reflect sympathovagal balance. It has also been established that beta-blockers and dihydropyridine-type calcium channel blockers (DHPCCB) influence the autonomic nervous system. This study tested the hypothesis that QTc interval length is a predictor of the blood pressure reduction induced by beta1-selective beta-blockers or DHPCCB. The predictive values of pretreatment heart rate and of the heart rate change with therapy were also evaluated. The authors conducted an historical reanalysis of 5 clinical trials that looked at the antihypertensive effects of beta-blockers (nebivolol) or DHPCCB (amlodipine, felodipine, isradipine, nifedipine). Correlation and quintile analyses were performed to measure the association between QTc interval, heart rate, or heart rate change and therapeutic blood pressure response. Separate analyses were undertaken for beta-blockers and DHPCCB. Seventy-three and 98 hypertensive subjects respectively were included in the beta-blocker and DHPCCB analyses. QTc interval, pretreatment heart rate, and heart rate change with therapy were not associated with therapeutic blood pressure response. In this study, QTc interval length, pretreatment heart rate, and heart rate change with therapy were not good predictors of the blood pressure response to beta1-selective beta-blockers or DHPCCB in hypertensive subjects.

Beat-to-beat analysis of the relation between RT and RR intervals in newborns

OBJECTIVE

To evaluate the dynamic RT (QRS apex-end of T wave) rate dependence in newborns.

STUDY DESIGN

A Digital Holter ECG was acquired on day 15 in nine full-term and eight preterm infants. Ten-minute periods were recorded during wakefulness and sleep. The accuracy of fit with RT-RR pairs was individually assessed by 14 regression formulas (r coefficient, Akaike score, residual analysis). The medians of RT and Bazett's RT correction were calculated for each 10 milliseconds of RR.

RESULTS

The mean RR and RT were 429+/-51 and 263+/-18 milliseconds. None of the prediction formulas were sufficiently accurate to describe RT over the whole range of RR (r<0.56). The Bazett correction produced differences of more than 50 milliseconds at different RR. Prematurity, sleep state and heart rate variability did not influence RT-RR relation.

CONCLUSION

None of the parametric formulas were found to be accurate in describing RT rate dependence in newborns.

Heart rate-dependent electrocardiogram abnormalities in patients with postural tachycardia syndrome

We recently published data suggesting the presence of an intrinsic sinus node abnormality in a subgroup of patients with the postural tachycardia syndrome (POTS). Based on the hypothesis that more widespread abnormalities of cardiac electrophysiologic properties may be present in POTS, we undertook a study to compare cardiac conduction and repolarization at different heart rate levels in patients with POTS and healthy controls. Eleven healthy controls and fourteen patients with POTS participated in the study. Acquisition of 12-lead electrocardiogram recordings were made during supine rest and during gradual head-up tilt. The heart rate of controls was titrated by isoproterenol infusion to match the heart rate of patients. Indices for cardiac conduction (PR interval, QRS duration, and R wave axis) and repolarization (QT interval, QTc interval, and T wave axis) were then compared at different heart rate levels. The PR interval decreased with increasing heart rate in controls more than in patients, resulting in a significantly longer PR interval in patients at the fastest heart rate level. The QT and QTc intervals were significantly shorter in POTS over the entire analyzed heart rate range. The T wave axis decreased with increasing heart rate in patients only. This resulted in a significantly lower T wave axis in patients at the fastest heart rate level. Our data suggest abnormalities of atrioventricular conduction and ventricular repolarization in patients with POTS. These findings may reflect intrinsic cardiac electrophysiologic abnormalities or may be secondary due to abnormalities of cardiac autonomic innervation.

Cardiac effects of chronic oral beta-blockade: lack of agreement between heart rate and QT interval changes

BACKGROUND

Although well established on the sinus node, the effects of beta-blockade on ventricular repolarization are still conflicting. The aim of the study was to investigate the effects of a chronic beta-blockade on sinus node and repolarization parameters and their relationship.

METHODS

Sixteen healthy volunteers (10 males, mean age: 40 +/- 6.7 years) were randomized to placebo or atenolol (100 mg). After 7 days, subjects were crossed over. Heart rate (HR) and HRV indices were calculated from long-term ECG recordings separately during the day and at night, together with ventricular repolarization parameters (QT interval duration and QT rate-dependence).

RESULTS

Mean R-R intervals were significantly and consistently increased after atenolol (Day: 916 +/- 103 ms vs. 712 +/- 89 ms, and Night: 1149 +/- 93 vs. 996 +/- 125 ms). HRV changes under atenolol were also consistent, with a significant decrease in sympathovagal ratio. In contrast, atenolol only lowered diurnal QT rate-dependence (0.123 +/- 0.032 vs. 0.190 +/- 0.065 on placebo, P < 0.001), but not the nocturnal pattern. After multivariate analysis QT rate-dependence changes induced by atenolol were correlated with pretreatment QT/RR relation (r = 0.65, P < 0.01) but not with any HR or HRV parameters.

CONCLUSIONS

In healthy subjects, repolarization changes following chronic beta-blockade cannot be predicted by HR or HRV changes, but are dependent on pretreatment rate-dependence.

Autonomic nervous system influences on QT interval in normal subjects

OBJECTIVES

We sought to determine whether the relationship between heart rate (HR) and QT interval (QT) differs as HR increases in response to exercise, atropine and isoproterenol.

BACKGROUND

Autonomic nervous system influences on repolarization are poorly understood and may complicate the interpretation of QT measurements.

METHODS

Twenty-five normal subjects sequentially underwent graded-intensity bicycle exercise, atropine injection and isoproterenol infusion. Serial 12-lead electrocardiograms were recorded at steady state during each condition and analyzed using interactive computer software. The HR-QT data were modeled linearly and the slopes (quantifying QT adaptation to HR) as well as the QT intervals at 100 beats/min for each intervention were compared by repeated-measures analysis of variance.

RESULTS

As HR increased, QT was longer for isoproterenol in comparison to exercise or atropine, which were similar. The HR-QT slope (ms/beats/min) was less steep for isoproterenol (-0.83 +/- 0.53) than for atropine (-1.45 +/- 0.21) or exercise (-1.37 +/- 0.23) (p < 0.0001). In comparison to men, women had more negative HR-QT slopes during all interventions. At 100 beats/min, the QT was 364 ms during isoproterenol, which was significantly longer than that during exercise (330 ms) or atropine (339 ms) (p < 0.0001). Isoproterenol produced a dose-dependent increase in U-wave amplitude that was not observed during exercise or atropine.

CONCLUSIONS

In comparison to exercise and atropine, isoproterenol is associated with much less QT shortening for a given increase in HR and, therefore, greater absolute QT intervals. Our findings demonstrate that autonomic conditions directly affect the ventricular myocardium of healthy subjects, causing differences in QT that are independent of HR.

QT-RR relationship in healthy subjects exhibits substantial intersubject variability and high intrasubject stability

Recently, it was demonstrated that the QT-RR relationship pattern varies significantly among healthy individuals. We compared the intra- and interindividual variations of the QT-RR relationship. Twenty-four-hour 12-lead digital electrocardiograms (ECGs; SEER MC, GE Marquette; 10-s ECG recorded every 30 s) were obtained at baseline and after 24 h, 1 wk, and 1 mo in 75 healthy subjects (42 women, 33 men, age 27.9 +/- 9.6 vs. 26.8 +/- 7.5 yr, P = not significant). QT interval was measured automatically in each ECG by six different algorithms, and the mean of the six measurements was analyzed. In each recording of each individual, QT-RR relationship was assessed by 10 different regression models including linear (QT = beta + alpha x RR) and parabolic (QT = beta x RR(alpha)) models. Standard deviations (SDs) of regression parameters alpha and beta of consecutive recordings of each individual were compared with SD of the individual means. Intrasubject stability and interindividual variability were further tested by ANOVA. With all models, intraindividual SDs of the regression parameters were highly significantly smaller than SD of individual means (P < 10(-5)-10(-9)). The intrasubject stability was further confirmed by ANOVA (P < 10(-19)-10(-30)). The QT-RR relationship exhibits substantial intersubject variability as well as a high intrasubject stability. This has practical implications for a precise estimation of the heart rate-corrected QT interval in which optimized subject-specific rate correction formulas should be used.

Cholinergic stimulation with pyridostigmine reduces the QTc interval in coronary artery disease

Parasympathetic dysfunction is an independent risk factor in patients with coronary artery disease; thus, cholinergic stimulation is a potential therapeutic measure that may be protective by acting on ventricular repolarization. The purpose of the present study was to determine the effects of pyridostigmine bromide (PYR), a reversible anticholinesterase agent, on the electrocardiographic variables, particularly QTc interval, in patients with stable coronary artery disease. In a randomized double-blind crossover placebo-controlled study, simultaneous 12-lead electrocardiographic tracings were obtained at rest from 10 patients with exercise-induced myocardial ischemia before and 2 h after the oral administration of 45 mg PYR or placebo. PYR increased the RR intervals (pre: 921 +/- 27 ms vs post: 1127 +/- 37 ms; P<0.01) and, in contrast with placebo, decreased the QTc interval (pre: 401 +/- 3 ms vs post: 382 +/- 3 ms; P<0.01). No other electrocardiographic variables were modified (PR segment, QT interval, QT and QTc dispersions). Cholinergic stimulation with PYR caused bradycardia and reduced the QTc interval without important side effects in patients with coronary disease. These effects, if confirmed in studies over longer periods of administration, may suggest a cardioprotection by cholinergic stimulation with PYR.

Influence of atenolol on the kinetics of RT interval rate adaptation in conscious dogs

The objective was to test an effect of atenolol independent of heart rate on electrocardiographic RT rate adaptation by investigating RT adaptation during spontaneous rate and after an abrupt change of atrial rate (study of RT delay). Digital electrocardiograms were recorded from eight conscious dogs. Analysis of RT interval (measured from QRS apex to end of T) was performed on a beat-to-beat basis. The protocol was repeated in the control state and after atenolol administration (2 mg/kg). Regarding spontaneous heart rate, an increased or decreased RR duration did not modify the beat-to-beat relative adaptation of RT to a change of RR (2.15 +/- 1% during control). Atenolol increased mean RR (p < 0.001) and decreased relative adaptation of RT (0.22 +/- 0.18%, p < 0.001). The inverse correlation between mean RR and the relative RT adaptation (r = -0.76, p < 0.05) disappeared after atenolol administration. Regarding RT delay, complete adaptation of RT required 3 min; 48 +/- 16% of this adaptation was observed after the first beat and 60 +/- 11% was observed after the 20th. Atenolol attenuated this adaptation during the first six beats following the abrupt cycle length change (p < 0.05). We concluded that the attenuation of RT rate adaptation after atenolol is related to heart rate modulation and to the time delay in RT rate adaptation.

Role of alpha1-blockade in congenital long QT syndrome: investigation by exercise stress test

Beta-blockade is widely reported to reduce the incidence of syncope in 75-80% of patients with congenital long QT syndrome (LQTS). However, despite full-dose beta-blockade, 20-25% of patients continue to have syncopal episodes and remain at high risk for sudden cardiac death. In some patients refractory to beta-blockade, the recurrence of arrhythmias is successfully prevented by left stellate ganglionectomy, and also by labetalol, a nonselective beta-blockade with alpha1-blocking action. These observations suggest that not only beta-adrenoceptors, but also alpha1-adrenoceptors, play an important pathogenic role, especially under sympathetic stimulation, in LQTS. The clinical effects of alpha1-blockade in congenital LQTS were investigated in 8 patients with familial or sporadic LQTS. Two measurements of the QT interval were taken, from the QRS onset to the T wave offset (QT) and from the QRS onset to the peak of the T wave (QTp). Using the Bruce protocol, an exercise test was performed after administration of beta-blockade alone and again after administration of alpha1-blockade. The following were compared: (1) Bazzet-corrected QT (QTc) and QTp (QTpc) intervals in the supine and standing position before exercise and in the early recovery phase after exercise; and (2) the slopes (reflecting the dynamic change in the QT interval during exercise) of the QT interval to heart rate were obtained from the linear regression during the exercise test. In the supine position before exercise, there was no change in the QTc before or after the addition of alpha1-blockade (498+/-23 vs 486+/-23 ms [NS]). However, in the upright position before exercise and in the early recovery phase after exercise, QTc was significantly shortened from 523+/-21 to 483+/-22ms (p<0.01), and from 521+/-30 to 490+/-39ms (p<0.01), respectively, by alpha1-blockade. The QTpc was unchanged in any situation. Consequently, QTc-QTpc was significantly shortened by alpha1-blockade in the upright position before exercise and in the early recovery phase after exercise (131+/-36 to 105+/-37ms (p<0.05), and 132+/-29 to 102+/-31 ms (p<0.01), respectively). The slopes of the QT interval-heart rate relation by linear regression became significantly steeper from -2.23+/-0.38 to -2.93+/-0.76 (p<0.01) with the addition of alpha1-blockade. The findings suggest that the addition of alpha1-blockade attenuated the exercise-induced prolongation of the QT interval and that the rate adaptation of the QT interval to heart rate during exercise was improved. This indicates that additional treatment with alpha1-blockade may be beneficial to prevent cardiac events in LQTS patients in whom ventricular arrhythmia is resistant to beta-blockade.

The effects of volatile anesthetics on the Q-Tc interval

OBJECTIVE

To examine the effects of halothane, isoflurane, and sevoflurane on Q-Tc interval (corrected for heart rate) during inhalation induction of anesthesia.

DESIGN

Prospective, double-blind, randomized study.

SETTING

Departments of Cardiology and Anesthesiology in a university hospital.

PARTICIPANTS

Patients undergoing noncardiac surgery.

INTERVENTIONS

A total of 65 American Society of Anesthesiologists physical status I-II patients, aged 16 to 50 years, undergoing general anesthesia, were randomly allocated to receive halothane, isoflurane, or sevoflurane.

MEASUREMENTS AND MAIN RESULTS

The time to reach the predetermined end-tidal concentrations of 3 minimum alveolar concentration was 6 to 10 minutes. When compared with preinduction values, heart rate decreased after halothane (p < 0.01) and sevoflurane (p < 0.05) administration; in contrast, heart rate increased after induction of anesthesia with isoflurane (p < 0.05). The mean QRS intervals were not significantly changed after halothane, isoflurane, or sevoflurane. The Q-Tc interval was increased with isoflurane compared with baseline (465 +/- 23 v 441 +/- 18 msec, p < 0.01), not changed with sevoflurane (441 +/- 17 v 434 +/- 19 ms, p > 0.05), and shortened with halothane (426 +/- 23 v 445 +/- 21 msec, p < 0.01).

CONCLUSION

Sevoflurane or halothane may be preferred to isoflurane in patients with conditions that are known to induce a prolonged Q-Tc interval. The effects of Q-Tc interval changes resulting from different anesthetic agents on morbidity and the incidence of arrhythmias during anesthesia warrant further investigation.