The ventricular paced QT interval--the effects of rate and exercise

Normative Heart-Rate Corrected Values for Repolarisation Length From Holter Recordings in Children and Adults

BACKGROUND

Normative values for heart-rate corrected repolarisation length are not available in children and are scarce in adults. We wished to define repeatability and normative values of Holter recording measurements of repolarisation length in healthy individuals using a commercially available system, and compare measurements with those from 12-lead electrocardiograms (ECGs).

METHODS

Twenty-four-hour (24-) Holter recordings were made on 99 Healthy volunteers: 52 children (7 months to 14 years) and 47 adults (≥15 yrs). Mean and peak values of QTc, and RTPc (R-wave to peak T-wave) were assessed. Bazett heart rate correction was employed for each measurement and only heart rates between 40 and 120 bpm were analysed. The end of the T-wave was defined from the zero-crossing point. QTc was also determined from 12-lead ECGs from the same population by manual measurement recording the longest QTc of leads 2 and V5. The tangent technique was used to define the end of the T-wave.

RESULTS

Interobserver repeatability: mean QTc ±15 ms (CI 3.5%), peak QTc ±25 ms (CI 4.5%), mean RTPc ±3 ms (CI 1%), peak RTPc ±44 ms (CI 11%). Mean values were very similar for <15 years and all females and were therefore amalgamated: mean (±2 SD); mean QTc 424 ms (394-454), mean RTPc 291ms (263-319). Values were lower in males ≥15 years; (mean QTc 408 ms (370-446), p<0.01; mean RTPc 274 ms (234-314), p<0.01. The highest mean QTc value was 467 ms in an adult female. QTc from 12-lead ECG: females <15 years 409 ms (384-434) males <15 years 408 ms (383-433), females ≥15 years 426 ms (401-451), males ≥15 years 385 ms (362-408).

CONCLUSIONS

Holter measurements of mean QTc and RTPc are highly repeatable. Males ≥15 years have shorter mean repolarisation length over 24 hours than males <15 years and all females. Mean QTc Holter values were on average 15-17 ms longer than QTc from 12-lead ECGs except in females >15 years.

Value of the New Spline QTc Formula in Adjusting for Pacing-Induced Changes in Heart Rate

AIMS

To determine whether a new QTc calculation based on a Spline fit model derived and validated from a large population remained stable in the same individual across a range of heart rates (HRs). Second, to determine whether this formula incorporating QRS duration can be of value in QT measurement, compared to direct measurement of the JT interval, during ventricular pacing.

METHODS

Individuals (N=30; 14 males) aged 51.9 ± 14.3 years were paced with decremental atrial followed by decremental ventricular pacing.

RESULTS

The new QTc changed minimally with shorter RR intervals, poorly fit even a linear relationship, and did not fit a second-order polynomial. In contrast, the Bazett formula (QTcBZT) showed a steep and marked increase in QTc with shorter RR intervals. For atrial pacing data, QTcBZT was fit best by a second-order polynomial and demonstrated a dramatic increase in QTc with progressively shorter RR intervals. For ventricular pacing, the new QTc minus QRS duration did not meaningfully change with HR in contrast to the HR dependency of QTcBZT and JT interval.

CONCLUSION

The new QT correction formula is minimally impacted by HR acceleration induced by atrial or ventricular pacing. The Spline QTc minus QRS duration is an excellent method to estimate QTc in ventricular paced complexes.

Predictive Power of f99 Repolarization Index for the Occurrence of Ventricular Arrhythmias

BACKGROUND

Defects of cardiac repolarization, noninvasively identifiable by analyzing the electrocardiographic (ECG) ST segment and T wave, are among the major causes of sudden cardiac death. Still, no repolarization-based index has so far shown sufficient sensitivity and specificity to justify preventive treatments. Thus, the aim of this work was to evaluate the predictive power of our recently proposed f99 index for the occurrence of ventricular arrhythmias.

METHODS

Our study populations included 170 patients with implanted cardiac defibrillator (ICD), 44 of which developed ventricular tachycardia and/or fibrillation during the 4-year follow-up (ICD_Cases) and 126 did not (ICD_Controls). The f99 index, defined as the frequency at which the repolarization normalized cumulative energy reaches 99%, was computed in each of the 15 (I to III, aVl, aVr, aVf, V1 -V6 , X, Y, Z) available ECG leads independently, and then maximized over the 6 precordial leads (f99_MaxV1 -V6 ), 12 standard leads (f99_Max12STD) and three orthogonal leads (f99_MaxXYZ) to avoid dispersion-related issues. Each index predictive power was quantified as the area under the receiving operating characteristic curve (AUC).

RESULTS

Median f99_MaxV1 -V6 , f99_Max12STD and f99_MaxXYZ values were significantly higher in the ICD_Cases than in the ICD_Controls (48 Hz vs. 35 Hz, P<0.05; 51 Hz vs. 43 Hz, P<0.05; 45 Hz vs. 31 Hz, P<10(-3) ; respectively), indicating a more fragmented repolarization in the former group. The AUC values were 0.62, 0.63 and 0.68, respectively.

CONCLUSIONS

The f99 represents a promising risk index for the occurrence of ventricular arrhythmias, especially when maximized over the three orthogonal leads.

QT Prolongation and Oncology Drug Development

Many pharmaceutical agents interact with cardiac ion channels resulting in abnormal ventricular repolarization and prolongation of the QT interval. In rare circumstances, this has resulted in the development of the potentially life-threatening arrhythmia, torsades de pointes. It is recognized, however, that accurate measurement of the QT interval is challenging, and it is a poor predictor for the development of this arrhythmia. Nevertheless, QT interval monitoring is an essential part of pharmaceutical development, and significant increases in the QT interval may prevent a drug from gaining approval.

Risk of QT/QTc Prolongation Among Newer Non-SSRI Antidepressants

Objective: To review QT prolongation potential with newer nonselective serotonin reuptake inhibitor (non-SSRI) antidepressants. Data Sources: A PubMed literature search was performed from 1982 through June 16, 2014. Search terms included bupropion, desvenlafaxine, duloxetine, levomilnacipran, mirtazapine, venlafaxine, and vilazodone in combination with each of the following terms: cardiac toxicity, QTc prolongation, QT prolongation, torsades de pointes, and TdP. Study Selection and Data Extraction: English-language human studies, case reports, package inserts, manufacturer electronic communications, and ArizonaCert database were utilized. Data Synthesis: Rare QT prolongation has been reported with venlafaxine at therapeutic doses and in overdose. Bupropion has also been linked to QT prolongation in overdose situations. In elderly patients with a variety of high-risk comorbidities, mirtazapine did demonstrate higher odds of sudden cardiac death and ventricular arrhythmias when compared with paroxetine. Largely because of a lack of available data, existing studies fail to demonstrate QT prolongation with desvenlafaxine, duloxetine, levomilnacipran, and vilazodone. Conclusion: Based on the current literature, risk of QT/QTc prolongation with the majority of newer non-SSRI antidepressants at therapeutic doses is low. The highest risk for QT prolongation appears to exist in overdose situations with venlafaxine and bupropion. Given the few to nonexistent controlled studies and confounding variables present in case reports, it is difficult to draw conclusions on QT prolongation risk with many of the newer non-SSRI antidepressants.

Acquired long QT syndrome: frequency, onset, and risk factors in intensive care patients

BACKGROUND

Acquired long QT syndrome is a reversible condition that can lead to torsades de pointes and sudden cardiac death.

OBJECTIVE

To determine the frequency, onset, frequency of medications, and risk factors for the syndrome in intensive care patients.

METHODS

In a retrospective chart review of 88 subjects, hourly corrected QT intervals calculated by using the Bazett formula were collected. Acquired long QT syndrome was defined as a corrected QT of 500 milliseconds or longer or an increase in corrected QT of 60 milliseconds or greater from baseline level. Risk factors and medications administered were collected from patients' medical records.

RESULTS

The syndrome occurred in 46 patients (52%); mean time of onset was 7.4 hours (SD, 9.4) from time of admission. Among the 88 patients, 52 (59%) received a known QTc-prolonging medication. Among the 46 with the syndrome, 23 (50%) received a known QT-prolonging medication. No other risk factor studied was significantly predictive of the syndrome.

CONCLUSIONS

Acquired long QT syndrome occurs in patients not treated with a known QT-prolonging medication, indicating the importance of frequent QT monitoring of all intensive care patients.

QT dynamics early after exercise as a predictor of mortality

BACKGROUND

Exercise and QT dynamics during ambulatory monitoring impact mortality in a variety of populations. Heart rate recovery (HRR) after exercise is a known strong predictor of mortality.

OBJECTIVE

This study assessed the independent prognostic significance of the QT response to changing heart rate (QT dynamics) during recovery from exercise.

METHODS

The cohort included patients referred for treadmill exercise stress testing over a 5-year period. Patients had to have at least 4 electrocardiographic tracings within 5 minutes of peak exercise. One had to be recorded 60 seconds into recovery to calculate the HRR. Linear regression of the QT-RR relation during recovery was used to predict the QT interval at cycle lengths of 500 and 600 ms (QT-500 and QT-600). Only studies with an R(2) > or = 0.9 (72%) were retained. Optimal binary cut points were chosen. All-cause mortality was determined from either the Social Security Death Index or hospital records.

RESULTS

A total of 2,994 patients met inclusion criteria; 228 (7.6%) died during an average follow-up of 7.6 +/- 1.9 years. Abnormal QT-500 (>316 ms) was the strongest univariate QT dynamics predictor in a Cox proportional hazards model (hazard ratio = 2.13, P <.001). It remained an independent predictor of mortality after adjustment for age, exercise capacity, medications, single photon emission computed tomography defects, and abnormal (<12 beats/min) HRR (hazard ratio = 1.46, P = .014).

CONCLUSION

An abnormal predicted QT interval at 500 ms (120 beats/min) during recovery from exercise independently predicts all-cause mortality. Because QT dynamics in recovery incorporate information on both repolarization and autonomic responsiveness, its role in risk prediction for sudden cardiac death should be further explored.

Do electrical parameters of the cardiac cycle reflect the corresponding mechanical intervals as the heart rate changes?

AIMS

The evoked QT interval can be detected beat by beat through an implanted pacing system. The correlation between the right ventricular paced QT interval and the left ventricular systolic interval is not known. The aim of our study was to collect data on the correlation between QT and systolic and diastolic indexes at different heart rates in patients with dual-chamber rate-responsive pacemakers.

METHOD AND RESULTS

The study involved 13 patients [67 +/- 9 years; ejection fraction (EF) 52 +/- 10%] with standard indication for dual-chamber pacing. Patients were evaluated at rest in the supine position. The AV delay was set at 130 ms, and the pacing rate was increased from 90 to 130 bpm (10 bpm steps for 3 min). At the end of each 3 min step, QT intervals were automatically evaluated in real time by means of pacemaker telemetry. We also evaluated heart performance by means of echo-2D (end-diastolic/end-systolic volumes, EF) and echo-Doppler measurements [left ventricular ejection time (LVET) and diastolic filling time (LVDFT), aortic velocity time integral, and systolic volume] and systemic arterial pressure. The QT interval progressively decreased from 330 +/- 20 to 280 +/- 10 ms as the pacing rate was increased from 90 to 130 bpm. The correlation between the QT interval and LVET as a function of the pacing rate was R(2) = 0.966, indicating a good and relatively parallel trend in these two parameters. The correlation between RR-QT (reflecting electrical diastole) and LVDFT (reflecting mechanical diastole) was R(2) = 0.975. The index LVET/QT (ratio between mechanical and electrical systole) was constant in the range 90-120 bpm, but significantly decreased at 130 bpm: the mechanical LVET shortens more than the electrical QT does at the highest heart rates.

CONCLUSION

In paced patients at rest and during artificially increased heart rates, QT interval dynamics is closely correlated with changes in ejection time, thus constituting an electrical parameter of systolic time. A similar correlation exists between RR-QT, as a diastolic electrical interval, and the DFT.

Heart rate and catecholamine contribution to QT interval shortening on exercise

BACKGROUND

QT interval shortens with exercise. Some of this shortening is due to an increase in heart rate, and some is due to other effects of exercise, probably mostly neuroendocrine effects. Data from subjects with cardiac transplants have suggested that non-heart rate-related changes in QT interval on exercise are due to the effects of circulating catecholamines.

HYPOTHESIS

We sought to determine whether changes in plasma catecholamine levels with exercise are an important contributor to non-heart rate-related QT interval shortening.

METHODS

Subjects with DDD pacemakers were recruited. Subjects had QT intervals measured at rest, during a low fixed level exercise test designed to increase heart rate to about 110 beats/min, and, after resting, during pacing at a heart rate of 110 beats/min. Catecholamine levels were measured at each stage of the study.

RESULTS

QT interval at rest was 420 +/- 12 ms, during pacing 366 +/- 16 ms, and on exercise 325 +/- 14 ms. This then gave the proportion of QT interval shortening due to heart rate as 68.6 +/- 9.3% of total QT shortening, with the range between 35 and 95.6%. There was no proportionality between the degree of QT interval shortening on exercise that was not due to increases in heart rate and changes in plasma catecholamine levels.

CONCLUSION

Two-thirds of exercise-induced QT interval shortening are due to an increase in heart rate, and one-third to other effects. Changes in plasma catecholamine levels on exercise were not closely related to changes in the QT interval on exercise.

QTc prolongation assessment in anticancer drug development: clinical and methodological issues

Cardiac safety assessments are commonly employed in the clinical development of investigational oncology medications. In anti-cancer drug development there has been increasing consideration for the potential of a compound to cause adverse electrocardiographic changes, especially QT interval prolongation, which can be associated with risk of torsades de pointes and sudden death. Irrespective of overt clinical toxicities, QTc assessment can potentially influence decision making at many levels during the conduct of clinical studies, including eligibility for protocol therapy, dose delivery or discontinuation, and analyses of optimal dose for subsequent development. Given the potential for serious and irreversible morbidity from cardiac adverse events, it is understandable that cardiac safety results can have broad impact on study conduct and patient management. The methodologies for risk management of QTc prolongation for non cardiac drugs have been developed out of experiences primarily from drugs used to treat non life-threatening illnesses in a chronic setting such as antibiotics or antihistamines. Extrapolating these approaches to drugs for treating cancer over an acute period may not be appropriate. Few specific guidelines are available for risk management of cardiac safety in the development and use of oncology drugs. In this manuscript, clinical and methodological issues related to QTc prolongation assessment will be reviewed. Discussions about limitations in phase-I design and oncology drug development will be highlighted. Efforts are needed to refine strategies for risk management, avoiding unintended consequences that negatively affect patient access and clinical development of promising new cancer treatments. A thoughtful risk management plan generated by an organized collaboration between oncologists, cardiologists, and regulatory agencies to support a development programme essential for oncology agents with cardiac safety concerns.

Cardiac arrest from acquired long QT syndrome: a case report

BACKGROUND

Many classes of medications initiated by clinicians can cause adverse events, such as cardiac disturbances. One such adverse outcome is that of acquired long QT syndrome, which can lead to arrhythmias and sudden death. When health care practitioners were surveyed about their knowledge of this condition, 20% indicated they knew very little about long QT syndromes and more than 30% failed to check on current therapy before prescribing QT-prolonging medications.

METHODS

A case will be presented to illustrate the importance of understanding this syndrome.

RESULTS

The causes and pathophysiology of acquired long QT syndrome are discussed, and the resources for clinicians to obtain more information and growing number of offending medications leading to acquired long QT syndrome are provided.

CONCLUSIONS

On-going education is needed to heighten awareness in the health care community to prevent the deleterious outcomes associated with medication induced acquired long QT syndrome.

An Improved QT Correction Method for use in Atrial Fibrillation and a Comparison with the Assessment of QT in Sinus Rhythm

BACKGROUND

Conventional QT corrections may be inappropriate inatrial fibrillation (AF) due to RR variability and QT lag. Existing formulashave been modified by the formula RRmod to account for this lag. Wedeveloped a novel correction formula for use in AF (QTAF) based onthe slope ∆QT/∆RRmod and report its performance in AF.We also compare QTAF obtained in AF with rate-independentcorrections in NSR.

MATERIALS AND METHODS

A total of 3063 RR/QT pairs from 28 patients with AFwere measured, 22 of whom also had measurements during sinus rhythm. QTc (theBazett equation), QTLC (the Framingham linear correction), and QTAFwere calculated utilizing RRmod, and the rate-independence of eachformula in AF tested. Mean QTAF values in AF were compared to QTintervals corrected with QTLC in normal sinus rhythm.

RESULTS

∆QTc/∆RRmod and∆QTLC/∆RRmod slopes were significantlynon-zero whereas ∆QTAF/∆RRmod was not. QTLCand QTc corrections were imperfect at extremes of RRmod whileQTAF was constant. QTAF corrections in AF were shorterthan QTc or QTLC corrections in NSR.

CONCLUSIONS

QTAF is a novel QT correction with adefined relationship to correction in NSR that performs better than existingstrategies.

Sample entropy of electrocardiographic RR and QT time-series data during rest and exercise

Sample entropy (SampEn) is a measure of the complexity of data. Few studies have compared the SampEn of electrocardiographic cardiac interval (RR) data (SampEn-RR) during differing physiological states, and none have examined SampEn for the corresponding QT interval (SampEn-QT). The aim of this study was to quantify SampEn-RR and SampEn-QT during rest and for a range of exercise workloads. Specific objectives were to assess both the utility of SampEn for discriminating between physiological states and the relationship of SampEn-RR with traditional measures of heart rate variability (HRV). Twelve males of similar age, mass and aerobic fitness participated. A three-lead ECG was recorded continuously during pre-exercise, progressive bicycle exercise and recovery, and beat-to-beat RR and QT intervals were quantified for sinus beats. SampEn and HRV were calculated within consecutive 1 min periods throughout. Consistent estimation of SampEn-RR and SampEn-QT was possible with an appropriate choice of SampEn parameters. SampEn-RR was sensitive to differing physiological conditions, but its discriminating ability was poorer than that of linear HRV indices. SampEn-RR was also negatively correlated with normalized LF and LF/HF parameters. We interpret changes in SampEn for RR and QT data in terms of the altered autonomic nervous system (ANS) control of either the atrial or the ventricular myocardium (or both) during discrete physiological states. We speculate that greater complexity in QT data might be explained by a direct ANS influence on the ventricular myocardium.

Analysis of the QT interval and its variability in healthy adults during rest and exercise

The aim of the present study was to quantify the variability of electrocardiographic QT and RR intervals during rest and dynamic physical exercise, and to interpret these variabilities in terms of relative autonomic modulation of the atrial and ventricular myocardium. We also sought to characterize the relationships between QT, heart rate-corrected QT (QT(c)) and RR intervals, and to consider their associations with differential autonomic regulation. Nine males and eight females of similar age (22.8 +/- 4.7 years), mass (75.5 +/- 13.0 kg) and aerobic fitness (43.6 +/- 7.7 ml kg(-1) min(-1)) (mean +/- SD) undertook progressive bicycle exercise. A three-lead Holter ECG was recorded continuously during pre-exercise, exercise and recovery, and mean values of RR, QT, QT(c), QT variability index (QTVI) and mean-normalized QT variance (QTVN) were determined. At the onset of exercise QTVI increased rapidly compared with rest and remained significantly elevated throughout exercise and recovery. There were significant differences between QT(a)VI and QT(e)VI (QT measured from Q wave onset to T wave apex (QT(a)) and T wave end (QT(e)), respectively) throughout the experimental protocol. QTVI was significantly reduced in males compared with females prior to exercise but was similar thereafter. We suggest that physical exercise perturbs the resting QT-RR relationship owing to the onset of differential parasympathetic modulation of the atrial and ventricular myocardium. QTVI can be used to quantify the relative autonomic influence on the atrial and ventricular myocardium during rest and exercise, and might be related to HR-dependent and HR-independent influences on the QT interval.

Lack of correlation between exercise and sibenadet-induced changes in heart rate corrected measurement of the QT interval

AIMS

We sought to investigate subject specific QT interval correction factors (SSCF) determined at rest and after exercise and to determine the validity of these factors after the administration of a probe drug known to increase heart rate without directly affecting cardiac repolarization.

METHODS

Thirty-two healthy volunteers underwent graded exercise, multiple recordings of electrocardiogram during rest over a day and a treatment phase administering inhaled placebo or sibenadet (a beta(2)-adrenoceptor/dopamine D(2)-receptor agonist) at 250, 500 or one of 750 or 1000 microg. SSCF were determined from linear regression of plots of log RR interval vs. log QT after exercise (QTcX), rest (QTcR), and combined data (QTcC). The SSCFs along with Bazett & Fridericia corrections were applied to the ECGs after inhalation of sibenadet.

RESULTS

SSCFs obtained from the combination of the exercise and resting day (mean QTcC = 0.41) and exercise alone (mean QTcX = 0.40) were similar with a good fit to the data (mean r(2) = 0.92 and 0.93, respectively) while data at rest resulted in a less pronounced slope (mean QTcR = 0.27) and poorer fit (mean r(2) = 0.52). After the administration of sibenadet, none of the SSCFs, Bazett or Fridericia corrections adequately corrected QT for heart rate induced changes.

CONCLUSIONS

Neither a SSCF from exercise, Bazett's or Fridericia's correction factors, adequately corrected the QT interval after the administration of a sympathomimetic agonist drug to increase heart rate in healthy volunteers demonstrating the potential need for QT/RR correction factors to be tailored for each drug studied.

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.

Changes in evoked QT intervals according to variations in atrioventricular delay and cardiac function in patients with implanted QT-driven DDDR pacemakers

In patients with implanted DDD pacemaker, cardiac output is maximal when atrioventricular (AV) delay is set to give the maximum QT interval (QTI). QTI is used as a sensor of a rate-responsive pacemaker and the evoked QTI (eQTI) is measured as the time duration from the ventricular pace-pulse and the T sense point, which is the steepest point of the intracardiac T wave. The relationship between the changes in eQTI according to AV delay variations and cardiac function was studied in 13 patients (74.2+/-9.3 [SD] years old) with an implanted QT-driven DDDR-pacemaker. A special software module was downloaded into the pacemaker memory and a personal computer equipped with the special software was connected to the programmer for eQTI date-logging. AV delay was set at 100, 120, 150, 180 and 210 ms. Delta eQTI was defined as maximal eQTI - minimal eQTI. The ejection fraction (EF) was measured by echocardiography. When the AV delay was prolonged, eQTI gradually increased and reached a peak, and then decreased. Delta eQTI in patients with reduced cardiac function (EF <40%) was significantly greater than that in normal cardiac function (EF >55%, 7.6+/-4.9 vs 2.7+/-9.8 ms, p<0.05). There was significant negative correlation between EF and delta eQTI (r=-0.63, p<0.05). The peak of changes in eQTI according to AV delay variations was steeper in patients with reduced cardiac function than in those with normal cardiac function. In conclusion, changes in eQTI according to AV delay variation are greater in patients with reduced cardiac function than in those with normal cardiac function, and the AV delay that gives the maximal eQTI can be easily determined in patients with reduced cardiac function.

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.

Assessment of frequency dependency of the class III effects of almokalant: a study using programmed stimulation and recording of monophasic action potentials and ventricular paced QT intervals

The aim of the present study was to assess the frequency dependency of the effects of almokalant, a selective class III antiarrhythmic drug, on ventricular repolarization using recordings of monophasic action potentials and measurements of ventricular paced QT intervals. Twenty male volunteers were studied during almokalant infusion aiming at plasma concentrations (Cpl) of 20, 50, 100, and 150 nmol/l. The duration of monophasic action potential at 90% repolarization (MAPD) was measured during incremental and premature ventricular extrastimulation. The ventricular paced QT interval was measured during incremental stimulation from the apical region (RV APEX) and the outflow tract (RVOT) of the right ventricle, and the frequency dependence was analyzed using a linear regression model. At an almokalant dose of Cpl > or = 50, there was a significant prolongation of the MAPD of 10-15%. The prolongation was of equal magnitude at all paced cycle lengths (CL). The MAPD of ventricular extrasystole increased in parallel over the range of coupling intervals studied and was significantly prolonged at Cpl 100 and 150. The ratio between the MAPD of the extrasystoles and preceding beats was unaltered after almokalant infusion. The ventricular paced QT intervals increased during almokalant infusion in a similar manner as that of the MAPD. During RV APEX stimulation, the prolongation was more pronounced at low heart rates, an effect that was not seen during RV OT stimulation. Almokalant significantly prolonged the MAPD at dose levels Cpl > or = 50. There was no evidence of a frequency dependence of this effect. The ventricular paced QT intervals were prolonged in a similar manner as that of the MAPD, and this effect exhibited a small reverse frequency dependence during RV APEX stimulation.

Effect of beta-adrenergic stimulation on the QT interval of children with syncope

The effect of intravenous bolus doses (0.25, 0.5, 1.0 microgram) of isoproterenol on the QT and RR intervals was reviewed in a group of 34 children undergoing autonomic testing for syncope. Twenty-one patients had a positive orthostatic test and 13 were negative. The two groups (positive and negative) were compared. Baseline QT and RR intervals were similar. The RR interval was shortened by isoproterenol in both groups. Isoproterenol shortened the QT interval in the negative group (as seen in normal persons), but produced QT prolongation in the positive group, although neither reached statistical significance when compared to baseline within the respective group. Comparing the values for RR and QT at each dose of isoproterenol (including baseline) between the two groups showed a significant difference in the QT interval after the 1.0-microgram dose of isoproterenol. Thus children with orthostatic positive neurocardiogenic syncope showed a different QT response to beta-adrenergic stimulation. This lends support to the theory of altered beta-adrenergic sensitivity being present in children with neurocardiogenic syncope.

New and combined sensors for adaptive-rate pacing

A new potential indication for cardiac pacing is chronotropic incompetence, that is, an inadequate cardiac rate response to exercise and other metabolic demands. Many patients who have been paced for indications such as complete heart block or sick sinus syndrome also have chronotropic incompetence. Such patients are not adequately treated when fitted with a constant rate pacemaker. Adaptive-rate pacemakers increase the pacing rate in proportion to signals derived from a biosensor which is sensitive to exertion and possibly to other metabolic requirements. These pacemakers have proven valuable for patients with overt chronotropic incompetence. However, no single sensor/algorithm is ideal and improvement has been sought by introducing new sensors, adjusting the algorithms by which biosensor signals are converted to the most appropriate pacing rate, or by combining sensors in such a way that a composite biosensor signal is derived which bears a close linear relationship with the appropriate heart rate. An example of a new sensor is the accelerometer, which is sensitive to a fuller range of movements than the piezo crystal. A successful new algorithm is the rate augmentation algorithm for use with minute ventilation, which provides a better initial pacing rate response. A combination of minute ventilation sensed by impedance changes and movement sensed with piezo crystals maintains the rapid response from the piezo crystal and overcomes its lack of proportionality. Another successful new combination of sensors is QT sensing from the evoked ventricular potential and motion sensing with a piezo crystal. As yet, these innovations have not been exhaustively tested and shown to confer clinical benefit but the improvements are such that an advantage can be expected.

Bradycardia-induced abnormal QT prolongation in patients with complete atrioventricular block with torsades de pointes

Fourteen patients with complete atrioventricular block with or without torsades de pointes (TdP) were included in this study. They were divided into 2 groups, 6 patients with TdP (TdP[+] group) and 8 patients without TdP (TdP[-] group). The patients were evaluated at 2 different periods, before (acute period) and after (chronic period) pacemaker implantation. In the acute period, the QRS and heart rate during the escape rhythm were not significantly different between the 2 groups; however, the QT and QTc intervals were significantly longer in the TdP(+) group than in the TdP(-) group: 753 +/- 57.5 vs 635 +/- 78.4 ms (p less than 0.01) and 585 +/- 44.8 vs 476 +/- 58.3 ms (p less than 0.01). In the chronic period (greater than 2 months after pacemaker implantation), we changed the pacemaker rate from 90 or 100 beats/min to 50 beats/min and examined the QT interval changes in relation to the heart rate. The QT interval in the TdP(+) group was significantly prolonged compared with the TdP(-) group when the pacing rate was decreased less than or equal to 60 beats/min: 551 +/- 40 vs 503 +/- 36 ms at 60 beats/min (p less than 0.05), and 700 +/- 46 vs 529 +/- 43 ms at 50 beats/min (p less than 0.001). Patients with complete atrioventricular block with TdP had a bradycardia-sensitive repolarization abnormality and this characteristic remained after pacemaker implantation. The critical heart rate that induced abnormal QT prolongation in the TdP(+) group was less than or equal to 60 beats/min.

Dynamics of ventricular repolarisation in the congenital long QT syndromes

Patients with congenital QT interval prolongation are at risk of ventricular arrhythmias and sudden death. It has been suggested that the susceptibility to arrhythmias in these syndromes may be related to the abnormal dynamics of ventricular repolarisation. The dynamics of ventricular repolarisation, including assessment of the effect of changing heart rate on the QT interval and the duration of the right ventricular monophasic action potential, were studied in eight patients with congenital long QT syndromes. The effects of altered sympathetic tone on these dynamics were investigated with isoprenaline, propranolol, and left stellate ganglion block. The rate adaptation of the QT interval was abnormal in only a few patients and in some patients this feature may be related to the severity of the condition. These abnormalities may be exaggerated by isoprenaline and lessened by propranolol and left stellate ganglion block. Monophasic action potential dynamics were normal in all patients. The hypothesis that impaired QT rate adaptation may play a role in the genesis of ventricular arrhythmias in these syndromes is not, in general, supported by the present data. However, in patients with impaired adaptation the normalisation of QT dynamics after beta blockade and left stellate ganglion block was consistent with the efficacy of these forms of treatment.

The relationship between heart rate and QT interval during atrial stimulation

The relationship between heart rate and QT interval was investigated during atrial stimulation (intrinsic effect of heart rate) in ten healthy male volunteers prior to and after administration of sotalol. The QT interval in the ECG (paper speed 200 mm/s) was determined at rates of 70, 85, 100, 115, 130, 145, and 160 beats/min and at pacing periods of 180 s each at 30, 60, 120, and 180 s. After a 15-minute period, 2.0 mg sotalol/kg body weight were administered iv and the stimulation protocol was repeated. The analysis of QT interval behavior reveals contradictions to the mathematical implications of Bazett's equation QT = QTc square root of 60/HR, so that the relationship between heart rate and QT interval is not adequately described under the given conditions. After examination of approaches reported in the literature and our own approaches, the expression QT = a e-b (HR-60) is used as a possibility differentially to describe the data by nonlinear regression. The parameters a and b may be interpreted as QT reference value and shortening parameter. The QT reference value a, a parameter in reference to heart rate of 60 beats/min, has a comparable significance to the expression QTc in the Bazett equation. A reduction in the shortening parameter b indicates whether substances influencing the QT interval additionally produce overproportional shortening of the QT interval with increasing heart rate. After administration of sotalol, an increase can be observed in both the QT reference value and also in the shortening parameter. The suggested approach is an attempt to provide a more precise assessment of the QT interval under different conditions.

QT sensing rate responsive pacing during subacute bacterial endocarditis: a case report

A 63-year-old woman treated with a QT sensing rate responsive pacemaker following aortic valve replacement developed late subacute bacterial endocarditis. During febrile periods, associated with systemic upset, pacing was physiological as evidenced by an increased heart rate during pyrexia and a decrease when afebrile.

An improved method to correct the QT interval of the electrocardiogram for changes in heart rate

The adequacy of the Bazett formula to correct for heart rate-induced changes in the QT interval of the electrocardiogram has been frequently questioned. In the present study, a simple linear equation was derived, which in anesthetized dogs corrects more adequately for changes in heart rate than the Bazett formula. Regression analysis of experimental data yielded the following equation: QTc = QT - 0.087 (RR - 1000) = QT - 87 (60/HR - 1). The reliability of this equation was investigated in experiments on anesthetized dogs with different cardioactive drugs with a known mechanism of action.

Long-term follow-up of patients treated with the QT interval sensing rate-responsive pacemaker

Thirty-seven patients (mean age 70 years) with QT-interval sensing (TX) rate-responsive pacemakers were followed for a mean of 27 months. This pacemaker measures the QT interval of the paced beat and, if the QT shortens, the pacing rate increases according to a programmed relationship, the 'slope'. With TX pacing the heart rate was 56% and the exercise tolerance 15% higher than with fixed-rate ventricular-inhibited (VVI) pacing during exercise. Holter ECG monitoring showed a physiological rate variability. Six patients (16%) have died during follow-up, and programming to the VVI mode was necessary in six patients (16%). The remaining patients were alive with a subjectively adequate TX function at the end of follow-up. Including the five patients who were on TX pacing at the time of death, satisfactory TX pacing was obtained in 84% of the patients. Although the TX pacemaker has been reliable and provides an adequate rate response during exercise, complaints of worsening angina pectoris and the somewhat complicated programming of the device are problems with the current models.

QT sensing rate responsive pacing and myocardial infarction: a case report

A 65-year-old man, treated with the QT sensing rate responsive pacemaker required to manage high degree AV block, sustained a transmural inferior wall myocardial infarction 6 months after the pacemaker implant. The rate response of the pacemaker during the acute phase of the infarction was physiological as evidenced by increased pacing rate during pain and with the gradual decrease in rate during the first postinfarction days. The underlying mechanisms are discussed.

Heart rate, PR, and QT intervals in normal children: a 24-hour Holter monitoring study

A dynamic electrocardiographic Holter monitoring study was performed in 32 healthy children (20 males and 12 females, age range 6-11 years old), without heart disease, according to clinical and noninvasive instrumental examination. We evaluated atrioventricular conduction time (PR), heart rate (HR), and QT interval patterns defining the range of normality of these electrocardiographic parameters. The PR interval ranged from 154 +/- 10 ms (mean +/- SD) for HR less than or equal to 60 to 102 +/- 12 ms for HR greater than or equal to 120 (range 85-180). The absolute mean HR was 87 +/- 10 beats/min (range 72-104), the minimum observed HR being 61 +/- 10 (range 51-79), the maximum 160 +/- 20 beats/min (range 129-186). Daytime mean HR gave a mean value of 93 +/- 10 (range 71-148), while during night hours it was 74 +/- 11 (range 54-98). The minimum QT interval averaged 261 +/- 10 ms for HR greater than 120 and the maximum 389 +/- 9 ms for HR less than or equal to 60; the corresponding mean value of QTc (i.e., QT corrected for HR) ranged from 388 +/- 8 for HR less than or equal to 60 beats/min to 403 +/- 14 ms for HR greater than 120 beats/min. The results of the present study provide data of normal children which can be readily compared against those of subjects in whom cardiac abnormalities are suspect or patient.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in QT and Q-aT intervals induced by mental and physical stress with fixed rate and atrial triggered ventricular inhibited cardiac pacing

We have investigated the influence of mental stress and physical stress, i.e., exercise, on the QT and Q-aT intervals (measured from the pacemaker stimulus to the end or the apex, respectively, of the T wave). The study was made on ten patients with high degree atrioventricular block treated with AV universal (DDD) pacemakers. These were programmed to a fixed rate ventricular (VVI) or an atrial triggered (VDD) function for different parts of the study. An arithmetic mental stress test and a bicycle exercise test were performed with each mode of pacing. In the VVI pacing mode, the atrial rate increased by 11% during mental stress and by 46% during exercise. There was a significant shortening of QT and Q-aT intervals with both types of stress. With VDD pacing, mental stress induced a 12% increase in rate and a significant shortening of QT and Q-aT. The paced rate increased by 50% during the exercise test. This increase in ventricular rate was associated with the most marked changes in QT and Q-aT intervals. Thus, both types of stress cause a significant shortening of the QT and Q-aT interval even in the absence of a simultaneous increase in ventricular rate. When the latter is allowed to increase during VDD pacing, both intervals shorten considerably more. There was a marked inter-individual variability in the response to both types of stress. These findings are of importance with regard to the QT sensing rate responsive pacemaker which can be expected to respond to mental stress in most patients, but that response might be unpredictable in the individual.

Hysteresis in the human RR-QT relationship during exercise and recovery

The present study was undertaken to test the hypothesis that the human RR-QT relationship during dynamic exercise differs markedly from that during the recovery phase. Fourteen subjects from the age of 16 to 71 years exercised on a treadmill according to the Bruce protocol. Electrocardiograms were recorded continuously on a magnetic tape, from 1 minute before exercise to 10 minutes into recovery. An exponential formula, proposed by us earlier, closely represented the exercise RR-QT data. However, it was not appropriate for the often S-shaped recovery curves which invariably deviated from the exercise curves, exhibiting hysteresis. Initially, all recovery QT intervals were shorter than the exercise values, but later in the recovery, some crossed the exercise curves from below, resulting in longer QT intervals. The recovery data were fitted by a third degree polynomial, and the hysteresis was calculated as the area between the exercise and recovery curves within a 150 ms range of the RR interval starting from its minimum value. The mechanisms for the occurrence of hysteresis are likely to involve the sympatho-adrenal activity in the early post-exercise period and the time course of QT interval adaptation to rapid changes in the RR interval.

Changes in QT interval during exercise testing in patients with VVI pacemakers

The changes in QT intervals were studied in nine patients with normal sinus node function who had VVI pacemakers. Though PP intervals uniformly shortened during exercise, the change in QaT* intervals during exercise was variable. The correlation between PP and QaT intervals varied from case to case. A good correlation was found in only two cases (r = +0.816 or +0.897); a fair correlation was found in four cases (r = +0.672, +0.615, +0.615, or -0.669) and in the remaining three, the correlation was poor (r = +0.494, +0.467 or -0.424). In patients who are candidates for VTI pacemaker implantation, changes in QaT intervals should be assessed during exercise stress testing to determine if the intervals shorten during exercise or not.

Changes in Q-T and Q-aT intervals at rest and during exercise with different modes of cardiac pacing

The influence of heart rate variation on the Q-T and Q-aT intervals (measured from the onset of the QRS to the end or the apex, respectively, of the T-wave) was studied both at rest and during exercise using different modes of pacing. The studies were made on 21 patients with high-degree atrioventricular block. In seven patients with programmable ventricular inhibited (VVI) pacemakers, an increase in pacing rate during rest produced significant shortening of both Q-T and Q-aT. During observations made at rest and during exercise in 14 patients with fixed rate VVI, atrial rate matched asynchronous (VVIm) or atrial triggered (VAT) pacing. Significant shortening of Q-T and Q-aT intervals occurred during exercise in all pacing modes, but was greatest with VVIm and VAT. The Q-T and Q-aT changes were almost parallel in all situations. For measurements made by two independent observers the coefficient of variation was lower for Q-aT than for Q-T (2.2 versus 2.5) and the correlation coefficient was higher (0.96 versus 0.93), indicating easier identification of Q-aT than of Q-T. This study indicated that changes in Q-T and in Q-aT are influenced by intrinsic factors in addition to the ventricular rate. Atrioventricular synchronization did not seem to influence these changes.

Ventricular tachyarrhythmias in the long QT syndromes

Marked prolongation of the electrocardiographic QT interval often is associated with a distinctive form of ventricular tachycardia characterized by the gradual oscillation around the baseline of the peaks of successive QRS complexes. This was named torsades de pointes, or "twisting of the points." This form of ventricular tachycardia tends to be rapid and self-terminating and often occurs in clusters, leading afflicted patients to present with recurrent dizziness and syncope. Ventricular fibrillation and sudden death are common.

An exponential formula for heart rate dependence of QT interval during exercise and cardiac pacing in humans: reevaluation of Bazett's formula

A new exponential formula to characterize the human RR-QT relation was evaluated in comparison with Bazett's formula in 16 subjects: 10 healthy, normal men (ages 18 to 30 years) who exercised on a stationary bicycle, and 6 patients (ages 50 to 80 years; 2 women and 4 men) with rate-programmable VVI pacemakers whose rates were changed by an external programmer. The RR and QT intervals for heart rate in the range of 50 to 180 beats/min were measured from electrocardiographic tracings recorded at a paper speed of 100 mm/s. The data from each subject were fitted separately by 4 formulas by an appropriate regression analysis using a statistical package program: (F1) QT = A1 - B1*Exp(-k1*RR); (F2) QT = A2[1-Exp-(-k2*RR)]; (F3) QT = A3* square root (RR) + B3; and (F4) QT = A4* square root (RR), where all A, B, and k are regression parameters. The relative goodness of fit of data by the 4 formulas was assessed by the mean-squared residual and the Akaike Information Criterion using Wilcoxon signed-ranks tests. This analysis confirmed that F1 is the best model among the formulas tested and F4 (Bazett's formula) is the least acceptable for both exercised and paced groups. The deviations from Bazett's formula were more striking for the paced group than for the exercised group, as reflected by the mean-squared residual values for F4 (715 +/- 86 for the paced group vs 384 +/- 41 for the exercised group, p less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

The long QT syndrome; effects of drugs and left stellate ganglion block

Four patients, two with congenital QT prolongation (Romano-Ward syndrome) and two with acquired idiopathic QT prolongation not related to bradycardia, drug toxicity, electrolyte imbalance, or neurological disorder were investigated for the onset of recurrent palpitations and/or syncope. The effects on the measured QT interval of intravenously administered propranolol (QTp), an infusion of isoproterenol (QTi) and left stellate ganglion block (QTs) were assessed at identical atrial paced rates and during sinus rhythm, corrected for rate change (QTc). Propranolol shortened the QTc in all patients. The QTp shortened only in those with congenital QT prolongation. Isoproterenol lengthened the QTc in the three patients studied. However, the QTi lengthened in the congenital syndrome whereas it shortened in the acquired syndrome. The QTs was uninfluenced by left stellate ganglion block in all patients. It is suggested that the congenital and acquired forms can be differentiated by pharmacological interventions and that the efficacy of propranolol in the former may result from its ability not only to increase the threshold for ventricular fibrillation, but also its ability to shorten the QT interval.