Spontaneous beat-to-beat variability of the ventricular repolarization duration

Information Flow Between Heart Rhythm, Repolarization, and the Diastolic Interval Series for Healthy Individuals and LQTS1 Patients

Using information theoretic measures, relations between heart rhythm, repolarization in the tissue of the heart, and the diastolic interval time series are analyzed. These processes are a fragment of the cardiovascular physiological network. A comparison is made between the results for 84 (42 women) healthy individuals and 65 (45 women) long QT syndrome type 1 (LQTS1) patients. Self-entropy, transfer entropy, and joint transfer entropy are calculated for the three time series and their combinations. The results for self-entropy indicate the well-known result that regularity of heart rhythm for healthy individuals is larger than that of QT interval series. The flow of information depends on the direction with the flow from the heart rhythm to QT dominating. In LQTS1 patients, however, our results indicate that information flow in the opposite direction may occur—a new result. The information flow from the heart rhythm to QT dominates, which verifies the asymmetry seen by Porta et al. in the variable tilt angle experiment. The amount of new information and self-entropy for LQTS1 patients is smaller than that for healthy individuals. However, information transfers from RR to QT and from DI to QT are larger in the case of LQTS1 patients.

Measures of repolarization variability predict ventricular arrhythmogenesis in heptanol-treated Langendorff-perfused mouse hearts

BACKGROUND

Time-domain and non-linear methods can be used to quantify beat-to-beat repolarization variability but whether measures of repolarization variability can predict ventricular arrhythmogenesis in mice have never been explored.

METHODS

Left ventricular monophasic action potentials (MAPs) were recorded during constant right ventricular 8 ​Hz pacing in Langendorff-perfused mouse hearts, in the presence or absence of the gap junction and sodium channel inhibitor heptanol (0.1, 0.5, 1 or 2 ​mM).

RESULTS

Under control conditions, mean action potential duration (APD) was 39.4 ​± ​8.1 ​ms. Standard deviation (SD) of APDs was 0.3 ​± ​0.2 ​ms, coefficient of variation was 0.9 ​± ​0.8% and the root mean square (RMS) of successive differences in APDs was 0.15 ​± ​0.14 ​ms. Poincaré plots of APDn+1 against APDn revealed ellipsoid morphologies with a SD along the line-of-identity (SD2) to SD perpendicular to the line-of-identity (SD1) ratio of 4.6 ​± ​2.1. Approximate and sample entropy were 0.61 ​± ​0.12 and 0.76 ​± ​0.26, respectively. Detrended fluctuation analysis revealed short- and long-term fluctuation slopes of 1.49 ​± ​0.27 and 0.81 ​± ​0.36, respectively. Heptanol at 2 ​mM induced ventricular tachycardia in five out of six hearts. None of the above parameters were altered by heptanol during which reproducible electrical activity was observed (KW-ANOVA, P ​> ​0.05). Contrastingly, SD2/SD1 decreased to 2.03 ​± ​0.41, approximate and sample entropy increased to 0.82 ​± ​0.12 and 1.45 ​± ​0.34, and short-term fluctuation slope decreased to 0.82 ​± ​0.19 during the 20-s period preceding spontaneous ventricular tachy-arrhythmias (n ​= ​6, KW-ANOVA, P ​< ​0.05).

CONCLUSION

Measures of repolarization variability, such as SD2/SD1, entropy, and fluctuation slope are altered preceding the occurrence of ventricular arrhythmogenesis in mouse hearts. Changes in these variables may allow detection of impending arrhythmias for early intervention.

Concomitant Evaluation of Heart Period and QT Interval Variability Spectral Markers to Typify Cardiac Control in Humans and Rats

The variability of heart period, measured as the time distance between two consecutive QRS complexes from the electrocardiogram (RR), was exploited to infer cardiac vagal control, while the variability of the duration of the electrical activity of the heart, measured as the time interval from Q-wave onset to T-wave end (QT), was proposed as an indirect index of cardiac sympathetic modulation. This study tests the utility of the concomitant evaluation of RR variability (RRV) and QT variability (QTV) markers in typifying cardiac autonomic control of humans under different experimental conditions and of rat groups featuring documented differences in resting sympatho-vagal balance. We considered: (i) 23 healthy young subjects in resting supine position (REST) undergoing head-up tilt at 45° (T45) and 90° (T90) followed by recovery to the supine position; (ii) 9 Wistar (WI) and 14 wild-type Groningen (WT) rats in unstressed conditions, where the WT animals were classified as non-aggressive (non-AGG, n = 9) and aggressive (AGG, n = 5) according to the resident intruder test. In humans, spectral analysis of RRV and QTV was performed over a single stationary sequence of 250 consecutive values. In rats, spectral analysis was iterated over 10-min recordings with a frame length of 250 beats with 80% overlap and the median of the distribution of the spectral markers was extracted. Over RRV and QTV we computed the power in the low frequency (LF, from 0.04 to 0.15 Hz in humans and from 0.2 to 0.75 Hz in rats) band (LF_RR and LF_QT) and the power in the high frequency (HF, from 0.15 to 0.5 Hz in humans and from 0.75 to 2.5 Hz in rats) band (HF_RR and HF_QT). In humans the HF_RR power was lower during T90 and higher during recovery compared to REST, while the LF_QT power was higher during T90. In rats the HF_RR power was lower in WT rats compared to WI rats and the LF_QT power was higher in AGG than in non-AGG animals. We concluded that RRV and QTV provide complementary information in describing the functioning of vagal and sympathetic limbs of the autonomic nervous system in humans and rats.

Towards a better understanding of QT interval variability

The International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) Guideline E14 recommends 'Thorough QT Study' as a standard assessment of drug-induced QT interval prolongation. At the same time, the value of drug-induced QTc prolongation as a surrogate marker for risk of life-threatening polymorphic ventricular tachycardia known as torsades des pointes remains controversial. Beat-to-beat variability of QT interval was recently proposed as an alternative metric. The following review addresses mechanisms of beat-to-beat QT variability, methods of QT interval variability measurements, and its prognostic value in clinical studies.

Frequency domain assessment of the coupling strength between ventricular repolarization duration and heart period during graded head-up tilt

We test the hypothesis that the degree of correlation between ventricular repolarization duration (VRD) and heart period (HP) carries information on cardiac autonomic regulation. The degree of correlation was assessed in the frequency domain using squared coherence function during an experimental protocol known to gradually induce a shift of sympathovagal balance toward sympathetic predominance (ie, graded head-up tilt). We observed a progressive decrease of squared coherence with tilt table inclination, thus confirming the working hypothesis. The VRD-HP uncoupling occurs in the high-frequency band, centered on the respiratory rate, thus suggesting that vagal withdrawal is responsible for the VRD-HP uncoupling.

Cardiac regulation and electrocardiographic factors contributing to the measurement of repolarization variability

Cellular and macroelectrical instability within the heart ventricles during repolarization is described as a potential triggering mechanism of life-threatening arrhythmias. Although this phenomenon was observed in animal and in vitro studies, significant efforts have been put into the design of computerized technologies to quantify very subtle variations of the repolarization signal from the surface electrocardiograms. These technologies aim at capturing repolarization instability of ventricular repolarization while controlling for the normal variability. Currently, the methods have focused on the autonomic regulation of the heart rate as a primary confounding factor (such as in the QT variability index). However, there are other factors that can influence the measurements of beat-to beat variability of the repolarization segment. Among them, the amplitude of the repolarization signal, the selected lead, and the heart vector orientation are very important and too often neglected in clinical investigations. We will discuss these factors and provocatively describe why they should be cautiously considered to avoid erroneous measurements of repolarization instability.

Assessing causality in normal and impaired short-term cardiovascular regulation via nonlinear prediction methods

We investigated the ability of mutual nonlinear prediction methods to assess causal interactions in short-term cardiovascular variability during normal and impaired conditions. Directional interactions between heart period (RR interval of the ECG) and systolic arterial pressure (SAP) short-term variability series were quantified as the cross-predictability (CP) of one series given the other, and as the predictability improvement (PI) yielded by the inclusion of samples of one series into the prediction of the other series. Nonlinear prediction was performed through global approximation (GA), approximation with locally constant models (LA0) and approximation with locally linear models (LA1) of the nonlinear function linking the samples of the two series, on patients with neurally mediated syncope and control subjects. Causality measures were evaluated in the two directions (from SAP to RR and from RR to SAP) in the supine (SU) position, in the upright position after head-up tilt (early tilt, ET) and after prolonged upright posture (late tilt, LT). While the trends for the GA, LA0 and LA1 methods were substantially superimposable, PI elicited better than CP the prevalence of causal coupling from RR to SAP during SU. Both CP and PI noted a marked decrease in coupling in both causal directions in syncope subjects during LT, documenting the impairment of cardiovascular regulation in the minutes just preceding syncope.

Measurement and regulation of cardiac ventricular repolarization: from the QT interval to repolarization morphology

Ventricular repolarization (VR) is a crucial step in cardiac electrical activity because it corresponds to a recovery period setting the stage for the next heart contraction. Small perturbations of the VR process can predispose an individual to lethal arrhythmias. In this review, I aim to provide an overview of the methods developed to analyse static and dynamic aspects of the VR process when recorded from a surface electrocardiogram (ECG). The first section describes the list of physiological and clinical factors that can affect the VR. Technical aspects important to consider when digitally processing ECGs are provided as well. Special attention is given to the analysis of the effect of heart rate on the VR and its regulation by the autonomic nervous system. The final section provides the rationale for extending the analysis of the VR from its duration to its morphology. Several modelling techniques and measurement methods will be presented and their role within the arena of cardiac safety will be discussed.

Bias of QT dispersion

BACKGROUND

Prolonged QT dispersion (QTD) is associated with an increased risk of arrhythmic death but its accuracy varies substantially between otherwise similar studies. This study describes a new type of bias that can explain some of these differences.

MATERIAL

One dataset (DiaSet) consisted of 356 subjects: 169 with diabetes, 187 nondiabetic control persons. Another dataset (ArrSet) consisted of 110 subjects with remote myocardial infarction: 55 with no history of arrhythmia and 55 with a recent history of ventricular tachycardia or fibrillation.

METHODS

12-lead surface ECGs were recorded with an amplification of 10 mm/mV at a paper speed of 50 mm/s. The QT interval was measured manually by the tangent-method. The bias depends on the magnitude of the measurement errors and the measurable part of the bias increases with the number of the repeated measurements of QT.

RESULTS

The measurable bias was significant for both datasets and decreased for increasing QTD in the DiaSet (P < 0.001) and in the ArrSet (P = 0.11). The bias was 2.5 ms and 1.9 ms at QTD = 38 ms and 68 ms, respectively, in the ArrSet, and 7.5 ms and 2.8 ms at QTD = 19 ms and 55 ms, respectively, in the DiaSet.

CONCLUSIONS

This study shows that random measurement errors of QT introduces a type of bias in QTD that decreases as the dispersion increases, thus reducing the separation between patients with low versus high dispersion. The bias can also explain some of the differences in the mean QTD between studies of healthy populations. Averaging QT over three successive beats reduces the bias efficiently.

Noise sensitivity of a principal component regression based RT interval variability estimation method

Ventricular repolarization duration (VRD) is controlled by neural regulatory system same way as heart rate and, thus, also VRD varies in time. Traditionally, VRD variability is assessed by determining the time differences between successive R and T-waves, i.e. RT intervals. We have recently proposed a method based on principal component regression (PCR) for quantifying RT variability. The main benefit of the method is that it does not necessitate T-wave detection. In this paper, the noise sensitivity of the PCR based method is evaluated by examining the effect of simulated Gaussian noise on the spectral characteristics of the estimated RT variability series.

Complexity of the dynamic QT variability and RR variability in patients with acute anterior wall myocardial infarction

QT and RR intervals' fluctuations are implicated in the development of malignant arrhythmias. Recent research has quantified repolarization lability using stochastic and linear methods. However, QT-RR intervals are nonlinearly coupled. To this end, QT and RR intervals were extracted from twenty four patients with acute myocardial infarction (AMI) and 13 controls, and a measure of local dimensional complexity (pointwise correlation dimension, PD2) was calculated. PD2 of QT intervals was significantly higher for the patients than for controls (4.83 +/- 0.81 versus 3.40 +/- 0.76, P =.0001), and vice versa for RR intervals (2.51 +/- 0.62 versus 2.91 +/- 0.42, P =.028). The RR and QT measures of complexity were highly correlated only among controls (r = 0.769, P =.0021). Our results support the presence of autonomic abnormalities during infarction and might complement existing tools for assessment of increased risk for sudden death after AMI.

Time and frequency analysis of beat-to-beat R-T interval variability in patients with ischaemic left ventricular dysfunction providing evidence for non-neural control of ventricular repolarisation

BACKGROUND

Determinants of temporal lability in ventricular repolarisation are not fully recognised. We aimed to analyse the sources of RT variability by comparing normal subjects and patients after myocardial infarction (MI) with either depressed or preserved left ventricular (LV) function.

METHODS

One hundred and nine patients (27 women, 82 men, aged 51 +/- 9 years) were divided into three groups: 24 patients (pts) with an uncomplicated angiographically proven coronary heart disease (CHD-group), 59 post-MI pts with preserved LV function (LVEF > 40%, PMI-N-group) and 26 post-MI pts with depressed LV function (LVEF < 40%, PMI-L-group). An ECG signal of low-noise 512 heartbeats was recorded using a computer-assisted amplifier (16 bit, 2 kHz). The onset and offset of the R-wave and T-wave were determined automatically. The magnitude of R-R and R-T variability was measured as the standard deviation of all intervals (SD-RR and SD-RT, ms, respectively). Their relationship was quantified by the correlation coefficient r(RT/RR). Power spectral density of RR or RT variability was estimated with the FFT (Welch's averaged periodogram, Hanning window) and frequency relation was quantified using a squared coherence spectrum (SCS). For all spectral and cross-spectral measurements two frequency ranges were considered: high (0.15-0.50 Hz, HF) and low (0.04-0.15 Hz, LF). Spectral power and SCS of RR and RT variability for both ranges (HF(RR), LF(RR), HF(RT), LF(RT), SCS(HF), SCS(LF)), and the ratios LF/HF(RR) and LF/HF(RT) were drawn for comparisons. The central frequency of HF(RR) was considered as the frequency of respiration (f(resp), Hz).

RESULTS

In the PMI-L group the SD-RT was significantly greater compared to the remaining groups and accounted for almost 10% of the SDRR. Also, the coefficient r(RT/RR) was weakest in this group. The spectral indices of RR variability were similar in all groups, while the greatest value of the HFRT was observed in the PMI-L group. The SCS(LF) was insignificant in this group, contrary to the CHD and PMI-N groups. Additionally, there were significant negative relationships between f(resp) and spectral indices of RT variability in PMI-patients with depressed LV function.

CONCLUSION

A greater beat-to-beat variation in RT interval duration along with increased power of its HF component indicates an important role of respiration in ventricular repolarisation control, while reduced time- and frequency RT-RR relationships seem to relate to an impaired process of ventricular duration adaptation.

Independent autonomic modulation of sinus node and ventricular myocardium in healthy young men during sleep

INTRODUCTION

The aim of this study was to investigate whether autonomic modulation of ventricular repolarization may spontaneously differ from that of the sinoatrial node.

METHODS AND RESULTS

Onset of P waves, QRS complexes, and the apex and end of T waves were detected beat to beat in high-resolution ECGs from nine healthy young men during the night. There were time-dependent fluctuations in the QT/RR slopes of consecutive 5-minute segments that could not be explained by the mean RR cycle length of the respective segment. Because the variability found in QT intervals could not be explained by either possible effects of rate dependence or hysteresis, autonomic effects were obvious. Power spectral analysis was performed for consecutive 5-minute segments of PP and QT tachograms. In a given subject, trends in the time course of low-frequency (LF) and high-frequency (HF) power in PP and QT often were similar, but they were quite different at other times. The mean LF/HF ratio for QTend (0.75 +/- 0.1) was different from that of PP (1.8 +/- 0.2; P = 0.002), indicating differences in sympathovagal balance at the different anatomic sites. Furthermore, at a given mean heart rate, averaged QT intervals were different on a time scale of several minutes to hours. The QT/RR slope of 5-minute segments correlated significantly with the HF power of QT variability but not with that of PP variability, indicating effects of the autonomic nervous system on ventricular action potential restitution.

CONCLUSION

These differences demonstrate that changes in sinus node automaticity are not necessarily indicative of the autonomic control of ventricular myocardium.

Increased QT variability in patients with panic disorder and depression

This study investigated beat-to-beat QT variability in patients with panic disorder and depression, and normal control subjects using an automated algorithm to compute QT intervals. An increase in QT variability appears to be associated with symptomatic patients with dilated cardiomyopathy and also with an increased risk for sudden death. QT(vm) (QT variability normalized for mean QT interval) and QT(vi) (a log ratio of QT variance normalized for mean QT over heart rate variability normalized for mean heart rate) were significantly higher in patients with panic disorder and depression in supine as well as standing postures (P=0.002 and 0.0001 for QT(vm) and QT(vi), respectively). In another analysis, QT(vi) was significantly higher in patients with panic disorder compared to control subjects in supine as well as standing postures during spontaneous breathing as well as 12, 15 and 20 per minute breathing (P=0.005). These findings are important especially in view of the recent reports of increased risk for cardiovascular mortality and sudden death in patients with anxiety and depression and the utility of QT(vi) as a noninvasive measure of temporal repolarization lability.

Using simulated noise to define optimal QT intervals for computer analysis of ambulatory ECG

The ambulatory electrocardiogram (ECG) is an important medical tool, not only for diagnosis of adverse cardiac events, but also to predict the risk of such events occurring. The 24-hour ambulatory ECG has certain problems and drawbacks because the signal is corrupted by noise from various sources and also several other conditions which may alter the ECG morphology. We have developed a Windows based program for the computer analysis of ambulatory ECG which attempts to address these problems. The software includes options for importing ECG data, different methods of waveform analysis, data-viewing, and exporting the extracted time series. In addition, the modular structure allows for flexible maintenance and expansion of the software. The ECG was recorded using a Holter device and oversampled to enhance the fidelity of the low sampling rate of the ambulatory ECG. The influence of different sampling rates on the interval variability were studied. The noise sensitivity of the implemented algorithm was tested with several types of simulated noise and the precision of the interval measurement was reported with SD values. Our simulations showed that, in most of the cases, defining the end of QT interval at the maximum of the T wave gave the most precise measurement. The definition of the onset of the ventricular repolarization duration is most precisely made on the maximum or descending maximal slope of the R wave. We also analyzed some examples of time series from patients using power spectrum estimates in order to validate the low level QT interval variability.

Assessment of the coupling between RTapex and RR interval as an index of temporal dispersion of ventricular repolarization

To evaluate the dynamic characteristics of the relationship between the RT and RR intervals we analyzed the RR/RTapex variability interaction with a dynamic parametric model whose parameters can be directly estimated from the beat-to-beat series RR and RTapex intervals. The model is designed to separate the fraction of RTapex variability driven by RR changes from that independent of RR variations and to quantify the gain and phase of the relationship between RR and RTapex intervals. The percentage of RTapex variability driven by RR variability was significantly greater in young normal subjects in comparison with postmyocardial infarction patients as well as with age-matched control subjects. This new approach based on the quantification of the RTapex variability dependent and independent of beat-to-beat RR interval changes could be used to quantify the degree of uncoupling between the two signals thus providing a new and noninvasive index of temporal dispersion of ventricular repolarization.

Quantifying electrocardiogram RT-RR variability interactions

A dynamic linear parametric model is designed to quantify the dependence of ventricular repolarisation duration variability on heart period changes and other immeasurable factors. The model analyses the beat-to-beat series of the RR duration and of the interval between R- and T-wave apexes (RT period). Directly from these two signals, a parametric identification procedure and spectral decomposition techniques allow RT variability to be divided into RR-related and RR-unrelated parts and allow the RT-RR transfer function to be calculated. RT variability is driven by RR changes at low frequency (LF, around 0.1 Hz) and high frequency (HF, at the respiratory rate), whereas, at very low frequencies, the RR-unrelated contribution to the total RT variability is remarkable. During tilt at LF the RR-related RT percentage power increases (p < 0.02), the RR-unrelated RT percentage power remains unchanged, the gain of the RT-RR relationship largely increases (p < 0.001), and the phase is not significantly modified. Both the RR-related and the RR-unrelated RT percentage powers at LF are not affected by controlled respiration, and an increase in the RT-RR gain at HF is observed (p < 0.02). The proposed analysis may help to describe the regulation of the ventricular repolarisation process and to extract indexes quantifying the coupling between heart period and ventricular repolarisation interval changes.

Performance assessment of standard algorithms for dynamic R-T interval measurement: comparison between R-Tapex and R-T(end) approach

Three automatic approaches to ventricular repolarisation duration measurement (R-Tapex, R-T(end threshold) and R-T(end fitting) methods) are compared on computer-generated and real ECG signals, in relation to their reliability in the presence of the most common electrocardiographic artefacts (i.e. additive broadband noise and additive and multiplicative periodical disturbances). Simulations permit the evaluation of the amount of R-T beat-to-beat variability induced by the artefacts. The R-T(end threshold) method performs better than the R-T(end fitting) one, and, hence, the latter should be used with caution when R-T(end) variability is addressed. Whereas the R-Tapex method is more robust with regard to broadband noise than the R-T(end threshold) one, the reverse situation is observed in the presence of periodical amplitude modulations. A high level of broadband noise dose not prevent the detection of the central frequency of underlying R-T periodical changes. Comparison between the power spectra of the beat-to-beat R-T variability series obtained from three orthogonal ECG leads (X,Y,Z) is used to assess the amount of real and artefactual variability in 13 normal subjects at rest. The R-Tapex series displays rhythms at high frequency (HF) with a percentage power on the Z lead (57.1 +/- 4.9) greater than that on the X and Y leads (41.9 +/- 4.6 and 46.1 +/- 4.9, respectively), probably because of respiratory-related artefacts affecting the Z lead more remarkably. More uniform HF power distributions over X,Y,Z leads are observed in the R-T(end threshold) series (31.8 +/- 3.8, 39.2 +/- 4.1 and 35.1 +/- 4.2, respectively), thus suggesting minor sensitivity of the R-T(end threshold) measure to respiratory-related artefacts.

Mental stress enhances the sympathetic fraction of QT variability in an RR-independent way

In 10 healthy male volunteers aged 19-20, spectral power of the beat-by-beat QT interval was measured at the Traube-Herring-Mayer (THM) band (0.05 to 0.15 Hz) when the subject was at rest and during atrial pacing. After resting in dorsal decubitus for 10 minutes, right atrial pacing was performed at a slight elevation above sinus rhythm as well as at 100 or at 110 beats per minute for 7 minutes each. In addition, during pacing at 100 or 110 bpm, the subject was required to perform a Kraepelin Arithmetic test. There was a statistically significant increase in QT spectral power at the THM band, while heart rate was maintained unchanged during the periods of mental stress versus rest. This experiment confirms the potential independence of the QT interval from heart rate and suggests that THM fraction of QT spectral power has a double subordination: during relaxed rest it follows mostly RR-fluctuation; during stress an RR-independent contribution is added, which presumably reflects the supra-normal sympathetic drive on the ventricles.

Adaptation of the QT interval to heart rate changes in isolated perfused guinea pig heart: influence of amiodarone and D-sotalol

The inadequacy of the QT interval to shorten following heart rate increase is a feature of the inherited long QT syndrome and may have a role in the genesis of the typical arrhythmias associated with this syndrome (torsade des pointes). The aim of our study was to evaluate whether drugs that prolong the QT interval, such as amiodarone and D-sotalol, may also impair the ability of the QT interval to adapt to sudden heart rate changes. Experiments were carried out on isolated perfused guinea pig hearts (Langendorff preparation). Driving frequency was changed, in steps, every two minutes (Hz: 2.5-3-2.5-3.75-2.5-5-2.5), while epicardial ECG was continuously recorded on magnetic tape. QT interval was automatically measured by means of a beat-by-beat analysis program. D-sotalol was added to the perfusion medium at a concentration of 4 micrograms ml-1, while amiodarone was administered, before in vitro evaluation, for seven days (50 mg kg-1 per day, intraperitoneally). In control experiments two phases of QT adaptation were identified: an abrupt QT shortening at the first beat after frequency change (QT1), followed by a gradual, exponential QT shortening that reached a new steady state in about 1 min (half life: 13 sec). The electrical restitution curve (the relation between QT1 and the corresponding diastolic interval) had a rate constant of 57 +/- 8 ms. Neither drug changed the slow component of QT adaptation. However, both drugs increased the ability of QT to shorten upon premature stimulation: D-sotalol by increasing the rate constant of the restitution curve and amiodarone by decreasing the y-intercept. Our results indicate that D-sotalol and amiodarone do not impair QT shortening during tachycardia but, on the contrary, they may favour QT adaptation, thus reducing the likelihood of the potentially lethal 'R on T phenomenon'. This may be an additional mechanism by which these drugs can exert their antifibrillatory action.

Beat-to-beat behavior of QT interval during conducted supraventricular rhythm in the normal heart

To assess beat-to-beat behavior of QT interval under different conditions, high resolution recordings and computerized beat-to-beat analysis of the electrocardiogram were performed at rest, during recovery after short exercise, and during atrial pacing. Beat-to-beat variations of QT interval during sinus rhythm at rest and after short exercise were measured in ten healthy men. In an additional three patients with supraventricular tachycardia, beat-to-beat QT changes were studied after abrupt sustained acceleration and deceleration of heart rate by atrial pacing. Beat-to-beat changes in RR interval at rest are followed by minimal changes of the QT interval. The measured proportional change of the QT interval compared with the change in RR interval (delta QT/delta RR) was 0.02. This value represents 10% of the value expected for QT changes from Bazett's formula. Following short exercise QT interval did not change for 15 seconds and reached a maximal value 80 seconds later as compared to the RR interval (192 vs 115 secs, P < 0.001). The steady state of the QT interval during sustained atrial pacing was achieved after 132, 135, and 133 seconds for pacing intervals of 600, 500, and 600 msec, respectively. Our data indicate a relatively slow adaptation of the QT interval to changes in heart rate.

Association between QT interval and coronary heart disease in middle-aged and elderly men. The Zutphen Study

BACKGROUND

Heart-rate-adjusted QT-interval (QTc) is prognostic of sudden death in myocardial infarction patients. So far, population studies have yielded conflicting results on the predictive value of QTc for coronary heart disease morbidity and mortality. Therefore, we investigated this in a longitudinal study of middle-aged and elderly men.

METHODS AND RESULTS

From 1960 to 1985, 877 middle-aged men were followed and repeatedly examined in the Zutphen Study. In 1985 the remaining cohort was extended to 835 elderly men from the same birth cohort and followed until 1990. Men with prolonged QTc (420 ms1/2 or more) had a higher risk of myocardial infarction and coronary heart disease death relative to men with QTc less than 385 ms1/2. Age-adjusted coronary heart disease mortality rate ratios were 4.3 (95% confidence interval, 1.3 to 13.8) in middle-aged men and 3.3 (95% confidence interval, 1.0 to 11.6) in elderly men. These associations could not be attributed to prevalent heart disease and were independent of other cardiovascular risk factors.

CONCLUSIONS

These results indicate that within the normal range of QTc in the general population, men with long QTc are at higher risk for coronary heart disease. Because QTc is easily determined, it may provide a valuable contribution to risk stratification.

Normal values for QT intervals in ECG during ramp exercise on bicycle

The relation between QT interval and heart rate during ramp exercise tests on a bicycle was investigated in 37 healthy individuals (21 women) without regular medication and with a normal thallium-201 exercise scintigram (mean age 52.9 +/- 8.3, range 38-68). The test started at 20 W and the load increased by 10 W min-1. A 12-lead ECG was recorded twice every min and mean complexes (during a 15 s period) were calculated by computer. At rest the QT interval (in s) corrected for heart rate (QTc) for women and men was 0.408 +/- 0.004 and 0.399 +/- 0.005, respectively, P > 0.05). During exercise there was no difference in QT interval between women and men or between younger (< 50 years) and older (> 50 years) individuals. A straight line was used to describe the relation between QT interval and heart rate (beats min-1; QT = 0.459-12.3 x 10(-4)*HR). A 95% prediction interval around the regression line was determined using a non-parametric statistical method. When QTc was calculated using Bazett's formula with a cut-off value of QTc = 0.46, 19 individuals (11 women) had a prolonged QT interval during exercise. It is concluded that the relation between QT interval and heart rate can during exercise be described by a straight line for normal individuals. It is not valid to use Bazett's formula for correction of QT intervals during ramp exercise tests.

Beat-to-beat measurement and analysis of the R-T interval in 24 h ECG Holter recordings

This study assesses the feasibility of beat-to-beat measurement of the R-T interval in Holter ECG recordings. The low sampling rate of the Holter system was increased by a specific interpolating filter, and the precision and accuracy of two T-wave fiducial point (T-wave maximum: Tm, T-wave end: Te) detection algorithms were compared. The results of the validation tests show better performance of the Tm measurement procedure in the presence of high noise levels. The overall process for the beat-to-beat R-T interval measurement was then tested on ECG Holter recordings collected during free and controlled respiration. Finally, the R-Tm and the corresponding R-R intervals were measured on 24 h ECG recordings of healthy subjects and the spectral analysis was applied to the constructed series. Both R-R and R-Tm spectra show two main frequency components (low-frequency approximately 0.1 Hz, high-frequency approximately 0.25 Hz) changing in their power ratios continuously throughout the 24 h period. The method described seems to provide a dynamic index of the sympatho-vagal balance at the ventricle that can be useful for a deeper understanding of ventricular repolarisation duration variability.