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 sotaloI. 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     , 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 QT, 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.     

Circadian Profile of QT Interval and QT Interval Variability in 172 Healthy Volunteers

BONNEMEIER, H., et al .: Circadian Profile of QT Interval and QT Interval Variability in 172 Healthy Volunteers. The limited prognostic value of QT dispersion has been demonstrated in recent studies. However, longitudinal data on physiological variations of QT interval and the influence of aging and sex are few. This analysis included 172 healthy subjects (89 women, 83 men; mean age   38.7 ± 15   years). Beat-to-beat QT interval duration (QT, QTapex [QTa], T_end[Te]), variability (QTSD, QTaSD), and the mean R-R interval were determined from 24-hour ambulatory electrocardiograms after exclusion of artifacts and premature beats. All volunteers were fully active, awoke at approximately 7:00 am , and had 6–8 hours of sleep. QT and R-R intervals revealed a characteristic day-night-pattern. Diurnal profiles of QT interval variability exhibited a significant increase in the morning hours (6–9 am ; P < 0.01) and a consecutive decline to baseline levels. In female subjects the R-R and T_end intervals were significantly lower at day- and nighttime. Aging was associated with an increase of QT interval mainly at daytime and a significant shift of the T wave apex towards the end of the T wave. The circadian profile of ventricular repolarization is strongly related to the mean R-R interval, however, there are significant alterations mainly at daytime with normal aging. Furthermore, the diurnal course of the QT interval variability strongly suggests that it is related to cardiac sympathetic activity and to the reported diurnal pattern of malignant ventricular arrhythmias. (PACE 2003; 26[Pt. II]):377–382)     

Dynamic Relationship Between the Q-aT Interval and Heart Rate in Patients with Long QT Syndrome During 24-Hour Holter ECG Monitoring

The purpose of this study was to investigate the dynamic relationship between heart rate and the Q-aT interval (the interval from the Q wave to the T wave apex) in patients with long QT syndrome. The QT to heart rate relation is useful for evaluating abnormalities of the ventricular repolarization, but its clinical application to the long QT syndrome requires accurate computer aided measurement of the QT interval and the sampling of a large number of beats. Therefore, the Q-aT interval was used on the basis of some reports that the heart rate dependency of the QT interval was concentrated in the Q-aT interval. Recent advances in the computer technology have allowed analysis of the relationship between the Q-aT and RR intervals on Holter ECG recordings. However, in addition to a prolonged QT interval, most patients with long QT syndrome have bizarre and variable T waves and the influence of this T wave morphology on the Q-aT to heart rate relation has not been clarified. We investigated the dynamic relationship between the Q-aT interval and heart rate in 10 patients with long QT syndrome and 11 control subjects using our original computer algorithm for the analysis of 24-hour Holter ECG recordings. The patients showed morphological T wave changes associated with heart rate changes during Holter recordings and these affected the Q-aT interval. The patients showed the following characteristics in the relationship between the major T wave peak and the RR interval: (1) a modestly decreased correlation between Q-aT and RR than in the control subjects (a median r value of 0.87 vs 0.93; P = 0.001); and (2) a steeper Q-aT/RR slope than in controls (a median slope of 0.24 vs 0.16; P < 0.05). Abnormal and variable T wave morphology in the long QT patients was closely related to a modestly decreased correlation between Q-aT and RR than in the control subjects. The steep Q-aT/RR slope might reflect unstable repolarization of the ventricle, which could act as a substrate for ventricular tachyarrhythmias.     

Circadian Rhythm of the Corrected QT Interval: Impact of Different Heart Rate Correction Models

SMETANA, P., et al .: Circadian Rhythm of the Corrected QT Interval: Impact of Different Heart Rate Correction Models . A reduced circadian pattern in the QTc interval has been repeatedly reported to provide prognostic information in cardiac patients. However, the results of studies in healthy subjects in which different heart rate correction formulas were used are inconsistent regarding the presence and extent of diurnal variations in QTc. This study compared the diurnal variations in QTc obtained with four frequently used heart rate correction models with those based on individually optimized heart rate correction. In 53 subjects (25 men aged 27 ± 7 years and 28 women aged 27 ± 9 years) 12-lead digital ECGs were obtained every 30 seconds during 24 hours. The QT interval was measured automatically by six different algorithms provided by a commercially available device. The QT/RR relation was estimated by four common heart rate correction models and by an individually optimized correction model, QTc = QT/RR^α. In each 24-hour recording, RR, QT, and QTc intervals of separate ECG samples were averaged over 10-minute intervals. Marked differences were found in the extent of the circadian pattern of QTc obtained with different formulas for heart rate correction. Under and overcorrection of the QT interval resulted in significant over- or underestimation of the circadian pattern. Thus, the extent of circadian variation in QTc depends highly on the heart rate correction formula used. To obtain proper insight regarding diurnal variation in QTc prolongation during pharmacologic therapy and/or to assess higher risk due to impaired autonomic regulation of ventricular repolarization, individualized heart rate correction is necessary. (PACE 2003; 26[Pt. II]:383–386)     

QT Interval Dispersion and its Clinical Utility

QT dispersion as a measure ofin-terlead variations of QT interval duration in the surface 12-lead ECG is believed to reflect regional differences in repolarization heterogeneity and thus, may provide an indirect marker of arrhythmogenicity. Methodology for determining QT dispersion and reproducibility of this parameter vary significantly between studies and, together with some other unresolved problems witb QT dispersion assessment, often lead to contradictory suggestions about potential clinical utility of this parameter. The results of our own study in 213 survivors of myocardial infarction, together with a comprehensive review of the literature, suggest that most of these inconsistencies reflect incomplete understanding of electrocardiographic correlates of both normal and abnormal ventricular repolarization. The application of more objective techniques, such as spectral analysis or combined assessment of different parameters (e.g., area beneath the T wave and its symmetricity) may add a new dimension to the noninvasive assessment of ventricular repolarization.     

Effect of lead exclusion for the manual measurement of QT dispersion

To investigate the effect of different lead exclusion criteria for the manual measurement of QT dispersion (QTd). Simultaneous 12-lead ECGs from three groups of 25 subjects were studied; healthy normal subjects, subjects with a myocardial infarction, and subjects with arrhythmias. Leads were excluded with (1) small absolute T wave amplitudes, (2) small relative T wave amplitudes, and (3) small and/or large relative QT measurements. QTd was calculated as the QT range and assessed for its ability to differentiate between the normal and pathological groups. With exclusion of no leads or low absolute amplitude T waves (< 50 microV) significant differences were observed only between normal and myocardial infarct groups (P < 0.05). Significant differences between normal and both pathological groups were observed when excluding the lead with the smallest amplitude T wave or shortest QT (P < 0.05), or when two leads of either type were excluded (P < 0.005). There was good agreement between leads excluded by amplitude or QT (P < 0.01). Lead exclusion for QTd is important. Exclusion of the two smallest amplitude or two shortest QT leads from each subject produced the greatest differences between the normal and pathological groups.     

QT Interval in Children with Sensory Neural Hearing Loss

EL HABBAL, M.H., et al. : QT Interval in Children with Sensory Neural Hearing Loss. Long QT syndrome was first described in children with congenital sensory neural hearing loss (SNHL). The deafness was attributed to abnormalities in potassium ion channels of the inner ear. Similar channels are present in the heart and its dysfunction causes long QT syndrome. Whether congenital SNHL is associated with prolonged QT is unknown. This study examined 52 patients (median age 8.35 years, range 0.21–17.42 years) with SNHL and compared them to 63 healthy children (median age 10.2 years; range 0.67–19 years). An observer, who was blinded from the presence or absence of SNHL, measured QT, QTc intervals and dispersions from a standard 12‐lead electrocardiogram. To assess the cardiac autonomic enervation, power spectral analysis of heart rate variability was determined using a 24‐hour ambulatory heart rate monitor and was expressed as high (HF) to low frequency (LF) ratio. Left ventricular size and functions were evaluated by using two‐dimensional echocardiography. The medians (and ranges) of QT intervals were 340 ms (230–420 ms) in patients and 320 ms (240–386 ms) in the control group (P < 0.01 ). The QTc was longer in patients with SNHL (median 417 ms, range 384–490 ms) than in controls (median 388 ms, range 325–432 ms, P < 0.001 ). QT dispersions in SNHL were higher (median .038 ms, range 00–11 ms) than controls (median 27 ms, range 00–52 ms, P < 0.001 ). T wave inversion (n = 16 ) and alternans (n = 3 ) occurred in patients with SNHL. Heart rates were similar in both groups. Some deaf patients (n = 8 ) had dizzy episodes with a QTc > 440 ms. The HF:LF ratio was 1.32 (0.516–2.33) in deaf patients and 1.428 (0.67–2.3) in the control group (P > 0.1 ). Left ventricular size and functions were similar and normal in deaf patients and controls. In children, congenital SNHL is associated with a prolonged QT interval.     

Association Between Stimulated QT Interval and Ventricular Rhythm Disturbances: Influence of Autonomic Nervous System

To examine the association between ventricular rhythm disturbances and changes in the pacemaker-induced stimulated T interval (STIM-T interval), we compared findings from monitoring of two patient groups. The first group consisted of 15 patients with QTX microprocessor pacemakers and the second group consisted of 198 patients with documented ventricular rhythm disturbances and coronary artery disease (CAD). In the first group, which was free of ventricular rhythm disturbances and manifest coronary artery disease, the STIM-T interval was measured every 4 hours over a 36-hour period at four pacemaker frequency settings (70, 80, 90, and 100) in order to observe the circadian variation of the STIM-T interval as a function of changes in autonomic nervous system (ANS) tone. The second group was comprised of patients with CAD and over 30 VES/hrs (Lown grade classification 1–5), and taking no antiarrhythmic medication. These patients were followed using 24-hour Holter monitoring over a minimum of 23 hours and with less than 5% artifact/recording. Information regarding mean hourly heart rate, total number of VES, VES pairs, VT runs, and ischemic episodes in this group was compared with changes in the STIM-T interval in the first group. The STIM-T interval was found to be shorter during the day and longer at night at all heart rate settings. The total frequency of VES, of VES pairs, VT runs, and ischemic episodes in the second group varies in a similar circadian fashion. The greatest total number of VES, of VES pairs, VT runs, and ischemic episodes was recorded in the waking hours, at the same time when the STIM-T interval is the shortest, while this number was significantly lower during sleep, when the STIM-T interval of the first group is the longest. This coincidence of circadian variation pattern between STIM-T interval in group I, and ventricular arrhythmias and ischemic episodes in group II, suggests that alterations in ANS tone reflected in the STIM-T interval may be an important factor in the occurrence of these untoward events.     

QT Interval and QT Dispersion Measured with the Threshold Method Depend on Threshold Level

Various computerized methods with multiple parameter options for measurements of the QT interval now are available. The optimum parameter setting for most algorithms is not known. This study evaluated the influence of the threshold level applied on the T wave differential on the QT interval and its dispersion measured in normal and abnormal electrocardiograms (ECGs). Seven hundred sixty ECGs recorded in 76 normal subjects and 630 in 63 patients with hypertrophic cardiomyopathy (HCM) (10 consecutive recordings in each individual) were analyzed. In each lead of each ECG, the QT interval was measured by the threshold method applied to the first differential of the T wave. The threshold level was varied between 5% and 30% of the T wave maximum in 1% steps, resulting in 26 different choices of QT measurements. With each choice the maximum QTc and the QT dispersion (QTd, standard deviation of the QT in all 12 leads) were obtained for each recording. The maximum QTc was significantly longer in HCM patients than in normal subjects (P < 0.001) at all threshold levels except between 5% and 7%. The QTd was significantly greater in HCM patients at all threshold levels. The QTc and QTd changed significantly with the threshold level. The maximum QTc varied up to 60 ms in normal subjects and up to 70 ms in HCM patients, depending on the threshold level. Thus, the QT intervai and its dispersion measured with the threshold method applied to the first T wave differential depended significantly on the threshold level in both normal and diseased hearts. All programmable options of available automatic instruments should be examined carefully before any study, and all algorithmic details should be systematically presented.     

QT Interval Variability and Adaptation to Heart Rate Changes in Patients with Long QT Syndrome

Background: Increased QT variability (QTV) has been reported in conditions associated with ventricular arrhythmias. Data on QTV in patients with congenital long QT syndrome (LQTS) are limited.
Methods: Ambulatory electrocardiogram recordings were analyzed in 23 genotyped LQTS patients and in 16 healthy subjects (C). Short-term QTV was compared between C and LQTS. The dependence of QT duration on heart rate was evaluated with three different linear models, based either on the RR interval preceding the QT interval (RR₀), the RR interval preceding RR₀ (RR_-1), or the average RR interval in the 60-second period before QT interval (mRR).
Results: Short-term QTV was significantly higher in LQTS than in C subjects (14.94 ± 9.33 vs 7.31 ± 1.29 ms; P < 0.001). It was also higher in the non-LQT1 than in LQT1 patients (23.00 ± 9.05 vs 8.74 ± 1.56 ms; P < 0.001) and correlated positively with QTc in LQTS (r = 0.623, P < 0.002). In the C subjects, the linear model based on mRR predicted QT duration significantly better than models based on RR₀ and RR_-1. It also provided better fit than any nonlinear model based on RR₀. This was also true for LQT1 patients. For non-LQT1 patients, all models provided poor prediction of QT interval.
Conclusions: QTV is elevated in LQTS patients and is correlated with QTc in LQTS. Significant differences with respect to QTV exist among different genotypes. QT interval duration is strongly affected by noninstantaneous heart rate in both C and LQT1 subjects. These findings could improve formulas for QT interval correction and provide insight on cellular mechanisms of QT adaptation.     

Henry Cuthbert Bazett (1885-1950)--the man behind the QT interval correction formula

The "Bazett formula" is used for correcting the observed QT interval and is named after Henry Cuthbert Bazett who was born in England in 1885. He studied medicine and worked in Oxford and served in the British Medical Corps during World War I. In 1920, Bazett published his seminal paper on the different intervals length of the electrocardiogram recordings in a small group of healthy individuals, and proposed a formula for correcting the QT length for heart rate. Later, he moved to the University of Pennsylvania, in Philadelphia, and from 1931 until his death in 1950, he served as head of the Department of Physiology and a leader of the American Society of Physiology. Bazett's scientific work was largely concerned with temperature control, circulation, and blood volume, and he contributed greatly to the study of circulation in humans by using invasive catheterization.     

QT interval prolongation

The QT interval is a function of ventricular repolarization time and is measured from the onset of the QRS complex to the end of the T wave. The length of this interval is inversely related to heart rate. A prolonged QT interval is most often secondary to the use of Type I antidysrhythmic medications (quinidine, procainamide). It is also associated with phenothiazines, organophosphates, hypocalcemia, liquid protein diets and the congenital long QT syndromes. QT prolongation is associated with a variety of ventricular dysrhythmias, most characteristically Torsades des pointes. Treatment consists of correction of the underlying metabolic disorder or discontinuation of the offending medication.     

Evaluation of Direct Respiratory Modulation of the QT Interval Variability

To investigate the direct respiration-mediated vagal modulation of the QT interval variability, spectral analyses of the RTp interval (from the R wave peak to the T wave peak) variability (RTpV) and the RR interval variability (RRV) were performed in 12 subjects with normal ventricular repolarization under three conditions while the respiration frequency was kept at 0.2 Hz: during sinus rhythm, during fixed atrial pacing, and during fixed atrial pacing with autonomic blockade. The cross-spectrum between the RRV and RTpV was quantified by the squared coherence. During sinus rhythm the RRV power spectrum showed two peaks: a broad peak in the low frequency (LF) band and a sharp peak at 0.2 Hz which corresponded to the controlled respiration frequency. The RTpV power spectrum showed corresponding peaks to the RRV peaks in both the LF and high frequency (HF) bands with high coherence (mean maximum values of the squared coherence in the LF band 0.59 ± 0.22, and in the HF band 0.74 ± 0.14). During atrial pacing mean total power of the RTpV decreased from during sinus rhythm (from 16.3 ± 5.6 ms² to 12.9 ± 5.4 ms², P < 0.05) and the RTpV spectral peaks were abolished in both the LF and HF bands concordant with disappearance of the RRV peaks. Autonomic blockade gave no additional change to the RTpV power spectrum independently of the RRV during fixed atrial pacing. The present study suggested that the direct respiration-mediated vagal modulation may not affect the short-term variability of the QT interval in subjects without repolarization abnormality.     

Paradoxically Shortened QT Interval after a Prolonged Pause

We analyzed Holter ECG recordings in 15 patients with episodes of prolonged RR intervals > 2.5 seconds. In 13 patients, the QT interval showed a linear prolongation when RR interval was < 1.5 seconds and became relatively flat at longer RR intervals. In the remaining two patients, the QT and RR intervals were correlated within physiological range of RR intervals. However, at longer RR intervals, the QT interval was unexpectedly shortened and constant. The paradoxically shortened QT interval observed in the present 2 cases may indicate an abnormal adaptation of repolarization time to an abrupt increase in the preceding RR intervals.     

Associations between electrocardiographic interval durations and coronary artery calcium scores: the Diabetes Heart Study

Background: The mechanisms underlying the associations between QT interval duration and risk of cardiovascular disease (CVD) remain unclear. It has been assumed that these associations are driven by abnormal myocardial repolarization. We examined the relationship between coronary artery disease, measured by coronary artery calcified plaque (CAC), and the duration of QRS, JT, and QT intervals, among predominantly type two diabetic participants.
Methods: The study sample included 1,123 subjects from the Diabetes Heart Study, of whom 85% had type 2 diabetes. Correlations between electrocardiogram interval durations and log-transformed coronary artery calcified (CAC) were assessed in univariate and sequential multivariable generalized estimating equation models adjusted for familial correlations, heart rate, age, race, gender, diabetes status, hypertension status, Body Mass Index (BMI), smoking status, systolic blood pressure, Low Density Lipoprotein (LDL) cholesterol, QT-prolonging medications, and use of exogenous estrogen.
Results: QT interval duration significantly correlated with the extent of CAC in univariate (r = 0.09, P = 0.01) and multivariable models (r = 0.08, P = 0.01). We observed strong correlations between the QRS duration and CAC in univariate (r = 0.23, P < 0.0001) and adjusted models (r = 0.10, P = 0.01). In contrast, the JT interval was not associated with CAC. A strong correlation existed between the QRS interval and CAC in men (QRS: r = 0.24, P ≤ 0.0001) and diabetics (QRS: r = 0.25, P ≤ 0.0001) but was absent in women and nondiabetics. These relationships were not modified by CVD, race, or presence of bundle branch block.
Conclusion: QT duration correlates with the amount of CAC in a predominantly diabetic population. The association between QT duration and CAC is driven by QRS and not JT interval duration.     

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.     

Relationship Between QT Interval Duration and Electrical Induction of Ventricular Tachycardia

The relation of inducible ventricular tachycardia (VT) to QT interval duration of ventricular paced rhythm has not been evaluated. To clarify this relation we measured corrected QT interval duration (QT_C) during sinus rhythm and QT interval duration during ventricular paced rhythm (QT-V) in patients with coronary artery disease without (non-VT group = group B) and with inducible VT (VT group = group A). Duration of QT-V was greater in the VT group (n = 20) compared with non-VT group (n = 20) during ventricular pacing at cycle lengths of 600 ms (424 ± 26 vs 396 ± 19 ms, P < 0.01), of 500 ms (407 ± 20 vs 383 ± 21 ms, P < 0.01), and of 400 ms (390 ± 21 vs 362 ± 17 ms, P < 0.001). During sinus rhythm the mean values of QT_C were similar in both groups (408 ± 25 vs 413 ± 20 ms, NSJ. During ventricular stimulation the percentage of patients with values of QT-V exceeding 380 ms was 35% in non-VT group and 95% in VT group (P <0.01) at cycle length of 500 ms and 5% versus 60%, respectively, (P < 0.01), at cycle length of 400 ms. Thus, a trend toward longer QT values of ventricular paced rhythm exists in patients with inducible VT.