Josef Kautzner

Short- and long-term reproducibility of QT, QTc, and QT dispersion measurement in healthy subjects.

The study investigated interobserver and intrasubject reproducibility of QT interval duration and dispersion measured in standard 12‐lead ECGs recorded at 25 mm/sec. Twenty‐eight healthy volunteers were studied. Each undenvent four ECG recordings, which were performed 1, 7, and 30 days apart. Two independent observers analyzed each ECG record. In each lead with a distinguishable T wave pattern, the RR interval, Q‐peak of T interval, and Q‐end of T interval were measured using a digitizing board with a 0.1‐mm resolution. From each recording the following measures were derived: the maximum, minimum, and mean QT interval; maximum, minimum, and mean heart rate corrected QT interval (QTc); QT and QTc dispersion (the difference between the maximum and minimum QT interval among the 12 leads); and adjusted QT and QTc dispersion (dispersion divided by the square root of the number of leads measured). The interobserver and short‐term (1 day) and long‐term (1 week and 1 month) reproducibility of individual indices was assessed by computing the relative errors and comparing them by a standard sign test. In addition, the distributions of maximum and minimum QTc values among electrocardiographicleads, and the differences between QT‐end and QT‐peak based measurements were investigated. The results showed that: (1) the measurement of the QT interval from standard ECG recordings is feasible and not operator dependent (interobserver relative error <4%); (2) the duration of the QT interval in healthy volunteers is stable and its short‐ and long‐term reproducibility is high (intrasubject relative error < 6%); (3) parameters that characterize dispersion of the QT interval in the 12‐lead ECG are highly nonreproducible, both between subsequent recording (relative error of 25%–35%) and between observers (relative errar 28%–33%), the reproducibility of QT dispersion is significantly lower than that of QT duration (P < 0.01); and (4) the duration of the entire QT interval correlates only weakly with the duration of the Q‐peak of T interval.

Arterial baroreflex sensitivity assessed from phase IV of the Valsalva maneuver

To assess the feasibility of 2 noninvasive methods for arterial baroreflex sensitivity testing based on phase IV of the Valsalva maneuver, the performance of a simple arterial baroreflex sensitivity index compared with a slope method and reproducibility of repeated measurements of either parameter were evaluated in 36 subjects without structural heart disease. The study showed feasibility and reproducibility of both these methods for arterial baroreflex sensitivity assessment and demonstrated the importance of an appropriate rest period between repeated Valsalva maneuvers, giving a basis for prospective testing of noninvasive determination of arterial baroreflex sensitivity together with phenylephrine method in survivors of myocardial infarction.

Interobserver Reproducibility of QT Interval Measurement and QT Dispersion in Patients After Acute Myocardial Infarction

Background: The study evaluated interobserver differences in the classification of the T‐U wave repolarization pattern, and their influence on the numerical values of manual measurements of QT interval duration and dispersion in standard predischarge 12‐lead ECGs recorded in survivors after acute myocardial infarction.

Day-to-day reproducibility of time-domain measures of heart rate variability in survivors of acute myocardial infarction

We conclude that in clinically unchanged conditions, the time-domain HR variability measures derived from 24-hour ambulatory recordings of AMI survivors are stable during the early convalescent phase, and the day-to-day differences have only little effect on the result. The only exception appears to be the pNN50 parameter, the use of which should be favorably substituted by the rMSSD measurement. Geometric estimates of HR variability are highly and consistently correlated with statistical measures of overall HR variability, and may be used as substitutes for each other.

The Effects of Reflex Parasympathetic Stimulation on the QT Interval and QT Dispersion

The effect of phenylephrine-induced reflex parasympathetic stimulation on QT interval and its dispersion was studied in 16 healthy subjects with a history of paroxysmal supraventricular tachycardia, both during sinus rhythm and during atrial pacing. Results demonstrate that rapid reflex parasympathetic stimulation does not influence QT interval duration or QT dispersion, and also emphasize the inappropriateness of Bazetts formula, the need for comparison of QT intervals during identical heart rates, and the importance of analyzing all 12 leads of a standard electrocardiogram when assessing the effects of various interventions on the QT interval.

Impact of Electrocardiogram Recording Format on QT Interval Measurement and QT Dispersion Assessment

FABER, T.S., et al.: Impact of Electrocardiogram Recording Format on QT Interval Measurement and QT Dispersion Assessment. The aim of this study was to determine the effect of recording conditions on the operator dependent measures of QT dispersion in patients with known and/or suspected repolarization abnormalities. Among several methods for risk stratification, QT dispersion has been suggested as a simple estimate of repolarization abnormalities. In a cohort of high and low risk patients, different components of the repolarization process were assessed in the 12‐lead ECG using three different paper speeds and amplifier gains. To assess measurement error and reproducibility, a straight line was repeatedly measured. The operator error was 0.675 ± 0.02 mm and the repeatability of the measurement error was 31 ± 6%. The QT interval was most frequently measurable in V2–V5. Depending on the lead selected for analysis, the incidence of visible U waves was greatest in the precordial leads with high amplifier gain and low paper speed, strongly affecting QT interval measurement. The timing of the onset of the QRS complex (QRS onset dispersion) or offset of the T wave was strongly dependent on the paper speed. Paper speed, but not amplifier gain, had a significant shortening effect on the measurement of the maximum QT interval. As QT interval measurement in each ECG lead incorporates QRS onset and T wave offset (depending on the number of visible U waves), the dispersion of each of these parameters significantly affected QT dispersion. Thus, QT dispersion appears to reflect merely the presence of more complex repolarization patterns in patients at risk of arrhythmias.

Identification of electrocardiographic patterns.

Computer-based electrocardiographs and electrophysiology systems enable ECG signals to be recorded and stored in a digital format with high precision. A sampling frequency of 1 kHz is usually nsed. which leads to high fidelity definition of individual ECG patterns. In order to utilize such patterns in various research and electrophysiological studies, their localization within the signal has to be established. Understandably, some ECC patterns, namely sharp depolarization peaks, are more easy to localize than others. For the localization and measurements of some patterns, e.g., the T waves appearing in the surface EGG leads, numerous technical approaches have been proposed and used in different studies. Frequently, such an ECG analysis depends on particular properties of the complex or wave that is addressed. Nevertheless, many features of pattern recognition and Identification are independent of the character of the signal. In this tutorial text, we discuss some of these common features and demonstrate their potential in example of T wave localizations in a surface EGG lead and of His-depolarization patterns in an intracardiac ECG recording.

Is Vagal Innervation to the Atrioventricular Node Impaired After Radiofrequency Ablation of the Slow Atrioventricular Nodal Pathway

To assess the potentially adverse effects of RF catheter ablation (RFCA) of the slow AV nodal pathway on the parasympathetic innervation to the AV node in patients with AV nodal reentrant tachycardia (AVNRT), AV nodal conduction was evaluated following vagal stimulation by means of a phenylephrine bolus injection (200 μg) before and after RFCA in ten patients (mean age, 37 ± 14 years). Nine patients with AV reentrant tachycardia (AVRT) due to a left free wall accessory pathway served as a control group (mean age of 37 ± 12 years). Whereas no prolongation of the AH interval was observed in the AVNRT group following the phenylephrine bolus during sinus rhythm, despite a significant slowing in sinus rate, phenylephrine administration in AVRT patients was associated with both slowing of the sinus rate and prolongation of the AH interval. Following successful RFCA, the same responses were observed. To delineate the indirect effect of heart rate on AV conduction in response to the phenylephrine bolus, the AH interval was also measured during fixed atrial pacing. A marked prolongation of the AH interval occurred in both groups following phenylephrine administration. This prolongation was biphasic in 50% of A VNRT patients before ablation, suggesting a predominant effect of vagal stimulation on the fast AV nodal pathway. RFCA was associated with disappearance of discontinuous AV conduction in all but one patient with AVNRT. Vagal stimulation caused the same amount of AH interval prolongation as before RFCA in both study groups. In conclusion, patients with AVNRT have a preserved modulation of AV nodal conduction in response to vagal stimulation during sinus rhythm. In addition, vagal stimulation seems to exert a predominant effect on the fast A V nodal pathway. RFCA of the slow AV nodal pathway in patients with A VNRT does not cause detectable damage to the vagal innervation to the AV node.

801-4 Prognostic Implications of QT and QU Interval Measures in Acute Myocardial Infarction

Prolongation of the QT interval corrected using Bazetts formula (QTc) has been reported as a marker for increased risk of arrhythmic events after acute myocardial infarction (AMI). However, the QU interval changes have not been examined. At the same time, QU interval may be of clinical significance, especially in the light of recent experimental evidence linking the U wave with the subpopulation of the so-called M cells within myocardial wall. To evaluate prognostic significance of QT and QU interval measures in AMI, we studied 512 survivors of acute phase of their first myocardial infarction. Patients with conduction defects and drugs likely to affect QT measures were noT included into the analysis. The following intervals were estimated in all the measurable leads on a standard predischarge 12-lead ECG (25 mm/sec paper speed) using a digitizing padxa0—xa0mean RR, mean and max QT, and mean QU. All QT and QU intervals were subsequently corrected for heart rate using Bazetts formula. At one year follow-up, 23 patients (Group I. 19 male. mean age 58.7xa0±xa08.9 years) suffered arrhythmic events (VT/VF or sudden cardiac death). This subset of patients was compared with arrhythmia-free group of 489 subjects (Group II, 385 male, mean age 56.1xa0±xa09.2 years). Statistical analysis was performed using unpaired t-test and ANOVA, results are expressed as meanxa0±xa0SD. Group QT mean QTc mean QT max QTc max QU mean QUc mean I 358.7xa0±xa031.5 426.6xa0±xa030.7 396.5xa0±xa038.5 472.8xa0±xa040.3 459.5xa0±xa058.7 535.2xa0±xa041.3 II 387.3xa0±xa044.1 423.9xa0±xa024 421.7xa0±xa051.5 467.9xa0±xa079.1 552.0xa0±xa073.9 585.7xa0±xa055.1 pxa0lxa0 0.002 NS 0.02 NS 0.001 0.01 The significant difference in QU and QUc, but not in QT intervals persisted even after elimination of the effect of heart rate (ANOVA: pxa0lxa00.007 and 0.011, respectively). Conclusion The differences in the QT but not QU interval measures in the 2 groups can be explained by differing heart rates. Shorter QU interval seemed to identify patients at risk of arrhythmic events after AMI. The pathophysiological basis for this finding is not clear, but could be related to differences in the subpopulation of M cells within myocardial wall.

Reproducibility of time- and frequency-domain indices of heart rate variability assessed after acute myocardial infarction

The most frequent clinical use of heart rate variability (HRV) is the identification of those survivors of acute myocardial infarction who are at risk of serious ventricular arrhythmias and/or sudden cardiac death. This study assessed day-to-day reproducibility of the whole spectrum of HRV parameters in survivors of acute phase of myocardial infarction. A 48 hour ambulatory ECG recording was performed in 21 patients on day 5-7 after hospital admission. The study revealed: (a) that under clinically stable conditions the reproducibility of different time-domain and frequency domain indices of HRV is high, and (b) that day-to-day differences in HRV assessment have presumably no effect on its predictive value. At the same time, individual subjects may exhibit marked day-to-day variation of HRV measures, especially those strongly related to the vagal tone. This should be considered when assessing natural course of the disease or the effects of therapeutic interventions.<<ETX>>