QT Measurement: A Comparison of Three Simple Methods

Comparison Between QT and Corrected QT Interval Assessment by an Apple Watch With the AccurBeat Platform and by a 12‑Lead Electrocardiogram With Manual Annotation: Prospective Observational Study

Background

Abnormal prolongation or shortening of the QT interval is associated with increased risk for ventricular arrhythmias and sudden cardiac death. For continuous monitoring, widespread use, and prevention of cardiac events, advanced wearable technologies are emerging as promising surrogates for conventional 12‑lead electrocardiogram (ECG) QT interval assessment. Previous studies have shown a good agreement between QT and corrected QT (QTc) intervals measured on a smartwatch ECG and a 12-lead ECG, but the clinical accuracy of computerized algorithms for QT and QTc interval measurement from smartwatch ECGs is unclear.

Objective

The prospective observational study compared the smartwatch-recorded QT and QTc assessed using AccurKardia’s AccurBeat platform with the conventional 12‑lead ECG annotated manually by a cardiologist.

Methods

ECGs were collected from healthy participants (without any known cardiovascular disease) aged >22 years. Two consecutive 30-second ECG readings followed by (within 15 minutes) a 10-second standard 12-lead ECG were recorded for each participant. Characteristics of the participants were compared by sex using a 2-sample t test and Wilcoxon rank sum test. Statistical comparisons of heart rate (HR), QT interval, and QTc interval between the platform and the 12-lead ECG, ECG lead I, and ECG lead II were done using the Wilcoxon sign rank test. Linear regression was used to predict QTc and QT intervals from the ECG based on the platform’s QTc/QT intervals with adjustment for age, sex, and difference in HR measurement. The Bland-Altman method was used to check agreement between various QT and QTc interval measurements.

Results

A total of 50 participants (32 female, mean age 46 years, SD 1 year) were included in the study. The result of the regression model using the platform measurements to predict the 12-lead ECG measurements indicated that, in univariate analysis, QT/QTc intervals from the platform significantly predicted QT/QTc intervals from the 12-lead ECG, ECG lead I, and ECG lead II, and this remained significant after adjustment for sex, age, and change in HR. The Bland-Altman plot results found that 96% of the average QTc interval measurements between the platform and QTc intervals from the 12-lead ECG were within the 95% confidence limit of the average difference between the two measurements, with a mean difference of –10.5 (95% limits of agreement –71.43, 50.43). A total of 94% of the average QT interval measurements between the platform and the 12-lead ECG were within the 95% CI of the average difference between the two measurements, with a mean difference of –6.3 (95% limits of agreement –54.54, 41.94).

Conclusions

QT and QTc intervals obtained by a smartwatch coupled with the platform’s assessment were comparable to those from a 12-lead ECG. Accordingly, with further refinements, remote monitoring using this technology holds promise for the identification of QT interval prolongation.

Improving the reproducibility of QT dispersion measures

BACKGROUND

The low reproducibility of the QT dispersion (QTD) method is a major reason why it is not used in clinics. The purpose of this study was to develop QT dispersion parameters with better reproducibility and identification of patients with a high risk of ventricular arrhythmia or death.

METHODS AND RESULTS

Three institutions using different methods for measuring QT intervals provided QT databases, which included more than 3500 twelve-lead surface ECGs. The data represented low and high risk subjects from the following groups: the normal population EpiSet (survivors vs dead from cardiovascular causes), acute myocardial infarction patients AmiSet (survivors vs dead) and remote myocardial infarction patients ArrSet (with vs without a history of ventricular arrhythmia). The EpiSet, AmiSet, and the ArrSet contributed with N = 122, 0, and 110 ECGs for reproducibility analysis, and 3244, 446, and 100 ECGs for the analysis of prognostic accuracy. The prognostic accuracy was measured as the area under the Receiver Operator Curve. The QT intervals were divided into six QT pairs; the longest pair consisted of the longest and the shortest QT intervals etc. The QT dispersion trend (QTDT) was defined as the slope of the linear regression of the N longest QT pairs after estimation of missing QT intervals by interpolation of measured QT intervals. The QTMAD and the QTSTD methods were defined as twice the mean absolute deviation and the standard deviation of the N longest QT pairs. The reproducibility was improved by 27% and 19% in the EpiSet and the ArrSet relative to the reproducibility of QTD. The accuracy improved for the EpiSet and the ArrSet and was maintained for the AmiSet.

CONCLUSIONS

By using the three longest and the three shortest QT intervals in QTDT, QTMAD, or QTSTD, the reproducibility improved significantly while maintaining or improving the prognostic accuracy compared to QTD.

The prognostic accuracy of different QT interval measures

BACKGROUND

The QT intervals accuracy for predicting arrhythmic death varies between studies, possibly due to differences in the selection of the lead used for measurement of the QT interval. The purpose of this study was to analyze the prognostic accuracy of all known ways to select the lead.

METHODS AND RESULTS

Three institutions that used different methods for measuring QT intervals provided their QT databases. They included more than 3500 twelve-lead surface ECGs. The data represented low- and high-risk patients of the normal population (survivors vs dead from cardiovascular causes), acute myocardial infarction (survivors versus death from all causes) and remote myocardial infarction (with vs without a history of ventricular arrhythmia). The prognostic accuracy was defined as the area under the Receiver Operator Curve (ROC-area). The most accurate standard leads were I and aVL and the least accurate was AVR. The most accurate precordial lead was V4. The prognostic accuracy of the longest QT interval was higher than for any standard lead. The prognostic accuracy of the mean of the three longest QT intervals was equal to or slightly lower than for the longest QT interval.

CONCLUSIONS

The highest prognostic accuracy is obtained with the longest QT interval. The accuracies of the lead selection methods are so different that it can explain a substantial part of the differences between otherwise similar studies in the literature. We recommend the use of the mean value of the three longest QT intervals.

Dispersion of the corrected QT and JT interval in the electrocardiogram of alcoholic patients

BACKGROUND

The aim of this study was to analyze the corrected QT interval (QTc), its dispersion (QTcd), and the QT interval index (QTcI) and to compare it with the corrected JT interval (JTc), its dispersion (JTcd), and the JT interval index (JTcI) in long-term alcoholic users, by investigating the ventricular activation until the completion of repolarization.

METHODS

The study of ECGs was performed on a selected group of 49 male white alcoholic patients, mean age 53.51+/-4.78 years. The control group was pair-matched. The analysis of ECGs was performed according to the Minnesota Code, and Bazett's formula gave the values of the corrected QT interval and JT interval.

RESULTS

The results of the descriptive statistics in the group of alcoholics showed a range of the QTcd of 12.0 to 103.0 milliseconds, mean 44.2+/-18.6 milliseconds, compared with 5.0 to 68.0 milliseconds, mean 23.8+/-11.1 milliseconds, in the control group (p < 0.0001). The range of JTcd in the alcoholic patients was 11.0 to 88.0 milliseconds, mean 43.0+/-18.3 milliseconds, compared with 5.0 to 66.0 milliseconds, mean 22.8+/-12.1 milliseconds, in the control group (p < 0.0001). The QTc from 422.0 to 480.0 milliseconds was obtained in 69.4% of the alcoholic patients and 32.6% of the controls. The QTc over 480.0 milliseconds was found in 24.5% of the alcoholic patients, but was not recorded among the controls at all (p < 0.0001). The QTcd over 50.0 milliseconds was registered in 34.7% of the alcoholic patients, while in the controls a QTcd above 50 milliseconds was found in 2.0% (p < 0.0001). The mean values of QT were markedly higher in the alcoholic patients (p < 0.01) than in the controls. The same was the case with the values of QTd, QT(I), QTcd, and QTcI (p < 0.0001). Also, the mean values of JTd, JT(I), JTc, JTcd, and JTcI were significantly higher in the alcoholic patients than in the controls (p < 0.0001). The mean value of JT, although higher in the alcoholic patients, did not significantly differ between the examined groups (p=0.1002). The odds ratio estimated for prolonged QTc and for enlarged QTcd in the alcoholics versus the controls was 31.625 and 25.500 (p < 0.0001), respectively.

CONCLUSIONS

Persons who consume various alcoholic beverages excessively and for a long time have significantly higher dispersions of the QTc and JTc, intervals and they have a significantly higher estimation of relative risk for the prolonged QTc interval and higher QTc dispersion than the control group, i.e., higher risk of arrhythmias.

An evaluation of the impact of oral magnesium lactate on the corrected QT interval of patients receiving sotalol or dofetilide to prevent atrial or ventricular tachyarrhythmia recurrence

BACKGROUND

Intravenous magnesium reduces the QTc interval of patients receiving ibutilide. Whether oral magnesium can reduce the QTc interval associated with oral sotalol and dofetilide is not known. This study was undertaken to evaluate the impact of oral magnesium on the QTc interval and whether an inherent intracellular magnesium deficiency exists among patients with arrhythmias.

METHODS

Participants receiving sotalol or dofetilide for atrial or ventricular arrhythmias were randomized to receive magnesium l-lactate (504 mg elemental magnesium daily, Niche Pharmaceuticals, Roanoke, TX) or placebo for 48 hours. A 12-lead electrocardiogram (ECG) was obtained at baseline, 3 hours, and 51 hours after dosing to correspond to the Tmax after oral ingestion. The QTc interval was measured from the ECGs and compared between groups. Intracellular magnesium concentrations were determined by energy-dispersive x-ray analysis at baseline and 51 hours after dosing (Intracellular Diagnostics, Inc., Foster City, CA).

RESULTS

The QTc interval reductions from baseline were greater in the magnesium group than placebo at 3 and 51 hours (P = 0.015 and P < 0.001, respectively). Sixty-three percent of patients (regardless of experimental group) had baseline intracellular magnesium concentrations below the normal reference range of 33.9-41.9 mEq/IU, with an average level of 32.6 +/- 2.2 mEq/IU.

CONCLUSIONS

Oral magnesium l-lactate raises intracellular magnesium concentrations and lowers the QTc interval of patients receiving sotalol or dofetilide.

Effect of epirubicin-based chemotherapy and dexrazoxane supplementation on QT dispersion in non-Hodgkin lymphoma patients

BACKGROUND AND OBJECTIVE

Aim of the present study was to assess the effect of epirubicin-based chemotherapy on QT interval dispersion in patients with aggressive non-Hodgkin lymphoma (NHL), and the effect of dexrazoxane supplementation. Prolongation of QT dispersion may not only represent a sensitive tool in identifying the first sign of anthracycline-induced cardiotoxicity, but it may serve also in identifying patients who are at risk of arrhythmic events.

METHODS

Twenty untreated patients, <or=60 years of age with newly-diagnosed aggressive NHL, eligible for a treatment with epirubicin-based chemotherapy were selected for the study. The patients were randomly allocated in two subgroups (N=10) to receive or not dexrazoxane hydrochloride (400 mg/m(2)) after epirubicin infusion. The patients underwent 12-lead electrocardiogram (ECG) before and after epirubicin infusion and after dexrazoxane supplementation. QT dispersion was defined as the difference between the maximum and the minimum QT interval occurring in any of the 12 ECG leads, corrected (QTc) for heart rate.

RESULTS

All the 20 patients showed increased QT dispersion (44.3 +/- 8.4 vs. 68.4+/-11.4 ms, P<0.001) and QTc dispersion (46.2 +/- 6.2 vs. 72.2 +/- 8.4, P<0.001) after chemotherapy infusion. The 10 patients who underwent supplementation with dexrazoxane exhibited a significant reduction of QT dispersion (67.4 +/- 8.1 vs. 49.5 +/- 4.2 ms, P<0.001) and QTc dispersion (71.2 +/- .7 vs. 51.4 +/- 4.3 ms, P<0.001), while the 10 patients not supplemented with dexrazoxane did not (QT dispersion: 69.3 +/- 7.6 vs. 64.2 +/- 6.9 ms; QTc dispersion: 72.8 +/- 8.1 vs. 67.3 +/- 7.2 ms, ns).

CONCLUSIONS

Epirubicin-based chemotherapy causes an early increase of the QT and QTc dispersion, which is attenuated by dexrazoxane supplementation. Therefore, dexrazoxane can reduce the arrhythmic risk in patients treated with epirubicin.

Increases in P-Wave Dispersion Predict Postoperative Atrial Fibrillation After Coronary Artery Bypass Graft Surgery

Atrial fibrillation (AF) is a common complication after coronary artery bypass graft (CABG) surgery. In this study we examined the effect of surgery on atrial electrophysiology as measured by P-wave characteristics and to determine the potential predictive value of P-wave characteristics on the incidences of postoperative AF in patients undergoing CABG surgery. Patients undergoing elective CABG surgery were monitored by continuous electrocardiogram (ECG) telemetry during the in-hospital period until discharge for the occurrence of postoperative AF. Differences in P-wave characteristics (P-wave duration, amplitude, axis, dispersion, PR interval, segment depression, and dispersion) were compared between the pre- and postoperative 12-lead ECG measurements, and also between patients with and without postoperative AF. The association of postoperative AF and potential clinical predictors and P-wave characteristics were determined by multivariate logistic regression. Postoperative AF occurred in 81 (27%) of 300 patients. Univariate analysis showed that patients who subsequently developed postoperative AF compared with those without AF were significantly older (mean age 68 +/- 8 versus 63 +/- 10 yr, P < 0.0001), had a larger body surface area (BSA) (2.03 +/- 0.24 versus 1.92 +/- 0.22 m(2), P = 0.0002), were more likely to have a history of AF (8 of 81 versus 1 of 219, P = 0.003), used preoperative antiarrhythmic medications more frequently (7 of 81 versus 4 of 219, P = 0.01), and had a more frequent rate of return to the operating room for postoperative complications (9 of 81 versus 9 of 219, P = 0.029). Furthermore, the postoperative P-wave duration decreased to a larger extent (mean change -11.3 +/- 0.1 ms versus -8.4 +/- 0.1 ms, P < 0.0001), and the P-wave dispersion increased postoperatively to a larger extent (3.1 +/- 15.5 ms versus -1.6 +/- 14.6 ms, P = 0.028) in those who subsequently developed AF compared with those without AF. Multivariate logistic regression showed age (odds ratio [OR] = 1.1, 95% confidence interval [CI]: 1.06-1.15, P < 0.0001), BSA (OR = 38.1, 95% CI: 8.2-176, P < 0.0001), and an increase in postoperative P-wave dispersion (OR = 1.03, 95% CI: 1.01-1.05, P = 0.01) to be independent predictors of postoperative AF. No surgical factor was identified to be responsible for this postoperative change in atrial electrophysiology.

IMPLICATIONS

In addition to clinical factors, such as advanced age and body surface area, we demonstrated that electrophysiologic changes involving an increase in P-wave dispersion postoperatively independently predict atrial fibrillation after coronary artery bypass graft surgery.

Correlation between dispersion of repolarization (QT dispersion) and ventricular ectopic beat frequency in patients with acute myocardial infarction: a marker for risk of arrhythmogenesis?

BACKGROUND

QT dispersion (QTd) has evoked a lot of interest in recent years as regards the basic concept of dispersion of repolarization, which it is supposed to reflect on a surface ECG, as being a marker or substrate for arrhythmogenesis. QTd has been shown to be high in patients with ventricular fibrillation and tachycardia. But there is still some debate about its possible role as a marker or substrate for arrhythmogenesis. We studied whether it has any correlation with simple benign ventricular ectopic beats (VEB) after acute myocardial infarction.

STUDY

We studied four different dispersion parameters (QTd, QTcd, JTcd, AQTd) on 2 different days after AMI and also obtained a 24-h ambulatory ECG on the 2nd day after admission in 64 out of a total of 90 patients. Patients were divided into five groups based on VEB frequency/h on a 24-h ambulatory ECG.

RESULTS

We found a gradual increase in dispersion parameters across the five groups with increasing frequency of VEB. A significant difference was noticed between group 1 (VEB 0.0-0.9/h) and group V (>30/h) on the day of admission: QTd 88.8+/-28.5 versus 123.3+/-23.4, P<0.02; QTcd 100.5+/-27.6 versus 160.3+/-30.7, P<0.01; JTcd 95.5+/-31.0 versus 160.4+/-30.9, P<0.01; AQTd 29.6+/-8.2 versus 48.6+/-13.7, P<0.01. We also noticed a significant positive correlation between VEB frequency and dispersion parameters on both days.

CONCLUSION

We hypothesize that with increasing dispersion of repolarization the chances or the frequency of ventricular arrhythmias increase. Our findings also point to a definite role of QTd as an arrhythmogenic marker or substrate.

Effects of intravenous magnesium sulfate on the QT interval in patients receiving ibutilide

STUDY OBJECTIVE

To determine the effect of intravenous magnesium sulfate on the QT and QTc intervals in patients receiving ibutilide for immediate chemical cardioversion of atrial flutter or fibrillation.

DESIGN

Prospective, randomized, double-blind, placebo-controlled trial.

SETTING

Hospital cardiology unit.

PATIENTS

Twenty patients (mean age 72 +/- 14 yrs) with atrial fibrillation (12 patients) or atrial flutter (8 patients) who were scheduled to receive ibutilide.

INTERVENTION

After determining that the patients' baseline QTc intervals were less than 440 msec and baseline serum magnesium levels were within normal limits (mean 2.1 +/- 0.29 mg/dl), the patients were randomly assigned to receive either a 10-minute infusion of magnesium sulfate 2 g in 50 ml of 0.9% sodium chloride or placebo immediately before ibutilide therapy. An additional 2 g of intravenous magnesium sulfate or placebo was given over 1 hour, 10 minutes after the first dose of ibutilide.

MEASUREMENTS AND MAIN RESULTS

QT interval duration was measured manually in all 12 leads by using a 0.5-mm-scale precision ruler and magnifying lens. The QT interval increased 29% from baseline at 30 minutes after ibutilide therapy in the placebo group (p=0.007), but no significant change from baseline occurred in the magnesium sulfate group. The 30-minute QTc interval in the placebo group was 18% higher than the baseline value (p=0.01) but did not change significantly in the magnesium sulfate group. QTc changes from baseline were greater in the placebo group than in the magnesium sulfate group at 30 minutes (75 vs 19 msec, respectively, p=0.04). Magnesium sulfate reduced the risk of an ibutilide-induced QTc interval increase of greater than 30 msec or greater than 60 msec and reduced the risk of a QTc interval value of more than 500 msec by 65%, 60%, and 68%, respectively (p=0.07, p=0.175, and p=0.160).

CONCLUSIONS

Prophylactic administration of intravenous magnesium sulfate prevents increases in the QT and QTc interval 30 minutes after the last infusion of ibutilide.

Electrocardiographic and hemodynamic effects of Panax ginseng

OBJECTIVE

To determine whether Panax ginseng ingestion can acutely or chronically alter electrocardiographic parameters: PR, QRS, QT, QTc, and RR intervals, and QT and QTc interval dispersion. Effects of P. ginseng on blood pressure and heart rate also were evaluated.

METHODS

This is a prospective, randomized, double-blind, placebo-controlled study of healthy adults at the University of Connecticut. Thirty subjects were randomly allocated to receive 28 days of therapy with either P. ginseng extract 200 mg or placebo. Baseline 12-lead electrocardiograms (ECGs) were obtained. Subsequent ECGs were performed following study drug ingestion at 50 minutes, 2 hours, and 5 hours on days 1 and 28. Blood pressure readings were taken with each ECG.

RESULTS

P. ginseng ingestion increased the QTc interval by 0.015 seconds on day 1 at 2 hours compared with the placebo group (p = 0.03). It also reduced diastolic blood pressure from 75 +/- 5 mm Hg at baseline to 70 +/- 6 mm Hg at the same time point (p = 0.02). The observed effects are not believed to be clinically significant. No other statistically significant changes were found in electrocardiographic or hemodynamic variables on days 1 or 28.

CONCLUSIONS

P. ginseng, at doses of 200 mg of the extract daily, increases the QTc interval and decreases diastolic blood pressure 2 hours after ingestion in healthy adults on the first day of therapy.

Effect of diuresis on P-wave duration and dispersion

STUDY OBJECTIVE

To evaluate the effects of changing volume status on P-wave duration and dispersion in patients with decompensated heart failure.

DESIGN

Prospective analysis.

SETTING

Hospital cardiology unit.

PATIENTS

Twenty-one patients with symptoms of decompensated heart failure who were treated with diuretics on admission.

INTERVENTION

Twelve-lead electrocardiograms were obtained at baseline and after diuresis. Average, minimum, and maximum P-wave duration and P-wave dispersion (minimum minus maximum duration) were determined.

MEASUREMENTS AND MAIN RESULTS

P-wave duration was measured manually in all 12 leads by using a 0.5-mm-scale precision ruler and magnifying lens. After 40+/-23 hours of diuresis, 3+/-1 L of fluid was removed. A significant correlation was found between average P-wave duration and amount of fluid removed (r = -0.59, p=0.015). Also, average and maximum P-wave duration were significantly decreased with diuresis (p=0.001 and 0.022, respectively). No other P-wave variables were significantly affected.

CONCLUSIONS

Diuresis may attenuate electrophysiologic changes caused by fluid overload.

Changes in P-wave dispersion and P-wave duration after open heart surgery are associated with the peak incidence of atrial fibrillation

BACKGROUND

Increased P-wave dispersion (P-disp) and maximum P-wave duration (P-max) predict the development of atrial fibrillation (AF) in the general population. The present study evaluates the time-dependent relationship of P-disp and P-max after open heart surgery.

METHODS AND RESULTS

P-disp and P-max were measured in the perioperative period of open heart surgery (one day before surgery through postoperative day 4). Compared with the baseline, P-max decreased immediately after open heart surgery (112.63 +/- 7.4 ms vs 106.9 +/- 8.2 ms, P =.005). An increase in P-disp was observed between postoperative days 1 and 2 (37.5 +/- 6.8 ms vs 43.1 +/- 4.5 ms, P <.05), and postoperative days 1 and 3 (37.5 +/- 6.8 ms vs 44.1 +/- 6.6 ms, P <.05). There was also an increase in the P-max between postoperative day 1 and 3 (103 +/- 8.3 ms vs 110 +/- 7.7 ms, P <.05).

CONCLUSIONS

Nonuniform atrial conduction (P-disp) is greatest on days 2 and 3 after open heart surgery, and the longest atrial conduction time (P-max) is greatest on day 3 after open heart surgery, findings that coincide with the time of greatest risk for AF. (Heart Lung((R)) 2001;30:466-71.)

An evaluation of the change in electrocardiographic P-wave variables after acute caffeine ingestion in normal volunteers

BACKGROUND

Caffeine's effect on supraventricular dysrhythmias is poorly understood, and establishing a marker to predict atrial fibrillation may help to explain supraventricular dysrhythmias caused by caffeine.

OBJECTIVE

To evaluate the effects of caffeine (mean 6.1 mg/kg) on maximum P-wave duration and P-wave dispersion in normal volunteers.

METHOD

A randomised, double-blind, placebo-controlled, crossover study in 10 healthy volunteers at least 17 years of age at the University of Connecticut. Participants abstained from caffeinated products for at least 2 days before study initiation and were randomly allocated on different days to receive placebo or caffeine 400 mg. For each of the study phases, a baseline 12-lead electrocardiogram (ECG) was performed and a subsequent 12-lead ECG was performed 3 h after ingesting the study drug.

RESULTS

No significant changes were found within intra- or intergroup comparisons for any P-wave variables measured.

CONCLUSION

Single dose caffeine (400 mg) does not affect average P-wave duration, maximum P-wave duration or P-wave dispersion in normal individuals.

Evaluation of electrocardiographic and hemodynamic effects of caffeine with acute dosing in healthy volunteers

STUDY OBJECTIVE

To evaluate the effects of moderate, single-dose caffeine consumption on electrocardiographic variables: PR, QRS, QT, QTc, and RR intervals, and QT and QTc interval dispersion. Effects of caffeine on blood pressure and heart rate also were evaluated.

DESIGN

Randomized, double-blind, placebo-controlled, crossover study.

SETTING

University school of pharmacy.

PATIENTS

Ten healthy volunteers aged 17 years or older.

INTERVENTION

Participants abstained from caffeinated products for at least 2 days before the study began and were randomly designated to receive placebo or caffeine 400 mg on various days. For each of the study phases, a baseline 12-lead electrocardiogram (ECG) was performed and a subsequent 12-lead ECG performed 3 hours after ingesting the study drug. Blood pressure readings were taken with each ECG.

MEASUREMENTS AND MAIN RESULTS

No significant changes in any intragroup or intergroup electrocardiographic variables occurred. Caffeine increased blood pressure (systolic blood pressure [SBP]/diastolic blood pressure [DBP]) from 118+/-5/75+/-6 mm Hg to 128+/-8/77+/-7 mm Hg versus baseline (p=0.0022 and p=0.0368 for SBP and DBP, respectively). After drug dosing, SBP in the caffeine group was significantly higher than in the placebo group (128+/-8 mm Hg versus 119+/-7 mm Hg, p=0.0174).

CONCLUSION

Moderate caffeine consumption by healthy young adults does not acutely affect PR, QRS, QT, QTc, and RR intervals, or QT and QTc interval dispersion. Caffeine-naive subjects experienced persistent elevations in SBP and DBP 3 hours after caffeine ingestion, indicating that longer caffeine abstinence than that which is recommended is necessary for blood pressure determination in the clinical setting.

Measurement, interpretation and clinical potential of QT dispersion

QT dispersion was originally proposed to measure spatial dispersion of ventricular recovery times. Later, it was shown that QT dispersion does not directly reflect the dispersion of recovery times and that it results mainly from variations in the T loop morphology and the error of QT measurement. The reliability of both automatic and manual measurement of QT dispersion is low and significantly lower than that of the QT interval. The measurement error is of the order of the differences between different patient groups. The agreement between automatic and manual measurement is poor. There is little to choose between various QT dispersion indices, as well as between different lead systems for their measurement. Reported values of QT dispersion vary widely, e.g., normal values from 10 to 71 ms. Although QT dispersion is increased in cardiac patients compared with healthy subjects and prognostic value of QT dispersion has been reported, values are largely overlapping, both between healthy subjects and cardiac patients and between patients with and without adverse outcome. In reality, QT dispersion is a crude and approximate measure of abnormality of the complete course of repolarization. Probably only grossly abnormal values (e.g. > or =100 ms), outside the range of measurement error may potentially have practical value by pointing to a grossly abnormal repolarization. Efforts should be directed toward established as well as new methods for assessment and quantification of repolarization abnormalities, such as principal component analysis of the T wave, T loop descriptors, and T wave morphology and wavefront direction descriptors.

Predictive power of increased QT dispersion in ventricular extrasystoles and in sinus beats for risk stratification after myocardial infarction

BACKGROUND

QT dispersion, commonly measured in sinus beats (QTd-S), can also be calculated in premature ventricular beats (QTd-V). To date, no studies have addressed the relation between these 2 variables.

METHODS AND RESULTS

In 148 patients with remote myocardial infarction and premature ventricular beats on a routine ECG, QT dispersion, defined as the difference between the maximum and the minimum QT interval across the 12-lead ECG, was calculated separately for the ventricular extrasystole and the preceding sinus beat. In the total group of patients, QTd-V was greater than QTd-S (83+/-33 versus 74+/-34 ms, respectively; P=0.001). During a follow-up period of 35+/-17 months, arrhythmic events (sustained ventricular tachycardia, ventricular fibrillation, or sudden death) were noted in 30 patients. A QTd-V of >/=100 ms was a stronger univariate marker of arrhythmic events than was a QTd-S of >/=100 ms, and multivariate analysis selected only prolonged QTd-V (hazard ratio 3.81, 95% CI 2.2 to 11.2) and low ejection fraction (hazard ratio 3.05, 95% CI 1.6 to 7.6) as independent predictors of arrhythmic events.

CONCLUSIONS

The magnitude of QTd-V was greater than that of QTd-S in the total group of patients. Prolonged QTd-V is associated with a significantly increased risk for arrhythmic events in postinfarction patients, and the prognostic significance of QTd-V exceeds that of QTd-S.

Measurement and interpretation of QT dispersion

QT dispersion was proposed as an index of the spatial inhomogeneity of ventricular recovery times. The results of studies that found significant correlation between dispersion of ventricular recovery times measured with monophasic action potentials and QT dispersion were interpreted as proof of the direct link between QT dispersion and the dispersion of ventricular recovery times. Later it was shown that QT dispersion is not a direct reflection of the spatial variation of the recovery times and cannot be used for quantification of this variation. The interlead variability of the QT intervals is a result of different projections of the spatial T-wave loop into the various electrocardiographic leads. The reliability of both manual and automatic measurement of QT dispersion is low and is often of the order of the differences of Qt dispersion between different patient groups. The measurement reliability is influenced by intrinsic factors (e.g., amplitude of the T wave) and extrinsic factors (e.g., noise, paper speed of recording, instruments for manual measurements, and type of algorithm and interalgorithmic settings for automatic measurement). There is very little to choose between the different indices of expression of QT dispersion, as well as between the different lead configurations used for its measurement. QT dispersion is not simply a result of measurement error, but a crude measure of abnormalities during the whole course of repolarization. Only grossly prolonged QT dispersion (e.g., > or =100 ms), must be interpreted simply as a sign of the abnormal course of the repolarization, and inferences about the actual dispersion of the ventricular recovery times should not be made. Newer concepts of assessment of the morphology of the T wave are already emerging and will probably be of higher clinical value.