QT dispersion magnitude is related to the respiratory phase in healthy subjects

Beneficial effect of deep breathing on premature ventricular complexes: can it be related to the decrease in QT dispersion?

It has been reported that deep breathing at 6 breaths/min is associated with reduction of premature ventricular complexes. The beneficial effect of deep breathing is supposed to be due to vagal modulation of sinoatrial and atrioventricular node. Beside the modulating effects of deep breathing, we believe that deep breathing, which is also used in yoga training, might have additional effects via decreasing QT dispersion.

Corrected QT Dispersion and Cardiac Sympathetic Function in Patients With Obstructive Sleep Apnea-Hypopnea Syndrome

STUDY OBJECTIVES

Hypoxemia increases corrected QT dispersion (QTcD), which is the difference between the maximum and minimum QT intervals and is a strong risk factor for cardiovascular mortality. The aim of this study was to investigate the QTcD in patients with obstructive sleep apnea-hypopnea syndrome (OSAHS), and the relationship between the QTcD and (123)I-metaiodobenzylguanidine (MIBG) cardiac imaging, which reflects cardiac sympathetic activity.

SETTING

A university hospital.

PATIENTS

Forty-eight OSAHS patients without cardiac diseases (mean [+/- SD] age, 45.9 +/- 10.8 years; apnea-hypopnea index [AHI] 51.9 +/- 18.5 events per hour) who underwent polysomnography before treatment and on the first night of nasal continuous positive airway pressure (nCPAP) treatment.

METHODS

Before and after nCPAP treatment was started, we measured the QTcD with computer software, before, during, and after sleep, as well as the washout rate of the MIBG administered for cardiac imaging. As a control, QTcD was also measured in the morning from 26 healthy subjects.

RESULTS

Before treatment, the mean QTcD during sleep (65.0 +/- 14.6 ms) was greater than that before sleep (57.0 +/- 13.5 ms; p < 0.0001). Meanwhile, after 1 night of nCPAP therapy, the QTcD during sleep (50.6 +/- 11.4 ms) decreased from that before treatment (p < 0.0001) and was smaller than the QTcD before sleep (56.2 +/- 13.3 ms; p = 0.003). Before treatment, the QTcD during sleep correlated with the AHI (r = 0.38; p = 0.009) and the percentage of time that SaO(2) was < 90% (SaO(2) < 90% time) [r = 0.34; p = 0.018]. The QTcD did not correlate with the body mass index or the washout rate of MIBG. However, the washout rate of MIBG correlated with the AHI and the SaO(2) < 90% time.

CONCLUSIONS

Nocturnal QTcD is increased in OSAHS patients but is decreased by nCPAP therapy independently of cardiac sympathetic function.

Effects of cardiovascular autonomic function tests on QT dispersion in the 12-lead electrocardiogram of healthy patients

Changes in autonomic tone modulate QT interval duration. How cardiovascular autonomic reflexes affect QT dispersion, a suggested marker of arrhythmia risk, is not well established. We studied 10 healthy young adult volunteer men during quiet and deep breathing, the Valsalva maneuver, sustained handgrip, hyperventilation, the cold pressor test, and mental stress. An automated method was used for measurement of QT-peak and QT-end intervals, and QT dispersion was defined as maximum-minimum of the measured intervals. QT-peak dispersion was greater on deep expiration than deep inspiration (49 +/- 20 ms vs 37 +/- 14 ms, P < .05). QT-end dispersion decreased in the tachycardia phase of the Valsalva maneuver (45 +/- 23 ms vs 35 +/- 21 ms, P < .05), but QT dispersion did not change during the other interventions. Rapid cardiovascular autonomic reflex adjustment does not change QT dispersion in healthy young adult men. However, large intrathoracic volume and intrathoracic pressure changes during forced respiratory movements might confound QT dispersion measurements.

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.

Changes in QT dispersion magnitude during respiratory phases: role of maximum inspiration and expiration

There is still controversy about the reliability and prognostic value of QT interval dispersion because of interobserver and intraobserver variability. The authors aimed to study the effect of respiratory phases on QT dispersion. Sixty healthy volunteers (38 men, 22 women, mean age 25+/-3 years) from the medical staff comprised the study group. Electrocardiograms were recorded by the same technician with a rate of 50 mm/s during normal breathing, maximum inspiration and expiration. QT dispersion was defined as the difference between the maximal and minimal QT interval measurement occurring among any of the 12 leads. Corrected QT (QTc) interval was calculated according to Bazzet's formula. There were no significant differences between QTc max interval during maximum inspiration and expiration compared with those in normal breathing (409+/-20 ms vs 417+/-26 ms, p>0.05 and 412+/-18 vs 417+/-26 ms, p>0.05 respectively). QTc dispersion during maximum inspiration and expiration was significantly lower than that of normal breathing (36+/-8 ms vs 44+/-9 ms, p<0.003 and 32+/-7 vs 44+/-9 ms, p< 0.003, respectively). And the QTc dispersion during maximum expiration was also lower than that during maximum inspiration (p<0.01). QT dispersion magnitude is affected by the respiratory phases in healthy subjects and decreases during both maximum inspiration and expiration as compared with normal respiration.

QT dispersion in patients with Duchenne-type progressive muscular dystrophy

BACKGROUND

A high degree of QT dispersion is a risk factor for arrhythmic sudden death in patients with myocardial infarction and cardiomyopathy. Duchenne-type progressive muscular dystrophy (DMD) is also associated with the development of ventricular arrhythmias. The purpose of this study was to determine the relationship between QT interval dispersion and ventricular arrhythmias in patients with DMD.

METHODS

Sixty-seven patients with DMD were studied. Standard 12-lead electrocardiograms and 24-hour Holter electrocardiograms were recorded, and the QT interval was determined in every lead of the standard electrocardiogram to determine the QT dispersion. QT dispersion was compared with the frequency of ventricular arrhythmias and the severity of skeletal muscle damage on the basis of the Swinyard and Deaver 8-stage scale.

RESULTS

QT dispersion in all 67 patients averaged 54 +/- 18 ms. The QT dispersion was 49 +/- 16 ms in stage 5 patients, 61 +/- 22 ms in stage 6 patients, 52 +/- 17 ms in stage 7 patients, and 56 +/- 17 ms in stage 8 patients. Ventricular arrhythmias of Lown grade III or higher were observed in 3 of 35 patients with QT dispersion <60 ms and in 14 of 32 patients with QT dispersion >/=60 ms. Logistic regression analysis demonstrated that QT dispersion is an independent risk factor for ventricular arrhythmias of grade III or higher in patients with DMD.

CONCLUSIONS

The incidence of ventricular arrhythmias of Lown grade III or higher was greater in patients with QT dispersion >/=60 ms than in patients with QT dispersion >60 ms. QT dispersion therefore is a risk factor for serious ventricular arrhythmias in patients with DMD.