Effect of age on diurnal changes of 24-hour QT interval variability

Normal Values of QT Variability in 10-s Electrocardiograms for all Ages

Aims: QT variability is a promising electrocardiographic marker. It has been studied as a screening tool for coronary artery disease and left ventricular hypertrophy, and increased QT variability is a known risk factor for sudden cardiac death. Considering that comprehensive normal values for QT variability were lacking, we set out to establish these in standard 10-s electrocardiograms (ECGs) covering both sexes and all ages.

Methods: Ten-second, 12-lead ECGs were provided by five Dutch population studies (Pediatric Normal ECG Study, Leiden University Einthoven Science Project, Prevention of Renal and Vascular End-stage Disease Study, Utrecht Health Project, Rotterdam Study). ECGs were recorded digitally and processed by well-validated analysis software. We selected cardiologically healthy participants, 46% being women. Ages ranged from 11 days to 91 years. After quality control, 13,828 ECGs were available. We assessed three markers: standard deviation of QT intervals (SDqt), short-term QT variability (STVqt), and QT variability index (QTVI).

Results: For SDqt and STVqt, the median and the lower limit of normal remained stable with age. The upper limit of normal declined until around age 45, and increased strongly in the elderly, notably so in women. This implies that a subset of the population, small enough not to have appreciable effect on the median, shows a high degree of QT variability with a possible risk of arrhythmias or worse, especially in women. Otherwise, sex differences were negligible in all three measurements. For QTVI, median, and normal limits decreased until age 20, and steadily went up afterwards except for the lower limit of normal, which flattens off after age 65.

Conclusion: We report the first set of normal values for QT variability based on 10-s ECGs, for all ages and both sexes.

Transfusion Decision Making in Pediatric Critical Illness

Transfusion decision making (TDM) in the critically ill requires consideration of: (1) anemia tolerance, which is linked to active pathology and to physiologic reserve, (2) differences in donor RBC physiology from that of native RBCs, and (3) relative risk from anemia-attributable oxygen delivery failure vs hazards of transfusion, itself. Current approaches to TDM (e.g. hemoglobin thresholds) do not: (1) differentiate between patients with similar anemia, but dissimilar pathology/physiology, and (2) guide transfusion timing and amount to efficacy-based goals (other than resolution of hemoglobin thresholds). Here, we explore approaches to TDM that address the above gaps.

Cardiac autonomic function and vascular profile in subclinical hypothyroidism: Increased beat-to-beat QT variability

BACKGROUND

Patients with subclinical hypothyroidism (SH) may have higher incidence of coronary heart disease and autonomic dysfunction.

DESIGN OF THE STUDY

Prospective case control study.

AIM AND OBJECTIVES

To evaluate beat-to-beat QT variability and vascular stiffness in patients with SH compared to normal controls.

MATERIALS AND METHODS

We compared linear and nonlinear measures of cardiac repolarization liability using beat-to-beat QT intervals derived from the surface electrocardiogram during supine posture and vascular indices including pulse wave velocity and ankle-brachial index (ABI) during supine posture between female patients with SH and age- and sex-matched normal controls. Spectral analysis was done at very low frequency (LF) (0.003-0.04 Hz), Low frequency (LF) (0.04-0.15 Hz), and high frequency (HF) (0.15-0.4 Hz). The HF represents vagal regulation (parasympathetic) and LF represents both parasympathetic and sympathetic regulation.

RESULTS

We recruited 58 women with a mean age of 31.83 ± 8.9 years and 49 controls with mean age of 32.4 ± 9.9 years (P = NS). QT variability index (QTvi) was higher in cases compared to controls (P = 0.01). The ratio of LF/HF of R-R interval which is an index of sympathovagal tone was significantly more in cases compared to controls (P = 0.02). The difference in the left minus the right ABI was significant between cases and controls (P = 0.03).

CONCLUSIONS

The cases had lower parasympathetic activity as compared to controls, and there was a predominance of sympathetic activity in cases. QTvi may be an important noninvasive tool in this group of patients to study the risk of cardiovascular mortality.

QT interval variability in body surface ECG: measurement, physiological basis, and clinical value: position statement and consensus guidance endorsed by the European Heart Rhythm Association jointly with the ESC Working Group on Cardiac Cellular Electrophysiology

This consensus guideline discusses the electrocardiographic phenomenon of beat-to-beat QT interval variability (QTV) on surface electrocardiograms. The text covers measurement principles, physiological basis, and clinical value of QTV. Technical considerations include QT interval measurement and the relation between QTV and heart rate variability. Research frontiers of QTV include understanding of QTV physiology, systematic evaluation of the link between QTV and direct measures of neural activity, modelling of the QTV dependence on the variability of other physiological variables, distinction between QTV and general T wave shape variability, and assessing of the QTV utility for guiding therapy. Increased QTV appears to be a risk marker of arrhythmic and cardiovascular death. It remains to be established whether it can guide therapy alone or in combination with other risk factors. QT interval variability has a possible role in non-invasive assessment of tonic sympathetic activity.

Influence of antenatal physical exercise on heart rate variability and QT variability

OBJECTIVE

We sought to characterise the influence of an antenatal exercise programme on ECG-derived cardiac variables.

METHODS

Fifity-one healthy pregnant women were recruited and randomly assigned (2 × 2×2 design) to an exercise group or a control group. Exercising groups attended weekly classes from the 20th week of pregnancy onwards. Cardiovascular assessments (heart rate variabiliy (HRV), QT, and the QT variability index (QTVI)) were performed at 12-16, 26-28, 34-36 weeks and 12 weeks following birth, during supine rest and exercise conditions.

RESULTS

Advancing gestation was associated with an increased maternal heart rate (p = 0.001), shorter QT interval (p = 0.003), diminished HRV (p = 0.002) and increased QTVI (p = 0.002). Each of these changes was reversed within 12 weeks postpartum (p < 0.004). The Exercise group displayed exaggerated changes for all variables (except QT) but only during supine rest in the third trimester (p < 0.029).

CONCLUSION

Advancing gestation is associated with a shift in HRV/QTVI towards values that have been associated with an elevated risk of arrhythmia. A 20-week exercise programme undertaken between mid and late pregnancy exaggerated these changes during rest in the third trimester of pregnancy.

QT Variability Index

The QT Variability Index (QTVI) is a non-invasive measure of repolarization lability that has been applied to a wide variety of subjects with cardiovascular disease. It is a ratio of normalized QT variability to normalized heart rate variability, and therefore includes an assessment of autonomic nervous system tone. The approach assesses beat-to-beat variability in the duration of the QT and U wave in conventional surface electrocardiographic recordings, as well as determines the heart rate variability (HRV) from the same recording. As opposed to T wave alternans, QTVI assesses variance in repolarization at all frequencies. Nineteen studies have published data on QTVI in healthy individuals, while 20 have evaluated its performance in cohorts with cardiovascular disease. Six studies have assessed the utility of QTVI in predicting VT/VF, cardiac arrest, or cardiovascular death. A prospective study utilizing QTVI to determine therapy allocation has not been performed, and therefore the final determination of the value of the metric awaits definitive exploration.

Circadian models of serum potassium, sodium, and calcium concentrations in healthy individuals and their application to cardiac electrophysiology simulations at individual level

In the article a brief description of the biological basis of the regulation of human biological clocks was presented in order to introduce the role of circadian rhythms in physiology and specifically in the pharmacological translational tools based on the computational physiology models to motivate the need to provide models of circadian fluctuation in plasma cations. The main aim of the study was to develop statistical models of the circadian rhythm of potassium, sodium, and calcium concentrations in plasma. The developed ion models were further tested by assessing their influence on QT duration (cardiac endpoint) as simulated by the biophysically detailed models of human left ventricular cardiomyocyte. The main results are model equations along with an electronic supplement to the article that contains a fully functional implementation of all models.

Relation between beat-to-beat QT interval variability and T-wave amplitude in healthy subjects

OBJECTIVES

Elevated beat-to-beat QT interval variability (QTV) has been associated with increased cardiovascular morbidity and mortality.The aim of this study was to investigate interlead differences in beat-to-beat QTV of 12-lead ECG and its relationship with the T wave amplitude.

METHODS

Short-term 12-lead ECGs of 72 healthy subjects (17 f, 38 ± 14 years; 55 m, 39 ± 13 years) were studied. Beat-to-beat QT intervals were extracted separately for each lead using a template matching algorithm. We calculated the standard deviation of beat-to-beat QT intervals as a marker of QTV as well as interlead correlation coefficients. In addition, we measured the median T-wave amplitude in each lead.

RESULTS

There was a significant difference in the standard deviation of beat-to-beat QT intervals between leads (minimum: lead V(3) (2.58 ± 1.36 ms), maximum: lead III (7.2 ± 6.4 ms), ANOVA: P < 0.0001). Single measure intraclass correlation coefficients of beat-to-beat QT intervals were 0.27 ± 0.18. Interlead correlation coefficients varied between 0.08 ± 0.33 for lead III and lead V(1) and 0.88 ± 0.09 for lead II and lead aVR. QTV was negatively correlated with the T-wave amplitude (r =-0.62, P < 0.0001). There was no significant affect of mean heart rate, age or gender on QT variability (ANOVA: P > 0.05).

CONCLUSIONS

QTV varies considerably between leads in magnitude as well as temporal patterns. QTV is increased when the T wave is small.

Chemoreceptors, baroreceptors, and autonomic deregulation in children with obstructive sleep apnea

Obstructive sleep apnea (OSA) is highly prevalent sleep disorder of breathing in both adults and children that is fraught with substantial cardiovascular morbidities, the latter being attributable to a complex interplay between intermittent hypoxia (IH), episodic hypercapnia, recurrent large intra-thoracic pressure swings, and sleep disruption. Alterations in autonomic nervous system function could underlie the perturbations in cardiovascular, neurocognitive, immune, endocrine and metabolic functions that affect many of the patients suffering from OSA. Although these issues have received substantial attention in adults, the same has thus far failed to occur in children, creating a quasi misperception that children are protected. Here, we provide a critical overview of the evidence supporting the presence of autonomic nervous system (ANS) perturbations in children with OSA, draw some parallel assessments to known mechanisms in rodents and adult humans, particularly, peripheral and central chemoreceptor and baroreceptor pathways, and suggest future research directions.

A pilot study of QT interval analysis in overweight and obese youth

This descriptive pilot study examined if manual corrected QT (QTc) interval measures obtained from a standard 12-lead electrocardiogram (ECG) correlated with automated 24-hour ambulatory Holter QTc measures in 30 overweight and obese youth aged 12-17 years. In addition, we sought to determine if a significant difference existed between the means of manual 12-lead ECG versus automated 24-hour ambulatory Holter measures. Spearman's rho correlation coefficient revealed there was little if any correlation between manual 12-lead ECG and automated 24-hour ambulatory Holter QTc measures (r = .179, p = .345). In addition, a significant difference existed between QTc measures obtained from the manual 12-lead ECG in comparison to the automated 24-hour ambulatory Holter measures (p = .01). The manual 12-lead ECG and automated 24-hour ambulatory Holter analysis methods should not be used for comparison of QTc measures in overweight and obese youth.

Gender differences and risk of ventricular tachycardia or ventricular fibrillation

BACKGROUND

Healthy women have longer QT intervals and more drug-induced proarrhythmia compared to men, yet those given implantable cardioverter-difibrillators (ICDs) for ischemic cardiomyopathy have fewer episodes of ventricular tachycardia/ventricular fibrillation (VT/VF) than men. The role of repolarization duration and stability in arrhythmogenesis in men and women with structural heart disease has not been explored.

OBJECTIVES

The purpose of this study was to analyze repolarization differences between men and women and their relation to the risk of VT/VF.

METHODS

Multicenter Automatic Defibrillator Trial II study patients underwent 10-minute, resting digitized recordings at study entry. QT and heart rate were measured for each beat with a semiautomated method. QT variance was normalized for mean QT (QTVN) or for heart rate variance (QTVI). Spectral analysis of heart rate and QT time series was performed; coherence was indexed to quantify consistency of heart rate and QT power spectra. The incidence of VT/VF was determined by ICD interrogation.

RESULTS

There were 805 usable recordings (142 females); 463 received ICDs (86 females). There was no gender difference in mean or median QT, QTc, or heart rate. QTVN and QTVI were slightly (but significantly) higher, and the mean coherence was lower in women. In a Cox multivariate analysis, increased QTVN or QTVI (top quartile) was associated with a significantly higher risk for VT/VF in men (QTVN hazard ratio (HR) 2.2; confidence interval [CI] 1.4-3.4; P = .001; QTVI HR 1.9; CI 1.2-3.0; P = .006) but not in women, while reduced coherence (bottom quartile) predicted VT/VF in women (HR 3.3; CI 1.2-9.0; P = .021) but not in men.

CONCLUSIONS

In post-myocardial infarcation patients with depressed ejection fraction, both women and men manifest increased temporal variability in the QT interval. In men, QT variability by itself raised arrhythmic risk. In women, however, QT variability dissociated from HR variability (low coherence) appeared to be a uniquely significant predictor of arrhythmic events.

Changes of the corrected QT interval in healthy boys and girls over day and night

AIMS

The study was designed to detect changes in corrected QT intervals over day and night in both sexes in healthy children.

METHODS AND RESULTS

The corrected QT interval was calculated from 24 h ECGs obtained from 282 healthy children aged 6 months to 18 years. The QTc interval as measured by the 24 h recording differs to the standard ECG measurement which is in average of 40-50 ms shorter. The QTc interval changes little over a 24 h period and is remarkably constant despite significant heart rate changes in healthy children.

CONCLUSION

The routine ECG-even if the calculated values differ markedly from those obtained over 24 h-seems to be a good screening method for the measurement of corrected QT intervals, because the corrected QT interval is kept constant over the whole day in healthy children.

Power spectrum analysis and cardiovascular morbidity in anxiety disorders

Spectral analysis of heart rate variability (HRV) and related measures has been shown to be a reliable noninvasive technique enabling quantitative assessment of cardiovascular autonomic regulatory responses to autonomic regulatory mechanisms; it provides a dynamic probe of sympathetic and parasympathetic tone, reflecting the interactions between the two. Over 20 studies reported abnormalities of HRV in anxiety, and patients with heart disease and anxiety are at increased risk for morbidity and mortality. Psychiatric drugs partly correct abnormalities of HRV and, recently, autonomic drugs (beta-blockers) have been studied in anxiety disorders. The authors call for further studies, especially in patients with co-existing anxiety disorders and heart disease, incorporating assessment of HRV.