![]() Different techniques exist for determining dispersion (including automated algorithms), and the results of one study are often difficult to compare with those of another in addition, the tests are sensitive to a variety of factors, including age, time of day, season of year, and even body position. Abnormally high QRS and QT dispersion have been correlated with risk for overall mortality and arrhythmic death in patients with various disorders, although the results are not consistent. Dispersion indices usually measure the maximum difference (shortest to longest) in the intervals of interest, which may be adjusted for heart rate and the number of leads sampled (e.g., when the T wave is flat in some leads for QT dispersion). One index of the heterogeneity of ventricular conduction is derived from the QRS complex duration on surface ECG leads, while heterogeneity of ventricular refractoriness can be found in differences in the length of the QT interval. Heterogeneity in refractoriness and conduction velocity is a hallmark of reentrant arrhythmias. QRS and QT Dispersion and T Wave Abnormalities Abnormal heart rate turbulence is a strong independent predictor of mortality in patients with coronary artery disease and dilated cardiomyopathy abnormal indices in some patients can be improved or normalized after treatment with beta blockers and statin drugs. Heart rate turbulence is a measure of reflex vagal control of the heart, whereas heart rate variability is more indicative of overall vagal tone. 9 In normal individuals the sinus rate initially accelerates and then slows this phenomenon is blunted or absent in patients with various heart diseases. ![]() Heart rate turbulence is an index of changes in the sinus discharge rate after a PVC that is followed by a compensatory pause. Even the simple measure of resting heart rate has been shown to be an independent cardiovascular risk factor (although a target “safe” heart rate has not been established), as has the heart rate obtained during and after exercise. New indices of heart rate variability are continually being evaluated. Reduced R-R interval variability, a marker of increased risk, indicates loss or reduction of the physiologic periodic sinus node fluctuations, which has many potential causes and may not necessarily represent a significant shift in autonomic modulation. High-frequency components of R-R interval variability reflect tonic vagal activity. 8 Similar results have been obtained in patients with dilated cardiomyopathy ( see Chapters 25 and 77). R-R variability predicts all-cause mortality, as well as left ventricular ejection fraction or nonsustained VT in patients after MI, and can be added to other measures of risk to enhance predictive accuracy. Frequency domain analysis resolves parasympathetic and sympathetic influences better than time domain analysis does, but both types of analysis are useful. Heart rate variability is used to evaluate vagal and sympathetic influences on the sinus node (inferring that the same activity is also occurring in the ventricles) and to identify patients at risk for a cardiovascular event or death. Detection of AF as a possible cause of cerebral symptoms has become a common indication for implantable monitors. In patients with unexplained syncope, a diagnosis can be made in up to 80% by long-term monitoring, many only after a long period (up to 18 months). The devices can be configured to store patient-activated episodes, automatically activated recordings (heart rate outside preset parameters), or a combination of these. Both P waves and QRS complexes can usually be identified. It is capable of recording several minutes of a single channel of the ECG before and after a symptomatic event and can have a service life of 2 to 3 years. ![]() These devices (smaller than a pack of chewing gum) are inserted under the skin at about the level of the anterior second rib on the left chest and are activated by passing a special magnet over the device. In such patients an implantable loop recorder can be used. Zipes MD, in Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, 2019 Implantable Loop Recorderįor patients with very infrequent symptoms, neither Holter recorders nor 30-day event recorders may yield diagnostic information.
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