Autonomic influences on ventricular repolarization in congestive heart failure

Sympathetic control of short-term heart rate variability and its pharmacological modulation

The static relationship between heart rate (HR) and the activity of either vagal or sympathetic nerves is roughly linear within the physiological range of HR variations. The dynamic control of HR by autonomic nerves is characterized by a fixed time delay between the onset of changes in nerve activity and the onset of changes in HR. This delay is much longer for sympathetically than for vagally mediated changes in HR. In addition, the kinetics of the HR responses shows the properties of a low-pass filter with short (vagal) and long (sympathetic) time constants. These differences might be secondary to differences in nervous conduction times, width of synaptic cleft, kinetics of receptor activation and post-receptor events. Because of the accentuated low-pass filter characteristics of the HR response to sympathetic modulation, sympathetic influences are almost restricted to the very-low-frequency component of HR variability, but the chronotropic effects of vagal stimulation usually predominate over those of sympathetic stimulation in this frequency band. Oscillations in cardiac sympathetic nerve activity are not involved in respiratory sinus arrhythmia (high-frequency component) and make a minor contribution to HR oscillations of approximately 10-s period (low-frequency component of approximately 0.1 Hz), at least in the supine position. In the latter case, HR oscillations are derived mainly from a baroreflex, vagally mediated response to blood pressure Mayer waves. Beta-blockers and centrally acting sympathoinhibitory drugs share the ability to improve the baroreflex control of HR, possibly through vagal facilitation, which might be beneficial in several cardiovascular diseases.

Beat-to-beat QT interval variability before and after cardiac surgery

Non-uniform recovery of excitability may be essential in triggering malignant ventricular tachycardia after cardiac surgery. Thirty-five channels ECG was recorded for 6 min in 27 patients before and after heart surgery and in 20 control subjects. Off-line analysis was performed. RR interval duration, RR SD, QT SD and power spectra of RR variability were computed from 256 s stable RR and QT interval series. When compared to controls, patients had decreased RR SD and increased QT SD before surgery (p<0.002 and p<0.0005, respectively); RR SD further decreased and QT SD increased after the surgery (p<0.0001 and p<0.0002, respectively). Increase of QT variability and decrease of RR variability after cardiac surgery may reflect disrupted electrophysiological stability of the myocardium and thus electrophysiological substrate for triggering malignant arrhythmia.

Beat-to-beat assessment of QT/RR interval ratio in severe heart failure and overt myocardial ischemia: a measure of electrical integrity in diseased hearts

The study was designed to assess the beat-to-beat variation of ventricular repolarization in patients with myocardial ischemia, hear failure, and in normal subjects. Autonomic tone may alter the dynamic QT/RR interval relation and thus may be involved in ventricular arrhythmia development, especially in the diseased heart. The study included 145 patients (age 16-86 years) with CHF (LVEF < or = 0.30) or unstable angina pectoris (LVEF > 0.60). The control group consisted of healthy volunteers giving physiological baseline measures for the evaluated parameters: cycle length, QT interval, and QT/RR interval ratio during three time periods. In patients with myocardial ischemia (LVEF > 0.60) and healthy subjects the QT/RR interval ratio did not reveal significant differences between both groups (QT/RR(CAD) = 0.36 +/- 0.77 vs QT/RR(controls) = 0.28 +/- 0.83; NS). In sharp contrast, in patients with severe heart failure, RR dependent instantaneous variation of the QT interval was almost missing and regression line analysis disclosed a QT/RR interval slope substantially enhanced by 196% (compared to normal subjects) and 131% (compared to CAD patients; P < 0.05) with a complete loss of circadian modulation (QT/RR(CHF) = 0.83 +/- 0.71 vs QT/RR(CAD) = 0.36 +/- 0.77 vs QT/RR(controls) = 0.28 +/- 0.83; P < 0.05). Beat-to-beat QT interval assessment provides a dynamic parameter of physiological and altered repolarization in defined study groups. Compared to other groups (preserved LVEF), patients with left ventricular impairment exhibited a significantly increased sensitivity of repolarization to cycle length (enhanced QT/RR interval ratio) and a blunted circadian modulation of the QT interval. This is consistent with concept that increased repolarization disparity may be deleterious being a potential pathophysiological basis for enhanced arrhythmic risk.

Cardiac effects of chronic oral beta-blockade: lack of agreement between heart rate and QT interval changes

BACKGROUND

Although well established on the sinus node, the effects of beta-blockade on ventricular repolarization are still conflicting. The aim of the study was to investigate the effects of a chronic beta-blockade on sinus node and repolarization parameters and their relationship.

METHODS

Sixteen healthy volunteers (10 males, mean age: 40 +/- 6.7 years) were randomized to placebo or atenolol (100 mg). After 7 days, subjects were crossed over. Heart rate (HR) and HRV indices were calculated from long-term ECG recordings separately during the day and at night, together with ventricular repolarization parameters (QT interval duration and QT rate-dependence).

RESULTS

Mean R-R intervals were significantly and consistently increased after atenolol (Day: 916 +/- 103 ms vs. 712 +/- 89 ms, and Night: 1149 +/- 93 vs. 996 +/- 125 ms). HRV changes under atenolol were also consistent, with a significant decrease in sympathovagal ratio. In contrast, atenolol only lowered diurnal QT rate-dependence (0.123 +/- 0.032 vs. 0.190 +/- 0.065 on placebo, P < 0.001), but not the nocturnal pattern. After multivariate analysis QT rate-dependence changes induced by atenolol were correlated with pretreatment QT/RR relation (r = 0.65, P < 0.01) but not with any HR or HRV parameters.

CONCLUSIONS

In healthy subjects, repolarization changes following chronic beta-blockade cannot be predicted by HR or HRV changes, but are dependent on pretreatment rate-dependence.

How to correct the QT interval for the effects of heart rate in clinical studies

Much inter- and intra-subject variability in the QT interval in health and disease is accounted for by differences in heart rate, leading to difficulties when determining the effects of disease and drugs on the QT interval. Traditionally, heart rate correction formulae have been used to overcome this problem in man. However, the commonly used Bazett's heart rate correction formulae (QT=QT(C) radical RR interval) does not remove the effect of heart rate; indeed, it overcorrects at high heart rates. Fredericia's formula (QT=QT(C)x(3) radical RR interval) does remove the effects of heart rate; this is the preferable formula, if one is to be used. However, all formulae make assumptions about the nature of the QT-heart rate relationship, assumptions that may not apply to those with disease or on drugs. A more intellectually rigorous approach to QT interval-heart rate correction is to determine the QT-heart relationship for each individual, using data obtained from exercise tests or 24-h Holter tapes. The best mathematical relationship (linear, exponential, etc.) is obtained from analysis of this data, and is used to determine the QT interval at a heart rate of 60 bpm, the QT(60). The QT(60) measure makes no assumptions about the nature of the QT interval-heart rate relationship, removes the dependence of QT interval on heart rate, and maintains genuine biological differences in the QT interval. It should become the standard in QT interval-heart rate correction.