Autonomic effects on QT-RR interval dynamics after exercise

Abstract

This study was designed to assess autonomic effects on the QT interval during recovery from exercise. Exercise is associated with an acute increased risk of sudden cardiac death. Evidence of impaired parasympathetic activity, such as low heart rate variability and heart rate recovery, and an increased QT interval are also associated with increased mortality. However, there is no clear pathophysiological link among these findings. Bicycle exercise testing was performed serially in 33 healthy volunteers (19 men; ages, 54 Ϯ 7 yr) under four conditions: 1) baseline, 2) ␤-adrenergic blockade-intravenous propranolol (0.2 mg/kg) administered during exercise, 3) parasympathetic blockade-intravenous atropine (0.04 mg/kg) administered during exercise, and 4) double blockade with propranolol and atropine. ECGs were obtained every minute in recovery for 10 min and then at the 15th and 20th min, from which the QT and RR intervals were measured. Linear regression analyses were used to assess the individual QT-RR relationships for each subject for each condition. Relative to baseline, the QT-RR relationship with parasympathetic blockade was shifted to the left and had a steeper slope. In contrast, the QT-RR relationship with ␤-adrenergic blockade was shifted to the right and had a less steep slope. The baseline and double-blockade QT-RR relationships were in the middle and essentially superimposable. There was a negative relationship between QT-RR slope and heart rate or RR interval recoveries, but it was significant only for the 1-and 2-min RR interval recoveries with low R 2 values of 0.124 and 0.114. The main parasympathetic effect in the postexercise recovery period is to counteract the sympathetically mediated QT prolongation. These data support the concept that parasympathetic tone may provide a natural antiarrhythmic effect during this time.parasympathetic; heart rate recovery; QT interval THE PATHOPHYSIOLOGY of sudden cardiac death is not completely understood. A variety of epidemiologic findings has provided key insights into the potential roles of ventricular repolarization, autonomic tone, and exercise. Although exercise has overall salutary effects, it has been shown that the risk of sudden death is increased dramatically during and immediately after exercise (3, 36) compared with sedentary periods. Although there are several possible mechanisms for this marked increased risk of sudden cardiac death, including the induction of myocardial ischemia, it may be related to the acute changes in autonomic tone that accompany exercise. The prognostic significance of abnormalities of autonomic tone has been established in multiple studies that have evaluated autonomic control of the heart rate predominantly at rest or during daily activities. These studies have linked diminished parasympathetic control with increased mortality (8,21,24,26,39,41). Finally, population studies have linked the QT interval on a 12-lead electrocardiogram (ECG) with an increased risk of ventricular arrhythmias and sudden death both in patients wit...

Abnormal cardiac repolarization in anabolic androgenic steroid users carrying out submaximal exercise testing

“…In subjects who submitted to moderate exercise testing, it was observed that a parasympathetic blockade might provide an arrhythmic effect during the post-exercise time, with a significantly increased QT interval. 20 In the present study, AAS users and non-user subjects showed a lengthening in QTc after exercise; however, the AAS group presented a higher increment in QTc interval than the control group. Furthermore, the AAS group presented increased QTd compared with the control group, with a rest value higher than 65 ms, suggesting abnormal ventricular repolarization in those AAS users.…”

“…26 Nevertheless, a measurement of QT dispersion > 90 ms identified patients at increased risk of cardiac death. 20 Salles et al 13 showed for patients with Chagas' disease, that a maximal dispersion of the QT interval greater than 65 ms and a maximum corrected QT interval longer that 465 ms are independent predictors of sudden death. In the present study, the AAS group had a rest QTd > 65 ms and although the maximum QTc was below 465 ms at rest, it increased to a value > 465 ms at the postexercise recovery time.…”

“…35 Because the decrease in heart rate during the first minute after the cessation of exercise is primarily related to parasympathetic reactivation, 34,35 it has been proposed that a delayed heart rate recovery might be a prognostic marker for abnormal or delayed parasympathetic reactivation. 20,36 The impairment of vagal activity is a risk factor for sudden death, 37 and thus, an attenuated heart rate recovery after exercise might be an important predictor of sudden death. 34,36 The results of the present study, showing decreased heart rate recovery in the subjects with AAS abuse could suggest an AAS-induced impairment of parasympathetic activity.…”

“…19 An increased QT interval has been associated with abnormally depressed parasympathetic activity after exercise. 20 Despite the known adverse cardiovascular effects of high-doses of AAS, few studies have used QT parameters to assess cardiovascular risk associated with AAS abuse. Some of them have reported short QT intervals, 21,22 whereas Chung et al 23 have found no change in the QTc interval of AAS users.…”

Comparison of the physiologic and prognostic implications of the heart rate versus the RR interval

“…(12) Briefly, participants had normal physical examinations, ECGs, hematocrits, and serum electrolytes, were without cardiac complaints, and were not taking cardioactive medications. The study was approved by the Northwestern University Institutional Review Board and all participants provided written, informed consent.…”