Myocardial electrotonic response to submaximal exercise in dogs with healed myocardial infarctions: evidence for β-adrenoceptor mediated enhanced coupling during exercise testing

Abstract

Introduction: Autonomic neural activation during cardiac stress testing is an established risk-stratification tool in post-myocardial infarction (MI) patients. However, autonomic activation can also modulate myocardial electrotonic coupling, a known factor to contribute to the genesis of arrhythmias. The present study tested the hypothesis that exercise-induced autonomic neural activation modulates electrotonic coupling (as measured by myocardial electrical impedance, MEI) in post-MI animals shown to be susceptible or resistant to ventricular fibrillation (VF).Methods: Dogs (n = 25) with healed MI instrumented for MEI measurements were trained to run on a treadmill and classified based on their susceptibility to VF (12 susceptible, 9 resistant). MEI and ECGs were recorded during 6-stage exercise tests (18 min/test; peak: 6.4 km/h @ 16%) performed under control conditions, and following complete β-adrenoceptor (β-AR) blockade (propranolol); MEI was also measured at rest during escalating β-AR stimulation (isoproterenol) or overdrive-pacing.Results: Exercise progressively increased heart rate (HR) and reduced heart rate variability (HRV). In parallel, MEI decreased gradually (enhanced electrotonic coupling) with exercise; at peak exercise, MEI was reduced by 5.3 ± 0.4% (or -23 ± 1.8Ω, P < 0.001). Notably, exercise-mediated electrotonic changes were linearly predicted by the degree of autonomic activation, as indicated by changes in either HR or in HRV (P < 0.001). Indeed, β-AR blockade attenuated the MEI response to exercise while direct β-AR stimulation (at rest) triggered MEI decreases comparable to those observed during exercise; ventricular pacing had no significant effects on MEI. Finally, animals prone to VF had a significantly larger MEI response to exercise.Conclusions: These data suggest that β-AR activation during exercise can acutely enhance electrotonic coupling in the myocardium, particularly in dogs susceptible to ischemia-induced VF.

Antiarrhythmic Effects of Melatonin and Omega-3 Are Linked with Protection of Myocardial Cx43 Topology and Suppression of Fibrosis in Catecholamine Stressed Normotensive and Hypertensive Rats

“…It is consistent with other arrhythmia prone models accompanied by disorders in Cx43 topology [14,30,54,57,58] that subsequently alters the electrotonic coupling and myocardial conduction [51,59]. Susceptible animals also exhibited enhanced cardiac β-adrenergic responsiveness [60].…”

Influence of Exercise Mode on Post-exercise Arterial Stiffness and Pressure Wave Measures in Healthy Adult Males

“…However, for at-risk populations in whom bouts of strenuous exercise are associated with acutely increased risk of cardiovascular events, the additional pressure imposed on the heart by augmented wave reflection resulting from whole-body RES may lead to an increased risk and/or incidence of a critical adverse event. Significant stimulation of the sympathetic-adrenergic system, post-exercise has been associated with an increased risk of myocardial arrhythmias (Del Rio et al, 2015 ), particularly in those susceptible to abnormal rhythms. Therefore, acute RES may amplify the risk for those with arrhythmia susceptibility or heightened sympathetic activity including chronic cardiovascular diseases (Parati and Esler, 2012 ).…”

Heart Rate Changes Before, During, and After Treadmill Walking Exercise in Normal Dogs

“…Auscultation using a stethoscope is economical and easy to apply and interpret while the patient is stationary but has not been examined during rehabilitative work. Various Holter monitors have been used in clinical settings on cardiac patients, but their economical use, ease of setup, and ability to assess data in real time during exercise may limit their feasibility in practice (22)(23)(24)(25). A HR monitor has been examined previously in dogs under stationary and trot conditions (26,27); however, the validity and HR range in normal dogs during TE (WALK) using a different waterproof HR monitor model have not been assessed (26,27).…”