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Cardiac sympathetic nerve activity and ventricular fibrillation during acute myocardial infarction in a conscious sheep model

Christchurch School of Medicine and Health Sciences, Christchurch Hospital, Christchurch, New Zealand

Submitted 18 November 2006 ; accepted in final form 13 March 2007

The association between cardiac sympathetic nerve activity (CSNA) and ventricular fibrillation (VF) during acute myocardial infarction (MI) has not been assessed in conscious animal models. During the first 60 min post-MI, mean blood pressure (MBP), heart rate (HR), and CSNA were recorded continuously in 20 conscious sheep. Resistant sheep (group A, n = 10) were compared with susceptible sheep (group B, n = 10) who developed fatal VF (n = 7) or sustained ventricular tachycardia (VT, n = 3). The mean time to VF/VT was 28.1 ± 3.3 min. In group B, MBP, HR, and CSNA were averaged at each consecutive minute from baseline at 14 min before the onset of VF/VT and compared with time-matched values in group A. When compared with those of group A, indexes of CSNA burst size increased before the onset of VF/VT: burst area/minute (F_13,208 = 2.17, P = 0.01) and burst area/100 beats (F_13,208 = 1.86, P = 0.04). By contrast, burst frequency indexes were not significantly different: burst frequency (F_13,208 = 1.6, P = 0.09) and burst incidence (F_13,208 = 1.48, P = 0.13). In group A, CSNA burst area/min and burst area/100 beats did not change across this time period (F_13,117 = 0.97, P = 0.5, F_13,117 = 0.96, P = 0.7) but increased with time in group B (F_13,91 = 2.3, P = 0.01; and F_13,91 = 2.25, P = 0.01). Between-group comparisons demonstrated no differences in time of onset of ventricular ectopic beats: 18.5 (range 12–24) in group A versus 15.0 min (range 7–22) in group B (Mann-Whitney U-test, P = 0.09). Pre-MI baroreflex slopes were similar: R-R slopes were 11.8 ± 2 and 15.6 ± 1.1 ms/mmHg (t₁₈ = –1.6, P = 0.14). CSNA slopes were –1.8 ± 0.3 and –2.3 ± 0.2%/mmHg (t₁₈ = –1.4, P = 0.2). An early increase in CSNA burst size indexes (before 60 min post-MI), mediated by an excitatory sympathetic reflex, is important in the genesis of VF/VT.

arrhythmia; cardiac muscle; sympathetic nervous system