Submitted on November 18, 2006
Accepted on March 13, 2007
Cardiac sympathetic nerve activity and ventricular fibrillation during acute myocardial infarction in a conscious sheep model
David L. Jardine1*, Christopher J Charles2, Christopher M Frampton3, and A M Richards4
1 General Medicine, Christchurch Hospital, Christchurch, New Zealand
2 Christchurch Cardioendocrine Research Group, Christchurch School of Medicine, Christchurch, New Zealand
3 Christchurch School of Medicine, New Zealand
4 Christchurch Cardioendocrine Research Group, Christchurch School of Medicine and Health Sciences, New Zealand
* To whom correspondence should be addressed. E-mail: david.jardine{at}cdhb.govt.nz.
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 minutes 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 to 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 minutes. In group B, MBP, HR and CSNA were averaged each consecutive minute from baseline at 14 minutes before the onset of VF/VT and compared to time-matched values in group A. Compared to group A, indices of CSNA burst size increased before the onset of VF/VT: burst area/min (F13,208 =2.17, p=0.01) and burst area/100beats (F13,208 =1.86, p=0.04). By contrast, burst frequency indices were not significantly different: burst frequency (F13,208=1.6, p=0.09), burst incidence (F13,208=1.48, p=0.13). In group A, CSNA burst area/min and burst area/100 beats did not change across this time period (F13,117=0.97, p=0.5, F 13,117=0.96,p=0.7) but increased with time in group B (F13,91 =2.3, p=0.01, F13,91 =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 minutes (range 7-22) in group B (Mann-Whitney U-test, p=0.09). Pre-MI baroreflex slopes were similar: RR slopes were 11.8±2 and 15.6±1.1 msec.mmHg-1 (t18= -1.6, p=0.14), . CSNA slopes were -1.8±0.3 and -2.3±0.2 %.mmHg-1 (t18= -1.4, p=0.2). An early increase in CSNA burst size indices (before 60 minutes post-MI), mediated by an excitatory sympathetic reflex, is important in the genesis of VF/VT.
