Effect of cold on fetal heart rate and its variability.
1. The effects of cold saline (25 mL) injected over the fetal skin on fetal heart rate (HR) and HR variability (HRV), measured as the coefficient of variation (CV) in pulse interval (PI) and by power spectral analysis (PSA), were measured in 10 chronically catheterized fetal sheep aged 140-144 days. To determine the extent to which effects on HR and HRV were mediated by the sympathoadrenal neuroendocrine axis and the cardiac vagus, experiments were performed before and after beta-adrenoreceptor blockade with propranolol (n = 12 fetuses) or before and after cardiac vagal blockade with atropine (n = 4 fetuses). 2. Injection of ice-cold saline over the skin caused an immediate rise in mean arterial pressure (MAP) from 46+/-1 to 55+/-1 mmHg (P < 0.001) and HR from 156+/-2 to 182+/-2 b.p.m. (P < 0.001). Heart rate variability, measured as CV of PI, rose from 3.5+/-0.2 to 8.0+/-0.2% (P < 0.001) and total power spectral density (PSD) increased from 78+/-6 to 278+/-16 units (P < 0.001) as measured by PSA. Within 100s, MAP, HR and HRV had returned to baseline. 3. Beta-adrenoreceptor blockade abolished all these changes in HR, HRV and PSD, but had no effect on changes in MAP. Atropine had no demonstrable effect on the responses to cold. 4. Therefore, the increase in fetal MAP, HR and HRV that occurred with stimulation of peripheral thermoreceptors was the result of increased activity of the sympathetic nervous system. Alterations in efferent cardiac vagal tone were not involved in the cardiac response to cold.