Heart rate variability in conscious neonatal swine: spectral features and responses to short-term intermittent...

Spectral analysis of the cardiac time series has been used as a tool for assessing levels of parasympathetic and sympathetic modulation of the sinoatrial node. In the present investigation we evaluated daily changes in heart rate variability spectra in conscious neonatal piglets that were either neurally intact (n = 5) or had undergone right stellate ganglionectomy (n = 5). The partial stellectomized animals and their intact litter mates were exposed to four days of intermittent hypoxia, each day comprising nine episodes of hypoxia alternating with nine episodes of normoxia. A time control group (n = 7) comprised animals from different litters that were not exposed to intermittent hypoxia. We hypothesized that exposure to intermittent hypoxia would increase sympathetic efferent neuronal modulation of heart rate variability spectra in neurally intact animals and in those with right stellate ganglionectomy, and that his effect would be observed in heart rate variability spectra computed from baseline recordings.

Overall, heart rate variability spectra during baseline conditions were dominated by high frequency activity, a reflection of parasympathetic efferent neuronal innervation and linkage to the ventilatory cycle manifested as respiratory sinus arrhythmia. Exposure to intermittent hypoxia did not alter daily baseline spectral features that would indicate an increase of sympathetic cardiac activity: low frequency (0.05 - 0.15 Hz) activity was unaffected and the ratio of low- to -high frequency activity remained less than unity indicating a predominance of high frequency activity. The resultant spectra were remarkably similar despite differences in cardiac sympathetic efferent neuronal innervation and experimental treatment. When spectra were computed from cardiac time series during representative hypoxic episodes, significant increases in activity across the low frequency region (0.05 - 0.15 Hz) of heart rate variability spectra were noted and were comparable in neurally intact animals and in those with right stellate ganglionectomy.

The findings of this investigation provided important information regarding sympathetic efferent neuronal innervation of the heart during the neonatal period. Both neurally intact animals and those with right stellate ganglionectomy had equivalent increases of activity in the low frequency region of heart rate variability spectra during hypoxic stimulation. Such a finding demonstrated the capability of residual cardiac sympathetic neuronal innervation to affect functionally appropriate changes in cardiac chronotropy.