Experimental physiology
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Experimental physiology · Aug 2011
Baroreflex control of renal sympathetic nerve activity and heart rate in near-term fetal sheep.
Late preterm infants, born between 34 and 36 weeks gestation, have significantly higher morbidity than neonates born at full term, which may be partly related to reduced sensitivity of the arterial baroreflex. The present study assessed baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in near-term fetal sheep at 123 ± 1 days gestation. At this age, although fetuses are not fully mature in some respects (term is 147 days), sleep-state cycling is established [between high-voltage, low-frequency (HV) and low-voltage, high-frequency (LV) sleep], and neural myelination is similar to the term human infant. ⋯ In contrast, the fall in BP after sodium nitroprusside was associated with a significant increase in HR during LV but not HV sleep, and there was no significant effect of hypotension on RSNA. These data demonstrate that in near-term fetal sheep baroreflex activity is only partly active and is highly modulated by sleep state. Critically, there was no RSNA response to marked hypotension; this finding has implications for the ability of the late preterm fetus to adapt to low BP.