Asphyxia and therapeutic hypothermia modulate plasma nitrite concentrations and carotid vascular resistance in preterm fetal sheep

Abstract

In this study, we tested the hypothesis that cerebral hypoperfusion after asphyxia and induced hypothermia is associated with reduced circulating nitrite levels as an index of nitric oxide synthase (NOS) activity. The preterm fetal sheep at 0.7 gestation (103-104 days, term = 147 days) received 25-minute umbilical cord occlusion, followed by mild whole-body cooling from 30 minutes to 72 hours after occlusion. Occlusion and induced hypothermia were independently associated with reduced carotid vascular conductance (CaVC) from 2 to 72 hours, and with transiently suppressed plasma nitrite levels at 6 hours. There was a significant within-subjects correlation (r(2) = 0.33, P = .002) between CaVC and plasma nitrite values in the first 24 hours after occlusion but not after sham occlusion. These findings suggest that in preterm fetal sheep, changes in NOS activity are an important mediator of changes in carotid vascular tone in the early recovery phase after asphyxia and may help mediate some of the vascular effects of induced hypothermia.

Keywords: blood flow; fetal sheep; nitric oxide; therapeutic hypothermia.

Figure 1.

Changes in fetal core temperature (°C), fetal heart rate (FHR, bpm), mean arterial blood pressure (MAP, mm Hg), and carotid blood flow (CaBF, % baseline) in the sham-normothermia (open circles), sham-hypothermia (closed circles), occlusion-normothermia (open squares), or occlusion-hypothermia (closed squares) groups. Umbilical cord occlusion or sham occlusion was performed for 25 minutes at t = 0 (marked by vertical dashed line, data not shown). Mild whole-body cooling (marked in gray) was initiated 30 minutes after the end of umbilical cord occlusion, and both core (esophageal) and extradural (not shown) temperatures decreased significantly within an hour after the onset of cooling compared to the normothermia groups. Temperature remained stable over the 72-hour cooling period and there was no significant difference in core and extradural temperatures between or within the hypothermia groups. Time is represented in 6-hour averages during the baseline period, 30-minute averages up to 12 hours, and 1-hour averages from 12 to 24 hours after (sham) occlusion, and 6-hour averages for the remainder of the experiment. Data are mean ± standard error of the mean (SEM). *P < .05, hypothermia versus normothermia. §P < .05, occlusion versus sham occlusion. @P < .05, sham-hypothermia versus sham-normothermia.

Figure 2.

Changes in plasma nitrite levels (% baseline) and carotid vascular conductance (CaVC, % baseline) in the sham-normothermia (open circles), sham-hypothermia (closed circles), occlusion-normothermia (open squares), or occlusion-hypothermia (closed squares) groups. Umbilical cord occlusion or sham occlusion was performed for 25 minutes at t = 0 (marked by dotted line, data not shown). Time is represented in 6-hour averages during the baseline period, 30-minute averages up to 12 hours and 1-hour averages from 12 to 24 hours after (sham) occlusion, and 6-hour averages for the remainder of the experiment. Data are mean ± standard error of the mean (SEM). # P < .05 versus baseline values, *P < .05 hypothermia versus normothermia. §P < .05, occlusion versus sham occlusion.

Figure 3.

Left: panels A and B, Relationship between changes in cephalic vasoconstriction as shown by the carotid vascular conductance (CaVC, % baseline) and plasma nitrite values (μmol/L) during the first 24 hours after occlusion or sham occlusion. Each fetus is represented by a unique symbol, while the plotted lines indicate the regression relationship for individual fetuses. The normothermia groups are shown by open symbols with dashed lines and the hypothermia groups by closed symbols with solid lines. Panel A, The sham-normothermia and the sham-hypothermia groups showed no significant within subjects correlation. Panel B, After elimination of variance from group and individuals, there was a significant correlation between nitrite values and CaVC within the occlusion-normothermia and occlusion-hypothermia groups (P = .002, r ² = 0.33). Right, panels C and D: Relationship between changes in cephalic vasoconstriction as shown by CaVC (% baseline) and plasma nitrite values (μmol/L) from 72 to 120 hours in the normothermia and hypothermia groups. Panel C, No significant correlation was seen in the sham-normothermia group, whereas in the sham-hypothermia group rewarming was associated with a significant relationship (P = .013, r² = 0.80). Panel D, There was no significant correlation within the occlusion-normothermia and occlusion-hypothermia groups during the rewarming (or sham rewarming) phase.