Two recent observational studies have examined how cerebral oxygenation is affected by therapies designed to increase blood pressure.
The first, by Hilde Bonestroo and co-workers, studied 71 infants less than 32 weeks gestation who had a mean arterial blood pressure in mmHg less than their gestational age in weeks (Bonestroo HJ, Lemmers PM, Baerts W, van Bel F. Effect of antihypotensive treatment on cerebral oxygenation of preterm infants without PDA. Pediatrics. 2011;128(6):e1502-10. Epub 2011/11/09). They excluded infants with an open ductus. Infants received a fluid bolus over 30 minutes, and then went on to get dopamine if they remained hypotensive. Treatment decision were not determined by protocol, but by the attending clinical team. This explains why, even though fluid boluses did not affect the blood pressure, there were 33 infants who did not go on to receive dopamine. The other 38 babies started dopamine at 5 microg/kg/min, and a few increased their dose thereafter. There was no effect on cerebral oxygenation as determined by NIRS, either from the fluid bolus or after treatment with dopamine. Even though some of the cerebral saturations were quite low before treatment, the plots show the same distribution of NIRS signals before and after treatment. Fluid boluses did not increase the blood pressure, but dopamine at 5 microg/kg/min increased the average mean BP by 5 mmHg.
The second study from Rachel Garner and David Burchfield (Garner RS, Burchfield DJ. Treatment of presumed hypotension in very low birthweight neonates: effects on regional cerebral oxygenation. Arch Dis Child Fetal Neonatal Ed. 2012. Epub 2012/07/12) had a different treatment threshold. In their institution infants less than 30 weeks gestation and less than 1500 g birth weight receive treatment if their mean BP is less than 30 mmHg. They gave a fluid bolus over 15 to 60 minutes and then started dopamine at 2.5 to 5 microg/kg/min. The differences in entry criteria and treatment probably explain the differences in BP responses to the previous study; this study showed an increase of about 2 mmHg with the fluid bolus, and about 2 mmHg with the dopamine. Cerebral oxygenation was unaffected by either maneuver.
These results are consistent with an older study using a different NIRS device, (Wardle SP, Yoxall CW, Weindling AM. Determinants of Cerebral Fractional Oxygen Extraction Using Near Infrared Spectroscopy in Preterm Neonates. J Cereb Blood Flow Metab. 2000;20(2):272-9). In this study the blood pressure thresholds to define hypotensive were derived from the Watkins charts. They did not examine the effects of treatment. They found no relationship between hypotension and cerebral fractional oxygen extraction.
All of which suggests that most hypotensive preterm babies, using any of these 3 common definitions of hypotension, do not have impaired cerebral oxygenation, or at least if they do, then treating the hypotension does not improve the situation. Is there any effect of treating low blood pressure on outcomes? We will have to wait for the result of randomized trials to answer that one… more later.