30
mm Hg in the CBF-MAP autoregulation curve; and (3) dopamine improves
both MAP and CBF. NeoNotes
Journal Club
Mark Hudak MD, Contributing Editor
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Cerebral Blood Flow
Hypotensive Extremely Low Birth Weight Infants Have Reduced Cerebral Blood Flow. PEDIATRICS Vol. 114 No. 6 December 2004, pp. 1591-1596. Munro MJ, Walker AM, and Barfield CP
Objectives. Whether extremely low birth weight (ELBW) infants are at risk of cerebral hypoperfusion is uncertain because key issues concerning their cerebral blood flow (CBF) and mean arterial pressure (MAP) are unresolved: (1) whether CBF is pressure-passive or autoregulated; (2) the normal level of MAP; and (3) whether inotropic drugs used to increase MAP might inadvertently impair CBF. We addressed these issues in ELBW infants undergoing intensive care.
Methods. CBF (measured by near-infrared spectroscopy) and MAP were measured in 17 infants aged 1.5 to 40.5 hours.
Results. Five infants remained normotensive (MAP 37 ± 2 mm Hg, [mean ± SEM]); twelve became hypotensive (MAP 25 ± 1 mm Hg) and were treated with dopamine (10–30 µg · kg–1 per min). CBF of hypotensive infants (14 ± 1 mL · 100 g–1 per min) was lower than the CBF of normotensive infants (19 ± mL · 100 g–1 per min). After commencement of dopamine in hypotensive infants, MAP increased (29 ± 1 mm Hg) and CBF also increased (18 ± 1 mL · 100g–1 per min). CBF was correlated with MAP in hypotensive infants before (R = 0.62) and during (R = 0.67) dopamine, but not in normotensive infants. A breakpoint was identified in the CBF versus MAP autoregulation curve of untreated infants at MAP = 29 mm Hg; no breakpoint was evident in dopamine-treated infants.
Conclusions. In
ELBW infants (1) cerebral autoregulation is functional in
normotensive but not hypotensive infants; (2) a breakpoint exists at 30
mm Hg in the CBF-MAP autoregulation curve; and (3) dopamine improves
both MAP and CBF.
Comment: It is remarkable how much of our pharmacological therapy in the NICU has been extrapolated from experience in adults and older children. An example that meets criteria for “standard of care” is dopamine therapy for neonatal “hypotension”. That initiation of a dopamine infusion increases mean arterial blood pressure is not in dispute; rather, the real questions are what MABPs (by gestational age) need to be treated and what treatments of “hypotension” will optimize long-term neurodevelopmental outcome? Prior work has raised the possibility that dopamine might actually reduce cerebral blood flow via its peripheral adrenergic effects. This study in 17 Australian premature infants provides us with some reassurance on that point. Using near-infrared spectroscopy methodology, a rise in MABP secondary to dopamine therapy also increased total CBF. However, even if this finding is confirmed in a larger study, an increase in CBF is only a surrogate outcome for an improved neurodevelopment outcome. Animal experiments have shown that changes in total CBF induced by a variety of techniques do not predict the magnitude or even the direction of changes in CBF in different regions of the brain. In addition, if the sick preterm infant cannot autoregulate CBF when MABP changes, levels of MABP above the physiologic norm, as can result when even small changes in dopamine infusion rate are titrated, might be operative in the pathogenesis of IVH. Until a randomized controlled trial of dopamine is accomplished in the preterm infant, not all of us will be highly confident that the result of treating “hypotension” in the otherwise clinically asymptomatic infant with dopamine has no deleterious effect on long-term outcome. –MLH
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