PEDIATRICS Vol. 115 No. 4 April 2005, pp. 1114-1115 (doi:10.1542/peds.2005-0009)
Preterm Circulatory Support Is More Complex Than Just Blood Pressure
Nick Evans, DM, MRCPCHDepartment of Neonatal Medicine
Royal Prince Alfred Hospital
Camperdown, New South Wales 2050, Australia
Department of Neonatal Medicine
University of Sydney
Sydney, New South Wales 2006, Australia
David Osborn, FRACP
Department of Neonatal Medicine
Royal Prince Alfred Hospital
Camperdown, New South Wales 2050, Australia
Martin Kluckow, PhD, FRACP
Department of Neonatal Medicine
Royal North Shore Hospital
St Leonards, New South Wales 2065, Australia
To the Editor.—
We agree with Munro et al1 that adequate cerebral blood flow (CBF) is important to protecting the preterm brain, but we disagree that this can be ensured solely on the basis of a mean blood pressure (MBP) of >30 mm Hg.
Our studies show a period of vulnerability to low systemic blood flow (SBF) within the first 12 hours; it affects all organs (not just the brain), it is not always shown by low MBP, and it relates strongly to adverse neurologic outcome.2,3 Age of the infant (even within the first 40 hours) is critical to understanding transitional hemodynamics, which is not really addressed by the pooled-regression analysis used by Munro et al. Their lack of data on any other cardiovascular variable means that the underlying cause of low CBF and hypotension is not addressed. The cardiorespiratory causes of early low SBF and hypotension are complex and include myocardial maladaptation to high vascular resistance, positive intrathoracic pressure, and ductal shunts.2 Simply increasing the MBP by using dopamine is unlikely to address the underlying problem and does not always produce sustained improvements in SBF.4
The bilinear-regression analysis in this study combines 2 small populations predefined by treatment criteria of a MBP above or below 30 mm Hg and derives a cut point that is very close to this selection criterion. Individual infants have not been demonstrated to have this cut point. For any given BP, there is a scatter of CBF values, and most CBF data points in the "hypotensive" group are in the same range as the "normotensive" group. CBF below this range was not reliably found until the MBP was <25 mm Hg. We also showed a relationship between MBP and SBF, but many infants with a MBP of <30 mm Hg have normal SBF. It is more concerning that we found infants with normal MBP who have low SBF, particularly in the early hours after birth. Other workers using near-infrared spectroscopy have found the same phenomenon.5 MBP in these infants, if it falls, does so after several hours, resulting in late treatment. That infants can have low CBF with normal MBP cannot be excluded on the basis of the 5 "normotensive" infants in this study.
Since the article of Miall-Allen et al,6 despite no therapeutic trial having been performed, many NICUs have had protocols for keeping MBP above a number such as 30 mm Hg. That ischemic brain injury is still a problem should suggest that circulatory support is more complicated than this. BP must be important, and we agree that lost autoregulation seems to be a problem in some infants. It is what causes that loss of autoregulation that is the critical issue, and we would suggest that postnatal (and probably also prenatal) ischemia may be an important factor. It is the cardiovascular system that drives blood flow to all organs including the brain. To advance our understanding of this complex area, studies need to be structured to postnatal age to involve sufficient subjects and incorporate cardiovascular measures apart from just BP.
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- Munro MJ, Walker AM, Barfield CP. Hypotensive extremely low birth weight infants have reduced cerebral blood flow.
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[Abstract/Free Full Text] - Kluckow M, Evans NJ. Low superior vena cava flow and intraventricular haemorrhage in preterm infants.
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[Abstract/Free Full Text] - Hunt R, Kluckow M, Reiger I, Evans N. Low superior vena cava flow and neurodevelopmental outcome at 3 years in very preterm babies. J Pediatr. 2004;145 :588 –592[CrossRef][Web of Science][Medline]
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PEDIATRICS (ISSN 1098-4275). ©2005 by the American Academy of Pediatrics
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- Preterm Circulatory Support Is More Complex Than Just Blood Pressure: In Reply
- Michael J. Munro, Adrian M. Walker, and Charles P. Barfield
Pediatrics 2005 115: 1115-1116.[Extract] [Full Text]
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