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Editor—Based on careful invasive blood pressure measurements in a cohort of 61 very low birthweight (VLBW) infants during the first 24 hours of life, Leeet al 1 report on blood pressure standards in this population. The quality of the blood pressure readings was assessed using continuous video recordings of real time wave forms. The inclusion of a relatively large number (n = 28) of extremely low birthweight infants is another strength of this study.
In an attempt to define normal blood pressure values in a population that by definition is not “normal”, the authors have excluded very ill infants and infants requiring inotropic support. They have also excluded infants who developed intraventricular haemorrhage (IVH) grades III and IV during the first week of life. They suggest that the lower limits of mean arterial blood pressure (MAP) for infants between 26 and 32 weeks of gestation are numerically similar to the gestational ages.
With the exclusion of infants who developed severe IVH, it became impossible to detect any association between blood pressure and IVH. Decreases in cerebral blood flow may play an important role in the pathogenesis of IVH, and, although blood pressure is not a reliable parameter to assess cerebral blood flow,2 at least two published studies3 ,4 have shown that the incidence of IVH was higher in infants with lower MAP. Perhaps the authors could indicate how many infants with IVH grades III and IV were excluded and what blood pressures were recorded in these infants. For similar reasons, it remains uncertain whether the proposed blood pressure standards are applicable to very ill infants or infants who require inotropic support. It is conceivable that cerebral vascular resistance in sick preterm infants differs from that in the study patients, and consequently the application of the suggested blood pressure standards to sick preterm infants would result in different—and possibly inadequate—cerebral blood flows.
The data presented add to our knowledge on blood pressure in VLBW infants, but do not allow the conclusion that the suggested blood pressure standards are safe for all VLBW infants.
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Drs Juin, Rajadurai, and Wee respond:
We would like to thank Dr Berger for his comments. Our objective was to define the normal range of blood pressures for very small infants and this would logically exclude those with IVH and those on inotropes, both of which could affect blood pressure.
We did not compare infants with and without IVH because that was not our aim. There may very well be a correlation between hypotension and IVH, but studies looking at this association were not optimal because there is controversy as to what constitutes hypotension. We hope that our standards will provide norms for such studies. Blood pressure is the practical surrogate for cerebral blood flow because with present technology we cannot continuously monitor the latter by the bedside. Tyszczuk et al 1-1determined that cerebral blood flow was independent of blood pressure in preterm infants, but again the selection of a mean arterial pressure of 30 mm Hg as the cut off is open to debate. We think that choosing such an arbitrary number was not appropriate and more such studies are needed with particular attention being paid to the very different normal blood pressures among preterm infants with various birth weights and gestational ages.
References
- 1-1.↵