Intra-arterial blood pressure reference ranges, death and morbidity in very low birthweight infants during the first seven days of life

Abstract

Objectives: We aimed to: (1) assess the association of average, low, high and variable mean blood pressure (mbp) on death and the common morbidities of very low birthweight infants, and in doing so, (2) to derive representative reference ranges for mbp in very low birthweight infants. Study design: This five year retrospective study assessed 1 min computer recordings of intra-arterial mbp in 232 very low birthweight infants over the first 7 days of life in a tertiary NICU. Four measures of mbp were assessed: average, variability, maximum (per time period), and percentage of time with a mean blood pressure less than the infant’s gestation. Correlation was made with death and the development of intraventricular haemorrhage (IVH), periventricular leukomalacia (PVL) and retinopathy of prematurity (ROP). Results: The mbp increased with increasing birthweight and postnatal age (though with a slight decrease on days 6 and 7). Birthweight, gestation and colloid support (adjusted for birthweight and gestation) were the only factors significantly associated with mbp. IVH was predominantly associated with a low and variable mbp on the day IVH was noted or the day before. PVL and ROP were not associated with blood pressure. Conclusions: These reference ranges include more infants and data than previously published and relate mbp in this cohort to morbidity and mortality. They could assist clinicians in judging appropriate mbp for birthweight.

Introduction

The very low birthweight (vlbw) newborn infant is poorly prepared for the cardiovascular demands of extrauterine life [1]. The immature myocardium and cardiovascular control may limit the ability of such infants to provide adequate perfusion to tissues which in utero, would not have had such a high demand. Clinical signs of reduced organ perfusion may be difficult to ascertain when associated with immaturity and disease: the heart rate has a limited ability to increase cardiac output by increasing rate [2]; autonomic immaturity may limit peripheral vasoconstriction [3]; and vasopressin may produce oliguria in infants with respiratory distress syndrome, independent of renal perfusion [4]. In addition, preferential blood flow within some organs may produce local watershed ischaemia even when overall perfusion to the organ appears to be adequate. It is therefore important to have adequate reference blood pressure values in vlbw infants.

There are problems in deriving ‘normal’ blood pressure data in very preterm infants: A significant majority will have disease as a result of their immaturity – these are usually closely monitored. Less sick individuals do not have such frequent or precise measurements. Reference blood pressure ranges so far provided for the vlbw infant have usually been based on small numbers of infants [5], [6], [7] or infrequently collected data [8], [9], [10], [11] and have mostly been limited to the first 24 to 48 hours of life. The majority of problematic vlbw infants who require blood pressure support have a longer period of critical illness.

Our aim was to create easy to use reference ranges for mean blood pressure in very low birth weight infants based on 5 years of minute by minute intra-arterial blood pressure data stored on a computerised data monitoring system. We included all infants admitted to our 12 bed neonatal intensive care unit during that period. Data were incorporated from the first seven days of life so as to cover the most critical period of illness for the majority of infants. In addition, we have assessed the influence of perinatal factors on blood pressure and the association between blood pressure and infant morbidity and mortality.