Presented at the Neonatal Society 2005 Spring Meeting.
Lee GJ1, Fox GF1, Durward A2
1 Neonatal Unit, Guy’s & St Thomas’ Hospital Trust, London, UK
2 Paediatric Intensive Care, Guy’s & St Thomas’ Hospital Trust, London, UK
Background: Identification of tissue acidosis (TA) is important in the management of sick, extremely low birthweight infants (ELBW). Traditionally, base deficit and lactate have been considered useful in the estimation of this. However, previous work has suggested that anion gap may be a more accurate reflection of tissue acidosis (1). Hypoalbuminaemia leads to underestimation of anion gap (2).
Aims: To estimate the incidence of hypoalbuminaemia in ELBW infants during the first 72 hours of life, and its effect on estimation of TA in a retrospective, observational study.
Methods: Infants with birthweight <1000g admitted to a tertiary neonatal intensive care unit were studied. Blood gas samples that were taken at regular intervals were pooled with the results of laboratory electrolyte, albumin and haematology assays at 0, 24, 48 and 72 hours of life. The base deficit and anion gap (including potassium) were calculated and then corrected for albumin using Figge’s formula (Corrected anion gap = anion gap + 0.25(40 – albumin g/L)). Tissue acids were quantified by strong ion gap + lactate, a method which has been validated previously (1), and TA defined as (strong ion gap + lactate) >3mEq/L.
Results: 104 samples were collected from 26 infants (median birthweight 708g, range 510-990g; median gestation 25+6 weeks, range 23+1 – 32+1). The incidence of hypoalbuminaemia (≤25g/l) on admission was 100% and extreme hypoalbuminaemia (≤20g/l) 50%. TA was demonstrated in 91.3% of samples. Base deficit had the weakest correlation with TA (r2=0.25, p<0.0001), whereas anion gap was better (r2=0.88, p<0.001). Correction of the anion gap demonstrated the best correlation with TA (r2=0.90, p<0.0001). Due to the high incidence of hypoalbuminaemia, the anion gap underestimated the corrected anion gap by 5.1mEq/L. Of the 95 pooled samples demonstrating TA, 46.3% had a base deficit >5mEq/L, whereas 75.8% had a corrected anion gap of >16mEq/L. Corrected anion gap identified TA on 28 occasions where base deficit was <5mEq/L.
Conclusions: Base deficit is a poor marker of TA in sick, ELBW infants. Hypoalbuminaemia is a universal finding in this patient group. Failure to correct the anion gap for hypoalbuminaemia may lead to underestimation of TA, which may have significant treatment implications.
References
1. Stewart PA. Can J Physiol Pharmacol 1983;61:1444-61
2. Durward A, Mayer A, Skellett S et al. Arch Dis Child 2003;88:0-4