Neonatal Blood Gases and Outcome Following Perinatal Asphyxia

Objective: To determine how well neonatal blood base excess and lactate predict outcome following perinatal asphyxia. Study Design: Over 68 months, with around 9000 deliveries, lactate was measured on all blood gases taken from neonatal admissions, as well as mineral base and organic acid whenever electrolytes were required. Infants were classified according to their outcome more than 10 years following perinatal asphyxia. Blood gases affected by other causes of acidosis were excluded. Group I 490 unaffected, including need for resuscitation Group II 22 unaffected following mild hypoxic ischaemic encephalopathy Group III 5 unaffected following moderate hypoxic ischaemic encephalopathy Group IV 11 disability after acute asphyxia Group V 8 disability after chronic intrauterine hypoxia Group VI 5 died from perinatal asphyxia Logarithmic regression equations of base excess, lactate and organic acid against postnatal age for the first 48 hours were calculated for each group. Results: Lactate equations increased progressively with the severity of perinatal asphyxia, with similar equations and outcome for groups II and III as well as for groups IV and V. Base excess equations did not increase uniformly and underestimated the severity of asphyxia in group V, because of high mineral base soon after delivery. Organic acid equations ran parallel with lactate but were also increased by low glomerular filtration. Conclusion: Blood lactate during the first 48 hours predicted the risk following perinatal asphyxia. Lactate above 10 mmol/L at 1 hour indicated risk of adverse outcome, as well as levels above 5 mmol/L after 4 hours, when other causes of acidosis had been excluded. Base excess correlated poorly with lactate because of wide variation in mineral base. It failed to detect chronic in utero hypoxia and was of no value in predicting risk after 8 hours. Neonatal Blood Gases and Outcome Following Perinatal Asphyxia keywords: Perinatal asphyxia, Hypoxic ischaemic encephalopathy, Lactate, Base excess, Mineral base, Organic acid InTRODuCTIOn Base excess measures the metabolic component of acid base balance and is defined as the amount of acid or base needed to restore pH to 7.4 at a normal PCO2 of 40mmHg and temperature 37°C [1]. In Hypoxic Ischaemic Encephalopathy (HIE), it is often taken as a proxy measure of the degree of lactic acidosis, with base deficit >20 mmol/L shown to predict death or disability [2]. In term infants with intra-partum asphyxia, base deficit at 30 minutes was shown to be as good as lactate in assessing the severity of asphyxia [3]. However, base deficit correlates poorly with lactate [3,4], and in preterm and term infants receiving intensive care, adverse outcomes were not predicted by base deficit, but were by lactate levels [4,5]. Studies over the last 40 years have consistently demonstrated the prognostic value of lactate measurements after delivery [3-6]. Moderate or severe HIE is likely to develop at lactate levels above 9 mmol/L at 30 minutes but not at levels below 5mmol/L [3]. Urinary lactate: creatinine ratio over 0.64 in the first 6 hours predicted HIE with 94% sensitivity and 100% specificity, and the degree of elevation predicted adverse outcomes at 1 year [7]. Mineral balance studies show that base excess is determined by both mineral base and organic acid, with organic acid but not mineral base correlating with outcome [8]. This means that only lactate and not base deficit should be used to measure the degree of lactic acidosis. However this recommendation has met with little enthusiasm, partly because of no long term follow up studies. Christopher Geoffrey Alexander Aiken and Rebecca J M Bardgett, Neonatal Blood Gases Perinatal Asphyxia www.ijnmr.net Indian Journal of Neonatal Medicine and Research. 2013 Oct, Vol-1(2): 1-4 2 In order to evaluate a new neonatal Total Parenteral Nutrition (TPN), blood gases obtained from a 5 years cohort of neonatal admissions were prospectively analyzed [9]. Those infants at risk from HIE, very low birth weight or other serious problems have now been followed up for more than 10 years. This study examines the relation between blood gas measurements and outcome following perinatal asphyxia in this cohort of term and preterm infants. MeThODS Over a 68 months period from 1996 to 2001, lactate was measured on all blood gas samples taken as clinically required from newborn babies admitted after delivery to the neonatal unit in New Plymouth, New Zealand. Whenever electrolyte measurements were also required, Na, K, Ca, Mg, Cl, PO4, protein, urea and creatinine were measured, all on the same blood sample, to allow mineral base and organic acid to be calculated [10]. These protocols were put in place in order to prospectively evaluate a new neonatal TPN regimen [9]. The results were recorded along with antenatal, delivery and postnatal events, clinical and laboratory details and treatments. Data on outcome was obtained by reviewing over 10 years of paediatric follow up of infants who suffered severe perinatal asphyxia or HIE. Results were expressed as mean ± sample Standard Deviation (SD) and analysis performed with standard parametric tests, linear, logarithmic and power regression analysis. ReSulTS Over this 68 months period with approximately 9000 deliveries, blood gases were obtained from 582 infants during the first 48 hours, when most episodes of lactic acidosis occurred. How well lactate correlated with base excess and organic acid was determined from arterial blood gases obtained in the first 8 hours from 307 of these infants. This avoided the organic acidosis, with normal lactate, that developed thereafter caused by low glomerular filtration rates [9]. Lactate correlated poorly with base excess because of wide variation in mineral base, as shown in [Table/Fig-1]. High lactate resulted in high mineral base, as shown in [Table/Fig-2]. Sodium bicarbonate and blood products, containing sodium citrate, given after delivery further increased mineral base. Lactate had the expected relationship to organic acid, with correlation coefficients as high as could be anticipated from the error of measuring organic acid [10]. To determine how well lactate and base deficit predict outcome after perinatal asphyxia blood gases from 41 infants presenting with other causes of acidosis were first excluded: