Carboxyhaemoglobin levels in infants with hypoxic ischaemic encephalopathy

Introduction

Hypoxic ischaemic encephalopathy (HIE) is a serious birth complication resulting from impaired cerebral blood flow and oxygen delivery in term infants [1]. Treatment response and outcome prediction are measured using a combination of the results of clinical examination, amplitude integrated electroencephalography (aEEG) and magnetic resonance imaging (MRI) of the brain [2]. The severity of the clinical picture, however, can be subjective depending on the assessor and the interpretation of aEEG can be affected by hypothermia treatment or anti-epileptic medication, while some infants are too unstable for transport to an MRI scanner [3]. As a result, the early predictive abilities of those modalities can be hindered.

The pathophysiology of HIE involves oxidative stress which is most marked during the secondary phase of injury occurring 6–15 h after the initial event, potentially as a result of reperfusion injury [4]. By-products of the oxidative processes may act as useful biomarkers in these infants, providing an additional non-invasive means of risk stratifying neonates with HIE [5]. A potential marker of oxidative stress in the neonatal population is carboxyhaemoglobin (COHb) [6] which is the product of the reaction between carbon monoxide and haemoglobin. COHb levels are available as part of routine blood gas analysis. Carboxyhaemoglobin levels have previously been shown to correlate with the length of invasive ventilation in paediatric intensive care [7] and the development of bronchopulmonary dysplasia and intraventricular haemorrhage in preterm infants [8]. We hypothesised that infants with severe HIE, that is those with abnormal MRI brain findings and/or who had clinical or electrical seizures, would have elevated COHb levels during the first days after birth compared to those with less severe HIE.

Subjects and methods

Results

During the study period, 67 infants (44 male) underwent therapeutic hypothermia for HIE. Their median (IQR) gestational age was 40 (38–41) weeks and birth weight was 3.39 (2.08–3.66) kg (Table 1).

Seizures (electrographic or clinical) were reported in 33/66 (50 %). MRI features consistent with HIE were present in 20/62 (30 %). Nine infants (13 %) died from HIE before discharge from neonatal care.

The median (IQR) COHb level on day three was higher in infants without EEG confirmed seizures compared to in infants with seizures (p=0.024, Table 2). The median (IQR) COHb on day five was higher in infants without MRI abnormalities compared to in infants with MRI abnormalities (p=0.032, Table 2). The COHb levels were not significantly different between the nine infants who died compared to those who survived.

The COHb levels on any study day were not significantly different between infants with or without meconium stained amniotic fluid or pulmonary hypertension or who need inotrope support (Supplementary Table 1). The COHb levels on any of the study days did not significantly correlate with arterial cord blood pH, base deficit, lactate, AST or creatinine levels (Supplementary Table 2).

Discussion

We have demonstrated that COHb levels were higher on day three in infants without seizures compared to infants with seizures and higher on day five in infants with normal MRI brain examinations compared to infants with abnormal MRI scans.

These findings do not support our hypothesis that infants with severe HIE would have higher COHb levels during the first days after birth compared to those with less severe HIE. It was interesting that on all three study days better outcomes (survival, no seizures, normal MRI examination) were associated with higher COHb levels compared to the unfavourable outcomes (death, seizures, abnormal MRI). We postulate that the consistency of these finding suggests that elevated COHb levels have a different mechanism in HIE than in our initial hypothesis.

Whilst carbon monoxide is known to be a toxic agent, recent evidence has suggested that when given at low doses it might have neuroprotective properties [17], [18], [19]. Elevated carboxyhaemoglobin levels may thus represent an increased endogenous neuroprotective production of carbon monoxide in response to the hypoxic-ischemic insult. This effect might be mediated by the up-regulation of enzymes responsible for the heme catabolism which produce carbon monoxide as a by-product. Indeed, animal studies have highlighted the protective effect of carbon monoxide supplementation in ameliorating hepatobiliary dysfunction during catabolism of heme molecules in endotoxemic livers [20]. Furthermore, exogenous carbon monoxide has also been shown to have anti-inflammatory, antiapoptotic and cytoprotective effects in animal studies [18, 21]. In an animal study where mice were exposed to carbon monoxide at a concentration of either 200 or 250 ppm for a period of 1 h after an hypoxic insult, it was demonstrated that 250 ppm of carbon monoxide were effective in preserving cortical volumes following a mild hypoxic-ischemic injury [3]. In further animal studies, Liu et al. also showed that carbon monoxide reperfusion relieved oxidative injury and inhibited apoptosis and autophagy [22]. Our results then are interesting in pointing towards carbon monoxide not as an index of oxidative stress, but as a potential neuroprotective pathway that could be further explored as highlighted in the aforementioned animal studies. It is interesting to note that there were no significant correlations between cord gas variables (pH, BE, lactate), AST, platelets and creatinine levels and COHb levels on any of the study days. Those observations also highlight that COHb may be of limited clinical utility as a marker of oxidative stress in HIE.

This is the first human study pointing to a possible protective role of carbon monoxide in hypoxic ischemic injury. A limitation to our study is the lack of control samples, however, COHb levels would not be measured serially in infants without any pathology. In our cohort we also report a relatively low mortality rate so we are underpowered to report significant differences relating to this outcome. Our mortality rate, however, is not dissimilar to those in previously published national reports [23].

Conclusions

In conclusion higher carboxyhaemoglobin levels were associated with normal MRI brain findings and the absence of electrical seizures in infants with HIE. Those findings are consistent with a possible cytoprotective effect of carbon monoxide. Trials are warranted to investigate the potential therapeutic effects of carbon monoxide in the context of HIE.