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Abstract
Background/aims Fetal haemoglobin (HbF) has an oxyhaemoglobin dissociation curve that may affect systemic oxygenation and the development of retinopathy of prematurity (ROP). The study aim is to characterise the effects of HbF levels on systemic oxygenation and ROP development.
Methods Prospective study conducted from 1 September 2017 through 31 December 2018 at the Johns Hopkins NICU. Preterm infants with HbF measured at birth, 31, 34 and 37 weeks post-menstrual age (PMA), complete blood gas and SpO2 recorded up to 42 weeks PMA, and at least one ROP exam were included.
Results Sixty-four preterm infants were enrolled. Higher HbF was associated with significantly higher SpO2, lower PCO2, lower FiO2 from birth to 31 weeks PMA and 31 to 34 weeks PMA (rs=0.51, rs=−0.62 and rs=−0.63; p<0.0001 and rs=0.71, rs=−0.58 and rs=−0.79; p<0.0001, respectively). To maintain oxygen saturation goals set by the neonatal intensive care unit, higher median FiO2 was required for HbF in the lowest tercile from birth compared with HbF in the highest tercile to 31 weeks and 31 to 34 weeks PMA; FiO2=35 (21–100) versus 21 (21–30) p<0.006 and FiO2=30 (28–100) versus 21 (21–30) p<0.001, respectively. Preterm infants with ROP had poorer indices of systemic oxygenation, as measured by median levels of SpO2 and PCO2, and lower levels of HbF (p<0.039 and p<0.0001, respectively) up to 34 weeks PMA.
Conclusion Low HbF levels correlated with poor oxygenation indices and increased risk for ROP. O2 saturation goals to prevent ROP may need to incorporate relative amount of HbF.
- retina
- neovascularisation
- physiology
- treatment other
Data availability statement
All data relevant to the study are included in the article or uploaded as online supplemental information. All data relevant to this study are included in the article.
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Introduction
Retinopathy of prematurity (ROP) is the leading cause of childhood blindness in developed countries. Since the 1950s, high oxygen exposure has been known to play a central role in the development of ROP. High supplemental oxygen exposure has been hypothesised to cause vasoconstriction and delay retinal vascular development, which on exposure to hypoxia, stimulates the release of angiogenic factors, such as insulin-like growth factor and vascular endothelial growth factor, that drive ROP development.1–3 Recognising this effect, many studies have aimed to identify the ideal oxygen saturation that prevents ROP. These clinical trials have focused on delivering the optimal supplemental oxygen concentration to preterm infants, and the resultant oxygen saturation that prevents ROP. Unfortunately, large randomised clinical trials, Benefits of Oxygen Saturation Targeting (BOOST), Surfactant, Positive Airway Pressure, Pulse Oximetry Randomized Trial (SUPPORT), BOOST II and Canadian Oxygen Trial (COT), were unable to provide clear clinical guidance on oxygen management for preterm infants and define the ideal oxygen saturation goal that would reduce/prevent onset of ROP while simultaneously reducing/preventing systemic morbidity and mortality in this vulnerable population. These studies have not considered how levels of circulating haemoglobin isoforms can influence tissue oxygenation.
Haemoglobin delivers oxygen to tissue. Fetal haemoglobin (HbF), which comprises 90% of total haemoglobin in preterm infants, plays a critical role in oxygen delivery. Physiologically, HbF differs from adult haemoglobin (HbA) due to its (1) decreased binding to 2,3 diphosphoglycerate (2,3 DPG) and therefore higher affinity for oxygen, (2) a leftward shift in the oxyhaemoglobin dissociation curve and (3) a steeper oxyhaemoglobin dissociation curve. Therefore, the haemoglobin composition may influence the oxygenation of preterm infants and the predisposition for ROP. We demonstrated in the Preterm Infants and Foetal Haemoglobin in ROP (PacIFiHER) Report No. 1 that low HbF levels were associated with increased ROP risk.4 At 31 weeks post-menstrual age (PMA), when compared with preterm infants without ROP, preterm infants with mild or severe ROP had significantly lower HbF levels (28.7% and 9.7% vs 67.1%, respectively) and higher HbA levels (69.9% and 87.7% vs 31.7%, respectively).4 This significant association persisted through 37 weeks PMA. Furthermore, preterm infants with the lowest tercile HbF levels at 31 weeks PMA were 7.6 times more likely to develop ROP than preterm infants in the first tercile, a risk that increased by 1.6-fold by 34 weeks PMA. Because the impact of HbF on systemic oxygenation was not assessed, in this follow-up study, we propose that higher HbF levels are associated with improved oxygenation indices in preterm infants that protect against ROP development.
Methods
This report is from the PacIFiHER study, a prospective, observational single-institution cohort study approved by the Johns Hopkins Institutional Review Board (IRB00135120). Preterm infants meeting guidelines for ROP screening were recruited.5 Written informed consent was obtained from the legal guardian/parent. Heel blood (0.5 mL) was collected from each infant at 31, 34 and 37 weeks PMA. Haemoglobin levels were measured using high-performance liquid chromatography on a D-10 Haemoglobin Testing System (Bio-Rad, Paris, France). ROP examinations were performed by experienced paediatric ophthalmologists (MXR, MC, CK). The ophthalmic examination was classified as no ROP, mild ROP (type 1 ROP or less) or severe ROP (type 2 ROP). Daily oxygen saturation (SpO2, %), partial pressure of oxygen (PaO2, mmHg), partial pressure of carbon dioxide (PCO2, mmHg) and fraction of inspired oxygen (FiO2, %) values were extracted from the electronic medical record during the infant’s hospitalisation. Oxygenation indices were created using the median and range of daily cumulative SpO2, FiO2, PaO2 and PCO2 values over each of the predetermined intervals of birth to 31 weeks PMA, 31 to 34 weeks PMA and 34–37 weeks PMA. These indices were then correlated to HbF levels at 31, 34 and 37 weeks PMA.
Descriptive statistics were used to explore differences in baseline characteristics between preterm infants who did and did not develop ROP. Analysis of variance was used to explore differences between those who did and did not develop ROP. The association of prognostic variables with ROP development was explored through Spearman correlation. A two-sided p value less than 0.05 was considered significant. All statistical analysis was performed on SPSS V.22.0.
Results
Baseline characteristics of preterm infants with and without ROP
We enrolled 64 preterm infants from 1 September 2017 through 31 December 2018. Four infants were excluded for not having haemoglobin studies; three infants were transferred before the 31-week PMA sample could be obtained, and one infant had the consent withdrawn prior to 31 weeks PMA. Preterm infants were categorised into three groups: no ROP (n=45), mild ROP (n=12) and severe ROP (n=3). Preterm infants who developed ROP had significantly lower gestational age (no ROP, 28.2±2.0 weeks; mild ROP, 25.5±1.5 weeks; severe ROP, 23.7±0.6 weeks; p<0.0001) and lower birth weight (no ROP=1000±300 g, mild ROP=800±200 g, severe ROP=700±100 g; p<0.02) compared with those infants without ROP (table 1). HbF was lower at 31, 34 and 37 weeks PMA in those with mild and severe ROP compared with those with no ROP (p<0.002) (table 1).
Baseline characteristics of preterm infants with and without ROP
Systemic oxygenation patterns correlate with fetal and adult haemoglobin
HbF levels correlated with oxygenation patterns of SpO2 and PCO2 measurements from birth through 37 weeks PMA for the cohort of patients with no ROP, mild ROP and severe ROP at the following time intervals: birth to 31 weeks PMA, 31 to 34 weeks PMA and 34–37 weeks PMA. Preterm infants who developed mild and severe ROP had significantly lower SpO2 levels and higher PCO2 levels from birth through 37 weeks PMA compared with those infants without ROP (table 2).
Oxygenation patterns in preterm infants with severe and mild ROP vs no ROP
At each time interval, high HbF levels correlated with low required FiO2 (rs<−0.63, p<0.0001), high SpO2(rs>0.51, p<0.0027) and low PCO2 levels (rs<−0.58, p<0.0001), and, conversely, high HbA levels correlated with high FiO2 (rs>0.64, p<0.0001), low SpO2 (rs<−0.50, p<0.0053) and high PCO2 levels (rs>0.58, p<0.0001) (table 3).
Correlation of SpO2, PCO2 and fetal and adult haemoglobin
Infants with low HbF levels required a higher inspired FiO2 from 31 to 37 weeks PMA than those in the higher terciles (table 4). Despite the higher inspired FiO2, the oxygen saturation was lower in infants in the lowest HbF tercile (table 4).
Levels of haemoglobin F and FiO2 and oxygenation saturation
Discussion
In PacIFiHER Report No. 1,4 we reported that preterm infants with severe and mild ROP had significantly lower HbF fractions compared with preterm infants without ROP and that the decrease in HbF at 31 and 34 weeks PMA preceded the onset of development of ROP. In this prospective, observational cohort study, we observed that higher HbF levels correlated with better oxygenation indices (higher SpO2, lower PaCO2) and a reduced likelihood of developing ROP. In particular, from 31 through 37 weeks PMA, HbF levels had a pronounced effect on systemic oxygenation and ROP development. Importantly, by examining physiological oxygen parameters during this vulnerable timeframe, we found that higher HbF levels strongly correlated with higher levels of SpO2 and lower levels of PCO2. Conversely, higher HbA levels correlated well with lower SpO2 and higher PCO2 levels. We hypothesise that the better oxygenation in preterm infants is due in part to the unique properties of HbF.
HbF accounts for 90% of total haemoglobin in the preterm infant and begins to sharply transition to HbA at 32–36 weeks GA.6 Importantly, this critical window of haemoglobin transition also corresponds with the onset of ROP. Although the transition from HbF to HbA is for the most part genetically driven, studies suggest that HbF levels increase as a compensatory mechanism for cardiopulmonary insufficiency, hypoxemia and severe anaemia in preterm infants.7–9 Alternatively, the shift from HbF to HbA can be delayed in maternal hypoxia, infants who are small for gestational age and infants of diabetic mothers.10–13 The relationship between haemoglobin isoforms, systemic oxygenation and ROP in preterm infants has not been previously studied. HbF differs from HbA in two polypeptide chains; HbF has two ɣ subunits while HbA has two β subunits. Compared with the HbA β subunit, the HbF ɣ subunit has a decreased binding affinity of 2,3-DPG in the deoxygenated state, which effectively raises its oxygen binding affinity.14 This results in the leftward shift of the oxygen saturation curve of HbF and higher affinity for oxygen.14 With supplemental oxygen, HbF more readily binds to oxygen and blood with higher HbF compared with HbA will have a higher oxygen concentration.14 For example, patients who suffer from sickle cell disease and fetuses with erythroblastosis fetalis with higher HbF levels are associated with better oxygenation.15 16 The potential disadvantage of the higher affinity for oxygen is likely offset by the Bohr effect, in which haemoglobin releases oxygen in response to increased CO2 and decreased pH. Thus, infants with blood enriched with HbF, relative to HbA, can carry more oxygen, which would protect against developing ROP at this vulnerable age. In this study, we demonstrated that higher HbF was associated with improved oxygenation patterns in preterm infants. The utility of this finding is not to replace monitoring O2 and CO2 levels with fetal haemoglobin levels, but rather to suggest that the O2 saturation target goals may need to consider the amount of HbF and HbA in this vulnerable population when setting those goals.
Limitations of this study include HbF sampling at predefined time points rather than more frequent sampling, which would allow for a more precise description of HbF levels and oxygenation parameters during this period. In addition, only three infants had type 1 ROP. However, we limited the number of blood draws as recommended by the NICU to avoid anaemia of prematurity. Fortunately, we found that total haemoglobin was similar across all groups at 31, 34 and 37 weeks PMA. Due to the small number of infants with severe ROP, we are unable to differentiate the effects of HbF and HbA on the development of mild and severe ROP at 34 and 37 weeks PMA. However, despite limiting our collection of the different isoforms of haemoglobin at pre-specified weeks and the small number of infants with severe ROP, we demonstrated in PacIFiHER Report No. 1 that there is a strong correlation between HbF at 31 and 34 weeks PMA and the development of ROP, and this increased risk of ROP preceded the development of severe ROP in the natural history studies.17
Conclusion
Current clinical oxygen saturation targets for the preterm infant do not, at present, account for the ratio of HbF and HbA. Herein, we demonstrated that higher HbF is associated with better oxygenation indices in the preterm infant and lower prevalence of ROP. A novel finding is that preterm infants without ROP have higher and better oxygenation indices from 31 to 37 weeks PMA, a critical time for ROP development. This observation also suggests that the standard oxygenation guidelines of maintaining oxygen at less than 95% in all infants less than 30 weeks PMA might be re-examined for infants between 31 and 37 weeks PMA to consider whether oxygen parameters should take into account the HbF:HbA ratio when optimising the oxygen concentration for ROP without jeopardising the infant’s health. We plan a larger, multicentre prospective study to investigate whether alterations in HbF levels can be used to strategically adjust systemic oxygenation parameters that maintain an optimal oxygen concentration, reduce the development of ROP, and minimise systemic morbidity and mortality.
Data availability statement
All data relevant to the study are included in the article or uploaded as online supplemental information. All data relevant to this study are included in the article.
Ethics statements
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References
Footnotes
Contributors Substantial contributions to the conception or design of the work; or the acquisition, analysis or interpretation of data for the work: KJ, MXR, JT, SWA, JS, MC, JC, MF, IB, MR, KS, JFA, PLG, JTH. Drafting the work or revising it critically for important intellectual content: KJ, MXR, JT, SWA, JS, MC, JC, MF, IB, MR, KS, JFA, PLG, JTH. Final approval of the version to be published: KJ, MXR, JT, SWA, JS, MC, JC, MF, IB, MR, KS, JFA, PLG, JTH. Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved: KJ, MXR, JT, SWA, JS, MC, JC, MF, IB, MR, KS, JFA, PLG, JTH.
Funding Knights Templar grant (KJ), Mr and Mrs Carol Sprague (KJ), ARVO Alcon Early Investigator Career Award (KJ), unrestricted grant to Wilmer Eye Institute from Research to Prevent Blindness (JTH), Robert Bond Welch Professorship (JTH); Research to Prevent Blindness, New York, New York, USA, and gifts by the J. Willard and Alice S. Marriott Foundation, the Gale Trust, Mr Herb Ehlers, Mr Bill Wilbur, Mr and Mrs Rajandre Shaw, Ms Helen Nassif, Ms Mary Ellen Keck, Don and Maggie Feiner, and Mr Ronald Stiff, J.W. Marriott Jr. Professorship (PLG).
Competing interests None declared.
Provenance and peer review Not commissioned; internally peer reviewed.
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