Effects of fetal haemoglobin on systemic oxygenation in preterm infants and the development of retinopathy of prematurity PacIFiHER Report No. 2

Background/aimsFetal 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.MethodsProspective 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.ResultsSixty-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.ConclusionLow 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.

Methaemoglobinaemia in the perioperative period with regional block

Methaemoglobin is a form of haemoglobin with oxidised ferric (+3) iron rather than ferrous (+2) iron, which causes a leftward shift in the oxyhaemoglobin dissociation curve and prevents oxygen delivery. Anaesthesiologists need to be familiar with this differential diagnosis for hypoxia given the use of drugs in the perioperative setting known to induce methaemoglobinaemia, including benzocaine and lidocaine, antibiotics such as dapsone and anaesthetic gases, including nitric oxide. This case report details an interesting case of symptomatic methaemoglobinaemia in the perioperative period in the setting of dapsone use and an erector spinae block performed with ropivacaine.

Simplified bedside assessment of pulmonary gas exchange in very preterm infants at 36 weeks’ postmenstrual age

IntroductionWe aimed to develop and validate a prediction table for a simplified measure of rightward shift of the fetal oxyhaemoglobin saturation (SpO2) versus inspired oxygen pressure (PIO2) curve as an objective marker of lung disease severity in very preterm infants, independent of unit altitude or oxygen prescribing policies.MethodsVery preterm infants (n=219) had an oxygen reduction test at median (IQR) test age of 354 (345–360) weeks’ postmenstrual age (PMA). Shift was derived from at least three paired SpO2 versus PIO2 measurements using a computer algorithm, using the fetal oxyhaemoglobin dissociation curve as the reference. Linear regression of resultant shift values enabled construction of a table to predict shift using a single paired SpO2 versus PIO2 measurement, validated subsequently in a separate infant cohort using Bland-Altman analysis. Receiver operating curve analysis provided threshold values equating to a clinical diagnosis of mild bronchopulmonary dysplasia (BPD) or moderate to severe BPD.ResultsThe median (IQR) age of 63 infants in the validation cohort was 360 (356–362) weeks’ PMA. Mean difference (95% CI) between predicted and measured shift was 2.1 (−0.8% to 4.9%) with wide limits of agreement (−20.7% to 24.8%). Predicted shift >10.1 kPa identified mild BPD with 71% sensitivity and 88% specificity while values>13.0 kPa identified moderate to severe BPD with 81% sensitivity and 100% specificity.DiscussionShift predicted from a single paired SpO2 versus PIO2 measurement using our validated table enables objective bedside screening of lung disease severity in very preterm infant cohorts at 36 weeks’ PMA.

Aviation medicine

Travel by air is a safe means of transport, but puts people at various physiological risks and is a potential means of spreading infectious disease. Physiological risks associated with flying include hypoxia, as atmospheric pressure falls with altitude. The minimum cabin pressure in commercial passenger aircraft (565 mm Hg, 75.1 kPa) brings a healthy individual’s arterial P along the plateau of the oxyhaemoglobin dissociation curve until just at the top of the steep part, but does not cause desaturation. By contrast, people with respiratory disease and a low arterial oxygen pressure may desaturate, which can be overcome by administering 30% oxygen, this being equivalent to breathing air at ground level. There is no evidence that the pressurized aircraft cabin itself encourages transmission of disease, and recirculation of cabin air is not a risk factor for contracting symptoms of upper respiratory tract infection.