Vol.:(0123456789) 1 3 Journal of Clinical Monitoring and Computing https://doi.org/10.1007/s10877-018-0168-6 LETTER TO THE EDITOR Using the fetal oxyhaemoglobin dissociation curve to calculate the ventilation/perfusion ratio and right to left shunt in healthy newborn infants Theodore Dassios 1  · Kamal Ali 1  · Thomas Rossor 2  · Anne Greenough 2,3 Received: 25 March 2017 / Accepted: 31 May 2018 © Springer Nature B.V. 2018 Oxygenation impairment can be non-invasively assessed by the degree of right-to-left shunt and ventilation/perfusion inequality. The relative rightwards displacement of an indi- vidual’s oxyhaemoglobin dissociation curve (ODC) can be used to determine the reduction of the ventilation to perfu- sion ratio and the degree of depression of the ODC can be used to quantify the level of right to left shunt (Fig. 1). Using paired samples of peripheral oxygen saturation and fraction of inspired oxygen, a computer software can analyse and ft the data and derive a curve which represents the best ft for each infant and calculate the shunt and V A /Q. These indexes have been described in sick newborn infants with pulmonary failure [1] and bronchopulmonary dysplasia [2–4]. We have recently reported normative values for these indices in healthy term infants which are essential to deter- mine the magnitude of the abnormality [5]. We studied 145 infants with a mean (SD) gestation of 39 (1.6) weeks at a median (range) postnatal age of 3 (1–8) days and reported a mean (SD) rightwards shift of the ODC of 5.5 (1.1) kPa, V A /Q ratio of 0.95 (0.21). None of the infants had a right- to-left shunt [5]. As with previous studies in sick newborns [1, 2, 4] we used the adult ODC as a reference curve. This curve however is positioned to the right compared to the fetal oxyhaemo- globin curve. The presence of fetal haemoglobin shifts the fetal ODC to the left, refecting the higher afnity of fetal haemoglobin to oxygen [6]. Thus, had we utilised a fetal curve we would have reported relatively higher values of shift and lower values of V A /Q (Fig. 1). We chose the adult haemoglobin because the relatively rapid rate of decline of fetal haemoglobin after birth [7], would mean the neonatal curve would be shifting to the right during the frst weeks, constituting a moving reference curve. The fetal ODC [8] for newborns is likely to be more accurate for use in neonates than the adult ODC, which is found after approximately 6 months [7]. We, therefore, reanalysed our data using the fetal curve as a reference curve. Diferences of V A /Q and shift calculated with the adult or the fetal ODC were assessed for signifcance using the Mann–Whitney U rank sum test. The mean (SD) rightwards shift of the ODC was 7.9 (1.5) kPa using the fetal curve compared to 5.5 (1.1) kPa using the adult ODC (p < 0.001). The mean (SD) V A /Q was 0.84 (0.18) using the fetal curve compared to 0.95 (0.21) using the adult ODC (p < 0.001). The mean (SD) right-to-left shunt was 5.0 (5.4)% using the fetal curve compared to 0% using the adult ODC (p < 0.001). Using the fetal rather than the adult curve as a refer- ence to calculate the V A /Q ratio and the right-to-left shunt in healthy newborns, leads to lower values of V A /Q and a detectable right-to-left shunt. * Theodore Dassios theodore.dassios@kcl.ac.uk 1 Neonatal Intensive Care Centre, King’s College Hospital NHS Foundation Trust, 4th Floor Golden Jubilee Wing, Denmark Hill, London SE5 9RS, UK 2 Division of Asthma, Allergy and Lung Biology, MRC-Asthma UK Centre in Allergic Mechanisms of Asthma, King’s College London, London, UK 3 National Institute for Health Research (NIHR) Biomedical Research Centre Based at Guy’s and St Thomas’ NHS Foundation Trust and King’s College London, London, UK