Supplementary Material for: Characterisation of the Oxygenation Response to Inspired Oxygen Adjustments in Preterm Infants

Background: Oxygen saturation (SpO₂) targeting in the preterm infant may be improved with a better understanding of the SpO₂ responses to changes in inspired oxygen (FiO₂). Objective: We investigated the first-order FiO₂-SpO₂ relationship, aiming to quantify the parameters governing that relationship, the influences on these parameters and their variability. Methods: In recordings of FiO₂ and SpO₂ from preterm infants on continuous positive airway pressure and supplemental oxygen, we identified unique FiO₂ adjustments and mapped the subsequent SpO₂ responses. For responses identified as first-order, the delay, time constant and gain parameters were determined. Clinical and physiological predictors of these parameters were sought in regression analysis, and intra- and inter-subject variability was evaluated. Results: In 3,788 h of available data from 47 infants at 31 (28-33) post-menstrual weeks [median (interquartile range)], we identified 993 unique FiO₂ adjustments followed by a first-order SpO₂ response. All response parameters differed between FiO₂ increments and decrements, with increments having a shorter delay, longer time constant and higher gain [2.9 (1.7-4.8) vs. 1.3 (0.58-2.6), p < 0.05]. Gain was also higher in less mature infants and in the setting of recent SpO₂ instability, and was diminished with increasing severity of lung dysfunction. Intra-subject variability in all parameters was prominent. Conclusions: First-order SpO₂ responses show variable gain, influenced by the direction of FiO₂ adjustment and the severity of lung disease, as well as substantial intra-subject parameter variability. These findings should be taken into account in adjustment of FiO₂ for SpO₂ targeting in preterm infants.