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Influence of pulmonary factors on pulse oximeter saturation in preterm infants
  1. JG Jones1,
  2. GG Lockwood2,
  3. N Fung3,4,
  4. J Lasenby3,
  5. RI Ross-Russell5,
  6. D Quine6,
  7. BJ Stenson6
  1. 1Department of Anaesthesia, Addenbrookes Hospital, Cambridge, UK
  2. 2Anaesthetic Department, Hammersmith Hospital, London, UK
  3. 3Signal Processing Group, Cambridge University Engineering Department, Cambridge, UK
  4. 4Biomedical Signals and Systems Group, Faculty of Electrical Engineering, Mathematics and Computer Science, University of Twente, Enschede, The Netherlands
  5. 5Department of Paediatrics, Addenbrookes Hospital, Cambridge, UK
  6. 6Neonatal Unit, Royal Infirmary of Edinburgh, Edinburgh, UK
  1. Correspondence to Professor JG Jones, Woodlands, Rufforth, York YO23 3QF, UK; johngareth423{at}btinternet.com

Abstract

Aim To describe how the stability of oxygen saturation measured by pulse oximetry (SpO2%) varies within and between infants with bronchopulmonary dysplasia (BPD).

Methods Clinically stable infants with BPD had SpO2 measured at different inspired oxygen concentrations (FIO2 expressed as %). A computer model of gas exchange, that is, ventilation/perfusion ratio (VA/Q) and shunt, plotted the curve of SpO2 versus FIO2 best fitting these data. The slope of this curve is the change in SpO2 per % change in FIO2, hence SpO2 stability, calculated at each SpO2 from 85% to 95%.

Results Data from 16 infants with BPD previously described were analysed. The dominant gas exchange impairment was low VA/Q (median 0.35, IQR, 0.16–0.4, normal 0.86). Median shunt was 1% (IQR, 0–10.5; normal <2%). Slope varied markedly between infants, but above 95% SpO2 was always <1.5. In infants with least severe BPD (VA/Q ≈0.4, shunt ≤2%) median slope at 85% SpO2 was 5.1 (IQR, 3.7–5.5). With more severe BPD (VA/Q ≤0.3) slope was flatter throughout the SpO2 range. The highest FIO2 for 90% SpO2 was in infants with the lowest VA/Q values.

Conclusions In infants with BPD, there was large variation in the slope of the curve relating SpO2% to inspired oxygen fraction in the SpO2 range 85%–95%. Slopes were considerably steeper at lower than higher SpO2, especially in infants with least severe BPD, meaning that higher SpO2 target values are intrinsically much more stable. Steep slopes below 90% SpO2 may explain why some infants appear dependent on remarkably low oxygen flows.

  • Fetal Medicine
  • Intensive Care
  • Monitoring
  • Physiology

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