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 (F_IO₂ expressed as %). A computer model of gas exchange, that is, ventilation/perfusion ratio (V_A/Q) and shunt, plotted the curve of SpO₂ versus F_IO₂ best fitting these data. The slope of this curve is the change in SpO₂ per % change in F_IO₂, 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 V_A/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 (V_A/Q ≈0.4, shunt ≤2%) median slope at 85% SpO₂ was 5.1 (IQR, 3.7–5.5). With more severe BPD (V_A/Q ≤0.3) slope was flatter throughout the SpO₂ range. The highest F_IO₂ for 90% S_pO₂ was in infants with the lowest V_A/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.