Unstable SpO in preterm infants: The key role of reduced ventilation to perfusion ratio.

Instability of peripheral oxyhemoglobin saturation (SpO) in preterm infants is correlated with late disability and is poorly understood. We hypothesised that a reduced ventilation to perfusion ratio (V/Q) is the key predisposing factor for SpO instability. We first used a mathematical model to compare the effects of reduced V/Q or shunt on SaO stability (SaO and SpO are used for model and clinical studies respectively). Stability was inferred from the slope of the SaO vs. inspired oxygen pressure ( O) curve as it intersects the 21 kPa O line (breathing air). Then, in a tertiary neonatal intensive care unit, paired hourly readings of SpO and O were recorded over a 24 h period in week old extremely preterm infants. We noted SpO variability and used an algorithm to derive V/Q and shunt from the paired SpO and O measurements. Our model predicted that when V/Q < 0.4, a 1% change in O results in >8% fluctuation in SaO at 21 kPa O. In contrast, when a 20% intrapulmonary shunt was included in the model, a 1% change in O results in <1% fluctuation in the SaO. Moreover, further reducing the V/Q from 0.4 to 0.3 at 21 kPa O resulted in a 24% fall in SaO. All 31 preterm infants [mean gestation (±standard deviation) 26.2 (±1) week] had V/Q < 0.74 (normal >0.85) but only two infants had increased shunt at 1.1 (±0.5) weeks' postnatal age. Median (IQR) SpO fluctuation was 8 (7)%. The greatest SpO fluctuations were seen in infants with V/Q < 0.52 ( = 10): SpO fluctuations ranged from 11%-17% at a constant O when V/Q < 0.52. Two infants had reduced V/Q and increased shunt (21% and 27%) which resolved into low V/Q after 3-6 h. Routine monitoring of O and SpO can be used to derive a hitherto elusive measure of V/Q. Predisposition to SpO instability results from reduced V/Q rather than increased intrapulmonary shunt in preterm infants with cardiorespiratory disease. SpO instability can be prevented by a small increase in O.