Unstable SpO(2) in preterm infants: The key role of reduced ventilation to perfusion ratio

Introduction: Instability of peripheral oxyhemoglobin saturation (SpO(2)) in preterm infants is correlated with late disability and is poorly understood. We hypothesised that a reduced ventilation to perfusion ratio (V(A)/Q) is the key predisposing factor for SpO(2) instability. Methods: We first used a mathematical model to compare the effects of reduced V(A)/Q or shunt on SaO(2) stability (SaO(2) and SpO(2) are used for model and clinical studies respectively). Stability was inferred from the slope of the SaO(2) vs. inspired oxygen pressure (P (I)O(2)) curve as it intersects the 21 kPa P (I)O(2) line (breathing air). Then, in a tertiary neonatal intensive care unit, paired hourly readings of SpO(2) and P (I)O(2) were recorded over a 24 h period in week old extremely preterm infants. We noted SpO(2) variability and used an algorithm to derive V(A)/Q and shunt from the paired SpO(2) and P (I)O(2) measurements. Results: Our model predicted that when V(A)/Q < 0.4, a 1% change in P (I)O(2) results in >8% fluctuation in SaO(2) at 21 kPa P (I)O(2). In contrast, when a 20% intrapulmonary shunt was included in the model, a 1% change in P (I)O(2) results in <1% fluctuation in the SaO(2). Moreover, further reducing the V(A)/Q from 0.4 to 0.3 at 21 kPa P (I)O(2) resulted in a 24% fall in SaO(2). All 31 preterm infants [mean gestation (±standard deviation) 26.2 (±1) week] had V(A)/Q < 0.74 (normal >0.85) but only two infants had increased shunt at 1.1 (±0.5) weeks’ postnatal age. Median (IQR) SpO(2) fluctuation was 8 (7)%. The greatest SpO(2) fluctuations were seen in infants with V(A)/Q < 0.52 (n = 10): SpO(2) fluctuations ranged from 11%–17% at a constant P (I)O(2) when V(A)/Q < 0.52. Two infants had reduced V(A)/Q and increased shunt (21% and 27%) which resolved into low V(A)/Q after 3–6 h. Discussion: Routine monitoring of P (I)O(2) and SpO(2) can be used to derive a hitherto elusive measure of V(A)/Q. Predisposition to SpO(2) instability results from reduced V(A)/Q rather than increased intrapulmonary shunt in preterm infants with cardiorespiratory disease. SpO(2) instability can be prevented by a small increase in P (I)O(2).