Unstable SpO2 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 O-I(2)) curve as it intersects the 21 kPa P O-I(2) line (breathing air). Then, in a tertiary neonatal intensive care unit, paired hourly readings of SpO(2) and P O-I(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 O-I(2) measurements.Results: Our model predicted that when V-A/Q < 0.4, a 1% change in P O-I(2) results in > 8% fluctuation in SaO(2) at 21 kPa P O-I(2). In contrast, when a 20% intrapulmonary shunt was included in the model, a 1% change in P O-I(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 O-I(2) resulted in a 24% fall in SaO(2). All 31 preterm infants [mean gestation (& PLUSMN;standard deviation) 26.2 (& PLUSMN;1) week] had V-A/Q < 0.74 (normal > 0.85) but only two infants had increased shunt at 1.1 (& PLUSMN;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 O-I(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 O-I(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 O-I(2).

Automated summary

In preterm infants, instability of peripheral oxyhemoglobin saturation (SpO(2)) is associated with late disability. This study found that a reduced ventilation to perfusion ratio (V-A/Q) is the main factor leading to SpO(2) instability.