Ventilation-perfusion (V/Q) mismatch is a prominent feature of preterm infants and adults with
lung disease. V/Q mismatch is known to cause arterial
hypoxemia under steady-state conditions, and has been proposed as the cause of rapid arterial
oxygen desaturation during
apnea. However, there is little evidence to support a role for V/Q mismatch in the dynamic changes in arterial oxygenation that occur during
apnea. Using a mathematical model, we quantified the effect of V/Q mismatch on the rate of desaturation during
apnea to ascertain whether it could lead to rates of up to 10%s(-1) as observed in preterm infants. We used a lung-body model for the preterm infant that incorporated 50 parallel alveolar-capillary units that were ventilated and perfused with the severity of V/Q mismatch (sigma) defined conventionally according to sigma=S.D. of the distribution of V/Q ratios. Average desaturation rate 10s from
apnea onset was strongly elevated with worsening V/Q mismatch as a result of an earlier desaturation of low V/Q units compared with high V/Q units. However, V/Q mismatch had little impact after
apnea onset, with peak desaturation rate only substantially increased if mismatching caused a lowered resting arterial O(2) saturation. In conclusion, V/Q mismatch causes a more immediate onset of desaturation during
apnea, and therefore places preterm infants and adults with
lung disease at risk of hypoxemic
dips. However, V/Q mismatch does not accelerate desaturation rate beyond
apnea onset and cannot, therefore, explain the rapid desaturation observed during recurrent
apnea in preterm infants.