Use of ventilation/perfusion mismatch to guide individualised CPAP level selection in preterm infants: a feasibility trial

Non-invasive versus invasive respiratory support in preterm infants

Respiratory insufficiency is almost ubiquitous in infants born preterm, with its incidence increasing with lower gestational age. A wide range of respiratory support management strategies are available for these infants, separable into non-invasive and invasive forms of respiratory support. Here we review the history and evolution of respiratory care for the preterm infant and then examine evidence that has emerged to support a non-invasive approach to respiratory management where able. Continuous positive airway pressure (CPAP) is the non-invasive respiratory support mode currently with the most evidence for benefit. CPAP can be delivered safely and effectively and can commence in the delivery room. Particularly in early life, time spent on non-invasive respiratory support, avoiding intubation and mechanical ventilation, affords benefit for the preterm infant by virtue of a lessening of lung injury and hence a reduction in incidence of bronchopulmonary dysplasia. In recent years, enthusiasm for application of non-invasive support has been further bolstered by new techniques for administration of exogenous surfactant. Methods of less invasive surfactant delivery, in particular with a thin catheter, have allowed neonatologists to administer surfactant without resort to endotracheal intubation. The benefits of this approach appear to be sustained, even in those infants subsequently requiring mechanical ventilation. This cements the notion that any reduction in exposure to mechanical ventilation leads to alleviation of injury to the vulnerable preterm lung, with a long-lasting effect. Despite the clear advantages of non-invasive respiratory support, there will continue to be a role for intubation and mechanical ventilation in some preterm infants, particularly for those born <25 weeks' gestation. It is currently unclear what role early non-invasive support has in this special population, with more studies required.

Unstable SpO2 in preterm infants: The key role of reduced ventilation to perfusion ratio

Introduction: Instability of peripheral oxyhemoglobin saturation (SpO₂) 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 SpO₂ instability. Methods: We first used a mathematical model to compare the effects of reduced V_A/Q or shunt on SaO₂ stability (SaO₂ and SpO₂ are used for model and clinical studies respectively). Stability was inferred from the slope of the SaO₂ vs. inspired oxygen pressure (P _IO₂) curve as it intersects the 21 kPa P _IO₂ line (breathing air). Then, in a tertiary neonatal intensive care unit, paired hourly readings of SpO₂ and P _IO₂ were recorded over a 24 h period in week old extremely preterm infants. We noted SpO₂ variability and used an algorithm to derive V_A/Q and shunt from the paired SpO₂ and P _IO₂ measurements. Results: Our model predicted that when V_A/Q < 0.4, a 1% change in P _IO₂ results in >8% fluctuation in SaO₂ at 21 kPa P _IO₂. In contrast, when a 20% intrapulmonary shunt was included in the model, a 1% change in P _IO₂ results in <1% fluctuation in the SaO₂. Moreover, further reducing the V_A/Q from 0.4 to 0.3 at 21 kPa P _IO₂ resulted in a 24% fall in SaO₂. 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) SpO₂ fluctuation was 8 (7)%. The greatest SpO₂ fluctuations were seen in infants with V_A/Q < 0.52 (n = 10): SpO₂ fluctuations ranged from 11%-17% at a constant P _IO₂ 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 _IO₂ and SpO₂ can be used to derive a hitherto elusive measure of V_A/Q. Predisposition to SpO₂ instability results from reduced V_A/Q rather than increased intrapulmonary shunt in preterm infants with cardiorespiratory disease. SpO₂ instability can be prevented by a small increase in P _IO₂.