Chicken or egg? Dangers in the interpretation of retrospective studies

Observational cohort study of changing trends in non-invasive ventilation in very preterm infants and associations with clinical outcomes

OBJECTIVE

To determine the change in non-invasive ventilation (NIV) use over time in infants born at <32 weeks' gestation and the associated clinical outcomes.

STUDY DESIGN

Retrospective cohort study using routinely recorded data from the National Neonatal Research Database of infants born at <32 weeks admitted to neonatal units in England and Wales from 2010 to 2017.

RESULTS

In 56 537 infants, NIV use increased significantly between 2010 and 2017 (continuous positive airway pressure (CPAP) from 68.5% to 80.2% in 2017 and high flow nasal cannula (HFNC) from 14% to 68%, respectively) (p<0.001)). Use of NIV as the initial mode of respiratory support also increased (CPAP, 21.5%-28.0%; HFNC, 1%-7% (p<0.001)).HFNC was used earlier, and for longer, in those who received CPAP or mechanical ventilation. HFNC use was associated with decreased odds of death before discharge (adjusted OR (aOR) 0.19, 95% CI 0.17 to 0.22). Infants receiving CPAP but no HFNC died at an earlier median chronological age: CPAP group, 22 (IQR 10-39) days; HFNC group 40 (20-76) days (p<0.001). Among survivors, HFNC use was associated with increased odds of bronchopulmonary dysplasia (BPD) (aOR 2.98, 95% CI 2.81 to 3.15) and other adverse outcomes.

CONCLUSIONS

NIV use is increasing, particularly as initial respiratory support. HFNC use has increased significantly with a sevenfold increase soon after birth which was associated with higher rates of BPD. As more infants survive with BPD, we need robust clinical evidence, to improve outcomes with the use of NIV as initial and ongoing respiratory support.

Physiological effects of high-flow nasal cannula therapy in preterm infants

OBJECTIVE

High-flow nasal cannula (HFNC) therapy is increasingly used in preterm infants despite a paucity of physiological studies. We aimed to investigate the effects of HFNC on respiratory physiology.

STUDY DESIGN

A prospective randomised crossover study was performed enrolling clinically stable preterm infants receiving either HFNC or nasal continuous positive airway pressure (nCPAP). Infants in three current weight groups were studied: <1000 g, 1000-1500 g and >1500 g. Infants were randomised to either first receive HFNC flows 8-2 L/min and then nCPAP 6 cm H₂O or nCPAP first and then HFNC flows 8-2 L/min. Nasopharyngeal end-expiratory airway pressure (pEEP), tidal volume, dead space washout by nasopharyngeal end-expiratory CO₂ (pEECO₂), oxygen saturation and vital signs were measured.

RESULTS

A total of 44 preterm infants, birth weights 500-1900 g, were studied. Increasing flows from 2 to 8 L/min significantly increased pEEP (mean 2.3-6.1 cm H₂O) and reduced pEECO₂ (mean 2.3%-0.9%). Tidal volume and transcutaneous CO₂ were unchanged. Significant differences were seen between pEEP generated in open and closed mouth states across all HFNC flows (difference 0.6-2.3 cm H₂O). Infants weighing <1000 g received higher pEEP at the same HFNC flow than infants weighing >1000 g. Variability of pEEP generated at HFNC flows of 6-8 L/min was greater than nCPAP (2.4-13.5 vs 3.5-9.9 cm H₂O).

CONCLUSIONS

HFNC therapy produces clinically significant pEEP with large variability at higher flow rates. Highest pressures were observed in infants weighing <1000 g. Flow, weight and mouth position are all important determinants of pressures generated. Reductions in pEECO₂ support HFNC's role in dead space washout.