The title is how I sometimes refer to HFNC, but one could ask whether that is all there is to high flow, is it just another way to deliver CPAP, but with no control, or knowledge, of the pressure delivered?
A new study examined the lung mechanics in infants receiving CPAP and HFNC, and they tried to match the actual distending pressure achieved between the groups. (Lavizzari A, Respiratory mechanics during NCPAP and HHHFNC at equal distending pressures. Archives of Disease in Childhood – Fetal and Neonatal Edition. 2014). In a randomized cross-over study of 20 preterm infants with mild RDS, babies received 2,4, or 6 cmH2O of CPAP or 2,4, or 6 L/min. They then measured the retropharyngeal pressure and compared lung mechanics between periods of CPAP and periods of HFNC which achieved matching pressures.
Not too surprisingly pressures achieved during HFNC were very variable. 5 of the babies did not achieve a pressure of more than 2 cmH2O even with 6 liters per minute. only 5 babies got up to 6 cmH2O, so the comparisons were done at 2 and 4 cmH2O. There were basically no differences found. Although all the inspiratory work of breathing estimates were a little higher with HFNC than with CPAP.
The babies had very low oxygen requirements, and fairly good lung compliance, I guess it is possible that the results might have been different with subjects that had stiffer lungs; but this study does suggest that HFNC is indeed a ‘poor man’s CPAP’, with CPAP actually delivered often being less than 2 cmH2O but, when CPAP is generated, the physiological effects are similar to traditional CPAP. Again suggesting that the only real benefit of HFNC is comfort.
Or is it? Another small cross-over trial evaluated comfort using the EDIN scale during 24 hours periods with either CPAP (at 4 to 5 cmH2O) or HFNC (at 5 or 6 L/min) in 20 preterm infants with mild respiratory distress. (Klingenberg C, et al. Patient comfort during treatment with heated humidified high flow nasal cannulae versus nasal continuous positive airway pressure: a randomised cross-over trial. Archives of Disease in Childhood – Fetal and Neonatal Edition. 2013). No difference in the discomfort scores was found. A nice aspect of this study was that the authors asked parents how they felt about the therapies, the parents clearly preferred the HFNC, they thought their child was more satisfied, they could do more in the care of the baby, and they had improved interaction with them.
Finally another small cross-over study compared pressures achieved with 2 different HFNC systems (Collins CL, et al . Comparison of the pharyngeal pressure provided by two heated, humidified high-flow nasal cannulae devices in premature infants. Journal of Paediatrics and Child Health. 2013). Basically, at the same flow the pressures were the same. And again, very variable, at the highest flows they tested, 7 and 8 L/minute, the pressures were a bit lower with the Fisher-Paykell device, probably because of the pressure limiting valve. At 8 liters per minute the average pressure was between 4 and 5 with each device, but the standard deviation was 2.2, meaning that 95% of the time the pressures were between 0 and about 9 (if they were truly normally distributed, which is unlikely to be exactly true). So poor man’s CPAP, with unreliable pressures, no apparent physiologic advantage, a possibility of having very high or very low distending pressures, but parents prefer it, and there is less nasal trauma. Clinical outcomes are not any worse, and babies who are failing on HFNC (which is likely to be sometimes due to them not actually getting any pressure) can often be rescued with CPAP.