To be sure there are fans of both HFNC and CPAP out there. I have often heard from other Neonatologists that they use HFNC and find positive results while other centres refuse to use it in favour of the tried and true CPAP. Turning to the literature you will find some conflicting results with some studies suggesting equity and others more recently favouring CPAP. There has been speculation as to why one would be superior to the other and now we appear to have some answers as to where the differences lie.
A Physiologic Study
Liew et al published Physiological effects of high-flow nasal cannula therapy in preterm infants this month in an elegant study of 40 infants. The study was fairly simple in design either randomizing infants <37 weeks to starting with nCPAP +6 and then transitioning to 8 l/min HFNC followed by stepwise reductions of 1 l/min until 2 l/min was reached or the reverse, starting with 2 l/min and working their way up and then transitioning to nCPAP+6. All infants were on one or the other modality at the start and were all at least 3 days old, they were randomized to one or the other arm regardless of where they started off. Physiologic measurements were taken at each step including the following:
Mv -Minute ventilation
pEEP – nasopharyngeal end-expiratory pressure
pEECO2 -nasopharyngeal end-expiratory CO2
RR – respiratory rate;
SpO2 – oxygen saturation
TCCO2 – transcutaneous CO2
Vt – tidal volume
A Fabian device was used to deliver either HFNC or CPAP at the different flows for all patients.
The authors certainly found some interesting results that I think shed some light on why comparisons of HFNC and CPAP have been so inconsistent.
Table 2 contains the results of the study and I will point out the main findings below.
1. Flow matters – Compared to nCPAP+6 which is fairly consistent flows below 6 l/min deliver pEEP that is below 6 cm H2O.
2. Keep the mouth shut – With CPAP whether the mouth is open or closed the Fabian device delivers +6 cm H2O. As you can see from the table, when the mouth is open transmitted pressures drop off substantially. The infant put on a flow of even 6-8 l/min of HFNC sees pressures less than +6 consistently.
3. As flows increase end expiratory CO2 decreases. HFNC seems to help wash out CO2
4. Low flow rates on HFNC do not seem to help with ventilation as much as higher flow rates. In order to maintain Mv these infants at 2 l/min flow become tachypneic. The low pressures produced likely cause some atelectasis and hence tachypnea.
Size matters! Beware of excessive pressures.
An additional finding of this study was that on “multiple linear regression, flow rate, mouth position, current weight and gestation but not prong-to-nares ratio significantly predicted pEEP and account for a significant amount of its variance (F(4431)=143.768, p<0.0001), R2=0.572, R2=adjusted 0.568).” Essentially, infants under 1000g in particular could see pEEP levels as high as 13 cm H2O with flows of 8 l/min. The variability in transmitted pressures with HFNC is shown nicely in this figure from the study.
As flows increase above 6 l/min the actual pressures delivered become less reliable.
Looking at this data, it becomes evident why HFNC may be failing in its attempt to dethrone nCPAP. In order to achieve higher pressures and provide comparable distending pressure to nCPAP you need higher flows. With higher flows though come the problem of greater variability in delivered pressure. While the average pressure delivered may be equivalent or even higher than a CPAP of +6, in some infants (especially those below 1000g) one may be delivering significantly higher pressures than intended which may help with oxygenation and preventing intubation but others may be seeing far less than needed.
What it comes down to is that nCPAP is better at delivering a consistent amount of pressure. Studies using lower flows of HFNC likely failed to show superiority to CPAP as they just didn’t deliver enough pressure. An example of this was the study by Roberts CT et al Nasal High-Flow Therapy for Primary Respiratory Support in Preterm Infants, in which flows of 6-8 l/min were used. Other studies using higher pressures could have been problematic due to open mouths, or larger babies not receiving as much benefit.
I am not saying that we should throw out HFNC entirely however. Depending on the unit you practice in you might not be able to use CPAP but HFNC may be allowed. If you had to choose between no support or HFNC I would likely go with the HFNC. For me at least, if I want to delivery reliable pressures in my tertiary care NICU I will be calling for the CPAP.