Abstract
Background:
The helmet has been used as a novel interface to deliver noninvasive ventilation without applying direct pressure on the face. However, due to its large volume, the helmet may predispose to CO2 rebreathing. The purpose of this study was to compare the gas flows and CO2 rebreathing of the two types of helmets on CPAP.
Methods:
We evaluated the interface-helmet Caster R NEXT (StarMed, Mirandola, Italy) and the Caster R NEXT with the high-flow nasal cannula (HFNC) Optiflow (Fisher & Paykel, Auckland, NZ) that dead-space washout can be expected. Breathing was simulated using an LUNGOO (Air Water Safety Services, Japan) lung simulator; Pmus 20 (tidal volume 500 mL), frequency = 20/min, lung compliance 80 mL/cm H2O, airway resistance 5 cm H2O/L/s, EtCO2 = 40 mm Hg. The upper respiratory tract model created on a 3D printer was connected to the LUNGOO and was covered with the Caster R NEXT. PEEP valve 5 and 10 cm H2O was attached to left port of the Caster R NEXT and source gas flow was set at 30 L/min to 60 L/min on right port or the Optiflow through closed port (right port was closed on this occasion) of the Caster R NEXT. Tidal volume was measured by the LUNGOO. EtCO2 and FICO2 were measured by the Capnostream 20 p (Covidien) at three points: The oral cavity, nasopharynx and trachea.
Results:
Although in each interface-helmet a progressive reduction in EtCO2 and FICO2 (oral cavity < nasopharynx < trachea) was observed with increasing gas flow, the effect was greatest with the Caster R NEXT with the Optiflow at all points. The FICO2 obtained with the Caster R NEXT with the Optiflow (PEEP 5, gas flow 30, nasopharynx) was almost at the same level as FICO2 obtained with the Caster R NEXT (PEEP 5, gas flow 60, nasopharynx). Tidal volume decreased in both interface-helmet (up to 10% on PEEP 5 and up to 29% on PEEP 10).
Conclusions:
The lower concentration of nasal CO2 obtained using the Optiflow suggests that it causes less rebreathing.
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