Evaluation of the capnographic monitoring and oxygen delivery functions of three variable oxygen delivery masks

Abstract

A number of commercial devices that couple oxygen delivery and capnographic monitoring have become available. Many of these devices have been released for clinical use without prior clinical evaluation. We decided to evaluate the oxygen delivery and capnographic monitoring function of two commercially available masks (Capnomask; Mediplus, High Wycombe, Bucks, UK and Oxymask; P3 Medical Ltd, Bristol, UK) as well as the Lifecare variable O2 delivery face mask (Lifecare Hospital Supplies Ltd, Harlow, Essex, UK) designed primarily for O2 delivery but with a CO2 sampling port attached to the side of the mask for the purpose of the study [1]. We conducted a manikin study to evaluate the oxygen delivery and capnographic functions of the three face masks. We constructed a model using a manikin head and a ventilator to simulate ventilation. The manikin ‘breathed’ with two tidal volumes (300 ml and 500 ml) and two expired CO2 concentrations (3.5% and 5%). The experiment was carried out at four respiratory rate frequencies (9, 16, 24, 30 breaths.min−1) and four oxygen flow rates (0, 4, 6 and 10 l.min−1). End-tidal CO2 and oxygen concentration were recorded using a Datex AS/3 monitoring system. The measured CO2 concentrations at an oxygen flow rate of 6 l.min−1 are shown in Fig. 1. Mean (SD) O2 concentrations (6 l.min−1 O2 flow) were a 43% (6%), 43% (5%) and 32% (1.4%) for Capnomask, Oxymask and Lifecare mask respectively over the full range of respiratory rates. ‘Expired’ CO2 recordings for the three dual function masks over the range of respiratory rates. Our findings suggest that the Capnomask recorded higher expired CO2 levels than the Oxymask and Lifecare variable oxygen delivery mask with the side CO2 sampling port. Oxygen delivery over the full range of respiratory rates was very similar for the Capnomask and Oxymask. The Lifecare mask delivered consistently lower inspired O2 concentration than the Capnomask and Oxymask for all of the selected respiratory rates. This may be due to the position of the CO2 sampling tube, which may have interfered with the flow of O2 within the mask. The oxygen delivery function of a mask may be adversely affected by the attachment of the CO2 sampling tube and this is best avoided. We conclude that commercially available face masks with the dual function should be evaluated in the laboratory and if satisfactory function is demonstrated then evaluation in the clinical environment should follow.