A flow-through capnometer for obstructive sleep apnea

Introduction: Capnogram is often distorted due to aspiration of expired gas when a sidestream capnometer is used for non-intubated, spontaneously breathing condition. The purpose of this study was to make a flow-through capnometer without aspiration and to check if this capnometer precisely detected apnea during obstructive sleep apnea (OSA). Methods: (1) Flow-through capnometer The capnometer consisted of a flow-through etCO₂ sensor, cap-ONE®, and an accompanying capnometer. The size of cap-ONE® was small enough to be fitted under the nose where gas expired from the nose and the mouth passed through. Thus, the expired gas to be measured is directly blown into the cap-ONE®. (2) The cap-ONE® using a spontaneously breathing model Capnograms obtained by the cap-ONE® and sidestream capnometers during nasal and oral breathing under normal and reduced ventilation were compared with a reference capnogram. (3) Clinical study with OSA patients With nineteen OSA patients capnograms during apnea events diagnosed as OSA by polysomnography were examined using the cap-ONE®. (4) Simulation study with an OSA model Apnea in which inspiratory flow was zero and small expiratory flows repeated was produced. Capnograms and apnea detection were compared between the cap-ONE® and sidestream capnometers. Results: In the spontaneouly breathing model capnograms and etCO₂ of the cap-ONE® during nasal and oral breathing were almost identical with the reference capnogram but those of sidetream capnometers during oral breathing were significantly reduced. In the clinical study 41% of total OSA events showed capnograms with prolonged and elevated phase with small ripples. In a simulation study reduction of CO₂ tension during no-inspiration was small and apnea was successfully detected with the cap-ONE®. However, with sidestream capnometers the reduction of CO₂ tension was large and apnea was not detected. Conclusions: We concluded that the cap-ONE® can record capnograms with minimum distortion and detect apnea reliably during OSA.