Evaluation of a nasal/oral discriminate sampling system for capnographic respiratory monitoring

Although continuous end-tidal carbon dioxide (PETCO2 mmHg) measurements permit the earliest detection of alveolar hypoventilation, apnea and/or obstruction, technical difficulties have thus far precluded its reliable implementation in the spontaneously breathing patient with a natural (e.g., nonartificially secured) airway. Among the technical difficulties is the fact that conventional sampling devices do not take into account the possibility that breathing may take place primarily through either the nose or the mouth. As a result, the efficacy of a new nasal/oral discriminate sampling system (NODSS) was examined for capnographic observation of respiratory adequacy. NODSS is unique because it provides the opportunity to select simultaneous or discriminate collection of carbon dioxide exhaled through the nose and/or mouth. Twenty-four American Society of Anesthesiologists physical status I to IV patients (ages 30-88 years) were capnographically monitored in the postanesthesia care unit following general anesthesia for various surgical procedures. All patients were extubated and breathing spontaneously. Simultaneously, direct arterial carbon dioxide (PaCO2 mmHg) determinations were made using an indwelling radial artery catheter to determine their correlation with PETCO2 obtained by NODSS. A comparison between PaCO2 values and noninvasive nasal and/or oral PETCO2 obtained by NODSS showed a positive correlation (r value) of 0.602 to 0.849 when statistically analyzed by Pearson's product-moment correlation coefficient. There was no significant difference between the mean (PaCO2-PETCO2) gradient derived through nasal sampling, as compared to the mean gradient derived by oral sampling with this device (P > 0.05). Noninvasive capnographic monitoring by NODSS is a convenient, reliable, effective, and accurate alternative to direct arterial blood gas determination that may be used for the early detection of respiratory inadequacy in the spontaneously breathing patient who has a natural airway.