Reliability of CO2 measurements from the airway by a pharyngeal catheter in unintubated, spontaneously breathing subjects.

Although several short communications have appeared describing attempts to record the concentrations of carbon dioxide (cCO2) from the unintubated airway by a catheter placed in the nose, so far only few reports have documented the reliability of the method. To evaluate the reliability of CO2 measurements by a catheter in the open, unintubated airway during spontaneous respiration, a 12 CH PVC catheter was forwarded through the nostril to the hypopharynx and connected to a capnograph in nine healthy volunteers. Another capnograph was connected to a tightly fitting face mask and simultaneous CO2 recordings were attained from the two parts of the airway during normoventilation, hyperventilation and rebreathing. A corresponding blood sample was drawn from the radial artery for blood gas analysis. The configurations of the capnograms recorded from the pharyngeal catheter were similar to those recorded from the face mask. The results were analysed by a multifactor analysis of variance. The carbon dioxide tension (pCO2) was significantly influenced by degree of ventilation (P < 0.0001), subject (P < 0.0001), measurement site (P = 0.030) and interaction subject-ventilation (P = 0.015). In spite of the significant influence of the measurement site, the difference between end tidal carbon dioxide tension (pCO2(ET)) and carbon dioxide tension in arterial blood (pCO2(a)) was small. The mean differences between paired measurements (pCO2(ET)-pCO2(a)) were -0.10 kPa +/- 0.41 kPa (mean +/- SD) for the catheter and -0.20 kPa +/- 0.43 kPa for the face mask. The study demonstrates that reliable recordings of CO2 concentrations during spontaneous respiration can be obtained by a thin catheter positioned in the hypopharynx.