Capnometry for continuous postoperative monitoring of nonintubated, spontaneously breathing patients

We continuously monitored spontaneous respiration after extubation by end-tidal CO2 tension (PETCO2) in 19 patients aged 20 to 72 years who had undergone major operations. The respiratory gas was sampled from the nasopharynx via a special nasal catheter and analyzed by a side-stream analyzer. In each case, optimal placement of the nasal catheter was determined by CO2 waveform and the capnograms were recorded for waveform analysis and trend monitoring. PETCO2 was compared with arterial CO2 tension (PaCO2) two to four times during the 2- to 19-hour observation periods by simultaneous measurements. For 65 simultaneous measurements, mean PETCO2 was 38.9 +/- 5.7 mm Hg (range, 26.3 to 48.3 mm Hg) and mean PaCO2 was 38.9 +/- 5.7 mm Hg (range, 26.8 to 46.0 mm Hg; r = 0.82; p less than 0.01). While the mean values for PETCO2 and PaCO2 were similar, several patients had large differences for PaCO2 to PETCO2. The differences of the individual patients did not differ significantly between the various times of measurement. We conclude that this form of capnometry is well suited for continuous, noninvasive monitoring of respiration in nonintubated, spontaneously breathing patients.