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Assessment of Gas Exchange in Spontaneously Breathing Patients
Hartmut Gehring, M.D., Norman Stein, Holger Matz, Ph.D., Ewald Konecny, Ph.D.
Department of Anesthesiology, Medical University SH, Campus Luebeck, Luebeck, Germany.
Introduction: Capnograpy and pulse oximetry are essential aspects in the monitoring of mechanically ventilated patents. Recommendations have now been made to employ CO2 monitoring also in spontaneously breathing patients (1). The goal of this prospective study was to evaluate two procedures undergoing continuing development, i.e. transcutaneous (tc) and endtidal (et) PCO2 measurement, regarding their applicability and precision compared to blood gas analysis (BGA). Methods: With the approval of the local ethics committee and after written informed consent was obtained, 30 patients of the ASA groups II and III with arterial access were monitored in the recovery room after extubation. The tc-system (TOSCA, Fa. Linde, Switzerland) was fixed with a combination sensor for CO2 and SpO2 at the ear lobes. The et-system Microcap plus was employed in a randomized manner with each of 15 patients fitted with the newly developed oral/nasal FilterLine (Fa. Oridion, Luebeck) for measuring PetCO2, and to each of 15 patients with a normal facial mask and CO2 sampling employing the sidestream procedure. In addition to respiratory rates and PetCO2 values, capnography curves were digitally displayed and analyzed with respect to the guidelines of (2). In the statistical evaluation of both procedures data from 5 measurement points were taken between administration at the recovery room and transferral to regular care wards.

Main results: Both systems could be used for monitoring in the recovery room. Respiration rate indicated by the et-monitor provided more sophisticated information about breathing activity than did PetCO2 values (given in table 1), which differed significantly. In contrast, using curve analysis and the fitting algorithm, reference curves could be established that represented PetCO2 values within the range of the PaCO2. The Tosca system overestimated PaCO2 significantly but could be calibrated using PaCO2 and provided a more effective monitoring amongst less cooperative patients.

Summary: Both procedures allow only a rough estimation of PaCO2. Because of the continuity of the data display, the tc-procedure is better suited as a trend monitor, while the et-method also display respiratory rate and thereby serve as a monitor for apnoea, a none too trivial advantage.

Literature: 1. Kober A et al.: Capnography in non-tracheally intubated emergency patients as an additional tool in pulse oximetry for prehospital monitoring of respiration. Anesth Analg 98:206-210, 2004. 2. Muranyi L et al.: Continuous registration of the CO2 contents in expired air (Capnography) in the inhalative provocation of children: I Acetycholine provocation of asthmatic school-age children. Acta Paed Acad Scient Hung 10:133-154, 1969

Anesthesiology 2004; 101: A1559
Results of main variables
n140140 140
mean42.047.7< 0.0127.8< 0.01
sd4.67.9 9.6
SaO2SpO2 (ear)pSpO2 (finger)p
n140140 140
mean95.396.3< 0.0197.5< 0.01
sd2.62.7 2.1