• Andrea Casati, Grazia Squicciarini, Giulia Malagutti, Marco Baciarello, Marta Putzu, and Andrea Fanelli.
• Department of Anesthesia and Pain Therapy, University of Parma, and Ospedale Maggiore di Parma, via Gramsci 14, 43100 Parma, Italy. acasati@ao.pr.it
• J Clin Anesth. 2006 Sep 1;18(6):436-40.

Study ObjectiveTo evaluate the accuracy and precision of estimation of partial pressure of carbon dioxide (Pa(CO2)) using end-tidal or transcutaneous CO2 (TcP(CO2)) measurements during mechanical ventilation in the elderly patient.DesignA prospective, observational study was conducted.SettingsThe study was done in the anesthesia department of a university hospital.PatientsSeventeen anesthetized, mechanically ventilated patients older than 60 years were studied.Interventions And MeasurementsDuring standard sevoflurane anesthesia, and after proper calibration and an equilibration time of 30 minutes with stable hemodynamic and respiratory variables, arterial (Pa(CO2)), end-tidal (Pet(CO2)), and transcutaneous (TcP(CO2)) CO2 partial pressures were determined. In each patient, 1 to 5 sample sets (Pa(CO2), Pet(CO2), and TcP(CO2)) were obtained.Main ResultsA total of 45 sample sets were obtained from the patients studied. The Pa(CO2) values ranged between 21 and 58 mm Hg. The Pa(CO2) - Pet(CO2) tension gradient was 6 +/- 5 mmHg (95% confidence interval, -3 to 16 mmHg), whereas the Pa(CO2) - TcP(CO2) tension gradient was 2 +/- 4 mmHg (95% confidence interval, -6 to 9 mmHg) (P = 0.0005). The absolute value of the difference between Pa(CO2) and Pet(CO2) was 3 mm Hg or less in 7 of 45 sample sets (15%), whereas the absolute value of the difference between Pa(CO2) and TcP(CO2) was 3 mm Hg or less in 21 of 45 sample sets (46%) (P = 0.003). Linear regression analysis for TcP(CO2) versus Pa(CO2) showed a slope of 0.84 (r(2) = 0.73), whereas the linear regression analysis for Pet(CO2) versus Pa(CO2) showed a slope of 0.54 (r(2) = 0.50).ConclusionTranscutaneous monitoring of CO(2) partial pressure gives a more accurate estimation of arterial CO(2) partial pressure than does Pet(CO2) monitoring.

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