To 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.A prospective, observational study was conducted.The study was done in the anesthesia department of a university hospital.Seventeen anesthetized, mechanically ventilated patients older than 60 years were studied.During 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.A 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).Transcutaneous monitoring of CO(2) partial pressure gives a more accurate estimation of arterial CO(2) partial pressure than does Pet(CO2) monitoring
