[The arterial-end tidal CO2 partial pressure difference during anesthesia]

Arterial CO2 partial pressure (PaCO2) can be continuously and noninvasively estimated by monitoring end-tidal CO2 partial pressure (PetCO2). However, the difference between the two (P(a-et)CO2) may vary considerably between patients. In 26 patients undergoing general anesthesia for various surgical procedures, P(a-et)CO2 was measured. We tested the hypothesis that certain clinical characteristics are associated with higher P(a-et)CO2 values in a given patient. In addition, we tested the hypothesis that P(a-et)CO2 remains constant during anesthesia by comparing the mean of eight P(a-et)CO2 values, which were measured at 15-min intervals, with the initial (P(a-et)CO2 value. Impaired lung function, defined as a lung score above 6, age above 50 years, and overweight (defined as greater than 120% of ideal body weight) were associated with increased P(a-et)CO2 (mean +/- SD: 6.27 +/- 1.85; 5.27 +/- 2.11; and 6.19 +/- 1.95 mmHg respectively) when compared to normal lung function (lung score less than or equal to 6), age below 50 years, and normal weight (less than 120% of ideal body weight, 3.87 +/- 2.31; 3.55 +/- 2.58; and 3.77 +/- 2.25 respectively). Although these differences are statistically significant, the standard deviations are large and do not allow the prediction of P(a-et)CO2 in a given patient. Initial P(a-et)CO2 correlated well with subsequent P(a-et)CO2 (R = 0.91, P less than 0.001). The mean difference of the subsequent P(a-et)CO2 values from the initial values was 0.05 +/- 0.80 mmHg (mean +/- SD) and was not significantly different from zero. In the patient population studied, P(a-et)CO2 thus remained generally constant during anesthesia.