CEREBRAL BLOOD-FLOW REACTIVITY TO CHANGES IN CARBON-DIOXIDE CALCULATED USING END-TIDAL VERSUS ARTERIAL TENSIONS

We retrospectively examined arterial and end-tidal estimations of CO2 tension used to calculate cerebrovascular reactivity in 68 anesthetized patients. CBF was measured using the intravenous Xe-133 technique at mean +/- SD P(a)CO2 values of 28.2 +/- 5.2 and 38.8 +/- 4.8 mm Hg. The correlation between all P(a)CO2 and end-tidal P(CO2) (P(et)CO2) values was y = 0.85x - 0.49 (r = 0.93, p = 0.0001). There was a moderate correlation between age and the difference between P(a)CO2 and P(et)CO2 (y = 0.11x + 0.79; r = 0.73, p = 0.0001). Cerebrovascular reactivity to changes in CO2 (ml 100 g-1 min-1 mm Hg-1) was similar (p = 0.358) when calculated by using either P(a)CO2 (1.9 +/- 0.8) or P(et)CO2 (1.8 +/- 0.8) and highly correlated (y = 0.86x + 0.23; r = 0.91, p = 0.0001). The CBF response to changes in CO2 tension can be reliably estimated from noninvasive measurement Of P(et)CO2.