Cerebral Blood Flow Reactivity to Changes in Carbon Dioxide Calculated Using End-Tidal versus Arterial Tensions

Abstract

We retrospectively examined arterial and endtidal estimations of CO₂ tension used to calculate cerebrovascular reactivity in 68 anesthetized patients. CBF was measured using the intravenous ¹³³Xe technique at mean ± SD P_aco₂ values of 28.2 ± 5.2 and 38.8 ± 4.8 mm Hg. The correlation between all P_aco₂ and end-tidal Pco₂ (P_etco₂) 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_aco₂ and P_etco₂ (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_aco₂ (1.9 ± 0.8) or P_etco₂ (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_etco₂.