Relationship between middle cerebral artery blood velocity and end-tidal PCO2 in the hypocapnic-hypercapnic range in humans

Abstract

This study examined the relationship between cerebral blood flow (CBF) and end-tidal Pco₂ (PetCO₂) in humans. We used transcranial Doppler ultrasound to determine middle cerebral artery peak blood velocity responses to 14 levels of PetCO₂ in a range of 22 to 50 Torr with a constant end-tidal Po₂ (100 Torr) in eight subjects. PetCO₂ and end-tidal Po₂ were controlled by using the technique of dynamic end-tidal forcing combined with controlled hyperventilation. Two protocols were conducted in which PetCO₂ was changed by 2 Torr every 2 min from hypocapnia to hypercapnia ( protocol I) and vice-versa ( protocol D). Over the range of PetCO₂ studied, the sensitivity of peak blood velocity to changes in PetCO₂ (CBF-PetCO₂ sensitivity) was nonlinear with a greater sensitivity in hypercapnia (4.7 and 4.0%/Torr, protocols I and D, respectively) compared with hypocapnia (2.5 and 2.2%/Torr). Furthermore, there was evidence of hysteresis in the CBF-PetCO₂ sensitivity; for a given PetCO₂, there was greater sensitivity during protocol I compared with protocol D. In conclusion, CBF-PetCO₂ sensitivity varies depending on the level of PetCO₂ and the protocol that is used. The mechanisms underlying these responses require further investigation.