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

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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.