Cerebral blood flow sensitivity to CO2 measured with steady-state and Read's rebreathing methods

The ventilatory response to carbon dioxide (CO2) measured by the steady-state method is lower than that measured by Read's rebreathing method. A change in end-tidal P-CO2 (PETCO2) results in a lower increment change in brain tissue P-CO2 (Pt-CO2) in the steady-state than with rebreathing: since Pt-CO2 determines the ventilatory response to CO2, the response is lower in the steady-state. If cerebral blood flow (CBF) responds to Pt-CO2, the CBF-CO2 response should be lower in the steady-state than with rebreathing. Six subjects undertook two protocols, (a) steady-state: PETCO2 was held at 1.5 mmHg above normal (isocapnia) for 10 min, then raised to three levels of hypercapnia, (8 min each; 6.5, 11.5 and 16.5 mmHg above normal, separated by 4 min isocapnia). End-tidal Pot was held at 300 mmHg; (b) rebreathing: subjects rebreathed via a 6 L bag filled with 6.5% CO2 in O-2. Transcranial Doppler-derived CBF yielded a higher CBF-CO2 sensitivity in the steady-state than with rebreathing, suggesting that CBF does not respond to Pt-CO2. (C) 2003 Elsevier Science B.V. All rights reserved.