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 (PET CO2) 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: PET CO2 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 P O2 was held at 300 mmHg; (b) rebreathing: subjects rebreathed via a 6 L bag filled with 6.5% CO2 in O2. 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.
Pandit
Jaideep J
JJ
Nuffield Department of Anaesthetics, John Radcliffe Hospital, Oxford OX3 9DU, UK.
Mohan
Ravi M
RM
Paterson
Nicole D
ND
Poulin
Marc J
MJ
D001944:Breath Tests / Q000379:methods; D002245:Carbon Dioxide / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000187:drug effects* / Q000502:physiology; D006801:Humans; D006935:Hypercapnia; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D012119:Respiration; D012125:Respiratory Center / Q000737:chemistry / Q000502:physiology*; D013990:Tidal Volume; D017585:Ultrasonography, Doppler, Transcranial
D002245:Carbon Dioxide; D010100:Oxygen