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J Appl Physiol 102: 870-877, 2007. First published November 16, 2006; doi:10.1152/japplphysiol.00906.2006
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Transcranial Doppler estimation of cerebral blood flow and cerebrovascular conductance during modified rebreathing

¹Department of Geriatric Medicine, Radboud University Nijmegen Medical Center, The Netherlands; and ²Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas and The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas

Submitted 16 August 2006 ; accepted in final form 12 November 2006

Clinical transcranial Doppler assessment of cerebral vasomotor reactivity (CVMR) uses linear regression of cerebral blood flow velocity (CBFV) vs. end-tidal CO₂ (PET_CO2) under steady-state conditions. However, the cerebral blood flow (CBF)-PET_CO2 relationship is nonlinear, even for moderate changes in CO₂. Moreover, CBF is increased by increases in arterial blood pressure (ABP) during hypercapnia. We used a modified rebreathing protocol to estimate CVMR during transient breath-by-breath changes in CBFV and PET_CO2. Ten healthy subjects (6 men) performed 15 s of hyperventilation followed by 5 min of rebreathing, with supplemental O₂ to maintain arterial oxygen saturation constant. To minimize effects of changes in ABP on CVMR estimation, cerebrovascular conductance index (CVCi) was calculated. CBFV-PET_CO2 and CVCi-PET_CO2 relationships were quantified by both linear and nonlinear logistic regression. In three subjects, muscle sympathetic nerve activity was recorded. From hyperventilation to rebreathing, robust changes occurred in PET_CO2 (20–61 Torr), CBFV (–44 to +104% of baseline), CVCi (–39 to +64%), and ABP (–19 to +23%) (all P < 0.01). Muscle sympathetic nerve activity increased by 446% during hypercapnia. The linear regression slope of CVCi vs. PET_CO2 was less steep than that of CBFV (3 vs. 5%/Torr; P = 0.01). Logistic regression of CBF-PET_CO2 (r² = 0.97) and CVCi-PET_CO2 (r² = 0.93) was superior to linear regression (r² = 0.91, r² = 0.85; P = 0.01). CVMR was maximal (6–8%/Torr) for PET_CO2 of 40–50 Torr. In conclusion, CBFV and CVCi responses to transient changes in PET_CO2 can be described by a nonlinear logistic function, indicating that CVMR estimation varies within the range from hypocapnia to hypercapnia. Furthermore, quantification of the CVCi-PET_CO2 relationship may minimize the effects of changes in ABP on the estimation of CVMR. The method developed provides insight into CVMR under transient breath-by-breath changes in CO₂.

blood pressure; carbon dioxide

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