Abstract
There is a need for improved understanding of how different cerebrovascular reactivity (CVR) protocols affect vascular cross-sectional area (CSA) to reduce error in CVR calculations when measures of vascular CSA are not feasible. In human participants, we delivered ∼±4 mm Hg end-tidal partial pressure of CO2 (PETCO2) relative to baseline through controlled delivery, and measured changes in middle cerebral artery (MCA) CSA (7 Tesla magnetic resonance imaging (MRI)), blood velocity (transcranial Doppler and Phase contrast MRI), and calculated CVR based on a 3-minute steady-state (+4 mm Hg PETCO2) and a ramp (-3 to +4 mm Hg of PETCO2). We observed that (1) the MCA did not dilate during the ramp protocol (slope for CSA across time P > 0.05; R2 = 0.006), but did dilate by ∼7% during steady-state hypercapnia (P < 0.05); and (2) MCA blood velocity CVR was not different between ramp and steady-state hypercapnia protocols (ramp: 3.8 ± 1.7 vs. steady-state: 4.0 ± 1.6 cm/s/mm Hg), although calculated MCA blood flow CVR was ∼40% greater during steady-state hypercapnia than during ramp (P < 0.05) with the discrepancy due to MCA CSA changes during steady-state hypercapnia. We propose that a ramp model, across a delta of -3 to +4 mm Hg PETCO2, may provide an alternative approach to collecting CVR measures in young adults with transcranial Doppler when CSA measures are not feasible. Novelty: We optimized a magnetic resonance imaging sequence to measure dynamic middle cerebral artery (MCA) cross-sectional area (CSA). A ramp model of hypercapnia elicited similar MCA blood velocity reactivity as the steady-state model while maintaining MCA CSA.
Highlights
Cerebrovascular reactivity (CVR) studies assess changes in cerebral blood flow to a known vasoreactive stimulus
The noteworthy findings of this study are that: (1) the middle cerebral artery (MCA) cross-sectional area (CSA) did not change during the ramp protocol with delta $À3 to +4 mm Hg of partielle de fin d’expiration de CO2 (PETCO2), but did increase with steady-state hypercapnia $+4 mm Hg of PETCO2), (2) blood pressure remained stable throughout duration of both ramp and steady-state protocols, and (3) CVR measures based on MCA blood velocity cerebrovascular reactivity was not different between ramp and steady-state protocols, but (4) MCA blood flow-based CVR was greater during steady-state compared with the ramp protocol
The ramp protocol seems to result in similar CVR values as those observed in the steady-state protocol, with the added advantage of having minimal and negligible effects on MCA CSA or mean arterial pressure (MAP) in the face of moderate elevations in PETCO2
Summary
Cerebrovascular reactivity (CVR) studies assess changes in cerebral blood flow to a known vasoreactive stimulus (e.g., changes in end-tidal partial pressure of CO2; PETCO2). An additional concern regarding quantification of CVR is the potential for changes in central hemodynamics during hypercapnia (elevated PETCO2) that could elevate cerebral blood flow due to changes in cardiac output (Shoemaker et al 2002) and blood pressure (Regan et al 2014; Shoemaker et al 2002) and not directly due to cerebrovascular dilation (Regan et al 2014) Another complicating factor in quantifying CVR between groups is potential variation in large cerebral artery reactivity, when comparing age differences (Stefanidis et al 2019). The “ideal” velocity based CVR protocols conducted using TCD would require: (1) minimal CSA changes by conducting CVR protocols that result in negligible change in CSA, and (2) minimal influence of confounding variables such as blood pressure
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