Cerebral vasomotor reactivity: steady‐state versus transient changes in carbon dioxide tension

Abstract

New FindingsWhat is the central question of this study?The relationship between changes in cerebral blood flow and arterial carbon dioxide tension is used to assess cerebrovascular function. Hypercapnia is generally evoked by two methods, i.e. steady-state and transient increases in carbon dioxide tension. In some cases, the hypercapnia is immediately preceded by a period of hypocapnia. It is unknown whether the cerebrovascular response differs between these methods and whether a period of hypocapnia blunts the subsequent response to hypercapnia.What is the main finding and its importance?The cerebrovascular response is similar between steady-state and transient hypercapnia. However, hyperventilation-induced hypocapnia attenuates the cerebral vasodilatory responses during a subsequent period of rebreathing-induced hypercapnia.Cerebral vasomotor reactivity (CVMR) to changes in arterial carbon dioxide tension () is assessed during steady-state or transient changes in . This study tested the following two hypotheses: (i) that CVMR during steady-state changes differs from that during transient changes in ; and (ii) that CVMR during rebreathing-induced hypercapnia would be blunted when preceded by a period of hyperventilation. For each hypothesis, end-tidal carbon dioxide tension () middle cerebral artery blood velocity (CBFV), cerebrovascular conductance index (CVCI; CBFV/mean arterial pressure) and CVMR (slope of the linear regression between changes in CBFV and CVCI versus ) were assessed in eight individuals. To address the first hypothesis, measurements were made during the following two conditions (randomized): (i) steady-state increases in of 5 and 10 Torr above baseline; and (ii) rebreathing-induced transient breath-by-breath increases in . The linear regression for CBFV versus (P = 0.65) and CVCI versus (P = 0.44) was similar between methods; however, individual variability in CBFV or CVCI responses existed among subjects. To address the second hypothesis, the same measurements were made during the following two conditions (randomized): (i) immediately following a brief period of hypocapnia induced by hyperventilation for 1 min followed by rebreathing; and (ii) during rebreathing only. The slope of the linear regression for CBFV versus (P < 0.01) and CVCI versus (P < 0.01) was reduced during hyperventilation plus rebreathing relative to rebreathing only. These results indicate that cerebral vasomotor reactivity to changes in is similar regardless of the employed methodology to induce changes in and that hyperventilation-induced hypocapnia attenuates the cerebral vasodilatory responses during a subsequent period of rebreathing-induced hypercapnia.

Blunted cerebral vascular responsiveness to hypercapnia in obese individuals

“…However, the present study included only rebreathing‐induced hypercapnia because recent research (Brothers et al . ) reported that hyperventilation‐induced hypocapnia attenuates the cerebral vasodilatory responses during a subsequent period of rebreathing‐induced hypercapnia. In order to assess the cerebral vasodilatory response without confounding factors caused by a preceding hypocapnic stage, hypocapnia‐induced cerebral vasoconstriction was not assessed in the present investigation.…”

“…Subjects then performed the rebreathing protocol as previously described (Brothers et al . ; Hurr et al . ,b).…”

“…Subjects breathed room air for 6 min of baseline data collection while CBV, MAP, HR and P ET,CO 2 were continuously collected. Subjects then performed the rebreathing protocol as previously described (Brothers et al 2014;Hurr et al 2015a,b). Briefly, they first performed a deep inspiration, upon which the Y-valve was switched so that they inspired into and expired out of the 5 litre bag.…”

Fast-food meal reduces peripheral artery endothelial function but not cerebral vascular hypercapnic reactivity in healthy young men

“…Medical grade oxygen was bled into the bag in order to maintain constant arterial normoxia (S p O 2 = ~97%) during the rebreathing period (Claassen et al. ; Brothers ; Hurr et al. ).…”

Dynamic cerebral autoregulation is preserved during isometric handgrip and head-down tilt in healthy volunteers

“…; Brothers et al. ), and variations in end‐tidal CO 2 (ETCO 2 ) significantly contribute to very low frequency (<0.04 Hz) CBF variability (Mitsis et al. ; Peng et al.…”