Papers by Christina Mackay
Experimental Physiology, 2015
What is the central question of this study? We investigated the effects of superimposed tilt and ... more What is the central question of this study? We investigated the effects of superimposed tilt and hypercapnia-induced cerebral arteriolar dilatation on anterior and posterior cerebrovascular CO2 reactivity using hyperoxic rebreathing in human participants. What is the main finding and its importance? The main findings are threefold: (i) cerebrovascular CO2 reactivity in the anterior and posterior cerebrovasculature is unchanged with tilt; (ii) cerebral autoregulation is unlikely responsible due to unchanging cerebrovascular resistance reactivity between positions; and (iii) cerebral blood flow is not pressure passive during tilt as it is with pharmacological or lower body negative pressure-induced changes in mean arterial pressure, suggesting that sympathetic activation or balanced transmural pressures during head-down tilt regulate cerebral blood flow. Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, the extent that the cerebrovasculature can autoregulate in the face of superimposed steady-state orthostasis-induced changes in MAP (e.g. head-up and head-down tilt; HUT and HDT) and CO2 -mediated arteriolar dilatation is unknown. We tested the effects of steady-state tilt on cerebrovascular CO2 reactivity in the middle and and posterior cerebral artery in the following five body positions: 90 deg HUT, 45 deg HUT, supine, 45 deg HDT and 90 deg HDT on a tilt table during a modified hyperoxic rebreathing test. Absolute and relative cerebrovascular CO2 reactivity [cerebral blood velocity (CBV)/CO2 ], cerebrovascular resistance (CVR) reactivity (CVR/CO2 ) and MAP reactivity (MAP/CO2 ) were quantified using linear regression. Mean arterial pressure was significantly elevated in 90 deg HDT compared with other positions during baseline steady-state tilt (P < 0.01). Absolute CBV/CO2 and CVR/CO2 were greater in the middle cerebral artery than the posterior cerebral artery (P < 0.01) in all body positions, but relative measures were not different (P = 0.143 and P = 0.360, respectively), nor was there any interaction with tilt position. In addition, there was no difference in absolute (P = 0.556) and relative MAP/CO2 (P = 0.308) between positions. Our data demonstrate that cerebral blood flow remains well regulated during superimposed steady-state orthostatic stress and dynamic changes in the partial pressure of end-tidal CO2 during rebreathing. Cerebral autoregulation is likely not the mechanism responsible, but rather sympathetic nervous system activation or a balanced cerebrovascular transmural pressure with HDT maintains resting cerebral blood flow and cerebrovascular CO2 reactivity during rebreathing.
Progress in brain research, 2014
Central respiratory chemosensitivity is mediated via chemoreceptor neurons located throughout bra... more Central respiratory chemosensitivity is mediated via chemoreceptor neurons located throughout brain stem tissue. These receptors detect proximal CO2/[H(+)] (i.e., controller gain) and modulate breathing in a classic negative feedback loop. Loop gain (responsiveness) is the theoretical product of controller (chemoreceptors), mixing/feedback (cardiovascular and cerebrovascular systems), and plant (pulmonary system) gains. The level of chemoreceptor stimulation is determined by interactions between mixing and plant gains. The extent to which steady-state changes in body position may affect central chemoreflex loop gain in response to CO2 is unclear. Because of the potential effects of tilt on pulmonary mechanics, we hypothesized that plant gain would be altered by head-up and head-down tilt (HUT, HDT) during hyperoxic rebreathing, which theoretically isolates plant gain by eliminating systemic arterial-tissue gradients. Sixteen subjects (eight females) underwent hyperoxic rebreathing t...
The potential differences in cerebrovascular responses between the anterior and posterior circula... more The potential differences in cerebrovascular responses between the anterior and posterior circulations to changes in CO₂ are unclear in humans. Using transcranial Doppler ultrasound, we compared the CO₂ reactivity of the (1) BA and PCA and (2) MCA and PCA during hyperoxic rebreathing in supine position. The reactivity in the BA and PCA was similar in both absolute (1.27 ± 0.5 and 1.27 ± 0.6 cm/s/Torr; P=0.992) and relative (3.98 ± 1.3 and 3.66 ± 1.5%/Torr CO2; P=0.581) measures, suggesting that the PCA is an adequate surrogate measure of reactivity for the BA. The MCA reactivity was greater than the PCA in absolute (2.09 ± 0.7 and 1.22 ± 0.5 cm/s/Torr CO₂; P<0.001), but not relative measures (3.25 ± 1.0 and 3.56 ± 1.6%/Torr CO₂; P=0.629). Our findings (a) confirm regional differences in the absolute reactivity in the human brain and (b) suggest that in cerebrovascular studies investigating functions mediated by posterior brain structures (e.g., control of breathing), the posterior vasculature should also be insonated.
Uploads
Papers by Christina Mackay