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New Findings r 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 CO 2 reactivity using hyperoxic rebreathing in human participants. r What is the main finding and its importance?The main findings are threefold: (i) cerebrovascular CO 2 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 CO 2 -mediated arteriolar dilatation is unknown. We tested the effects of steady-state tilt on cerebrovascular CO 2 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 CO 2 reactivity [cerebral blood velocity (CBV)/CO 2 ], cerebrovascular resistance (CVR) reactivity (CVR/CO 2 ) and MAP reactivity (MAP/CO 2 ) 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/CO 2 and CVR/CO 2 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/CO 2 (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 CO 2 during rebreathing. Cerebral autoregulation is likely not the mechanism responsible, but rather sympathetic nervous system

One of the most effective ways of engaging students of physiology and medicine is through laboratory demonstrations and case studies that combine 1) the use of equipment, 2) problem solving, 3) visual representations, and 4) manipulation and interpretation of data. Depending on the measurements made and the type of test, laboratory demonstrations have the added benefit of being able to show multiple organ system integration. Many research techniques can also serve as effective demonstrations of integrative human physiology. The "Duffin" hyperoxic rebreathing test is often used in research settings as a test of central respiratory chemosensitivity and cerebrovascular reactivity to CO2. We aimed to demonstrate the utility of the hyperoxic rebreathing test for both respiratory and cerebrovascular responses to increases in CO2 and illustrate the integration of the respiratory and cerebrovascular systems. In the present article, methods such as spirometry, respiratory gas analysis, and transcranial Doppler ultrasound are described, and raw data traces can be adopted for discussion in a tutorial setting. If educators have these instruments available, instructions on how to carry out the test are provided so students can collect their own data. In either case, data analysis and quantification are discussed, including principles of linear regression, calculation of slope, the coefficient of determination (R(2)), and differences between plotting absolute versus normalized data. Using the hyperoxic rebreathing test as a demonstration of the complex interaction and integration between the respiratory and cerebrovascular systems provides senior undergraduate, graduate, and medical students with an advanced understanding of the integrative nature of human physiology.