Acute Gravitational Stress Selectively Impairs Dynamic Cerebrovascular Reactivity in the Anterior Circulation Independent of Changes to the Central Respiratory Chemoreflex

Figures

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  Figure 1. Single-exponential regression model, t0; the response time latency from induction of carbon dioxide administration to change [CO2-response delay], y0, baseline value; and G, gain term, and τ; time constant of the fitted curve of exponential regression during CO2 administration. Time 0 (t0 = 0) refers to the start of CO2 administration. The τ is the time (unit: second) from t0 to reach 63.2% of the steady-state value.

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  Figure 2. Panel A: Continuous recording of predicted partial pressure of arterial CO2 (PaCO2) responses to CO2 administration (5% CO2) during supine (gray line) and 50° head-up tilt (HUT; black line) in one representative participant. The dash-dotted and smooth curve represent the exponential lines at supine and 50° HUT, respectively. Panel B: Group-averaged gain (G) of predicted PaCO2 exponential fitting curves during supine and 50° HUT. Panel C: Grouped sum of CO2-response delay (t0) and time constant (τ) of predicted PaCO2 exponential fitting curves during supine and 50° HUT. The predicted PaCO2 was derived from PETCO2 and Vt using the following equation (Jones et al., 1979); Predicted PaCO2 = 5.5 + 0.9*PETCO2–0.0021*Vt. Grouped data are shown as median and interquartile range with individual data points.

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  Figure 3. Dynamic cerebrovascular carbon dioxide (CO2) reactivity and central respiratory chemoreflex were characterized using a single-exponential regression model. Panels A–C: Continuous recordings of middle and posterior cerebral artery mean blood velocities (MCA Vmean, A and PCA Vmean, B) and pulmonary ventilation (V̇E, C) response to CO2 administration (5% CO2) during hypercapnia during supine (gray line) and 50° head-up tilt (HUT; black line) in one representative participant. The dash-dotted and smooth curve represent the exponential lines at supine and 50° HUT, respectively. Panels D–F: Grouped gain (G, D), CO2-response delay (t0, E), and time constant (τ, F) of MCA Vmean, PCA Vmean, and V̇E exponential fitting curves during supine and 50° HUT. Grouped data are shown as median and interquartile range with individual data points.