End tidal CO2 (mmHg) and time (min) before and after upright leaning. Data are presented as means ± sem over time. Subjects were in the supine position at -1 and 0 and then stood up, leaning their upper backs against a wall without moving their legs. Both controls and CFS patients without orthostatic intolerance showed a small decline in eTCO2 over time. In contrast, the fall in those with orthostatic hypocapnia was substantial.