Plot of 2 hypothetical hemoglobin dissociation curves derived from the Hill equation and p50 values of 29.6 (solid line) and 28.6 (dotted line). The horizontal and vertical axes have been interrupted to highlight the small differences in oxygen saturation observed under both arterial and cerebral capillary pO₂ values. OEF can be calculated as the difference in oxygen saturation under arterial oxygen conditions (pO₂ 95 mm Hg) and at the cerebrovascular capillary level (pO₂ 33 mm Hg). The left-shifted hemoglobin dissociation curve (dashed line) yields a lower OEF (41.4%) than the unshifted curve (OEF 43.3%), despite identical tissue pO₂ levels. Thus, OEF, in isolation cannot be considered a marker of cerebrovascular risk.