The Robin Hood phenomenon describes what happens when PaCO2 is decreased?

The Robin Hood phenomenon describes what happens when PaCO2 is decreased?

• A. Dilation of vessels around ischemic tissue, increasing blood flow to ischemic areas
• B. Constriction of vessels around non-ischemic tissue, diverting blood flow to ischemic areas
• C. Vasoconstriction of all cerebral vessels equally
• D. No effect on cerebral blood flow

Answer: (B)

The key idea is how CO2 levels affect cerebral blood vessels. Lowered PaCO2 (hypocapnia) causes cerebral vessels in normal, non-ischemic brain tissue to constrict. In ischemic areas, however, the vessels are already maximally dilated to compensate for reduced blood flow and cannot constrict further. So as normal tissue constricts, the blood that would have gone there is redirected toward the already-dilated ischemic zones. This redistribution, where blood flow shifts from non-ischemic to ischemic tissue during hypocapnia, is the Robin Hood phenomenon.

The key idea is how CO2 levels affect cerebral blood vessels. Lowered PaCO2 (hypocapnia) causes cerebral vessels in normal, non-ischemic brain tissue to constrict. In ischemic areas, however, the vessels are already maximally dilated to compensate for reduced blood flow and cannot constrict further. So as normal tissue constricts, the blood that would have gone there is redirected toward the already-dilated ischemic zones. This redistribution, where blood flow shifts from non-ischemic to ischemic tissue during hypocapnia, is the Robin Hood phenomenon.