"Perfusion" a very rare word, peak use 1984, almost unseen anywhere before 1940. "Menation" a rare word, peak use also 1984, still, an old word.
Both words are off-putting in context, texts related to first aid should strive for clarity in the vernacular of the day. Pefusion means blood flow, mentation means reasoning.

As to the "three stages of shock": wow, also confusing as worded, except for the last. Irreversible meaning irreversible organ damage due to low blood flow is happening. The two other stages are low blood flow being compensated for by the bod (increased heart rate, tightened blood vessels, kidneys retaining fluid) and the middle level being lowered blood flow not being adequated compensated by, and at this stage the brain gets foggy, passing out can occur.