Background on rCBF
- Neurologic deficits occur when relative cerebral blood flow falls below ischemic threshold (22 mL per 100 g per minute).
- The infarct threshold is 10 mL/100 g/minute. Ischemic penumbra is the target tissue for prompt reperfusion therapy because it is impaired but above the infarct threshold (10 to 22).
- Untreated penumbra quickly converts into infarction.
Golden Rule Discussion (Refutation)
- Imaging-based hypoperfusion indicates ischemia.
- Hypoperfusion of parenchyma does not necessarily indicate tissue at risk.
- This is a reflection of lower relative cerebral blood flow and can be due to any number of causes.
- In essence, hypoperfused tissue lies in the spectrum of ischemia, penumbra and infarct.
- Mean transit time abnormality indicates tissue at risk.
- The spectrum of terms of ischemia, penumbra, oligemia and infarct should not be used interchangeably because oligemia tissue is not necessarily at risk for infarct (chronically hypoperfused tissue).
- Specifically, chronic proximal arterial occlusion is commonly seen and could result in a decreased mean transit time without risking tissue.
- In fact, chronically hypoperfused “starving” parenchyma may tolerate lower cerebral blood flow more than normal parenchyma.
- Diffusion weighted imaging of normality indicates infarction.
- This is not necessarily true because there can be spontaneous resolution of hypoperfusion. There are cases of oligemia that recover without treatment.
- Perfusion weighted imaging/diffusion weighted imaging mismatch indicates ischemic penumbra.
- Mean transit time-diffusion weighted imaging mismatch includes only ischemia, penumbra and infarction core.
- There is a fixed therapeutic window.
- The concept of therapeutic window has a lot of variance.
- The window for treatment should be individualized to the patient and depends on relative cerebral blood flow.
- Contralateral rCBF should be considered in evaluation.
- “Time is brain”.
- The article specifically states that inappropriate revascularization of the infarct core can convert the golden rule “time is brain“ into “time is blood”.