Background

‘Time is brain’ is a slogan that has been used by stroke advocacy groups to raise public awareness that stroke is a medical emergency which requires timely treatment.1 Similarly, the acronym FAST (face, arm, speech, time) has been advertised as an easy way to remember the most common stroke symptoms and the need to act quickly.2

The time dependent nature of brain injury following an ischemic event has been characterized in animal experiments. Early studies have shown that brain tissue gradually undergoes irreversible damage following an acute ischemic stroke. These studies suggest that infarct growth can be described by a logarithmic or a sigmoidal function. The rate at which tissue transitions from reversible to irreversible injury is dependent on the depth of the ischemia; with mild reductions in cerebral blood flow, brain tissue remains viable for a relatively long duration, whereas cerebral injury becomes irreversible rapidly when cerebral blood flow is severely impaired (figure 1).3 ,4 Another factor that determines the rate at which tissue undergoes irreversible ischemic injury is the type of tissue that is involved. For example, gray matter, which has a higher metabolic demand, is more susceptible to ischemia than white matter.5 The average rate of neuronal loss in humans is estimated to be 1.9 million neurons per minute during the first 10 h after stroke onset.1 These data indicate that stroke treatments aimed at restoring blood flow are most effective when they are initiated early.