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The brain has high energy requirements combined with an inability to store substrates critical for this tissue metabolism. This precarious balance results in a vital organ that is highly dependent on constant blood flow, providing oxygen and glucose via tissue perfusion. Although the brain only comprises 2% of total body weight, it receives 15% of cardiac output (700 ml/min) at rest, and accounts for 20% of oxygen consumption, and an even greater proportion of glucose utilization. Even brief interruptions in blood flow can trigger acute cerebral dysfunction, whether loss of consciousness from global hypoperfusion (e.g. syncope from non-perfusing cardiac arrhythmias or hypotension) or focal neurological deficits relating to ischemia from thromboembolism or vasospasm.