Hypertension, or high blood pressure, is a well known risk factor for stroke. Reducing blood pressure with blood pressure medications, such as an angiotensin receptor blocker (ARB), has been shown to reduce the risk of future stroke.

So, reducing blood pressure is a good thing for strokes, right? Apparently many physicians think so, despite solid evidence to the contrary.

If you are unlucky enough to actually have a stroke, reducing blood pressure with medications can be harmful. Although elevated blood pressure is a tip-off to the physician that you are having a stroke, reducing the blood pressure to “normal” often does more harm than good. This idea has been prominent since 1985, when a JAMA article was published entitled Hypertension in Acute Ischemic Stroke: Not to treat. High dose nimodipine resulted in more deaths compared to placebo in a radomized trial of acute ischemic stroke. A more recent trial involving ischemic stroke patients randomized to an ARB (candesartan) or placebo, showed no benefit to the ARB, and a trend towards worsened outcomes from blood pressure lowering. Another study compared whether to stop or continue previously prescribed antihypertensive medications during an acute ischemic stroke. There was no benefit from blood pressure lowering. Current guidelines do not recommend reducing the blood pressure of ischemic stroke victims in the first 24 hours unless the blood pressure is above 220/120.

Therefore the take home points for acute ischemic strokes are: 1) do not try to normalize blood pressure. 2) do not continue the already prescribed blood pressure medication.

Efforts to correct blood pressure during ischemic stroke (the most common kind) worsen brain damage from lack of blood flow, also called ischemia. Rose and Mayer propose that blood pressure normalization with medications exacerbate ischemia in part because of impaired autoregulation of the arteries in the brain areas around the stroke. Autoregulation is the adjustment of vasoconstriction or vasodilation of brain arteries that usually optimizes flow and oxygen delivery to the brain.

What if, instead, the changes in blood pressure that occur during many strokes, in fact, help the injured brain tissues surrounding an area of stroke (the penumbra) receive sufficient blood and oxygen?

It is possible that higher than normal blood pressure during a stroke might be the brain’s way of “making the best of a bad job.” If so, that adjustment in blood pressure might be an adaptation that preserves blood flow to damaged brain after infarction (stroke because of a blood clot) or injury. This “new normal” blood pressure finding during stroke could be an adaptation that evolved because of natural selection.

The idea that physicians should increase the blood pressure during stroke with drugs has also been considered and tested here, and here. These small pilot studies showed an improvement in function with increased blood pressure in some patients after receiving phenylephrine. Insufficient evidence exists at this time to advocate for the routine use of this therapy.

This does not mean that patients at high risk for stroke should stop taking their blood pressure medications. Quite the opposite. We are only talking about what physicians should do when a patient is in the earliest stages of having a stroke. We will highlight several other possible “new normals”, adjustments of physiology that allow the body to cope with some new challenge, over the next few weeks.

Late addition: It is common after head injury to try to maintain the intracranial pressure to 20 mm mercury or less,  in the thinking that elevated ICP prevents adequate blood flow to injured parts of the brain. As in ischemic strokes, it  has been proposed that autoregulation of blood flow is impaired in head injured patients. Contrary to dogma, however, a recent randomized controlled trial showed no benefit in those patients undergoing intensive monitoring and treatment to maintain ICP of 20 mm Hg. It also remains unknown whether 20mm Hg is normal or ideal after head injury. In the absence of a mass lesion, it is possible that some increased ICP may not be as harmful as once supposed.

JA

Categories: Adaptation

Joe Alcock

Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine