In patients with intracranial steno-occlusive disease, blood vessels distal to the stenosis are in a state of maximum compensatory vasodilation to maintain cerebral perfusion. With a global cerebral vasodilatory stimulus such as hypotension or hypercapnia, blood vessels in the non-affected areas often vasodilate, causing intracerebral “steal” (ICS) of the blood flow away from the affected area, increasing the risk of cerebral ischemia (and subsequent stroke).1 Mapping of ICS could potentially be used as a tool to assess patients for the risk for stroke.1,2

Figure
figure 1

Graph shows changes in the partial pressure of end-tidal carbon dioxide (PETCO2) over time (top) along with the corresponding cerebrovascular reactivity (CVR) determined using blood oxygen level-dependent magnetic resonance (BOLD-MR) imaging (bottom). Arrows indicate the level of PETCO2 where the corresponding cerebrovascular reactivity (CVR) images below were obtained using blood oxygen level-dependent magnetic resonance imaging. The color coding depicts the CVR (blue areas: intracranial steal; red areas: normal reactivity). A) BOLD-MR CVR image when PETCO2 is between 35 and 42 mmHg. Intracranial steal has developed on the left side with normal reactivity on the right. B) BOLD-MR CVR image when PETCO2 is between 42 and 52 mmHg. Note the intensification of intracranial steal on the left side with increasing PETCO2. C) BOLD-MR CVR image when PETCO2 is between 40 and 52 mmHg. With the return to normocapnia, reverse steal is occurring on the right side of the brain along with improved reactivity on the left side

Full size image

Using blood oxygen level-dependent magnetic resonance (BOLD-MR) imaging as a surrogate of cerebral blood flow (CBF) and CO2 as a vasodilatory stimulus, we developed a non-invasive method to map ICS. Here, we present the cerebrovascular reactivity – ∆ CBF/∆ end-tidal CO2 (PETCO2) – map of a 29-yr-old patient with 90% stenosis of the left proximal middle cerebral artery. The BOLD MR images were obtained while PETCO2 was controlled in a ramp sequence (see Figure) from 35 to 52 mmHg using a sequential breathing circuit and a computer-controlled gas blender (RespirAct™, Thornhill Research, Inc., Toronto, ON, Canada).3 Using custom software, colour cerebrovascular reactivity maps are generated that indicate areas of normal (red) and paradoxical (i.e., steal) (blue) reactivity. The cerebrovascular reactivity values were measured as % BOLD-MR signal intensity per mmHg change in PETCO2.

As the PETCO2 increases from 35 to 42 mmHg, steal (blue areas) develops on the left side of the brain, with normal reactivity on the right side (Figure, MR image A). With further increases in PETCO2 (from 42 to 52 mmHg) more steal appears on the left side (Figure, MR image B). Finally, a return to normocapnia results in a symmetrically opposite effect, with reverse steal on the healthy right side and increased blood flow to the compromised vascular bed on the left (Figure, MR image C). Understanding ICS may be important during perioperative care of patients with intracranial stenosis as both hypercapnia and hypotension could lead to hemodynamically mediated stroke.