What finding at a depth of 50 mm indicates significant vasospasm of the middle cerebral artery?

A local increase in mean velocity at a depth of 50 mm indicates significant vasospasm of the middle cerebral artery because it reflects the body's response to reduced blood flow due to spasm of the vessel. When a vessel constricts, such as in vasospasm, the narrowing leads to an increase in blood flow velocity at that point, which can be detected during Doppler ultrasonography.

In the context of the middle cerebral artery, an elevation in mean velocity suggests that the blood flow is being funneled through a narrower passage, characteristic of vasospasm. Clinicians monitor these changes in flow dynamics because they can correlate with vascular events such as strokes or transient ischemic attacks.

Other options, such as an increase in diastolic flow, may not necessarily correlate with vasospasm and can even indicate a different hemodynamic condition or response. A bidirectional flow pattern typically suggests turbulence or complex flow dynamics that are not specific to vasospasm. The absence of flow signal could indicate a complete blockage or severe narrowing, but such findings may occur in more extreme conditions than what would usually be identified as significant vasospasm alone. Therefore, a local increase in mean velocity is the most direct indicator of vasospasm in this