From the abstract: The author has witnessed impairment of the velocity of arterial blood flow of the occipital artery, unilaterally or bilaterally, as a direct result of a cervical subluxation in over 15,000 patients. This impairment is documented by Doppler examination and is the virtual 100 percent concomitant of a cervical subluxation. Clinical correction of the subluxation, especially at the C-1,C-2 level, restores the blood velocity through the occipital artery, virtually 100 percent of the time. Failure to correct the subluxation commonly fails to relieve the impaired blood velocity in this artery. It is thus an effective monitor of the efficacy of the administered adjustment.
Risley, WB. Journal American Chiropractic Assoc June 1995. pp. 61-63.