To the Editor:
THE review article about strokes in surgical patients1 omitted an important, although rare, cause of strokes. Patients undergoing shoulder surgery in the sitting or beach chair position risk a hypotensive/hypoperfusion ischemic stroke because the mean arterial pressure (MAP) in the brain is significantly lower than the cuff blood pressure measured at the arm (heart level). This intraoperative event is especially devastating because it occurs in relatively healthy patients who usually have no risk factors for stroke; they are simply undergoing shoulder surgery to improve their quality of life.
How does this happen? The beach chair position sits the patient at about 70 degrees. The brainstem MAP is about 20 to 40 mmHg lower (depending on the patient’s height) than the measured cuff blood pressure. The additional height to the cerebral cortex lowers brain MAP another 6 to 9 mmHg. Every inch of vertical height from the blood pressure cuff’s position on the arm to the brain reduces MAP 2 mmHg.2 This principle was well understood when anesthetizing neurosurgical patients for sitting craniotomies decades ago; appropriate adjustments were made to maintain adequate MAPs in the brain.3 This correction seems to have been forgotten or no longer taught. In 2005, a report of four cases called attention to this rare, but tragic, complication of brain death/strokes.4 In 2009, the Anesthesia Patient Safety Foundation called for more clinical and experimental research.5 We now better understand the physiologic mechanisms, etiology, prevention, and anesthetic management of this problem.6
The lower limit of autoregulation to maintain cerebral blood flow was revised upward from 50 mmHg to 70 to 80 mmHg.7 When patients are positioned upright under general anesthesia with positive pressure ventilation, blood pressure usually decreases well below the patient’s baseline or preoperative level to MAPs of 60 to 70 mmHg. If these low cuff blood pressures are not restored toward baseline levels and may even drift down further to MAPs in the 40- to 60-mmHg range, cerebral perfusion pressure will be in the 20- to 50-mmHg range.
Therefore, it is critically important, when evaluating the etiology of brain death or stroke in these patients, to account for the gravitational effect on cerebral perfusion pressure in order to include severe hypotension leading to brain damage in the differential diagnosis of the ischemic stroke. Thus, it is recommended that cuff blood pressure be maintained at or near baseline to better protect cerebral perfusion.6,8,9
As Drummond states,10 “We cannot take assurance from the notion that at any given time, ‘some’ of the brain is not ischemic. It would be slim consolation to the devastated patient or their families to know that blood flow continues to some portions of the nervous system while disabling damage was evolving in others.”
The author declares no competing interests.