What Causes Cognitive Impairment After Coronary Bypass Surgery?

Abstract & Commentary

By Matthew E. Fink, MD, Vice Chairman, Professor of Clinical Neurology, Weill Medical College, Chief of Division of Stroke and Critical Care Neurology, NewYork-Presbyterian Hospital. Mr. Fink reports no consultant, stockholder, speaker's bureau, research, or other relationships related to this field of study.

Synopsis: A large difference between preoperative blood pressure and intraoperative perfusion pressure increases the risk of postoperative cognitive impairment and stroke.

Source: Gottesman RF, Hillis AE, Graga MA, et. al. Early postoperative cognitive dysfunction and blood pressure during coronary artery bypass graft operation. Arch Neurol 2007; 64: 1111-1114.

Neurological impairment is the most feared complication of cardiac surgery, and great efforts have been made to reduce this complication. Stroke is reported in < 1% of low risk patients, but risk increases with increasing age, prior stroke or TIA, presence of carotid or vertebral artery stenosis, or the presence of hypertension and diabetes. Early cognitive impairment has been reported to occur in as many as 60% of patients, but the vast majority recover without long-term sequelae.

What causes these complications and how can they be prevented? The study by Gottesman and colleagues, from Johns Hopkins Hospital, sheds light on a possible mechanism for brain ischemia during heart surgery - a differential between preoperative blood pressure and cardiopulmonary pump pressure during surgery.

Gottesman et al. prospectively studied 15 patients who had on-pump coronary artery bypass surgery and measured Mini-Mental State Examination (MMSE), Trails A and B, and the Rankin score, before and after surgery, and correlated changes in these measures with changes from preoperative mean blood pressure to mean cardiopulmonary bypass pump perfusion pressure. They found a significant correlation between change in MMSE and the magnitude of BP change in the early postoperative period. A drop in mean arterial pressure predicted a decline in the MMSE, and the greater the preoperative mean arterial blood pressure, the greater the decline in MMSE. At one month postoperatively, most patients had recovered, but the few who had BP drops of >27 mm Hg had persistent declines in MMSE scores. None of the other measures had significant differences.

In addition, 13 patients had postoperative brain MRI studies, and 6 were found to have at least one DWI positive lesion. There was a trend that suggested a higher risk of brain lesions for patients with the largest drops in blood pressure, but the small number of patients did not allow for valid statistical analysis.

Commentary

Microemboli from the aortic arch as well as intracardiac air and particle emboli have been implicated as a cause of intraoperative stroke, but mechanical improvements in bypass techniques have reduced these risks. Blood flow alterations during cardiopulmonary bypass have been postulated as a cause of brain ischemia, but studies comparing "off-pump" surgery to cardiopulmonary bypass have shown no difference in stroke rate between the 2 groups (N Eng J Med 2003;348:394-402). The report by Gottesman et al points to a preventable mechanism, a drop in blood pressure during bypass, that can be ameliorated. Efforts should be made to reduce and stabilize the patient's blood pressure before surgery, and maintain maximum tolerable pressure during bypass, to avoid brain ischemia.