Patients who develop postoperative delirium after cardiac surgery are at risk for an overall decline in cognitive function and a prolonged period of impairment during the following year, according to a study published online July 4 in the New England Journal of Medicine.
Compared with cardiac surgery patients who do not develop postoperative delirium, those who do are significantly less likely to return to their preoperative level of cognitive performance within 6 months, said Jane S. Saczynski, Ph.D., of the division of geriatric medicine and Meyers Primary Care Institute at the University of Massachusetts, Worcester, and her associates.
"Our findings suggest that the development of postoperative delirium should be added to the list of risk factors for prolonged impairment after cardiac surgery, which includes the development of atrial fibrillation, a history of depression, a lower level of education, and preexisting cerebrovascular disease," they said.
Delirium is the only one of these factors that is potentially preventable, the investigators noted.
Delirium is common following cardiac surgery, affecting up to 75% of patients by some estimates. Dr. Saczynski and her colleagues followed 225 patients aged 60 years or older for 1 year after they underwent CABG (coronary artery bypass graft) or valve replacement, to assess the effect of postoperative delirium on later cognitive performance.
The study subjects were treated at two academic medial centers and a Veterans Affairs hospital. They were interviewed daily while they were hospitalized to assess delirium, and their records were reviewed for evidence of the clinical features of the condition. After discharge, the subjects were again interviewed in their residences at 1, 6, and 12 months to assess their cognitive function.
Delirium was rated as present or absent based on results of the CAM (Confusion Assessment Method), which evaluates four key features: acute change with a fluctuating course, inattention, disorganized thinking, and altered level of consciousness. This assessment was supported with results on the MMSE (Mini-Mental State Examination), the digit-span test, and the Delirium Symptom Interview.
The patients also were evaluated using the Charlson comorbidity index, which estimates the burden of illness based on the presence and severity of 17 medical conditions.
The mean age of the study subjects was 73 years; approximately 25% were women, and almost all were white and non-Hispanic.
Postoperative delirium developed in 46% of the study subjects. It lasted 1-2 days in 65% of these patients and for 3 or more days in the remaining 35%.
Subjects who developed delirium were significantly older and less educated than were those who did not; were more likely to have a history of stroke or TIA (transient ischemic attack); to have higher scores on the comorbidity index; and to have a lower level of cognitive function before the cardiac surgery.
Surgery type (whether CABG only, valve replacement only, or valve replacement with CABG) was not significantly associated with postoperative delirium.
In the study population as a whole, cognitive function declined significantly (by 4.6 points on the MMSE) between baseline and postoperative day 2. This initial drop was followed by significant increases in cognitive function, by 1 point per day, on days 3-5. The rate of improvement then slowed considerably and stabilized at approximately 6 months, showing no significant change thereafter.
However, this pattern changed when patients were stratified by the presence of delirium. Those who developed delirium showed a much greater decline in cognitive function in the immediate postoperative period, with a drop of 7.7 points vs. 2.1 points on the MMSE. They then showed more rapid recovery on days 3-5, and slightly greater recovery over the next 6 months.
"Patients without delirium returned to their preoperative level of cognitive function by approximately 1 month after surgery, whereas patients with delirium had not returned to their preoperative level of function by 1 year postoperatively," Dr. Saczynski and her associates said (N. Engl. J. Med. 2012 July 4 [doi:10.1056/NEJMoa1112923]).
"In addition, the proportion of patients who did not return to their preoperative level of function was significantly higher in the group with delirium than in the group without delirium through 6 months postoperatively (40% vs. 24%), but this proportion was not significantly different at 12 months (31% and 20%, respectively)," they noted.
This suggests that delirium, "which was once thought of as a short-term transient cognitive disorder, may have longer-term observed effects on cognitive function in patients who have undergone cardiac surgery," the investigators said.
The researchers did point out some potential caveats in their conclusions. The study did not have a nonsurgical group as a comparative control. In addition, the lower level of cognitive function at baseline in the group with postoperative delirium, as compared with those patients without postoperative delirium, could be caused by a trajectory of decline from a greater burden of preexisting disease that was not fully addressed in the multivariate models.