Post-operative delirium after elective cardiac surgery : risk factors and cognitive long-term outcome : results from a clinical study

Abstract

Delirium is a serious problem in the older patient. Delirium is associated with a significant independent morbidity and mortality that increases hospital costs. Due to the increasing elderly population, delirium is becoming an important problem in the western countries. In the clinical practice, however, delirium is often overlooked or misdiagnosed. Thus, knowledge of delirium, its presentation, causes, and consequences is important to improve detection rates and diminish its consequences.

Elective cardiac surgery is a well-known procedure to treat coronary artery disease (CAD). During the last decades, an increasing number of elderly patients have been treated with cardiac surgery. Several complications have been associated with this procedure. One of these complications is post-operative delirium. However, data on prevalence and consequences of delirium after cardiac surgery is still limited. The main problem has been the lack of a consistent diagnostic method. Another less studied complication is the long-term cognitive status in patients who undergo cardiac surgery. Therefore we here chose to study patients who underwent elective cardiac surgery with a longitudinal follow-up, comparing patients who developed delirium post-operatively with those who did not.

The aim of the first study was to identify pre-operative risk factors for delirium in patients undergoing elective cardiac surgery, using clearly defined diagnostic criteria for delirium, and a thorough clinical assessment. The objective of the second study was to prospectively assess the cognitive outcome in elderly patients with and without post-operative delirium after elective cardiac surgery during a two-year period.

In study I, data were collected prospectively from 107 patients 60 years and older who underwent elective cardiac surgery. None of the patients included suffered from dementia or delirium pre-operatively. Pre-operative cognitive function in all patients was assessed using the Mini Mental State Examination (MMSE). Delirium was diagnosed using the Confusion Assessment Method (CAM). Post-operative clinical and cognitive assessments were carried out for all patients. Twenty five patients (23.4%) developed delirium post-operatively. Clinical parameters, including age, gender, co-morbidities, medications and peri-operative parameters were similar in patients with and without post-operative delirium. Patients with pre-operative subjective memory complaints and lower MMSE scores, undergoing valve operation or valve + coronary artery bypass grafting (CABG), exhibited an increased risk of developing post-operative delirium. At discharge, all patients were delirium free. However, the patients who had been delirious post-operatively had significantly lower MMSE scores at discharge compared with those who did not develop post-operative delirium.

In study II, we studied the patients who developed post-operative delirium (n= 24) as well as those who did not (n= 80) from study I at discharge from the hospital (baseline) and 3, 12 and 24 month follow-up. Ninety-four patients completed the 2-year follow-up, drop outs were 20 % in the delirious group and 6.3 % in the non-delirious group. At the four time-points, cognitive function and possible presence of delirium was assessed using MMSE and CAM respectively. Furthermore, data on medical and neurological assessments, body mass index and number of drugs were collected and showed no significance between the groups during follow-up. The MMSE scores within the delirious, and the non-delirious group, respectively, did not change significantly during the two year follow-up. However, the delirious group had significantly (p < 0.001) lower MMSE-scores as compared with the non-delirious group at all time-points (baseline, 3, 12 and 24 months).

CONCLUSIONS: The prevalence of delirium after cardiac surgery in this study was 23.4%. The main pre-operative risk factors for developing post-operative delirium were subjective memory complaints, mild cognitive impairment and type of cardiac surgery, such as valve procedures. The cognitive impairment found in persons who developed post-operative delirium did not further decline during the two-year follow-up.

These studies suggest that cognitive evaluation (e.g. using MMSE and direct questions about memory problems) should be administered routinely to identify high-risk patients before surgery. Although delirium was not associated with further cognitive decline after 2 years, we should stress that longer follow-ups are needed to confirm our results.