This study of older patients undergoing CABG surgery analyzed preoperative memory and executive function performance on neuropsychological measures as risk factors for delirium after CABG. A CFA model was created to develop memory and executive function composites, and the association between these composites and development of postoperative delirium was validated. After adjustment for age, sex, education, comorbidity, baseline cognition, and memory, poor performance on measures of executive function was independently associated with greater risk of developing delirium. The effect of memory on delirium was significant until executive function was adjusted for, suggesting that executive function may mediate the effect of memory on delirium. Adjustment for baseline mental status did not change the mediating effect of executive function on delirium.
Previous studies of delirium have determined that dementia is a risk factor for delirium.6
Criteria for dementia require impairment in memory and another cognitive impairment,26
but those with memory impairment or other cognitive impairments have not been studied with respect to delirium. Although established dementia is a risk factor for delirium, the current data suggest that impairments in preoperative performance on measures of executive function may also be predictive.
The association between impairment in executive function and delirium may have a pathophysiological basis. Atherosclerosis is known to impair cognitive abilities, particularly in measures of frontal lobe function.11
In addition, risk factors for atherosclerosis are known to affect performance on measures of executive function adversely.11,12
In the CABG population, atherosclerosis has been associated with delirium.27
Delirium and atherosclerosis share common risk factors such as older age,2
and peripheral vascular disease.2
On the basis of these reports and these findings, it may be that the underlying atherosclerosis pathology that affects the frontal lobes impairs preoperative performance of executive functions. Preoperative executive function impairment predisposes patients to postoperative difficulty processing reorienting stimuli, which may present as deficits in attention or altered thought patterns that are required for the diagnosis of delirium.
This study has several advantages. The subjects were prospectively enrolled and did not have delirium at baseline. The neurocognitive battery was chosen in accordance with published concensus13
and has been used in other studies to detect cognitive deficits.30
Daily follow-up with a reliable delirium battery maximized capture of postoperative delirium.
There are also several limitations to this study. The population had symptomatic atherosclerosis, a known risk factor for executive function deficits. This may limit the generalizability of the findings to other medical and surgical populations. Delirium was not assessed for on postoperative Day 1, which would lead to a conservative estimate of the incidence of delirium in the population. Despite this, the incidence of delirium after CABG in this population is at the upper limit of published studies. Some subjects were unable to complete the full neuropsychological test battery because of illness, time commitments, and other wishes. Most of the subject recruitment was performed in the hospital, on the night before surgery; under these circumstances, compliance with the neuropsychological battery was gratifyingly high.
Within the battery, some neuropsychological measures assess multiple cognitive domains and therefore may not be specific for executive function or memory. The neuropsychological measures were assigned a priori to the predominant cognitive domain. The neuropsychological battery consisted of multiple measures of executive function, but the indicators of memory function both derive from the HVLT. Thus, the memory factor is underspecified. This analysis would benefit from an additional measure of visual memory function, which was not performed in the study. Finally, this study evaluated the independent effect of executive function and memory in terms of delirium risk; it was not designed to evaluate the relative contribution of executive function and memory on delirium risk.
This study has important clinical implications. Efforts to target populations for screening, intervention, or prevention of delirium should consider executive function impairment as a risk factor. The high incidence of delirium associated with CABG surgery may have some relationship to underlying microvascular disease of the brain that affects frontal function. Further studies in medical and surgical populations are needed to examine the role of mild executive function impairments in the development of delirium.