Cognitive performance after cardiac operation: implications of regression toward the mean

The purpose of this study was to assess the influence of regression toward the mean on different definitions of cognitive dysfunction after coronary artery bypass graft operation.

A total of 120 patients who underwent nonemergency coronary bypass operation and who were involved in a randomized trial of an anti-inflammatory agent were assessed prospectively with a battery of 10 psychometric tests covering a variety of cognitive domains. The battery was administered before the operation and 5 days and 3 months after the operation. Data from 2 representative tests, the Rey Auditory Verbal Learning Test and the Trail Making Test (part A), were used. The influence of regression toward the mean on 3 commonly used single-case definitions of cognitive impairment (1 SD method; one-half SD method; 20% method) was analyzed.

Group mean performance deteriorated on the Rey Auditory Verbal Learning Test at the discharge assessment (P <.001) and remained below baseline at 3 months (P =.03). Mean performance on the Trail Making Test (part A) showed a near-significant decline at discharge (P =. 06), followed by improvement at 3 months (P <.01). Regression toward the mean was demonstrated on both tests by classifying the preoperative scores into low, moderate, and high-performance categories. Applying the different definitions of dysfunction resulted in substantially larger numbers of patients in the high-performance group being classified as impaired.

Single-case definitions of cognitive dysfunction are influenced strongly by regression toward the mean. Disproportionate numbers of high-baseline performers are classified as impaired, thereby questioning the validity of established definitions. Group mean analysis with controls is potentially the most reliable method for detecting real change or differences.