Stress-induced plasma beta-endorphin immunoreactivity may predict postoperative morphine usage

The relationship between stress and human behavior is studied using general surgery as a stress paradigm. As indices of arousal, presurgery and surgery plasma β-endorphin and cortisol immunoreactivity are assessed. Behavioral analysis is restricted to the measurement of total morphine usage during the first 24 postoperative hours under standard PRN (as needed) clinical orders. Individual patient morphine requirements vary widely (12-56 mg). Both presurgery and mean surgery plasma β-endorphin levels significantly predict morphine requirement, and similar, although not so strong, correlations are found for cortisol. Patient age is also found to be negatively correlated with morphine requirement. When multiple regression analysis is used, the variables of plasma β-endorphin and age predict 70% of the variance in individual morphine requirements.