Modifiers of patient-controlled analgesia efficacy.  I. Locus of control. 
The effectiveness of patient-controlled analgesia (PCA) depends upon the patient's appropriate response to a strong aversive stimulus (i.e., pain) with subsequent reinforcement (i.e., opiate injection).  Each patient may have psychological characteristics that modify this response to aversive stimuli.  To test for such characteristics, 76 female patients undergoing abdominal gynecologic procedures were given psychological tests (i.e., the Chance External, Powerful Others External, and Internal subscales of the Multidimensional Health Locus of Control; the Activities of Daily Living scale; and the Hypochondriasis, Depression, and Hysteria scales of the Minnesota Multiphasic Personality Inventory).  Patients utilized PCA for postoperative analgesia.  At the completion of PCA, patients were given a questionnaire assessing both the level of pain and degree of satisfaction with pain relief.  Correlational analysis compared the level of pain and degree of satisfaction with results of psychological testing.  Results showed that female patients with an external locus of control had higher levels of pain and greater dissatisfaction with PCA.  An internal locus of control was predictive of lower pain scores and increased satisfaction.  PCA effectiveness, as measured by the level of pain and degree of patient satisfaction, correlated with results of psychological testing.  The delineation of these and other possible modifiers of PCA efficacy may define populations that are optimally responsive to PCA.