Effect of demand frequency on lockout period

Abstract

SummaryAn Abbott Provider 5500 Patient Controlled Analgesia machine was noticed to have developed a prolonged lockout period when frequent analgesic demands were being made. This study testedjive other machines and demonstrated that the duration of the lockout period was influenced by the frequency of analgesic demands. This finding could have clinical implications and we recommend that the duration of the lockout period should be speciJically examined during the testing of patient-controlled machines. Key wordsAnalgesia; patient-controlled. Equipment; patient-controlled analgesia machine.Patient-controlled analgesia (PCA) is now a well established form of postoperative pain relief, and its widespread use has been paralleled by an increase in the number of PCA devices currently available. However, concern has been expressed about the inadequacy of testing procedures for PCA machines [ 1,2], which may have exposed patients to unwarranted risks [ 1, 31.We were referred a patient who, after surgery, was found by nursing staff to be experiencing considerable pain. The patient was using an Abbott Provider 5500 PCA system which contained our standard solution of morphine sulphate 1 mg.ml-' in 0.9% saline. The machine had been programmed to deliver a standard regimen of a 1 mg bolus of morphine, delivered over 15 s, with a lockout period of 5min. The patient was making very frequent demands for analgesia and a nurse noticed that the lockout period of the machine appeared longer than the Smin period that had been programmed by the anaesthetist. A member of the Acute Pain Team (J.M.) was called to see the patient and he confirmed the observation.Previously published testing protocols have focused on the accuracy of volume delivery or the physical properties of the device but not the accuracy of the lockout period [4-71. We therefore decided to test five Abbott Provider 5500 machines to establish whether demand frequency had an effect on the duration of the lockout period. MethodsFive Abbott Provider 5500 machines (Fig. l), which had been in regular use at Nottingham City Hospital, were randomly selected from a pool of 20 for testing. Each was loaded with new batteries and giving sets and attached to a 100 ml bag of 0.9% saline solution. Each machine was programmed to deliver a 1 ml bolus with a lockout period of 5 min. A stopwatch was used for all timings and demands were triggered, using the handheld button, by the authors. The buttons were depressed until the confirmation signal was heard from the machine.Following a successful demand the machines should complete delivery of a bolus within 15 s. The lockout time begins at the completion of the 15 s delivery period and with standard programming has a duration of 5 min (300 s).We performed two sets of tests. First, to ascertain proper functioning of each device we instituted a 'control cycle': after a normal bolus of 1 mg delivered over 15 s a further demand was made immediately, followed by additional demands made at 1 min intervals for a 5 min period. After completion ...