When initiating PCA, the drug to be employed must be selected and several dosing parameters must be established.13 The settings that must be chosen when starting PCA include a loading dose, a demand dose, a lock-out interval, and a bolus dose 73 III ACUTE PAIN
(an additional dose that can be delivered by nursing personnel if pain is uncontrolled). In addition, a continuous infusion and 1- or 4-hour maximum dose limit can be selected if desired.
Medications Used in PCA
Several opioids are currently available for use in PCA. The most commonly employed agents include morphine, fentanyl, hydro- morphone, and meperidine. Methadone, oxymorphone, and alfen- tanil have also been administered via PCA. Factors that may influence the selection of opioid include patient disease states that might influence drug metabolism or enhance the risk for opioid- related toxicity and any history of adverse effects from prior expo- sure to a particular opioid. In general, meperidine is not used by most pain management practitioners for PCA owing to potential adverse effects from one of its metabolites, normeperidine, with repeated exposure to this drug. Normeperidine accumulation can result in neuroexictatory activity, including seizures. The risk of such events occurring increases when meperidine is administered in large doses (>600 mg in 24 hr) and when it is repeatedly given over a period exceeding 24 hours. The risk of toxicity from this drug is most significant among patients with renal insufficiency, and therefore, use of meperidine in this patient population is not advised.
Loading Dose
A loading dose is typically administered at the time of initiation of PCA as a means of quickly achieving a serum opioid concentration that provides effective pain control. After administration of a load- ing dose, patients can then self-administer additional opioid doses via the demand mode to maintain satisfactory analgesia. A loading dose should generally be prescribed when patients are experiencing significant pain upon starting PCA because use of the demand feature alone is unlikely to allow for timely establishment of suffi- cient analgesic serum opioid concentrations.
Loading doses can be given before PCA is initiated as multiple small opioid doses, repeated at frequent intervals until adequate analgesia is achieved (e.g., in the postanesthesia care unit [PACU]). It is vitally important to understand that PCA should be initiated only after the patient has achieved pain relief that is ‘‘in the ballpark of adequate analgesia.’’
Demand Dose
The demand dose is the dose of opioid delivered each time the patient activates the PCA button, except during the established lock-out interval. The size of the demand dose selected may be influenced by factors such as age, weight (in pediatric patients), and any preexisting opioid tolerance. In general, elderly patients may respond well to lower demand doses, whereas chronic opioid users will require higher demand doses than the average adult patient. Adjustment of the demand dose may be required if evaluation of the patient’s utilization of the PCA reveals a high rate of demands.
Bolus Dose
A bolus dose is an extra dose of medication that can be delivered by nursing personnel for inadequately controlled pain. A typical scenario in which a bolus dose may need to be administered is movement-associated pain, such as when a patient participates in physical therapy. A bolus may also be necessary when a long period has elapsed between demand activations of the PCA
device, as might occur with sleep. Effective bolus doses usually amount to two to three times the programmed demand dose. Bolus doses may be given as often as necessary. If frequent bolus doses are needed, adjustment of the demand dose should be considered.
Lock-out Interval
A lock-out interval is the period of time that the PCA device is unable to deliver further demand doses after activation of the demand button. This is usually set in a range of 5 to 10 minutes. Setting a lock-out interval allows the patient to appreciate the effects of the delivered opioid dose prior to administration of another dose. The lock-out serves as an integral safety feature of PCA in that it prevents rapid stacking of doses on top of one another, as might otherwise occur if dosing was achieved every time a patient attempted to activate the device. This significantly limits the poten- tial for inadvertent drug overdosage that could result from robust attempts to achieve rapid analgesia by repeatedly activating the demand button. When assessing the patient’s pattern of analgesic use, caregivers should observe the frequency of demands the patient is attempting to obtain in addition to the actual opioid consump- tion. This will typically appear as ‘‘demands’’ or ‘‘attempts’’ when reviewing the PCA device history feature. If the number of ‘‘demands’’ or ‘‘attempts’’ appears high compared with the actual number of doses delivered, the patient may need to be reeducated about how PCA works, with a reminder about the lock-out interval. If both ‘‘demands’’ or ‘‘attempts’’ and actual number of opioid doses administered appear frequent, the demand dose may need to be increased to allow for enhanced analgesia.
Continuous (Basal) Infusion
A continuous (basal) infusion can be added to PCA if desired. This will be delivered regardless of patient demands. Few studies indicate a distinct advantage to the use of continuous infusions, although some believe analgesia may be better maintained with the use of basal infusions, particularly when patients are not able to activate the demand feature such as during periods of sleep. Total opioid consumption may be increased in the presence of basal infusions. There is some concern that use of continuous infusions increases the likelihood of PCA-related adverse events, with particular con- cern for the possibility of respiratory depression. The primary indi- cation for use of basal opioid infusions is as a substitution for chronic baseline opioids when opioid-tolerant patients are unable to continue their regular pain medications.
One- or 4 -Hour Maximum Dose
Limits can be placed on the total opioid delivered in a specified time interval. Most commonly, a 4-hour maximum dose is selected when such restrictions in dosing are desired. Setting a dose limit may be of particular importance when meperidine is employed in PCA. Owing to the potential for accumulation of normeperidine, as pre- viously described, doses of meperidine should not exceed 600 mg in 24 hours. If dosing limits are not set, it might be fairly easy to exceed such a total daily dose with the typical dosing parameters selected for this agent when initiating PCA. Setting a 1- or 4-hour dosing limit may also provide some protection against overdosage in the event of other programming errors. However, 1- or 4-hour limits can also result in inadequate analgesia in patients requiring large amounts of opioid because restrictive limits may cause them to be ‘‘locked-out’’ from further doses for extended periods and can cause such individuals to be dissatisfied with their analgesic therapy (Table 10–1).