Most infusion systems allow settings for both basal and patient controlled analgesia (PCA) opioid doses.* Basal doses are set as X mg/h. PCA doses are ordered as Y mg qX minutes. For SC systems, breakthrough intervals should be no more often than q15 minutes. Every 20 to 30 minutes may be preferable to ensure adequate drug delivery before giving an additional dose.
Once a steady-state basal dosage has been determined, the PCA dosage and interval can be calculated. For stable, chronic pain in accord with the palliative care note above, a good dose is often one that results in a doubling of the serum opioid blood level with peak effect. Dosing intervals should be set such that stacking of PCA doses does not occur. Thus, a patient on 6 mg basal SC morphine might have 2 mg q20 minutes ordered for a PCA dose. In contrast, if a patient is unable to push the PCA button (due to illness or altered mental status, such as dementia), either the basal rate must be emphasized or nursing staff (or family) instructed to assess and administer the PCA dose as needed.
As is true for oral breakthrough doses, patients usually need occasional PCA boluses, but generally these should be less than three to five per day. Such incidents may be anticipated (for example, before nursing care or dressing changes) or unanticipated. If more boluses than this are required, pain should be reassessed and consideration given to raising the basal dose.
It is usually safe to increase the basal dose administered over 24 hours by the amount of PCA doses given over the prior 24 hours. For example, a patient on 6 mg/h basal morphine infusion and a PCA of 2 mg q20 minutes has required 24 injections for a total of 48 mgs of PCA dosing. Dividing this total by 24, this is the equivalent of 2 mg per hour. Thus, at a minimum, the basal dose should be increased by 2 mg/h, from 6 to 8 mg per hour (if pain was well controlled with these PCA doses). If pain was not well controlled with this combination of basal and PCA dosing, a higher basal dose, 10-12 mgs, for example, may be needed.
If no PCA boluses are required and the patient is pain free, the basal dose may be too high. This should especially be considered in a patient who appears sedated or talks with slurred speech. If opioid excess is suspected, holding administration of the opioid for a couple of hours and then lowering the dose will usually suffice. Rarely is naloxone administration required. Naloxone should generally be reserved for patients who are beginning to show signs of respiratory depression or hemodynamic compromise, usually associated with bradycardia. Even here, in all but the most extreme cases, consideration should be given to administering small boluses, 0.1-0.2 mg (1/4-1/2 an ampule) incrementally as needed to avoid complete opioid reversal and concomitant pain exacerbation and chemical withdrawal.
Unstable pain changes rapidly up and down. opioids require frequent adjustment. Rapidly lessening pain may be seen in most acute pain syndromes, in which pain naturally decreases over time. Pain following surgery or trauma is usually of this nature. Chronic pain, such as is found in many cancers, may also rapidly decrease in certain situations, such as when a painful focal metastasis is radiated.
When pain is either rapidly increasing or decreasing, proper opioid therapy requires less reliance on basal opioid doses and more on PCA doses. In the extreme, many surgeons treat postoperative pain exclusively with PCA doses. Such therapy has the advantage of minimizing the chance of opioid excess that would result from unnecessary amounts being administered to a patient with lessening pain and a decreasing need for opioids. Excessive reliance on PCA dosing risks making the patient overly dependent on pushing the PCA button on time. Such patients may state that pain is well controlled while awake, but when they fall asleep (and thus cannot push the button) they may suddenly awaken in pain and play "catch-up" with PCA doses. Sleep is thus disturbed, which is an impediment to healing.
Often, what works best in such situations is the use of a PCA:basal hourly ratio that is greater than the 1:1 ratio described for chronic, stable pain. Such a ratio will allow both rapid titration up (in the case of increasing pain) or down.
All machines with PCA:basal drug administration capabilities I have used have a means of recording how many PCA doses were given, when they were given, and often how many attempts were made to administer a dose (a crude measure of desperation). Machines may have a button labeled "history" that allows the clinician to scroll through the PCA history over the past several hours. This data should be used in making adjustments as discussed above.
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Palliative Care Perspectives
James L. Hallenbeck, M.D.
Copyright © 2003 by Oxford University Press, Inc.
The online version of this book is used with permission of the publisher and author on web sites affiliated with the Inter-Institutional Collaborating Network on End-of-life Care (IICN), sponsored by Growth House, Inc.