The use of patient-controlled analgesia (PCA) is a cornerstone of analgesia for patients requiring high dose opioids for acute pain control. Age has been accepted as the best predictor of opioid requirements in the acute postoperative setting following research published in 1995 by Macintyre and Jarvis [1], with many current textbooks and guidelines [2] supporting this method of dose estimation (using this formula of dose in IV morphine = 100 – age). Advances in the management of acute pain since this dose formula include regular use of multimodal analgesia (including NSAIDs, atypical opioids, neuropathics and ketamine) and regional anaesthesia has improved the management of acute pain significant. Testing of the age formula in the current acute pain service (APS) environment to ensure accurate dosing for patients in the acute phase of use.

 

A retrospective 12-month audit was undertaken at the Queen Elizabeth Hospital (QEH) from December 2022-23, with all patients receiving PCA being analysed. Routine data collected on APS rounds included all analgesia prescribed, dose of opioid used via PCA, any regional technique or adjuvant use (e.g. ketamine infusion) and scores for pain, functional activity scale, sedation, nausea/vomiting and pruritis. All patients receiving PCA were reviewed by the APS team within 24 hours of prescription. All opioid doses were converted using the Faculty of Pain Medicine Opioid Dose Equivalence Calculation Table and online ‘Opioid Calculator’ [3] to equivalent intravenous morphine doses to allow for analysis against patient age.

 

Subgroup analysis was undertaken for post-operative patients under the age of 70 years old, replicating the conditions of the original study by Macintyre and Jarvis [1].

 

Ethics: Central Adelaide Local Health Network Human Research Committee Approval HREC 19611

614 patients were prescribed PCA during the period of analysis with 472 patients using a fentanyl PCA and 141 using an oxycodone PCA. Of these patients, a total of 362 (267 fentanyl and 96 oxycodone) were under the age of 70 and prescribed their PCA for the management of acute postoperative pain. Average opioid PCA dose for the total group was 53.9mg (range 0-284mg) and for the under 70 surgical group was 61.7mg (range 0-284mg). The difference between expected opioid use (using the 100 – age formula) and actual opioid use was 12mg more in both the total group and the under 70 surgical group. Patients using fentanyl PCA on average used 18mg IV morphine equivalent more than predicted and oxycodone PCA patients used 8mg IV morphine equivalent less than predicted.

 

Both the total group and under 70 surgical group received multimodal analgesia including paracetamol (98% for both groups), NSAIDs (32% and 42% respectively), atypical or pre-existing opioids (17% and 20% respectively), neuropathics (7% for both groups), regional (6% and 5% respectively) and ketamine infusion (21% and 25% respectively).

The results of this audit suggest that despite the advances in acute pain management and regular use of multimodal analgesia that age may underestimate opioid requirement in the first 24 hours of PCA use and the agent used may influence dose requirements. Larger studies may help elucidate both if the agent used influences total opioid requirements and the relationship between age and opioid requirements in the first 24 hours after surgery.

 

Conflicts of interest:
No conflicts of interest to declare.

 

1. Macintyre PE, Jarvis DA. Age is the best predictor of postoperative morphine requirements. Pain. 1995;64(2):357-64.

 

2. Macintyre, PE, Schug, SA. Acute Pain Management: A Practical Guide (5th edition). CRC Press. https://doi.org/10.1201/9780429295058

 

3. Faculty of Pain Medicine 2021, Opioid Dose Equivalence Calculation Table, Australian and New Zealand College of Anaesthetists, viewed 1st March 2024, https://www.anzca.edu.au/getattachment/6892fb13-47fc-446b-a7a2-11cdfe1c9902/PS01(PM)-(Appendix)-Opioid-Dose-Equivalence-Calculation-Table.