PCA for Opioid Tolerant Patients

Principles

Patients who are already established on opioid analgesia and who now require a PCA for an acute exacerbation of their pain or for unrelated acute pain will require higher PCA doses with a background infusion.  The background will replace their usual opioid requirement and prevent opioid withdrawal, while a larger than usual bolus will account for their opioid tolerance.   

However, the extent of their tolerance will be unpredictable so they remain vulnerable to opioid toxicity ( Opioid Toxicity ). The patient must be monitored according to the OUH PCA policy, with blood pressure, heart rate, respiratory rate, sedation and nausea recorded. 

 The pain team can give advice on conversion to a PCA, but the following is a guide: 

  1. add up the usual total daily dose of opioid 

  1. convert to a morphine equivalence if necessary 

  1. divide by 3 for parenteral conversion (bioavailability of oral morphine is approx 30%) 

  1. divide by 24 to give an hourly dose  

  1. give 50% of this calculation as the PCA background 

  1. estimate the PCA bolus dose as higher than or the same as the background 

Example: Morphine

Morphine sulphate modified release (e.g. MST, Morphgesic and Zomorph ) 60mg bd with breakthrough morphine immediate release (e.g. Oramorph ) 20mg x 3 per day 

 i.Total daily dose of morphine:120 + 60 = 180mg po morphine in 24 hrs 

ii.Morphine equivalence= 180mg po morphine in 24 hrs 

iii.Divide by 3 to account for oral bioavailability = 180/3 = 60mg iv in 24 hrs 

iv.Divide by 24 to give an hourly rate= 60/24 ≈ 2.5mg/hr iv 

v.50% of this becomes the PCA background=1.5mg/hr to prevent withdrawal and cover “usual” pain 

The PCA bolus should be the same or more than the background rate: 

vi. PCA bolus1.5mgfor “new pain” and to account for tolerance 

Example: Fentanyl

Fentanyl patch 50mcg/hr with breakthrough oxycodone immediate release (e.g. oxynorm) 20mg x 2 per day 

 i.Fentanyl patch 50mcg/hr ≡ morphine 120-180mg in 24hr;  

oxynorm twice the strength of oramorph = 20mg x 2 x 2 = 80  

ii.Total daily dose of morphine:180 + 80 = 260mg po morphine in 24 hrs 

iii.Divide by 3 to account for oral bioavailability =260/3 ≈  86mg iv in 24 hrs 

iv.Divide by 24 to give an hourly rate= 86/24 ≈ 3.5mg/hr iv 

v. 50% of this becomes the PCA background=2mg/hr to prevent withdrawal and cover “usual” pain 

The PCA bolus should be the same or more than the background rate: 

vi. PCA bolus2mgfor “new pain” and to account for tolerance 

Importance of frequent review

The extent of a patient's tolerance will be unpredictable so they remain vulnerable to opioid toxicity ( Opioid Toxicity ). The patient must be monitored according to the OUH PCA policy, with blood pressure, heart rate, respiratory rate, sedation and nausea recorded. 

 The PCA settings may need to be adjusted according to the following questions: 

1.Does the patient have significant constant pain?  Assessing pain at rest. 

If they still have constant pain despite optimizing their bolus (6 or more boluses per hour), then consider increasing the background. 

2) Does the bolus provide adequate pain relief?  Assessing pain on movement.  

Consider increasing the bolus, e.g. by 1mg, if the patient feels that the bolus is insufficient.