6.4 Equianalgesic Conversion and Route Changes

6.4 Equianalgesic Conversion and Route Changes

Equianalgesic conversion is the highest-yield arithmetic on CHPN Domain 2. The skill is not memorizing a branded app. It is: sum 24-hour use, convert through oral morphine, apply the agency ratio, reduce 25–50% when the opioid changes, split the new basal, and write a 10–15% rescue. Every table is approximate. Receptor biology, incomplete cross-tolerance, and cachexia all move the real dose. Always check the hospice agency equianalgesic table and the prescriber's order; the numbers below are the standard hospice-nursing teaching set, not a license to freelance.

Standard ratios (teaching table)

These oral and parenteral figures are the ones hospice nurses are expected to handle without a specialist. They are single-dose equivalences used to build a 24-hour total, not a promise that 30 mg of morphine will feel identical in every patient.

Read the morphine line as the anchor: 30 mg oral morphine ≈ 10 mg IV or SQ morphine. Subcutaneous morphine and hydromorphone are treated as equianalgesic to their IV milligram doses in usual hospice teaching (bioavailability of a well-absorbed SQ bolus is high). That is why a patient losing the oral route can move from oral morphine to a subcutaneous morphine infusion using 3:1, without inventing a new opioid.

Incomplete cross-tolerance: the 25–50% reduction

Mu receptors are not identical across morphine, oxycodone, hydromorphone, and fentanyl. A calculated equianalgesic dose of a new opioid is often too strong. When pain is controlled (or you are rotating for toxicity), reduce the calculated 24-hour dose of the new opioid by 25–50%, then titrate up with rescue doses. If pain is raging and the rotation is for uncontrolled pain, some protocols reduce less; CHPN-level default remains build in a 25–50% safety cut when the opioid changes, then use 10–15% rescue aggressively rather than starting at 100% of the table dose.

Same opioid, new route (oral morphine to SQ morphine) uses the 3:1 ratio without an automatic 25–50% cross-tolerance cut, then you still reassess because absorption and peak change. New opioid always gets the cut.

Transdermal fentanyl is not a titration tool

Fentanyl 25 mcg/h ≈ 60 mg oral morphine per 24 hours is the CHPN approximation. Manufacturer ranges are wider; agency tables win. Additional patch rules:

• Use only in opioid-tolerant patients with stable 24-hour requirements. A patch needs 12–24 hours to take hold and about 72 hours to approach steady state. You cannot chase a pain crisis by changing the patch every shift.
• Do not start a patch as the first move in a cachectic, unstable, actively dying patient. Low subcutaneous fat makes absorption erratic; fever and heating pads dump extra fentanyl; the next 72 hours may be the last 72 hours of the life. Use SQ morphine or hydromorphone (or concentrated oral/SL solution if still absorbing) for unstable dying pain.
• Do not cut most patches unless a specific product and agency policy allow it. Do not apply heat. Used patches still contain drug; fold and dispose per policy.
• If a conservative table maps a stable 60 mg oral-morphine TDD to 25 mcg/h, that is a match for a stable patient with adequate tissue. If you also owe a 25–50% incomplete-cross-tolerance reduction because you are leaving morphine, the practical start may be the next lower patch (12 mcg/h) plus IR rescue, not a jump to 50 mcg/h.

Methadone is specialist care

Methadone is a mu agonist plus an NMDA-receptor antagonist with SNRI activity. It can help refractory and neuropathic pain, but it is not a bedside conversion. Half-life is long and highly variable (often quoted from about 15 to more than 60 hours, sometimes longer), so accumulation appears on day 3–5. Conversion ratios change with the prior morphine milligram total (higher prior dose, more potent methadone relatively). It prolongs the QT interval. CHPN action: recognize that methadone is specialist-prescribed, never milligram-for-milligram with morphine, and never the crash conversion when you are unsure.

Routes: PO, SL, SQ, IV, rectal, transdermal

Pick the route the patient can use today:

Worked conversion 1: oxycodone ER to oral morphine, with the reduction

A patient is comfortable on oxycodone ER 20 mg every 12 hours (no PRN yesterday). The oral route still works; the team must switch to morphine because oxycodone is unavailable.

1. TDD oxycodone = 20 × 2 = 40 mg/day.
2. Ratio: oral oxycodone 20 mg ≈ oral morphine 30 mg, so morphine milligrams = 40 × (30/20) = 60 mg oral morphine / 24 h.
3. Pain is controlled, and this is a new opioid → reduce 25–50%.
   • 25% off 60 mg = 45 mg/day
   • 50% off 60 mg = 30 mg/day
4. Start in that window: morphine ER 15 mg every 12 hours (30 mg/day) if very conservative, or 15 mg and 30 mg split to total 45 mg/day if you want the milder cut. A typical teaching start is 30–45 mg oral morphine/day.
5. Breakthrough = 10–15% of the new TDD: for 30 mg/day that is 3–4.5 mg IR (practically 5 mg oral solution); for 45 mg/day that is 4.5–6.75 mg (practically 5 mg).
6. Recheck at oral peak (~60 minutes) after the first IR, and reassess the basal after 24 hours. Do not start at 60 mg/day morphine just because the table said 60.

Worked conversion 2: oral morphine to subcutaneous hydromorphone, with the reduction

A patient with dysphagia has been on morphine ER 45 mg every 12 hours (90 mg/day) plus morphine IR 15 mg twice yesterday (30 mg). Pain is mostly controlled but swallow is gone. Convert to SQ hydromorphone (new opioid and new route).

1. Current oral morphine TDD = 90 + 30 = 120 mg/day.
2. Parenteral hydromorphone ratio through the table: 30 mg oral morphine ≈ 1.5 mg SQ hydromorphone, so SQ hydromorphone milligrams = 120 × (1.5/30) = 6 mg SQ hydromorphone / 24 h.
3. New opioid → reduce 25–50%.
   • 25% off 6 mg = 4.5 mg/day
   • 50% off 6 mg = 3 mg/day
4. Choose about 3–4.5 mg/24 h as a continuous SQ infusion (for example 0.15 mg/h, which is 3.6 mg/day, in the middle of the window).
5. Breakthrough SQ bolus = 10–15% of the new TDD: 10–15% of 3.6 mg ≈ 0.4–0.5 mg SQ as needed, after the SQ peak (~30 minutes), per protocol.
6. If the oral route had remained, the oral hydromorphone equianalgesic would have been 120 × (7.5/30) = 30 mg oral hydromorphone/day, then 25–50% off to 15–22.5 mg/day oral hydromorphone — same reduction logic, different route.

A same-opioid contrast, so you do not over-cut: 120 mg oral morphine to SQ morphine is 120 ÷ 3 = 40 mg SQ morphine / 24 h (~1.7 mg/h) without a mandatory 25–50% species cut, plus a 10–15% SQ rescue.

Conversion is a sequence, not a vibe: TDD → table → 25–50% if the opioid changed → split the new basal → 10–15% rescue → agency table always wins → methadone and unstable fentanyl patches are not that sequence.