Opioid metabolism in the liver can be slowed in older people due to normal physiological changes of aging.
Some opioids (like morphine) are only partially metabolized and have active metabolites. Some (like hydromorphone) are nearly 100% metabolized in the liver, making it an ideal choice for older people. All are cleared by the kidney which also has reduced function in the older population.
The following graph of time vs plasma concentration is important to master when managing opioid administration. You will notice that the time to peak concentration and therefore peak effect of IV opioids is around 6 minutes (may be up to 15 minutes in older people). This is true of both beneficial effects (pain relief) and negative effects (sedation, hypotension, respiratory suppression). At the end of 15 minutes, if the person is still in pain, then she needs more opioid! If she is still breathing, alert, and normotensive, nothing worse is going to happen to her!
One barrier to effective analgesia in end-of-life or palliative patients in the ED is that they are often receiving MUCH higher doses than Emergency physicians are accustomed to. It’s important to remember that:
- The cause of their pain is steadily increasing (unlike other conditions we treat) AND
- Opioids have a habituating effect with a steady need for increased dose to produce the same effect.
If the patient is in the ED because of uncontrolled pain, aside from assessing treatable causes of that increase, it will be necessary to increase the dose of opioid. To do that successfully an understanding of equianalgesic conversions is necessary.
There are many conversion tables (and online conversion tools) each with subtle differences. This one, recommended by Cancer Care Ontario, is clear, simple, and accurate.
Equinanalgesic Table For Conversion From One Narcotic to Another
|OPIOID||PO||SC/IV (divide po in half)|
|Morphine||10 mg||5 mg|
|Hydromorphone||2 mg||1 mg|
|Fentanyl 25 ucg patch||Equal to 45 – 130 mg morphine po over 24 hours. We commonly use conversion of 60 mg PO morphine over 24 hours equals 25 ucg patch. Never use in opioid naïve patient or if pain not well controlled.|
Converting From One Opioid to Another
MEDD (Morphine Equivalent Daily Dose)
The dose of morphine that is equivalent in strength to the current opioid. It is usually calculated for a 24 hour period.
When converting from one opioid to another, or one route to another, it is helpful to calculate the Morphine Equivalent Daily Dose (MEDD), the dose of morphine that is equivalent in strength to the current opioid. It is usually calculated for a 24 hour period.
For example, a patient who is taking 4 mg of oral Hydromorphone every four hours is receiving 24 mg of oral Hydromorphone in a 24-hour period. The MEDD of this dose is 120 mg. When converting to a new oral opioid or converting to a new administration route one should use a dose equivalent to 120 mg of morphine.
You can also use MEDD as a safe basis for:
- Establishing break through doses (usually 10% of MEDD given every hour prn)
- Escalating daily doses (usually increase by 10% of the MEDD per day)
Cross-tolerance: Individuals have varying tolerance to different opioids. When switching from one opioid to another, it is safest to decrease the MEDD of the new opioid by 25% with a clear plan for breakthrough doses as needed.
Side Effects of Opioids
Take Home Message
Be the doctor you would want if YOU were in pain!
Respiratory depression is the biggest concern for Emergency physicians that hinders effective use of opioids. It’s important to know that somnolence ALWAYS precedes respiratory depression. The SAFEST way to control pain in the ED is with IV administration and close observation since maximum effect will occur within 15minutes.
Allergy. True anaphylaxis is rare. Itch and urticaria are common short-term side effects which can be controlled with anti-histamines.
Nausea, constipation, drowsiness, are NORMAL expected effects of almost all opioids which can be treated symptomatically (see the following pages). Tolerance usually develops within 2-4 days except for constipation which is universal and chronic.