Therapeutic Effects: Analgesia

Anesthesia Implications

Post-op analgesia – IV Morphine should be given 30-45 minutes prior to the end of surgery to provide post-op analgesia.

2-Chloroprocaine – antagonizes mu and kappa receptors and, as such, will reduce the efficacy of epidural morphine.

Side effects – meiosis, drowsiness, euphoria, respiratory depression (dose-dependent), cough reflex suppression, constipation, nausea and vomiting (CTZ activation), and urinary retention. Reduces peripheral vascular resistance (drops blood pressures) but has little to no affect on cardiac indexes. Morphine, however, does decrease cerebral blood flow, ICP, and CMRO2. Histamine release can contribute to hypotension, reflex tachycardia, and pruritis. At high doses, the adrenocortical system is blunted to stress. Endogenous morphine acts via the µ 3-receptor located on immune cells. This means that morphine can have immunosuppressive effects.

Dose Reduction – doses should be reduced in the hypovolemic, elderly, and those receiving other CNS depressants.

TENS units – PCA morphine requirements can be reduced 50-80% with the use of Transcutaneous Electrical Nerve Stimulation (TENS) units.

Herpes Simplex Reactivation – neuraxial administration of morphine has been known to potentially reactivate herpes simplex.

Biliary Spasm – this may be induced by morphine administration. Reversal using Naloxone (0.2-0.4 mg) can be used to reverse these affects but administration of naloxone can cause catecholamine release and onset of pain previously covered by morphine/opioids.

Nonsteroidal Anti-inflamatory drugs – PCA morphine requirements can be reduced 30 – 50% using NSAIDS such as ketorolac (toradol).

Drug interactions – CNS depressants will be potentiated by morphine. Diuretics used to treat congestive heart failure may have a decreased effect. Analgesia can be enhanced/prolonged by alpha 2 agonists (eg. clonidine, epinephrine). If epinephrine is added to neuraxial doses, the side effects will be amplified.

OB – Morphine crosses the placenta. Usage during labor can cause respiratory depression in the neonate. Make sure to have naloxone available if used.

IV push dose

Adult analgesia dose: 2-5 mg IV
Adult High-dose Induction technique: 1-2 mg/kg.

Pediatric analgesia: 0.05 – 0.20 mg/kg (MAX: 15 mg)

Slow push to avoid hypotension

IV infusion dose

Adult: 0.5 – 10 mg/hr
Pediatric: 10-100 mcg/kg/hr

Typically prepared by diluting 20 mg of morphine in 100 ml of Normal saline (0.2 mg/ml)

IM dose: Adult analgesia dose: 2.5-20 mg
Adult high-dose narcotic technique: 0.1 mg/kg

Pediatric analgesia dose: 0.05 – 0.2 mg/kg (MAX: 15 mg)

Epidural bolus dose: 2-5 mg (40-100 mcg/kg)

Epidural maintenance rate: 0.1 – 1.0 mg/hr (2-20 mcg/kg/hr)
Typically prepared by diluting 10 mg of morphine in 100 ml of local anesthetic or normal saline (0.1 mg/ml) – all should be preservative free

Spinal bolus dose: 200-400 mcg (preservative free).
High incidence of pruritis, nausea, and vomiting.

PCA dosing: -IV-
Bolus: 0.5 – 3.0 mg (10-60 mcg/kg)
Infusion: 0.5 – 10.0 mg/hr (15-200 mcg/kg/hr)
Lockout: 5-20 min

-Epidural-
Bolus: 50-200 mcg (1-4 mcg/kg)
Infusion: 0.1 – 0.4 mg/hr (2-8 mcg/kg/hr)
Lockout: 10 min

Reversal: Naloxone 0.2 – 0.4 mg (See naloxone post). Administration of naloxone can cause catecholamine release and onset of pain previously covered by morphine/opioids.

Pruritis caused by histamine release may be treated using antihistamines (eg diphenhydramine IV/IM 12.5 – 25 mg PRN every 6 hours)

Nausea and vomiting sometimes caused by morphine can be treated using metoclopramide (10 mg PRN every 6 hours)

Classification: Opioid, Narcotic

Time to Onset: IV: less than 1 min
IM: 1-5 min
Epidural: 15-60 min

Time to Peak: IV: 5-20 min
IM: 30-50 min
Epidural: 90 min – 5 hrs

Duration: IV and IM: 2-7 hours
Epidural and Spinal: 6-24 hours

Method of Action: Hydrophilic opioid agonist. Works primarily on the central nervous system and organs with smooth muscle.

Metabolism: Hepatic
Active metabolite: morphine­-6 glucuronide. This can accumulate in patients with renal impairment.