ACUTE PAIN MANAGEMENT

Acute pain = pain that occurs as direct result of tissue damage, <3 mos in duration

•  Can occur as a result of trauma, surgical insult, labor, infection, or inflammation

•  Acute postoperative pain → inflammation is a major cause

Treatment

•  Includes opioid & nonopioid meds

•  Opioids commonly used—can be given parenterally, helpful immediately postop

Opioid Rotation

•  Definition: Using different opioids interchangeably & via different routes

•  Presence of major organ dz (liver/kidney dysfx) can markedly ↓ dose required for adequate clinical response

•  Lack of complete cross tolerance between different opioids

→ Essential to use lower than equianalgesic doses

•  Oral & parenteral doses often dissimilar (due to pharmacokinetic differences)

•  Opioid-associated itching: Dilute naloxone & benadryl → can both be used

• Naloxone 400 mcg in 1 L of LR or NS given at 75 mL/hr

• Avoid benadryl in advanced age

MULTIMODAL APPROACH TO PAIN CONTROL (ALSO SEE CHAPTER 27)

Analgesic Delivery Systems

•  PO

• Not optimal for immediate postop pain (delayed time to peak effect)

• Opioids commonly combined with COX inhibitors

•  SC/IM

• Less desirable routes (pain on injection & erratic absorption)

• Cyclical period of sedation → analgesia → inadequate analgesia common

•  Intravenous administration

• Requires close respiratory monitoring

• Common in PACU, ICU, & specialized units

•  Patient-controlled analgesia (PCA)

• Allows pt to self-administer opioids with button push

• Physician specifies dose, minimum time period between doses (lockout), basal infusion rate, max delivered dose

Neuraxial Analgesia

•  Intrathecal or epidural routes; effective for postop pain after abd, pelvic, thoracic, & orthopedic surgeries of lower extremity

•  Opioid infusions often combined with local anesthetics (bupivacaine, lidocaine, ropivacaine)

•  Clonidine & buprenorphine also used epidurally

Epidural Opioids

•  Site of action = pre- & postsynaptic receptors in dorsal horn substantia gelatinosa

•  Opioid drugs enter CSF at rate dependent on physicochemical properties

• Molecular weight, pKa, oil:water solubility

•  Direct transport via spinal cord blood supply can occur

•  Diffusion through dural cuff regions to spinal cord can occur

•  Lipid-soluble opioids (sufentanil, fentanyl) enter spinal cord faster

• Also eliminated faster by vascular uptake, leading to a short duration of action

•  Morphine = water-soluble opioid (slower action onset, longer duration of action)

Epidural Clonidine

•  Dose = 3–5 mcg/kg; can be added to epidural mix (local or opioid)

•  Prolongs duration of epidural local anesthetic by ≈50%

•  ↓ epidural opioid requirements ≈30%

•  Side effects: Severe bradycardia (if used >1 mcg/kg/hr)

Epidural Placement Levels

Signs of an Inadequate Epidural

•  Pain at rest & with movement (pain score >5)

•  Tachycardia

•  ↑ respiratory rate

•  For thoracic & abd cases

→ Inability to breathe deeply, cough, use incentive spirometer

Testing of Epidurals

•  2% lidocaine with 1:200,000 epinephrine or 0.25% bupivacaine with 1: 200,000 epinephrine

• Give 3 mL of either via epidural → check BP, motor block & pain relief

•  May repeat 3 mL after 3–5 min

•  Consider replacing epidural if no pain relief after 8 mL of test dose over 10 min

“Splitting” Epidural with IV PCA

•  Technique:  Add IV PCA in addition to epidural infusion (must simultaneously remove opioids from epidural infusion)

•  May be necessary for

• Pt with opioid dependence

• Incomplete incisional coverage from epidural

• Upper & lower body surgeries (trauma victims)

• Large surgical incision

• Lower level epidural catheter placement

Intrathecal Opioids

•  Act on substantia gelatinosa of the spinal cord

•  Potent analgesia due to central localization of receptors

•  Side effects are mediated by mu receptors (in brain & brainstem)

Mixed Agonists/Antagonists

•  Block effects of high doses of morphine-like drugs

• By competing with morphine-like drugs to bind to mu opioid receptor

•  Produce partial agonist effects at κ (kappa) and/or δ (delta) opioid receptors