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A26 Anesthesia
Pain Management
Toronto Notes 2019
Table 16. Opioids (continued)
Agent
Remifentanil
Methadone (opioid agonist)
Buprenorphine (opioid agonist antagonist)
Relative Dose to 10 mg Morphine IV
100 μg IV
Morphine to methadone conversion is variable based on patient’s morphine dose. Ranges from 1/4 between 1/20
Varies depending on route of administration (pill/ film, transdermal)
Moderate Dose
0.5-1.5 μg/kg IV
15-40 mg/ day in divided doses
Film: 2mg up to max of 24mg
Onset
Rapid (1-3 min)
Rapid (8 mins)
Moderate (30 min)
Duration
Ultra short (<10 min)
15 - 90 (24h average)
6 –8 h
Special Considerations
Only use during induction and maintenance of anesthesia
Can only be prescribed by federally/ provincially licensed physicians. Acts through both NMDA and mu receptors
Challenging due to variable equianalgesic dose and half-life After titration, accumulates in tissue for once/twice daily dosing Metabolized by CYP3A4
Caution with high doses, may cause QT prolongation, baseline ECG required
For moderate to severe chronic pain and opioid addiction
Ceiling effect for respiratory depression but not analgesia
High affinity to mu-opioid receptors, very resistant to reversal with opioid antagonists
PCA 4 h Maximum
30 mg 6 mg 400 μg
Epidural vs. Non-Epidural or No Analgesia in Labour
Cochrane Database Syst Rev 2011;(12):CD000331 Purpose: To assess all modalities of epidural analgesia (including combined-spinal-epidural) versus non-epidural analgesia and no analgesia during labourin terms of maternal and neonatal outcomes.
Methods: Systematic review of randomized controlled trials comparing all modalities of epidural analgesia to any form of pain relief not involving regional blockade, or no pain relief. The Cochrane Pregnancy and Childbirth Group’s Trials Register was searched up to end March 2011. Primary analysis was by intention to treat.
Results: 38 studies involving 9658 women were included; 33 studies compared epidural analgesia with opiates. Epidural analgesia was associated with better pain relief (risk ratio (RR) 0.05, 95%
CI 0.02 to 0.17), reduced risk of acidosis (RR
0.80, 0.68 to 0.94), and reduced risk of naloxone administration (RR 0.15, 0.10 to 0.23), but was also associated with increased risk of assisted vaginal birth (RR 1.42, 1.28 to 1.57), maternal hypotension (RR 18.23, 5.09 to 65.35), motor-blockade (RR 31.67, 4.33 to 231.51), maternal fever (RR 3.34, 2.63 to 4.23), urinary retention (RR 17.05, 4.82 to 60.39), longer second stage of labour (MD 13.66 minutes, 6.67 to 20.66), oxytocin administration (RR 1.19, 1.03 to 1.39) and caesarean section for fetal distress (RR 1.43, 1.03 to 1.97). No significant differences were found in terms of overall risk of caesarean section (RR 1.10, 0.97 to 1.25), long- term backache (RR 0.96, 0.86 to 1.07), Apgar score less than seven at five minutes (RR 0.80, 0.54 to 1.20), and maternal satisfaction with pain relief (RR 1.31, 0.84 to 2.05).
Conclusions: While epidural analgesia appears effective for pain control during labour, it places women at an increased risk of an instrumental delivery, and is not associated with significant differences in C-section risk, maternal pain relief satisfaction long-term back ache or Apgar scores.
Nociceptive Pathways in Labour and Delivery
Labour
• Cervical dilation and effacement stimulates visceral nerve fibres entering the spinal cord at T10-L1
Delivery
• Distention of lower vagina and perineum causes somatic nociceptive impulses via the pudendal nerve entering the spinal cord at S2-S4
In general, parenteral route is 2-3x more potent than oral
Table 17. Opioid PCA Doses
Agent
Morphine Hydromorphone Fentanyl
PCA Dose
1 mg
0.2 mg 25-50 μg
PCA Lockout Interval
5 min 5 min 5 min
Opioid Antagonists (naloxone, naltrexone)
• indication: opioid overdose (manifests primarily at CNS, e.g. respiratory depression)
• mechanismofaction:competitivelyinhibitopioidreceptors,predominantlyμreceptors
■ naloxone is short-acting (t1/2 = 1 h); effects of narcotic may return when naloxone wears off; therefore, the patient must be observed closely following its administration
■ naltrexone is longer-acting (t1/2 = 10 h); less likely to see return of opioid effects
• side effects: relative overdose of naloxone may cause nausea, agitation, sweating, tachycardia,
hypertension, re-emergence of pain, pulmonary edema, seizures (essentially opioid withdrawal)
Neuropathic Pain
• seeNeurology,N41 Chronic Pain
• chronicpain:greaterthan3mo,orrecurrentpainthatoccursatleast3timesthroughoutthreemonth period
• painofdurationorintensitythatpersistsbeyondnormaltissuehealingandadverselyaffects functioning
• mayhavenociceptiveandneuropathiccomponents;dysregulationofanalgesicpathwaysimplicated • intheperioperativeperiod,considercontinuingregularlong-actinganalgesicsandaugmentingwith
regional techniques, adjuvants, additional opioid analgesia and non-pharmacological techniques