Article Notes

• Hide
  • Pearls
  • Summaries
  • Comments

reference
1
Collection: Remifentanil
A. Physiochemistry
• pKa - 7.3 (58% nonionised @ 7.4)
• Octanol water coeff - 18
• phenylpiperidine opioid
• contain 2 ester bonds so hydrolysed by non-specific tissue esterases.
• Preparation contains 'glycine', so cannot be used epidurally.
• White powder for reconstitution with water - 1, 2, 5 mg packs
B. Pharmacokinetics
1. Dose: (100x morphine potency, ~equal to fent)   - TCI: 3-8 ng/mL     - (up to 15 ng/mL for very stimulating procedures)     - Spontaneous ventilation returns @ 1-2 ng/mL   - 0.1-0.3 mcg/kg/min infusion (with propofol 80 mcg/kg/min (= 34 mL/h for 70 kg).
   • 0.01-0.05 mcg/kg/min spont vent
   • dilute 1 mg to 50 mL = 20 mcg/mL, or 5 mg in 50 mL = 100 mcg/mL.
   • paeds: 0.03 mg/kg in 50 mL then 1 mL/h = 0.01 mcg/kg/min.
   • Or paediatric whole-ampoule dilutions when advanced pumps are unavailable:
   • 1mg in 16.7mLs
   • or 2mg in 33.3 mLs
   • or 3mg in 50mLs 
   • → to give a dilution of 60mcg/mL
   • then for a patient of XYkg running at X.Y mLs/hr is 0.1mcg/kg/min. eg. for a 42kg patient running at 0.1mcg/kg/min will be 4.2mLs/hr which over 4 hrs uses 16 mL so a 1mg ampoule would be sufficient.
   • 1 mcg/kg IV bolus to blunt pressor resp to intubation, better than fentanyl. (equiv. fent 2 mcg/kg, alfent 20 mcg/kg)
   • 3-5 mcg/kg for intubation with propofol 2 mg/kg.
   • 0.2-0.8 mcg/kg bolus for PCA analgesia (++SEs: sedation, desaturation)
2. Absorption - IV
3. Distribution - 0.5 L/kg (small)
4. Protein binding - 70-90%
5. Onset 1-4 min; Offset 4 min (offset due to metab not redist)
6. Metabolism - ß½ ~10 min. (CSHT-8h only 4 min!) Metabolised by non-specific plasma esterases to almost-inactive metabolites (GR90291: 1/4600 activity! / t½ 2h).
   • Minor pathway - N-dealkylation. NOT metabolised by plasma cholinesterase.
7. Clearance - 42 mL/min/kg (30-50% CO)
C. Pharmacodynamics
1. Mech - highly selective mu agonist.
2. CVS - dec MAP & HR 20-30%. (? low dose glycopyrrolate to attenuate brady).
   • No histamine release.
3. CNS
   • max MAC reduction ~ 85% (0.1-0.2 mcg/kg/min = 60-70% MAC reduction).
   • To avoid awareness keep propofol @ at least 80 mcg/kg/min or volatile 0.3 MAC.
   • Sedation.
   • Beware rapid Opioid Induced Hyperalgesia.
4. Resp - ⇣ RR & MV; apnoea. Spontaneous respiration occurs at blood concentrations of 4 to 5 nanogram/mL in the absence of other anaesthetic agents; for example, after discontinuation of a 0.25 microgram/kg/minute infusion of remifentanil, these blood concentrations would be reached in two to four minutes.
5. GIT - dec CTZ stimulation as rapidly metabolised; no ion trapping.
6. Muscle - muscle rigidity similar to alfentanil, though more than fentanyl.
   • May cause chest wall rigidity (inability to ventilate) after single doses of > 1 microgram/kg administered over 30 to 60 seconds or infusion rates > 0.1 microgram/kg/minute.
   • Administration of doses < 1 microgram/kg may cause chest wall rigidity when given concurrently with a continuous infusion of remifentanil.
7. Foetal - little effect as rapidly metabolised by foetus.
  Daniel Jolley
reference
1
Collection: Buprenorphine
A. Physiochemistry
• Semi-synthetic thebaine derivative (like oxycodone).
• Partial µ-agonist.
B. Pharmacokinetics
1. Dose: 0.5 mg q6h IV/IM
   • 30x morphine potency
   • 200mcg-400mcg sublingual qid  for analgesia
2. Absorption - IV, IM, s/l, epidural (po undesirable as ++ 1st pass met)
3. Distribution - 3 L/kg
4. Protein binding - 96%
5. Onset 30 min; Offset 4 h (longer latency & duration than morph)
6. Metabolism - ß½ 5 h; hepatic dealkylation & glucuronidation. Excreted in bile & hydrolysed by GIT bacteria.
7. Clearance - 14 mL/min/kg (dec 30% by GA)
C. Pharmacodynamics
1. Mechanism: µ partial agonist.
   • 50x greater mu rec affinity than morphine.
   • May be used to treat heroin/morphine dependence.
   • Greater lipid solubility than morphine.
   • Ceiling effect to both analgesia & respiratory depression.
   • Long duration as slow to dissociate from receptor & thus difficult to reverse.
  Daniel Jolley
reference
1
Collection: Ketamine
Ketamine is a dissociative anaesthetic & potent analgesic.
• "Dissociative anaesthesia" refers to dissociation of thalamocortical and limbic systems on the EEG.
A. Physiochemistry
• phenylcyclidine (PCP) derivative
• pKa 7.5, weak acid (like thiopentone 60% nonionised @ pH 7.4)
• highly lipid soluble (4x thio)
• ampoule: 200 mg in 2 mL
• acidic solution of i) ketamine hydrochloride with ii) benzethonium chloride (preservative - neurotoxic !)
• 2 optical isomers - S(+)d ketamine has i) more rapid emergence due to higher metab, ii) less emergence SEs, iii) less cardiac depression, iv) 3x analgesic potency.
B. Pharmacokinetics
1. Dose - 1.5-2 mg/kg IV, 10 mg/kg IM
   • oral premed: 6-7 mg/kg po (15-30 min onset)
   • Rx: asthma 20 mcg/kg/min
   • analgesia: 0.1-0.3 mg/kg/h (no dysphoria @ 0.1, sometimes pleasant dreams @ 0.2 mg/kg/h). -[HPH 400mg in 50mL]
   • TIVA: 10-50 mcg/kg/min
2. Absorption - IV, IM, oral or PR
3. Distribution - 8 L/kg
4. Protein binding - 25% (thiopentone 75%, propofol 98%)
5. Onset IV: 45-60s, peak 60s; IM: 3-5 min; Offset 15-30 min
6. Metabolism - alpha∆ 11 min, ß ∆ 2.5 h. Hepatic p450 to N-demethylation to norketamine, hydroxylated to hydroxynorketamine, conjugated to water sol glucuronide derivatives.
   • Norketamine has 1/5 activity of ketamine (? post-op S/Es).
7. Clearance - 18 mL/kg/min (prop 25, thio 4 mL/kg/min)
C. Pharmacodynamics
1. Mech - non-competitive NMDA antagonism (PCP site on NR1 subunit); anti-muscarinic; anti-monaminergic; inhibits peripheral reuptake of catecholamines; S+ enantiomer has some mu receptor activity; inhibits NO synthesis; inh non-NMDA glutamate rec.
2. CNS - analgesia, amnesia, dissociative anaesthetic (thalamocortical - limbic system); inc CBF, CMRO2, ICP & IOP.
3. CVS - direct cardiac depressant, but inc SNS outflow - inc CO, HR, MAP. Variable Vc & Vd.
4. Resp - unaltered response to CO2; bronchodilator; inc salivary secretions; airway reflexes intact.
5. GIT- inc BSL
6. SEs - PONV, emergence delerium, ++ secretions, uterine hypertonicity at > 1.5 mg/kg
7. Interactions - halothane prolongs duration by delaying its redistribution and metabolism.
Ketamine produces a dissociative state (unconsciousness where in cataleptic state, disconnected from surroundings associated with functional and electrophysiological dissociation between thalamo-neocortical and limbic system)
• Characteristically : eye open, slow nystagmus, varying purposeful movement and hypertonus unrelated to stimuli
• Advantages: sympathetic stimulation with preservation of BP esp in hypovolaemic state, preservation of airway reflexes, bronchodilation and intense analgesia
• Disadvantages: can theoretically precipitate myocardial ischaemia (increasing both workload and O2 requirements) increases CBF, increases PVR, emergence delirium (also anaesthetic end-point unclear and uncontrolled movements).
  Daniel Jolley
pearl
1
Prone CPR is an acceptable choice in the setting of COVID-19 where turning a patient supine may create delays or added risk to patient or staff.
Douma MJ, MacKenzie E, Loch T et al. Prone cardiopulmonary resuscitation: A scoping and expanded grey literature review for the COVID-19 pandemic. Resuscitation. 2020 Oct 1; 155: 103111103-111.
  Daniel Jolley
pearl
1
Although exposure to surgery/anesthesia is associated with a small cognitive decline among the elderly, there is no impact from the presence or absence of nitrous oxide.
Sprung J, ASA Abcejo, Knopman DS et al. Anesthesia With and Without Nitrous Oxide and Long-term Cognitive Trajectories in Older Adults. Anesth. Analg. 2020 Aug 1; 131 (2): 594604594-604.
  Daniel Jolley
pearl
1
Peri-operative intravenous lidocaine infusion does not appear to offer meaningful clinical benefit in bariatric surgery.
Plass F, Nicolle C, Zamparini M et al. Effect of intra-operative intravenous lidocaine on opioid consumption after bariatric surgery: a prospective, randomised, blinded, placebo-controlled study. Anaesthesia. 2021 Feb 1; 76 (2): 189-198.
  Daniel Jolley
pearl
1
Propofol has a dose-dependent effect on pupillary diameter, which is positively correlated with BIS.
Sabourdin N, Meniolle F, Chemam S et al. Effect of Different Concentrations of Propofol Used as a Sole Anesthetic on Pupillary Diameter: A Randomized Trial. Anesth. Analg. 2020 Aug 1; 131 (2): 510-517.
  Daniel Jolley
pearl
1
Chewing gum pre-operatively for 2 minutes may reduce post-operative sore throat after general anaesthesia with the SLIPA supraglottic airway.
Wang T, Wang Q, Zhou H et al. Effects of Preoperative Gum Chewing on Sore Throat After General Anesthesia With a Supraglottic Airway Device: A Randomized Controlled Trial. Anesth. Analg. 2020 Dec 1; 131 (6): 1864-1871.
  Daniel Jolley
pearl
1
Recovery after total knee arthroplasty was similar for spinal and general anaesthesia across broad post-operative indicators.
Palanne R, Rantasalo M, Vakkuri A et al. Effects of anaesthesia method and tourniquet use on recovery following total knee arthroplasty: a randomised controlled study. Br J Anaesth. 2020 Nov 1; 125 (5): 762-772.
  Daniel Jolley
pearl
1
The McGrath video laryngoscope improves glottis visualisation in the morbidly obese.
Ruetzler K, Rivas E, Cohen B et al. McGrath Video Laryngoscope Versus Macintosh Direct Laryngoscopy for Intubation of Morbidly Obese Patients: A Randomized Trial. Anesth. Analg. 2020 Aug 1; 131 (2): 586-593.
  Daniel Jolley