Knowledge

Collections of knowledge containing reference notes.

  • Pethidine/Meperidine

     

    Pethidine (Meperidine) is a phenylpiperidine synthetic opioid first synthesized in 1938. Although widely used in the 20th century, it has fallen out of favour over the past decade due to abuse potential, limited advantages over other opioids and the existence of toxic metabolites.

    A. Physiochemistry

    • pKa - 8.5 (9% nonionised @ 7.4)
    • Octanol water coefficient - 39 (so 40x lipid solubility of morphine)
    • phenylpiperidine opioid

    B. Pharmacokinetics

    1. Dose - 25-100 mg (10% morphine potency). Limit 1000 mg 1st day, then 600 mg/day there after.
    2. Absorption - IV, IM, epidural, po (55% biov)
    3. Distribution - Vdss 4.5 L/kg. Crosses placenta - foetal 80% of maternal.
    4. Protein binding - 60%
    5. Onset 10 min ; Offset 2-3 h
    6. Metabolism - ß½ 3 h; N-demethylation to norpethidine and then hydrolysis to norpethidinic acid; also direct hydrolysis to pethidinic acid. Renal elimination.
      • Norpethidine - ß½ 15 h; 50% analgesic properties, 2x convulsant effects.
    7. Clearance - 20 mL/kg/min (same as morph & fentanyl)

    C. Pharmacodynamics

    1. Mech - mu and kappa agonist, causing potent spinal and supraspinal analgesia.
    2. CNS - more euphoria, less N/V than morphine. No miosis, but may cause mydriasis (pupil dilation -atropine-like kappa action). No EEG changes like morphine. ⇡ latency & amplitude of SSEPs.
      • NB: has LA action, so can be used as sole agent for neuroaxial block.
      • anti-shivering effect (kappa)
    3. CVS - ⇣ MAP (> than morphine) due to histamine release & alpha adrenergic blockade (vasodilation). Inc HR (atropine like effect). Large doses depress myocardial contractility. May cause hypertensive crisis in those on MAOIs.
      • vasodilation
      • tachycardia
      • depress myocardial contractility
    4. Resp - potent resp depressant - greater effect on TV than RR. Histamine release. Chest wall rigidity.
    reference

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  • Methohexitone

     

    A barbiturate derivative (Brevital™, Brietal™) intravenous anaesthetic agent, no longer available in Australia although still used in other parts of the world.

    Preferred for use in electroconvulsive therapy for its pro-seizure effects and comparatively short duration.

    Compared to thiopentone

    • Oxybarbiturate
    • Made up in 50 mL to 1% solution
    • 3x more potent
    • 3x clearance (12 mL/kg/min)
    • tß½ 3 h (STP 8h)
    • Greater ionised proportion
    • Less protein binding (65%)
    • More rapid recovery: 2-3 min (smaller fat compartment, no active metabolites, ⇡ clearance)
    • Higher incidence of pain on injection
    • Pro-convulsant/epileptiform EEG (excitatory in 30%)
    • PONV (30%)
    • Less dec MAP, more inc HR than STP
    • More pronounced resp depression
    reference

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  • Propofol

     

    A. Physiochemistry

    • A highly lipid-soluble alkylphenol.
    • 2,6 di-isopropyl phenol
    • 20 mL ampoules contain:
      1. 200 mg 1% propofol
      2. 10% soybean oil (solubiliser)
      3. 1.2% egg lecithin (emulsifier)
      4. 2.25% glycerol (make isotonic)
      5. Sodium hydroxide (buffer)
    • pKa 11, pH 7
    • 90% non-ionised @ pH 7.4
    • weak acid
    • stable at room temp, not light sensitive
    • 1 mL = 0.1 g fat = 1.1 kcal

    B. Pharmacokinetics

    1. Dose
      • 2 mg/kg induction -> 2-6 mcg/mL
      • 3-4 mg/kg in children
      • 1 mg/kg load then: 10, 8, 6 mg/kg/h infusion (10m, 10m, cont) after 1 mg/kg loading - aims for blood conc of 3 ug/mL.
      • Children: 15 mg/kg/h for 15 min, 13 mg/kg/h for 15 min, 11 mg/kg/h for 30 min then 9 mg/kg/h for 1-2 h, then 9 mg/kg/h for 2-4 h -> 3 ug/mL.
      • Sedation 25-100 mcg/kg/min
      • Plasma levels:
      • major surg 4 mcg/mL (4-8 ug/mL)
      • minor surg 3 mcg/mL
      • 50% wake @ 1.07 mcg/mL (decrement lvl: 1.2 mcg/mL on TCI)
      • 50% orientated @ 0.95 mcg/mL
      • Psychomotor perfomance pre-op levels @ 0.3 mcg/mL
    2. Absorption - IV
    3. Distribution - Vdcc 0.5 L/kg, Vdss 2-10 L/kg
    4. Protein binding - 98% albumin
    5. Onset < 60s, peak 60-90s (slightly slower than thio: peak 30-60s); Offset 5-10 min (faster than thio).
    6. Metabolism - alpha1∆ 2 min, tß∆ 1h, CSHT-8h: 30 min. Conjugated to glucuronide & sulphate - water sol and renally excreted. 0.3% excreted unchanged.
    7. Clearance - 30 mL/kg/min.
      • Children - larger central vol; longer CSHT (10m@1h & 20m@4h cf. 7m@1h & 10m@4h for adults); slower recovery; but require higher infusion rates and have higher clearance (req. same blood (=effect) conc as adults).
      • NB: children have primarily pharmacokinetic differences not pharmacodynamic.
      • Women - higher clearance.

    C. Pharmacodynamics

    1. Mech - potentiates GABA inhibition.
    2. CNS - anaesthetic, anticonvulsant (?), antiemetic, antipruritic, amnesic.
      • Not ant-analgesic like thio.
      • Inc interthreshold range for temp
    3. CVS - 25-45% dec MAP, dec CO, dec SVR (dec SNS outflow; direct effect on veins, dec intracellular Ca mobilisation), HR unchanged (resets barorec response).
    4. Resp - resp depression (apnoea in 30% alone, 100% + narcotic), dec TV, inc RR, bronchodilation (slight), dep laryngeal reflexes.
    5. Renal - dec RBF, green urine.
    6. GIT - antiemetic, no hepatic effects.
    7. Haem - intralipid dec platelet aggregation.
    8. SEs - anaphylaxis rare; sig hypotension in volume depleted; hallucinations; abuse.
    reference

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  • 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).
    reference

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  • Etomidate

     

    Etomidate (Amidate™) is short-acting intravenous anesthetic agent first developed in 1964. It is available and used in the United Kingdom, Europe, New Zealand, United States, but not Australia.

    Advocates highlight etomidate's hemodynamic stability when used for induction. Critics point to the well-established adrenocortical suppression, and wide-range of suitable alternatives (propofol, ketamine, thiopentone) in trained hands.

    A. Physiochemistry

    • Carboxylated imidazole
    • 2 isomers - only R(+) hypnotic
    • Haemodynamic stability, minimal respiratory depression, cerebral protection, wide margin of safety.
    • Originally formulated in propylene glycol (painful), now in soybean lipid.

    B. Pharmacokinetics

    1. Dose - 0.3 mg/kg (0.1-0.4 mg/kg)
    2. Absorption - IV
    3. Distribution - 4 L/kg
    4. Protein binding - 75% (like thiopentone)
    5. Onset 30-60s ; Offset
    6. Metabolism - alpha1 ½ 2.5m, alpha2 ½ 30m, tß½ 3.5h; hepatic ester hydrolysis of ester side chain.
    7. Clearance - 20 mL/kg/min

    C. Pharmacodynamics

    1. Mech - probably by GABAa receptors.
    2. CNS - hypnosis; no analgesic action; ⇣ CBF and CMRO2
    3. CVS - stable; may have slight dec MAP 15% due to ⇣ SVR.
    4. Resp - minimal; sometimes brief hypoventilation or apnoea post-induction.
    5. Endo - adrenocortical suppression - inhibits 11ß-hydroxylase (11-deoxycortisol → cortisol). Temporary & reversed by vit C.
      • ⇡ ICU mortality when used for sedation.
    6. SEs - excitatory phenom, involuntary muscle movement (50%), PONV (30%), thrombophlebitis (20%), pain on injection.
    reference

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