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12 Cards in this Set
- Front
- Back
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Barbiturates - Structure, Classification & Prototype |
- Structure; heterocyclic derivatives of barbituric acid (condensation reaction from malonic acid & urea). Classified according to C2 substitution (Sulfur = THIO, Oxygen = OXY) - Prototype; Hexobarbitone 1934 - first rapid onset short acting |
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Ideal IV induction agent qualities |
- Physicochemical; water-soluble & stable, stable on light exposure, long shelf-life, no pain on IV inj, non-irritant with SC, cheap - Pharmacokinetic; Rapid onset, short duration, rapid redistribution to vessel-rich CNS tissue, rapid clearance & metabolism with active metabolites - Pharmacodynamic; High therapeutic ratio, min CVS/RESP/GIT (Emetic)/Endocrine or histaminic effects, no hangover, no involuntary movements, analgesic at sub-anaesthetic, no emergence nightmares
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Structural Activity Relationships |
- Increasing length of C5 chain incr hypnotic potency up until 6 C, after that incr pro-convulsant - Thio has higher lipid solubility vs oxy - Lipid solubility can be increased w methyl or ethyl N1 substitutions but can incr excitatory phenomena - Stereospecificity; l isomers are more potent than d isomers - racemic mixtures
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Describe the GABA-A channel |
- GABA primary inhibitory neurotransmitter in CNS - Ligand gated chloride channel; pentameric glycoprotein subunits - beta site for barbiturate (& GA) binding, (gamma site for BDZ) - Activation leads to hyperpolarisation by incr chloride conductance & inhibition of further action potentials |
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Explain the mechanism of action of barbiturates |
- Binds to GABA-A receptor binding site; allosteric modulation, enhances & mimics action of GABA and decr GABA dissociation -- At high doses they may activate receptor directly to produce 'barbiturate anaesthesia'
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Thiopentone - Indications |
- Induction agent - Barbiturate anaesthesia - Decr CMRO2 in head injury - Anti-convulsant |
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Thiopentone - Presentation, Preparation, ADME |
- Presentation; Formulated sodium salt in alkaline (pH 10.5) solution which is also bacteriostatic. DO NOT RECONSTITUTE with lower pH solutions i.e. NaCl 0.9, LR. Tauterism. - Rapid onset, Duration 5-10min - A; - D;- Rapid onset of action; due to redistrib vessel rich CNS & high lipid solubility. pKa 7.6 -60% unionised at pH 7.4. PPB 80% - therefore only 12% immediately available (unbound/unionised). Rapid offset from redistrib to muscle. - M; Hepatic oxidatin to mainly inactive metabolites, pentobarbitone excepted. With an infusion metab can convert to saturation kinetics. - E; 3.4 ml/min/kg & t1/2 11.5hr |
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Thiopentone - Dynamics, ADR, contraindications |
- Dynamics; CNS - dose related EEG suppression, decr CMRO2 55%, decr CBF/ICP, decr IOP. CVS - Venodilation (incl pulmonary), SVR usually maintained, decr sympathetic outflow, sl depress myocardial contractility, Resp - central depression, risk laryngospasm, sl hypersalivation, laryngeal reflexes more intact vs propofol - ADR; depress WCC in ICU, intra-arterial ppt to crystal and ischemia/pain (Rx Papaverine/LA Block/Dilution), can ppt Porphyria crisis ... careful dosing in shock/hypovolemic patients due to compartment changes, not suitable for LMA due to laryngospasm |
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Propofol - Physico-chemical & preparation |
- Phenolic derivative (2, 6 Di-isopropylphenol) - Highly lipid soluble (emulsified in soyabean & egg phosphatide) - Weak organic acid, pKa 11 (near entirely unionised at pH 7.4)
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Propofol - Indications & effects |
- IV Induction - TIVA - Sedation (ICU) - Anti-convulsant
- CNS; excitatory in 10% but not thought to be true seizure - CVS; MARKED decr SVR without compensatory tachycardia due to decr sympathetic outflow, decr contractility - Resp; Apnea & diminished laryngeal reflexes - GIT; anti-emesis ?D2 antagonist - Metabolic; fatty infiltration of organs - Bizarre; green urine & hair |
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Propofol - ADME |
- A; IV preparation - D; 98% PPB, HUGE Vd ~4L/kg, rapidly redistributed from plasma to vessel rich tissues. Onset within 30s endpoint is loss of verbal contact - M; Hepatic metab to inactive metabolites. ?Extra-hepatic metabolism also (clearance exceeds HBF) - E; Renal clearance, Cl 30-60 ml/kg/min, t1/2 5-12h. Incr context sens t1/2 in prolonged infusion, (fat redistribution) |
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What is propofol infusion Sdr What problems does an egg allergy pose to administration |
- Described in young ventilated children w unresponsive bradycardia, metabolic acidosis & lipaemia, recommended to not be used in <16 - Egg allergy is usually due to albumin or protein. The phosphatide in propofol is lecithin and is unlikely to produce allergy. The soya bean component is devoid of protein.
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