Inhalation and Intravenous Anaesthetics Flashcards
What three things are anaesthetics trying to achieve?
Immobility; hypnosis/amnesia; autonomic areflexia
Idea of balanced anaethesia
How are inhaled anaesthetics washed into the blood stream and out?
Lipid solubility?
Diffusion gradient created by Pbr being lower than that of the PInspired. When anaethetic switched off gradient reverses.
The drug is inhaled and exhaled. Exhalation is mode of getting rid of the drug
The more lipid soluble a drug, the lower the dose required (more potent) however unsure and could be due to GABA and glycine modulation.
What is the MAC?
Note using a vaporizer, the fraction of inspired anaesthetic must be higher than the amount desired in the alveoli. Titrate against end tidal vapor monitoring
The minimal alveolar concentration producing immobility on standard surgical stimulus in 50% of patients.
Thus a lower MAC means more potent
A way of describing potency and dose referenced to a clinical standard.
Pharmacodynamics of modern agents:
CNS, CVS, RS
CNS: hypnosis, immobility; decrease CMRO2, may increase CBF and ICP
CVS: vasodilators, lowers BP, only desflurane increases HR
RS: depresses resp system, bronchodilators
Halothane
Desflurane
Sevoflurane
Isoflurane
Halothane: 0.86% MAC
Isoflurane: 1.1 % MAC CVS stability. Cardiac theatres
Desflurane: 6%, rapid onset and offset. Good or long cases
Sevoflurane: 2% MAC pleasant to breath, often used in children
Types of intravenous agents
Barbiturates (Thiopentone) Phenols (Propofol) Imidazoles (etomidate) Phenylcycline derivatives (ketamine) Benzodiazepines (midazolam)
Mechanism of IVA’s
Thiopetone,propofol, etomidate, midazolam all ehance GABA, Cl- current prolonged, hyperpolarisation.
Ketamine: Bind to PCP receptors so antagonise glutamate, less excitation.
Pharmcokinetics of IVA’s?
Redistribution?
Highly lipid soluble
Drugs taken up by IV go to well perfused organs first, e.g. the brain. The offset of the drug action after a single dose is often due to redistribution.
Thiopentone: onset and offset; clearance; metabolised; cvs and rs
Onset: Very rapid 10s Offset: rapid to redistribution Clearance: Slow Metabolised: Liver CVS: lowers PVR and BP RS: depressed
Propofol: onset and offset; clearance; CVS
Note used as standard against thiopentone, as cleared faster, used as maintenance.
Both have CVS instability
Onset: Mod rapid 20s
Offset: rapid to redistribution
Clearance: Fast so no accumulation, maintenance. No hangover
CVS: sig. decrease in BP
Etomidate: CVS; clearance; negatives
CVS: Very stable
Clearance; Rapid
BUTS: Adrenocortical inhibition (lack of stress hormones), myoclonus and epileptogenic
Ketamine: Special properties; CVS; CNS; RS; Onset and but
Special: is analgesic
CVS: is a stimulant, good for shocked patient
CNS: increases CMRO2, CBF and ICP (bad for neurosurgery)
RS: preserves airway reflexes, keeps em breathing
Onset: Slow and dysphoria
