Volatile agents


Basic Pharmacology

Basic pharmacologic overviews of the volatile agents are available at anaesthetist.com. Also the WFSA site has the following overviews: Volatile Anaesthetic Agents (1995) , Volatile Update 11 (2000) and inhalational agents (includes nitrous – 2008). This OSU Vet Sciences PDF is a very good PDF!

MetroHealth/AnaesthesiaWiki provide basic phamacology notes and details for each agent.

Jason Sewell’s Anaesthetic Structure Database has the physicochemical properties of all the volatile anaesthetic molecules – not just those in current practice – with a rotatable 3D image that needs java of the molecule and its surrounding electrical fields. Searching by name is the easiest way to find the agent of choice.

Eger has a long article about maintenance agents in Medscape (worth registering).

Drs. Nickalls and Mapleson have published a set of graphs showing iso-MAC values for the common anaesthetic agents with and without nitrous for patients of different ages.

Physics of gases and vapours: WFSA , my notes on vapourisers

Halothane

MAC 0.7. Predisposes to arrhythmias with high levels of circulating catecholamines and nodal rhythms in children. Profoundly impairs autoregulation of cerebral blood flow. Can initiate malignant hyperthermia.

  • Wikipedia, AnaesthesiaWiki
  • Hepatotoxicity of inhaled agents – brief lecture notes from Chris Thompson
  • Trey’s 1969 A&A syndrome of halothane hepatitis paper
  • Notes about halothane-ether azeotropes

Enflurane

Mac 1.68. Less soluble than halothane (more rapid onset and offset), fewer arrhythmias and much less chance of MH. Impairs cerebral blood flow autoregulation but to a lesser extent than halothane, can infrequently trigger seizure activity, is metabolised to fluoride in levels that infrequently are significant.

Product information from Wikipedia, FRCA and MetroHealth.

 

Isoflurane

Mac 1.14. Similar to enflurane but less soluble (more rapid again) and much less metabolism, can be used in closed circuit. Less effect on cerebral blood flow autoregulation than Enflurane of Halothane. More irritant than Sevoflurane

  • Product information from NZ Govt Medsafe, also Wikipedia, MetroHealth

Sevoflurane

Compared to isoflurane, is less irritant and has more rapid onset and offset, but is metabolised to a greater extent. Mac approx. 2.6 for adults. Excellent for mask induction. Sevoflurane would be ideal for long cases except that it is more expensive than isoflurane and that concerns exist about potential dose and time dependent nephrotoxicity, so that low flows cannot be used for extended periods.

Sevoflurane is metabolised to fluoride ion in a dose*time dependent fashion. Additionally, potentially nephrotoxic by-products (compound A etc.) are formed when exposed to warm moist hydroxide-containing soda lime. Rate of production is increased with pH, low flows, high CO2’s and long MAC*hr exposures.

Current US recommendations are to not use flows less than 1 litre/minute, and to not exceed 2 MAC*hours exposures at flows less than 2 litres/minute with standard soda lime.

Fluoride and compound A etc. are not produced to any significant extent if Amsorb or similar NaOH-free is used instead of ordinary soda lime – so you can use any flow you like.

  • US FDA information and regulation (see warnings)
  • Basic pharmacology from NZ Govt MedSafe Wikipedia and MetroHealth RxList.
  • Pub Med abstract of Amsorb in reducing compound A production. PubMed abstract showing in clinical use 1.0 l/min FGF seems OK.
  • Eger Stanford review

Desflurane

The most insoluble volatile agent with fastest onset and offset, causes sympathetic activation with rapid increases in inhaled levels, irritates airways, requiresspecial electronic vapouriser.

  • Basic profiles from MedSafe NZ, MetroHealth Wikipedia
  • Effect on SEF95, BIS. MF

Malignant Hyperthermia and volatile anesthetics

  • Malignant Hyperthermia by A.Urwyler. (4K)
  • Altered sodium current response to intracellular fatty acids in halothane-hypersensitive skeletal muscle. Wieland, Steven J., Qi-Hua Gong, Jeffrey E, (2K)
  • basic info from Wikipedia
  • Link to other MH info in this textbook

CNS effects

  • EEG effects of desflurane – lowers spectral edge like all the others
  • Paediatric EEG effects of Sevoflurane
  • The details will be in the full SNACC bibliography (members only)
  • Effects of halothane, hypocapnia and hypercapnia on cerebrocortical laser-doppler flow oscillations following inhibition of nitric oxide synthase.

Apparatus

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