Halothane Flashcards
How is halothane stored?
Unstable when exposed to light and corrodes certain metals.
Stored with 0.01% thymol to prevent the liberation of free bromide.
Dissolves into rubber and may leach out into breathing systems after the vaporiser is turned off.
Respiratory effects
Minute volume depressed largely due to a decreased TV.
The normal response to hypoxia and hypercarbia is blunted. More pronounced MAC >1
Bronchiolar tone reduced.
Sweet, non-irritant odour- can be used in induction.
Cardiovascular effects
Significant effects on the heart!
Bradycardia- increased vagal tone, depressed SA and AV activity.
Direct myocardial depressant- reduces cardiac output.
Sensitises the heart to catecholamines which can lead to arrhythmias (esp ventricular and bradyarrhythmias)
SVR reduced = increased cutaneous blood flow.
Due to a reduced cardiac output, renal and liver flow is reduced.
What is the max amount of adrenaline allowed to be given alongside halothane use?
Less than 100 micrograms per 10 minutes
What drugs should be avoided with halothane?
Drugs that specifically reduce AV conductivity e.g. verapamil
CNS effects
Increases cerebral blood flow more than any of the other volatiles leading to a significant increase in ICP above 0.6 MAC.
Cerebral O2 requirements are reduced.
How is halothane metabolised?
Up to 25% undergoes oxidative metabolism by cytochrome P450 to produce trifluoroacetic acid, Br- and Cl-
Reductive metabolism producing F- and other reduced metabolites predominate when the liver becomes hypoxic. Not involved in halothane hepatitis
How does it cause reversible hepatic damage?
Often subclinical and associated with a rise in transaminases. Probably due to hepatic hypoxia.
Halothane hepatitis
Trifluoroacetyl chloride (an oxidative metabolite of halothane) may behave as a hapten and bind covalently with hepatic proteins. This induced antibody formation.
Diagnosis of exclusion.
1/2500-35000 incidence.
Risk factors: multiple exposures, obesity, middle age and female.
Mortality 50-75%
In theory, other agents could cause this (esp enflurane) but unlikely as rate of metabolism much lower.
When should halothane be avoided?
If administered within the previous 3 months/ history of previous adverse reaction/ pre-existing liver disease.
Molecular weight
197
Boiling point
50.2
SVP at 20 degrees
32.3
MAC %
0.75
Blood: gas
2.40
