Inhaled Anaesthetics

Jason Koerber


Definitions
Gas
A substance above its critical temperature.
Volatile
A substance in a gaseous state but below its critical temperature.

Definitions - Anaesthesia
“Anaesthesia” was first used by the Greek philosopher Dioscorides in the 1st century to describe the narcotic effect of the plant mandragora.

The word reappeared in the 1771 Encyclopaedia Britannica, where it was defined as “privation of the senses”.

After introduction of ether by Morton in 1846, Oliver Wendell Holmes coined the word to describe the new phenomenon that made surgical procedures possible.


Definitions - Anaesthesia 2
We measure the response to stimulation.
Does the patient move when their name is called?
Does the response to incision suggest conscious perception?
Does the heart of BP go up in response to surgical manipulation?
Does the patient remember events, conversations, or pain?
Regional or General?


Definitions - Anaesthesia 3
Anaesthetic agent: reversible.
General consists of the following components:UnconsciousnessAmnesiaAnalgesiaMuscle relaxation
Regional: blockade of pain transmission.


MAC
Measure of potency.
Minimum alveolar concentration at steady state that prevents reaction to a standard surgical stimulus (skin incision) in 50% of subjects at sea level.
Meyer-Overton hypothesis




Name the Anaesthetic Agent!


H H H H
H C C O C C H
H H H H

Diethyl ether
Ethanol and sulphuric acid.
Flammable/explosive.
High blood:gas and oil:gas coefficients.
10% metabolised to ethanol and aldehydes.
Nausea and vomiting.
Seizures can occur.
Increased salivation.
Increased gluconeogenesis

Name the Anaesthetic Agent!


F H F
H C O C C F
F Cl F
Isoflurane
Halogenated ethyl methyl ether.
Resp: dep, pungent, bronchodil.
CVS: dec SVR, tachy,
? coronary steal.
CNS: best balance of reduction cerebral consumption and minimal increase in cerebral blood flow.
Met: 0.2% (trifluroacetic acid, F-, CO)


Name the Anaesthetic Agent!


F F H
H C O C C F
F F Cl

Enflurane
Halogenated ethyl methyl ether.
Resp: most dep, non-irritant
CVS: dec SVR, tachy, heart not sensitized.
CNS: high conc in presence of hypocarbia produce 3Hz spike and wave pattern. Increase ICP.
Met: 2% (F-)

Name the Anaesthetic Agent!


Cl F
H C C F
Br F
Halothane
Halogenated hydrocarbonThymol 0.01% preservative.Dissolves into rubber.
Resp: dep, sweet, bronchodil.
CVS: dec SVR, brady, sensitizes to catecholamines.
CNS: increases cerebral blood flow more than other volatiles.
Met: 25% (trifluroacetic acid, Cl-, Br-, F-). Hepatitis?


Name the Anaesthetic Agent!


F H F
H C O C C F
F F F

Desflurane
Fluorinated ethyl methyl ether.Tec 6 vaporizer.
Resp: dep, pungent, coughing and breath holding.CVS: Similar to isoflurane but at conc above 1 MAC may produce stimulation.
CNS: increased cerebral blood flow
Met: 0.02% (Trifluroacetic acid)


Name the Anaesthetic Agent!


F CH3
H C O C H
H CH3

Sevofluorane
Polyfluorinated isopropyl methyl ether.Achiral - unlike other volatiles.
Resp: dep, pleasant odour bronchodilationCVS: dec SVR, HR unchanged, heart not sensitized.
CNS: increases cerebral blood flow. ? Post-op agitation in children
Met: 3.5% (F-), compound A


Name the Anaesthetic Agent!


H F Cl
H C O C C H
H F Cl

Methoxyflurane
Halogenated ethyl methyl ether.
BP 105 degrees.
Very soluble in blood.Blood:gas coefficient 13.MAC 0.2.
Fluoride ion caused renal failure.



Nitrous oxide (N2O)
lManufacture: heating ammonium nitrate to 250 degrees. May contain impurities NH3, N2, NO, NO2, HNO3. These are removed by scrubbers, water and caustic soda.
lBlue cylinders. Filling ratio.
lConcentration effect, second gas effect and diffusion hypoxia.
lResp: small fall in VT and increase in rate.
lCVS: mild direct cardiac depressant. Normally counterbalanced by central sympathetic stimulant effects.
lCNS: increases cerebral blood flow. Produces analgesia.
lToxicity.


Xeon
lInert, odourless gas.
lMakes up 0.0000087% of atmosphere.
lFastest onset.
lResp: increases VT and decreases rate.
lCVS: no effect on contractility but may cause small decrease in HR.
lCNS: increases cerebral blood flow. May be used to enhance CT images of brain.Produces analgesia.
lNot metabolised.


The bloody table!!