Pharmacology for Anaesthesia and Intensive Care

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Section IICoredrugs in anaesthetic practice

Monitoring:chemiluminescence or electrochemical analyzers should be used

and are accurate to 1 ppm. Methaemoglobinaemia is only very rarely

significant and is more likely in paediatric patients or those with

methaemoglobin reductase deficiency but levels should be checked before

and after starting NO, and daily thereafter.

Exposure:Environmental NO levels should not exceed 25 ppm for 8 hours

(time-weighted average). Scavenging is not required in a well ventilated unit.

Contraindications:Methaemoglobinaemia (bleeding diathesis, intracranial

haemorrhage, severe left ventricular failure).

Helium

Helium (He) is an inert gas presented as either Heliox (79% He, 21% O 2 )inbrown

cylinders with white shoulders or as 100% helium in brown cylinders at 137 bar. It

does not support combustion.

Itskey physical characteristic is its lower density (and hence specific gravity) than

both air and oxygen.

Helium Heliox Oxygen Air

Specific gravity 0.178 0.337 1.091 1

Therefore, during turbulent flow the velocity will be higher when Heliox is used.

This will reduce the work of breathing and improve oxygenation in patients with an

upper airway obstruction such as a tumour. Helium/oxygen mixtures are also used

for deep water diving to avoid nitrogen narcosis. The lower density of helium/oxygen

mixtures produces higher frequency vocal sounds, giving the typical squeaky voice.

Carbon dioxide

Carbon dioxide (CO 2 )isacolourless gas with a pungent odour at high concentrations.

Itis stored as a liquid at 51 bar at 20◦Cingreycylinders (C = 450 litres up to E = 1800

litres).

Physiochemical properties


Boiling point−78.5◦C


Critical temperature 31◦C


Critical pressure 73.8 bar

Uses

Itis used as the insufflating gas during laparoscopic procedures and occasionally to

stimulate respiration following general anaesthesia. It is also used in cryotherapy.