The anaesthetic technique that was used at PMH at that time was safe and simple – but limited. It consisted of an initial injection of thiopentone (pentothal) to put the person to sleep. (Many older people will remember having been asked to count to ten as they were given such an injection.) Thiopentone is a short-acting drug – not a good agent to keep the patient anaesthetized for very long – so a switch must be made to another anaesthetic.
Source: wiktionary