Anaesthesia

5. Induction of General Anaesthesia

5. Induction of General Anaesthesia When it has been decided to give the patient general anaesthesia, induction of anaesthesia (i.e . the process o f transferring th e patient fro m th e consciou s t o th e uncon scious state) may be achieved by the following routes : (a) Inhalation . (b) Intravenous . (c) Intramuscular . (d) Other s (rectal, enteral). It shoul d b e realized tha t th e induction o f general anaesthesi a an d renderin g th e patient unconsciou s wil l immediatel y necessitat e th e institutio n o f al l thos e proce dures that are essential for the care of the unconscious patient. The basis of all these are still (1) the institution and maintenance of a free airway, (2) continued respiratio n (either spontaneou s o r artificial ) an d (3 ) a n adequat e conten t o f oxyge n i n th e inspired air at all times. Inhalation Agents Gases or vapours are used. A. Gases (1) Nitrous oxide N 2 0. Chemica l name : Nitroge n monoxide . Officia l preparation : Nitrous oxide (B.P. and U.S.P. ) The best way to use nitrous oxide for induction of anaesthesia is giving the patient pure oxygen t o breathe by facemask fro m th e anaesthetic machine fo r th e first 2- 3 minutes. Nitrous oxid e i s then adde d (i n a percentag e o f approximately 70%) . The patient will be induced more easily and more safely as the so called 'pre-oxygenation ' has cause d de-nitrogenation . Th e uptak e o f N 2 0 wil l b e greatl y facilitate d an d equilibrium more rapidly established without the danger of hypoxaemia . Used fo r induction , thi s method i s fairly rapi d (especiall y afte r pre-oxygenation ) and no t to o unpleasant , thoug h hallucination s an d som e strugglin g i n th e secon d stage may occur . Althoug h arteria l bloo d become s abou t 90 % saturated wit h N 2 0 after 2 0 minutes, all tissues will take hours to reach tha t point . Afte r discontinuin g nitrous oxide, consciousness returns early (mainly due to a rapid drop in the arterial blood tension) but the expired air will continue containing small amounts of the gas for severa l hours. 'Diffusion hypoxia ' i s caused by the displacement o f oxygen fro m the alveol i b y returnin g amount s o f nitrou s oxid e carrie d b y th e bloo d fro m th e 66 Anaesthesia Management saturated tissues . An oxyge n enriche d atmospher e shoul d therefore , b e given t o th e patients to inhale for 20-30 minutes following th e prolonged use of nitrous oxide. Nitrous oxide is now being used with increasing frequency i n a number o f institu tions fo r it s effec t a s a 'sedative' . Denta l clinics , childre n examinatio n room s an d similar location s ar e turning t o 'nitrou s oxid e and oxyge n sedation' , a n offshoo t o f 'Relative Analgesia ' (R.A.) . Unlik e R.A. , th e concentratio n o f th e N 2 0 i n 0 2 i s limited to below 25% so as not to produce unconsciousness (Griffin, 1982) . (2) Cyclopropane C 3H6 CH2 A฀ H2C CH 2 Particularly usefu l i n childre n an d th e ver y il l where hig h oxyge n concentratio n may be an advantage. Its use will be discussed in Chapter 7. B. Vapours Any volatile anaesthetic agent is capable of inducing anaesthesia, though some are more suitabl e tha n others . Th e method s use d ar e eithe r th e 'ope n dro p method ' using 1 2 gauze layers over a 'Schimmelbusch Mask ' (or similar device), or the 'dra w over' method . Ai r o r anothe r ga s (e.g . nitrou s oxide ) flows o r i s draw n ove r (o r through) th e volatil e anaestheti c containe d i n a bottl e o r a speciall y constructe d vaporizer (ofte n thermostaticall y controlle d to ensure exact vaporization) . In the past, ethyl chloride, chloroform an d diethyl ether were used for this purpose, especially in...