1. Intravenous versus Inhalational Induction
Dr James Hayward
Worthing Anaesthetic department

2. Introduction Transition from the awake to the anaesthetised state.
Changing situation
Hypotension and arrhythmias
Hypoventilation and apnoea
Aspiration
Laryngospasm, hiccups
Adverse drug reactions
Involuntary movements

3. Stages of anaesthesia (1)
Guedel (1937) – inhalational induction using diethyl ether.
Stage 1 (analgesia)
Normal reflexes
Ends with loss of eyelash reflex and unconsciousness
Stage 2 (excitement)
Irregular breathing, struggling
Dilated pupils
Vomiting, coughing and laryngospasm
Ends with loss of eyelash reflex and onset of automatic breathing

4. Stages of anaesthesia (2)
Stage 3 (surgical anaesthesia)
Plane I
Until eyes central and loss of conjunctival reflex
Swallowing and vomiting depressed
Pupils normal to small
Lacrimation increased
Plane II
Regular deep breathing
Loss of corneal reflex
Pupils larger
Plane III
Shallow breathing
Light reflex depressed
Laryngeal reflexes depressed
Lacrimation depressed
Plane IV
Diaphragmatic paralysis
Carinal reflexes depressed
Stage 4 (overdose)
Apnoea
Dilated pupils

5. Inhalational Children Airway obstruction Difficult intubations
Unable to cannulate/needlephobes

6. An ideal anaesthetic gas
Predictable in onset and emergence
Provide muscle relaxation
Provide bronchodilation
Be cardio-stable
Not trigger malignant hyperpyrexia
Inflammable
Undergoes no transformation in vitro
Easily estimated at site of action

7. Halothane

8. Sevoflurane

9. Isoflurane

10. Desflurane

11. Potencies MAC MAC is highest at Decreases with Unaffected by
Concentration at one atmosphere that abolishes motor response to a painful stimulus in 50% of patients
MAC is highest at
Aged term to 6 months
Decreases with
Increasing prematurity
Increasing age
Reduction in body temperature (2-5% per Celsius)
Hyponatraemia, barbiturates, Ca- blockers, pregnancy.
Unaffected by
Hypocarbia, hypercarbia, gender, thyroid, hyperkalaemia.

12. MAC Isoflurane Desflurane Enflurane Halothane Sevoflurane Nitrous 1.15
6.0
Enflurane
1.7
Halothane
0.77
Sevoflurane
Nitrous
104

13. Factors affecting rapidity of inhalational induction
Increasing alveolar concentration
High flow breathing circuit
Increasing minute ventilation
High cardiac output slows induction by decreasing partial pressure at the alveolus
If the partial pressure of anaesthesia in the pulmonary artery and vein is equal then the alveolar partial pressure increases more rapidly.
Second gas effect

14. Second gas effect Should speed induction.
Nitrous oxide is rapidly absorbed from the alveoli and so causes an abrupt rise in the alveolar concentration.
In practice only a minimal effect.

15. Which anaesthetic agent has been shown to be teratogenic in animals?
Nitrous oxide, administered to pregnant rats at over 50% concentration for over 24hrs has been shown to increase skeletal abnormalities.
Inhibition of methionine synthesis
Impaired uterine blood flow caused by nitrous oxide

16. Which anaesthetic gas is associated with the greatest frequency of dysrhythmias?
Halothane
Increases the sensitivity of the myocardium to epinephrine, resulting in more frequent premature ventricular contractions and dysrythmias.

17. An ideal IV induction agent?
Water soluble, stable in aqueous solution and compatible with iv fluids.
Rapid onset (within 1 arm-brain circulation time)
Rapid and predictable recovery.
Properties
Anticonvulsant
Antiemetic
Analgesic
Amnesic
Should not impair
Renal function
Hepatic function
Steriod synthesis
Have no teratogenic properties

18. IV agents
Action?
Not sure
“Modulate GABAergic neuronal transmission, thereby interfering with transmembrane electrical activity.”
With increasing age volume of distribution increases, and elimination reduces
Older patients are more sensitive to intravenous anaesthetics.
Dose calculations should be based on lean body mass

19. Thiopentone Barbiturate Induction dose
3-5mg/kg
Dose dependent CNS depression with hypnosis and amnesia
Loss of of consciousness within 15 seconds.
Recovery
5-10 minutes.
Metabolised
99% by liver
1% excreted by kidneys.
Elimination half-life
6-12hrs.
Accumulates with repeated dosing – not used for maintenance.
Driving impaired for 8hrs.

20. Propofol 2,6 diisopropylphenol Induction Loss of consciousness
2-2.5mg/kg
Loss of consciousness
within 1 minute
Recovery
4-6 minutes
Clearance by redistribution to fatty tissues and metabolism by the liver.
Incidence of post-op nausea and vomiting is less.
During induction
Rapid arterial and venous dilation
Mild negative inotropic effects
BP drop 20-30%
Reduced effect by slow rate of infusion and pre-induction volume loading.

21. Etomidate Carboxylated imidazole dissolved in 35% propylene glycol.
Induction
mg/kg
Recovery within
3-12 minutes.
Rapid redistribution and hepatic metabolism
Veno-irritation.
Myoclonus may occur due to disinhibition of subcortical neuronal activity.
High incidence of nausea and vomiting.

22. Ketamine
Phencyclidine derivative available as a racemic mixture of two isomers.
10 times more lipid soluble than thiopental.
Rapid CNS depression
Sedation, amnesia and analgesia
Doses
1-2mg/kg IV
10mg/kg IM
Stun dose 4mg/kg for uncooperative patients.
Elimination half life is 3hrs.
Stimulates cardiovascular system (non-dose dependent)
Causes smooth muscle relaxation
S/E
Bad trips
Salivation
Tricyclic interaction – hypertension and cardiac dysrythmias.

23. Best outpatient agent? Propofol Rapid on and off
Low incidence of nausea and vomiting

24. Best trauma agent?
Ketamine recommended for use in acutely hypovolaemic patients
Caution in chronically or critically ill patients.
Depleted catecholamines may result in severe hypotension.
In this instance etomidate may be a better choice.

25. Which agents raise ICP and which lower ICP?
Reduce
Etomidate
Thipopentone
Propofol
Fentanyl
Raise
Ketamine

26. Intravenous Advantages: Disadvantages: Rapid onset Dose titratable
Depression of laryngeal reflexes (good for laryngeal mask airways)
Anti-emetic and anti-convulsive properties
Disadvantages:
Venous access required
Risk of hypotension
Apnoea common
Risk of anaphylaxis
Loss of airway control

27. Inhalational Advantages: Disadvantages: Avoids venepuncture
Respiration maintained (avoids being unable to ventilate)
Slower depression of reflexes
End-tidal CO2 can be measured
Rapid recovery
Upper oesphageal tone maintained
Disadvantages:
Slow process
Potential excitement phase
Irritant and unpleasant
Pollution
May cause rise in intracranial pressure/intraocular pressure

28. Thanks!