Intravenous versus Inhalational Induction Dr James Hayward Worthing Anaesthetic department

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

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

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

Inhalational Children Airway obstruction Difficult intubations Unable to cannulate/needlephobes

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

Halothane

Sevoflurane

Isoflurane

Desflurane

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.

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

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

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.

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

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.

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

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

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.

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.

Etomidate Carboxylated imidazole dissolved in 35% propylene glycol. Induction 0.2-0.4mg/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.

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.

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

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.

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

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

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

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