INHALATIONAL ANAESTHETIC AGENTS Dr Jyeshtharaj Patangankar Assist Prof

INHALATIONAL ANAESTHETIC AGENTS Dr. Jyeshtharaj Patangankar Assist. Prof. Dept. Of Anesthesia

LEARNING OBJECTIVES At the end of the lecture, the student shall be able to n Define MAC n Describe Guedel’s classification of stages of anaesthesia n Enumerate pharmacological actions, indications & side effects of nitrous oxide, halothane, isoflurane, sevoflurane & desflurane n Describe historical importance of ether & chloroform n Enumerate important side effects of enflurane, methoxyflurane & trichloroethylene

GENERAL PRINCIPLES n n n Volatile anaesthetic agents are liquid with low boiling point so that they evaporate easily at room temp. Volatile agents are inhaled & enter circulation via alveolar pulmonary capillaries. GA is the result of a rise in the partial pressure of the agent in the brain

MAC n n MAC is the minimum alveolar concentration required to abolish the response to skin incision in 50% of the patients. MAC represents the potency of an anesthetic agent. Higher the MAC, less potent is the drug. MAC value decreased hypothermia, opioids, sedatives, hypotension MAC value increased hyperthyroidism, hyperthermia, anxiety

CLINICAL STAGES OF ANAESTHESIA n n n Guedel demonstrated in ether anaesthesia Stage 1 Analgesia From start of inhalation of anaesthetic agent to loss of eyelash reflex i. e. unconsciousness Stage 2 Excitement From loss of consciousness to beginning of regular smooth resp. Events seen in 2 nd stage are breath holding, coughing, limb movements, laryngospasm.

Cont…. n n Stage 3 Surgical anaesthesia (Plane I, III, IV) Corneal reflex lost, fixed eyeballs. Laryngeal reflexes lost in plane 3, so endotracheal intubation can be done in this plane Diaphragmatic paralysis occurs in plane 4. Stage 4 – Overdose & Apnoea

INDIVIDUAL AGENTS n n n n n Nitrous oxide Halothane Isoflurane Sevoflurane Desflurane Enflurane Methoxyflurane Ether Trichloroethylene

NITROUS OXIDE n n n Also called as laughing gas Sweet smelling, non irritating, colourless gas supplied in a blue coloured cylinder as a liquid under pressure Nonflammable Weak anaesthetic - MAC 104 Low solubility in blood Excreted unchanged via lungs

ADVANTAGES n n n Good Analgesic Speeds up induction with Volatile agents Decreases requirement of volatile agents Used to reduce awareness Premixed N 2 O/Oxygen: Entonox 50: 50 mixture of gaseous nitrous oxide and oxygen Uses Obstetric analgesia, Dental surgery, Wound dressing, Chest physiotherapy

UNDESIRABLE EFFECTS n n n Diffusion into gas containing spaces Increases volume or the pressure of the gas spaces Interaction with Vitamin B 12 Megaloblastic anaemia & peripheral neuropathy Teratogenecity CVS – Small reduction in myocardial contractility Diffusion hypoxia Post operative nausea & vomiting

HALOTHANE n n n Clear, colourless, sweet smelling liquid Supplied in amber coloured glass bottle to protect against decomposition by light. Potent anaesthetic – MAC 0. 75 High blood gas solubility so induction & recovery are slower No analgesic property

CONT… n CVS 1. Fall in arterial BP due to myocardial depression & significant vasodilatation. 2. Sinus Bradycardia due to increased Vagal tone. 3. Sensitization of myocardium to catecholamine leading to arrhythmias.

CONT… n n CNS 1. Increases cerebral blood flow in high concentration (>1 MAC). 2. Non epileptogenic RS 1. Depresses respiration by decreasing tidal volume 2. Causes bronchodilatation preferred in patients with Asthma 3. Laryngoscopy and intubation can be performed under deep halothane anaesthesia.

CONT…. Uterus Dose related relaxation increasing chances of PPH n Temperature Skin temperature falls as peripheral vasodilatation aids heat loss shivering is common n Liver Can cause post op. liver dysfunction n

SIDE EFFECTS n n Risk of triggering malignant hyperthermia especially in patients with myopathies. Post Op. shivering (Halothane shakes) Avoided in patients with jaundice or history of jaundice after previous halothane anaesthetic. Repeated use of halothane Halothane Hepatitis

ISOFLURANE n n n n Colourless liquid with a pungent smell. Can cause laryngospasm during induction so not used for inhalational induction. Induction as well as Recovery is rapid Least myocardial depressant Causes less hypotension Does not sensitize the heart to the action of catecholamine Do not increase CBF in conc. < 1 MAC Agent of choice for neurosurgery, cardiac surgery & laparoscopic surgeries

SEVOFLURANE n n n n Non irritant, sweet smell Useful for Inhalational induction in children Rapid induction & recovery due to low blood solubility so useful for Day care surgery. Offers good hemodynamic stability Depresses laryngeal reflexes, so we can intubate without muscle relaxants causes bronchodilatation Do not increase CBF Not to be used with soda lime for prolonged period as it can cause nephrotoxicity due to compound A formation

DESFLURANE n n n Newer anesthetic agent. Because of low boiling point (23. 50 C) it is present as gas at room temperature hence requires special vaporizer for its use. Cardio stable. Increases CBF & ICP Rapid induction & recovery due to low blood solubility so useful for Day care surgery. Highly irritant, increased bronchial secretions, high incidence of coughing, laryngospasm. Not suitable for inhalational induction.

ETHER n n It is mainly of historical importance but is still being used as an anesthetic agent in some countries Introduced in clinical practice by WTG Morton in 1846 Safest inhalational anesthetic agent. Sympathetic stimulation maintains the BP with low incidence of arrhythmias & cardiac depression.

Cont… n n n Bronchial smooth muscle relaxation Products of metabolism of ether Relatively nontoxic Can not be used in presence of electro cautery due to risk of explosion. Increases salivary & bronchial secretions, so anticholinergic premedication is necessary. High incidence of postoperative nausea & vomiting.

OTHER AGENTS n n n Enflurane Harmful in patients with H/O convulsions and renal failure Methoxyflurane – Causes renal failure Trichloroethylene (Trilene) Good analgesic Can not be used with soda lime as it forms neurotoxic gas phosgene.

OTHER AGENTS OF HISTORIC INTEREST n n Chloroform Cyclopropane

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