Class on General Anaesthetics

1. General Anaesthetics
Dr Farhan

3. General Anaesthesia
 It refers to drug induced reversible loss of consciousness and
all sensations.
 There is no single agent that can produce all the above
effects. Hence we have an anaesthetic protocol
 Premedication, Induction of anaesthesia, Maintenance of
anaesthesia, skeletal muscle relaxation, Analgesics as
premedication-during and post operatively, Drugs to reverse
effects

4. GUEDEL’S STAGES OF ANAESTHESIA
 Stage 1 – Analgesia or disorientation
 Stage 2 – Excitement or delirium*
 Stage 3 – Surgical Anaesthesia# – Plane 1,2,3 & 4
 Stage 4 – Medullary Depression
 *Induction & #Maintenance

5. CLASSIFICATION
 INHALATIONAL ANAESTHETICS –
GAS – Nitrous Oxide
LIQUID – Ether, Halothane, Fluranes
 INTRAVENOUS ANAESTHETICS –
INDUCERS – FAST – Thiopentone (Barb) Propofol, Etomidate
(Non – Barb)
INDUCERS – SLOW – Diazepam, Lorazepam, Midazolam
(Benzos)
OPIOIDS – Fentanyl
NEUROLEPTIC AGENT – Droperidol

6. Inhalational Agents
 Minimum Alveolar Concentration (MAC) – It is the minimum
concentration of inhalational anaesthetic in the alveoli that is required to
produce immobility in response to a painful stimulus in 50% patients. It
indicates the potency of the inhalational general anaesthetic. Greater
is the MAC, lesser is the potency.
Nitrous oxide – High MAC, Least Potent
Methoxyflurane – Low MAC, Most Potent
 Blood Gas Partition Coefficient – It determines the solubility of an agent
in the blood. It indicates the speed of onset and recovery of an
anaesthetic drug. Greater is the blood gas partition coefficient, lesser is
the speed of onset and recovery
Methoxyflurane – High BGPC, Slow Acting
Desflurane – Low BGPC, Fastest Acting

7. Ether
 Highly inflammable, Pungent smell, Explosive agent
 Effective analgesic, Muscle relaxant
 Slow inducer
 Good bronchodilator. Safer in asthmatics
 Inexpensive
 C/I – Diabetics (causes Hyperglycaemia)

8. Nitrous Oxide
 Colourless, Non Irritating, Non Inflammable gas – Laughing Gas
 Effective analgesic at low concentration – Labour Pain
 Weak Muscle Relaxant, Fast Inducer & Faster recovery due to High BGPC
 It takes time to produce surgical anaesthesia and hence combined with others (50-
65% N20 + 33% O2)
CONCENTRATION EFFECT – When N2O is administered in high dose, then it diffuses
from alveoli to blood that generates negative pressure in the alveoli that results in
more gas removal from the cylinder or source
SECOND GAS EFFECT – When N2O is given along with other inhalational agent like
halothane, then due to generation of negative pressure, the second gas is also taken in
from the cylinder or source
 ADVERSE EFFECTS – If N2O is suddenly stopped while recovery from anaesthesia
then more negative pressure is generated in the alveoli which removes alveoli from
blood, results in diffusional hypoxia. It can be prevented by giving 100% oxygen a
few minutes before discontinuing the nitrous oxide
Presence of impurities cause methemoglobinemia and laryngospasm
Bone Marrow Suppression + Megaloblastic Anaemia = Subacute Combined
Degeneration of spinal cord

9. Halothane
 Colourless, Non Irritant, Non Inflammable, Pleasant smelling volatile liquid
 High therapeutic efficacy with weak analgesic effect
 Excellent choice for induction of anaesthesia in children – rapid & smooth
 Agent of choice in asthma
 Agent of choice in internal version and manual removal of placenta as is
relaxes the placenta. C/I during labour
 ADVERSE EFFECTS – Pruritis, sensitizes the heart to catecholamines and
can cause arrhythmias C/I in pheochromocytoma; Cardio depressant effect
resulting in hypotension and bradycardia
Malignant Hyperthermia; Post anaesthetic chills and shivering

10. FLURANES
 Enflurane, Isoflurane, Desflurane, Sevoflurane
 ISOFLURANE – Isomer of enflurane. Medium rate of onset and
recovery.
 Maintains cardiac output, produces least intracranial tension
and hence an agent of choice for cardiac and neurosurgery
Used for producing controlled hypotension
Safe in pheochromocytoma
Lacks proconvulsive property
 Adverse effects – Coronary steal phenomenon – divert blood
away from myocardium – ischemia in patients with CAD

11. INTRAVENOUS ANAESTHETICS
 Thiopentone – Ultra short acting barbiturate and is the most widely used.
3-5mg/kg; 5-10 min
 High lipid solubility, rapidly crosses BBB but terminates its action very
quickly due to redistribution
 Recovery time – 5 minutes
 Safe in pregnancy
 Decreases ICP – agent of choice for cerebral protection, also reduces
cerebral oxygen consumption
 Hyperalgesia at subanaesthetic doses
 Adverse effects – Hypotension, respiratory depression, apnoea
 C/I – Acute intermittent Porphyria
 VTIG - SH

12. INTRAVENOUS ANAESTHETICS
 Propofol – It has fast onset within 15 sec and lasts for 5-10 min because of
redistribution. 2mg/kg
 IV Drug of choice for day care surgery, for sedation in ICU, and in patients with
malignant hyperthermia
 Used with ALFENTANIL to produce TIVA
 It has antiemetic action
 Indicated for those procedures where return to a preoperative mental status is
desirable
 Bronchodilator property
 Cerebroprotective activity; no muscle relaxing property
 Can cross placenta but safe in pregnancy
 Anti convulsant property
 Adverse effects – Apnoea and pain at the site of injection, marked fall in BP,
Bradycardia

13. INTRAVENOUS ANAESTHETICS - KETAMINE
 It is a phencyclidine derivative that is often abused
 Its primary site of action is the cortex and limbic system. It
acts by blocking the action of Glutamate at NMDA receptor
 It is a highly lipophilic drug
 After its fast diffusion into brain it is subsequently
redistributed in various body compartments with
simultaneous hepatic metabolism followed by urinary and
biliary excretion.
 Its usual induction dose is 1-2mg/kg

14. INTRAVENOUS ANAESTHETICS - KETAMINE
 It produces dissociative anaesthesia which is characterized by a feeling of
dissociation from the surroundings, profound analgesia, immobility and
amnesia with light sleep.
 It is a potent analgesic, potent bronchodilator
 It significantly increases HR, BP and cardiac output (can be used in
SHOCK) along with marked increase in cerebral blood flow and ICP. Hence
it is C/I in patients of hypertension, IHD, Schizophrenia and Epilepsy.
 It is the drug of choice for induction in children. At Sub doses, it is used
for painful procedures of short duration like dressing of burn,
radiotherapy, minor orthopaedic procedures
 Adverse Effects – Emergence delirium characterized by vivid dreams,
hallucinations, disorientation and sensory illusions. (use diazepam or
midazolam)

15. NEUROLEPT ANALGESIA
NEUROLEPT ANAESTHESIA
 A fixed dose combination of opioid analgesic FENTANYL (50mcg/ml) and a
neuroleptic DROPERIDOL (2.5mg/ml) is used to produce a state of neurolept-
analgesia. It is diluted in a glucose solution and infused IV over 10-15 minutes.
 It is characterized by a state of calmness, psychic indifference to the
surroundings and profound analgesia without any loss of consciousness.
 Alfentanil, Sufentanil, Remifentanil can also be used in place of Fenatanyl
 This is utilized for endoscopies, angiographies, burn dressings and minor
surgical procedures.
 Neurolept-analgesia + 65% nitrous oxide + 35% oxygen = Neurolept-
anaesthesia

16. PRE ANAESTHETIC MEDICATION
comfort and less adr to patient
 Antianxiety drugs – diazepam (5-10mg oral) lorazepam (2mg IM)
midazolam (70-100 mcg/kg IV)
 Sedative Hypnotics – Promethazine (25mg IM)
 Opioid Analgesics – Morphine (8-12mg IM) Pethidine (50-100 mg IM)
 Anticholinergics – Atropine (0.5 mg IM) Hyoscine (0.5mg IM)
Glycopyrrolate (0.1-0.3 mg IM)
 Anti emetics – Metaclopramide (10 mg IM) Domperidone (10 mg oral)
Ondansetron (4-8 mg IV)
 H2 Receptor blockers – Ranitidine (150-300 mg oral) omeprazole (20mg)