General Anaesthetics Flashcards
Definition of general anesthesia?
GAs are drugs which produce reversible loss of all sensations and consciousness. Cardinal features are
1. Loss of sensation especially pain
2. Sleep and amnesia
3. Immobility and muscle relaxation
4. Abolition of somatic and autonomic reflexes.
Stages of Anaesthesia
- Stage of analgesia: pain abolished, dream state, reflex and respiration normal
- Stage of delirium: hyperexcitable state, heart rate and BP rise.
- Surgical anaesthesia: 4 planes
Plane 1: roving eyeballs, eyes becomes fixed as plane ends
Plane 2: loss of corneal and laryngeal reflex
Plane 3: pupil dilates and light reflex lost
Plane 4: intercostal paralysis, dilated pupil, shallow respiration - Medullary paralysis: breathing stops, coma, death
Example of second gas effect
N2O
Classification of general anaesthetics
Inhalational and intravenous
Inhalational:
1. gas: Nitrous oxide
2. Volatile liquid: Halothane, isoflurane, enflurane, sevoflurane, desflurane
Intravenous:
1. Fast acting: Thiopentone Sod, propofol
2. Short acting: further divided to three:- a. Opioids: Fentanyl, b. dissociative anaesthetic: Ketamine, c. benzodiazepine: lorezapam, diazepam
Halothane
Volatile liquid, sweet odor, no irritation, non inflammable.
Potent anaesthetic.
2-4% for induction
0.5-1% for maintenance
Actions of Halothane
- CVS:-
Direct depression of myocardial contractility by reducing intra cellular Ca2+ concentration.
Cardiac output reduced
BP falls 20-30mmhg
Heart rate reduced
No baroreceptor reflex
Sensitize heart to arrythmogenic action of adrenaline
2.RESP:-
Depression of respiration
Breathing shallow and rapid
Ventilators support with oxygen required
- Kidney
Low bp leads to reduced blood flow results in reduced GFR - Inhibit intestinal and uterine contractions.
- Pharyngeal, laryngeal and cough reflex abolished
Advantages of Halothane
- Potent, non-inflammable anaesthetic.
- Induction is smooth and rapid - in 2-5 min surgical anaesthesia can be produced
- Non-irritant- therefore, does not augment salivary or bronchial secretions.
- Recovery is rapid
- Postoperative nausea and vomiting ared low incidence.
Disadvantages of halothane
- Not a good analgesic
- Not a muscle relaxant
- Depressed CO, heart rate decreased, adrenaline arrhythmias
- Hypotension
- Malignant Hyperthermia
- Hepatitis
- Low GFR
- Increases cerebral blood flow hence increases intracranial pressure
Status in anaesthesia is Halothane
Not used worldwide due to poor analgesic and no muscle relaxation. But still used in India. Preferable in paediatric group as well
Enflurane
Same as Halothane
No liver toxicity
Causes seizures
Isoflurane
Newer version of enflurane
No liver and kidney toxicity
No seizures
Fall in BP mainly due to vasodilation
No cerebral vasodilation hence preferred for neurosurgeries
Good maintenance anaesthetic
Ether like odor hence irritable and not preferred in children
Desflurane
Rapid action
Pungent odor
Sevoflurane
Not pungent
Induction and emergence from anesthesia is rapid, hence preferred for day case/outpatient surgeries and in children
Fall in BP due to vasodilation
Respiratory depression
No seizures
Chemically unstable hence degraded by CO2 absorbants like soda like into compound A which causes nephrotoxicity.
Malignant hyperthermia in some
Malignant hyperthermia?
Susceptible individuals have abnormal Ryanodine receptor calcium channels at sarcoplasmic reticulum of skeletal muscles. Channels triggered by halothane to release massive amount of Ca2+ which causes persistent muscle contractions and increased heat production
Propofol
Propofol is an oily liquid with several advantages making it the most preferred IV anaesthetic.
Advantages of propofol
- Quick induction: 15-45 seconds
- Rapid recovery
- Can be used to outpatient surgery
- Safe in pregnancy
- Has antiemetic property
Actions of propofol
Cause CNS depression by GABA receptor mediated neuronal inhibition
No airway irritability hence preferred in asthmatics
Fall in BP due to Vasodilation and decreased PR
Respiratory depression
Disadvantages of Propofol
- Respiratory depression
- Hypotension
- Direct negative inotropic effect
- Pain on injection m
- Prolonged use can cause propofol infusion syndrome: skeletal muscle necrosis, metabolic acidosis, lipaemia, Hyperkalemia, renal failure and heart failure
What is Preanaesthetic medication
It refers to use of drugs before anesthesia to make it safe and less unpleasant.
Aims are:-
1. Relief of anxiety and apprehension preoperatively to facilitate smooth induction
2. Perioperative amnesia
3. Supplement analgesic action and potentiate the anesthetic so less amount is needed
4. Anti emetic
5. Decrease secretions and vagal stimulations
6. Decrease acidity and volume of gastric juice so there is no injury if aspirated
Various pre anesthetic medications?
- Sedative-anti anxiety drugs:
Benzodiazepines like diazepam and lorazepam: produce tranquility and smoothen induction, anti emetic action and slight amnesia
Midzolam: good amnesic
Promethazine: antihistamine with sedative, anti emetic and anticholinergic action - Opioids: Morphone Pethidine
Prevent anxiety and produce analgesia so less amount of anesthetic needed
Disadvantage: causes resp depression, fall in bp - Anticholinergic: Atropine, Hyosine, Glycopyrrolate
To reduce salivary and bronchial secretions, prevent vagal bradycardia and hypotension
Hyoscine also has anti emetic and amnesia - Neuroleptics: chlorpromazine, triflupromazine, haloperidol
Anti anxiety and anti emetic - H2 blockers/ PPI: ranitidine, pantoprezole, omeprezole
Decrease acid secretion hence less chance of regurgitation - Antiemetics: metoclopramide, ondansetron, domperidone
