P11: General Anaesthetics Flashcards
What are the 3 main effects of GA
Sedative
Hypnotic
Analgesic
What are sedatives, hypnotics and analgesics
- Sedative - compound that reduces irritability or excitation
- Hypnotic - compound that induces sleep
- Analgesic - compound that reduces pain sensation without loss of consciousness
Name typical GAs in use
Desfluorane sevofluorane Fentanyl Propofol Thiopentone Isofluorane
What are the 4 stages of Anaesthesia
- Analgesia Stage
- Excitement Stage
- Surgical Anaesthesia Stage
- Medullary Depression Stage
What happens in the 3rd plane of the Analgesia Stage
Complete analgesia and amnesia, disorientation, vertigo/ataxia, increased respisation, BP and HR
What happens in the excitement stage of anaesthesia
Loss of consciousness to automatic breathing
What can happen to respiration in the surgical anaesthesia stage
Automatic respiration to respiratory paralysis
What happens in the first plane of surgical anaesthesia
Cessation of eye movements, loss of swallowing reflex
What happens in the second plane of surgical anaesthesia
Laryngeal reflex lost, tear secretion increases, regular deep breathing, response to skin stimulation lost
What happens in the third plane of surgical anaesthesia
Progressive intercostal paralysis, diaphragmatic respiration persists, pupils dilated and light reflex lost
What happens in the fourth plane of surgical anaesthesia
Complete intercostal and diaphragmatic paralysis (apnea)
What happens in the medullary depression stage
Stoppage of respiration to death - medullary paralysis = respiratory depression, vasomotor collapse and death
What can be used to prevent many of the changes found in the stages of anaesthesia
Neuromuscular blockers
What is the lipid theory of GA mechanisms
- Relationship between lipid solubility and anaesthetic potency
- Anaesthesia occur if solubilisation of the GA in the lipid bilayer causes a redistribution in membrane lateral pressure
- Ion channels in particular are highly sensitive to membrane lateral pressure
- Increased pressure prevents channels opening limiting neural excitation
What is the protein theory of GA mechanisms
- Specific targeting of CNS receptors
- GABA, Glycine, 5-HT and ionotropic glutamate receptors and voltage gated ion channels
What are the 2 major classes of GA and give examples of each
Intravenous - propofol, thiopnetone
Inhalation -
- Gaseous - Nitrous Oxide
- Voltaile liquids - desfluorane, sevofluorane, isofluorane, halothane
Define the Minimum Alveolar Concentration (MAC)
Steady state partial pressure (%) of an inhalational agent required for immobility of 50% of subjects exposed to a noxious stimulus (surgical incision)
What is the Minimum Alveolar Concentration (MAC) used for
- Provides a means to compare the potency of the various inhalational agents
- Serves as a guide to determining dose
50% of the MAC dose (MAC awake) gives what response from patients
50% of patients can be woken
100% of the MAC (MAC) dose gives what response from patients
50% of patients wont move at surgical incision
130% of the MAC dose (ED95) gives what response from patients
95% of patients will not move at surgical incision
150-200% of the MAC dose (MAC-BAR) gives what response from patients
50% of patients have blocked autonomic responses
Describe the characteristics of sevoluorane and its MAC value
Rapid-acting Volatile liquid anaesthetic Non-irritant Rapid recovery MAC - 2.1% NB - has largely replaced halothane
Describe the side effects of sevofluorane
- Can trigger malignant hyperthermia
- Can increase intracranial pressure
- Little effect on heart rhythm compared with other volatile anaesthetics
Describe the mechanism of sevofluorane
- Positive allosteric modulator of GABAa receptors
- NMDA receptor antagonist
- Potentiates glycine receptor activity
- Inhibits nicotinic ACh and 5-HT3 receptors
Describe the characteristics of isofluorane and its MAC value
- Rapid-acting volatile liquid anaesthetic
- Analgesic and Muscle relaxer
- Usually used to maintain anaesthesia induced by other agents
MAC - 1.15%
What are the vascular effects produced by isofluorane
- Increased incidence of coronary ischaemia
- Heart rate can rise, particularly in younger patients
- Systemic vascular resistance can decrease, reducing arterial pressure and cardiac output
What are the potential side effects of isofluorane
- Bronchial secretions = coughing and laryngospasm
- Malignant hyperthermia
- Post-operative cognitive dysfunction
What type of anaesthetics are triggering factors for malignant hyperthermia
Most inhalational anaesthetics
What is the first sign of malignant hyperthermia
Elevated CO2 production
What is malignant hyperthermia characterised by
- Hypermetabolism in skeletal musculature
- Muscle rigidity
- Muscle injury
- Increased sympathetic nervous system activity
- Hyperkalaemia
How is malignant hyperthermia treated
Intravenous administration of dantrolene (suppresses exciation-contraction coupling in muscles) and supportive therapies (cooling, oxygen)
What is the MAC of nitrous oxide
104% - would cause asphyxia
Where is Nitrous oxide used
In combination with other anaesthetics - allows a reduction in dosage
Dental/obstetric preparation - entonox, a 50:50 mix with oxygen
Describe the characteristics of nitrous oxide as an anaesthetic
Inhaled and excreted via the lungs Rapid onset of action Good analgesic actions Causes euphoria No signicicant effects on respiration, liver, kidney or GI tract
What are the most important injection anaesthetics
Propofol and thiopentone
How is IV injection better than other injection routes
Faster, more stable and more reliable
Describe the characteristics of propofol
Rapid onset and metabolism (hepatic)
N.B. Used for induction and maintenance of anaesthesia
What is given with propofol
Given alongside lidocaine
Supplemented by nitrous oxide or opioids
What are the side effects of Propofol
- Respiratory Depression
- Hypotension due to peripheral vasodilation
- Induction of cardiac dysrhythmia
- Can induce priapism in males
Describe the characteristics of Thiopentone
- Barbiturate with high lipid solubility
- No analgesic properties
- Smooth and rapid induction of anaesthesia
Describe the mechanism of Propofol
Potentiates inhibitory GABAa receptor activity, slowing the channel-closing time and by blocking voltage gated sodium channels
How is the action of thiopentone terminated
Terminated by redistribution into adipose tissue, rapid recovery from anaesthesia but produces prolonged sedation
What are the side effects of thiopentone
- Induces respiratory depression and hypotension
- Lack of analgesic effects may result in increased SNS on recovery: tachycardia, sweating, tachypnea, raised BP, pupil dilation
- Narrow margin between anaesthesia and cardiovascular depression
Describe the mechanism of Thiopentone
Binds GABAa Nicotinic 5-HT3 Glycine receptors (all ion channels)
What kinds of pre-medications are used before GAs
Anti-emetics
Opioid analgesics
Benzodiazepines
Give examples of anti-emetics and where the act
Droperidol, Domperidone (DA D2 antagonists)
Acts in CTZ in brainstem
Give examples of Opioid analgesics used as pre-medications and what they do
Alfentanil, fentanyl, remifentanil
Pre-surgical pain relief, sedation, reduction in GA dosage
Side effects of opioid analgesics as pre-medications
Respiratory and cardiovascular depression, emesis
Give examples of Benzodiazepines used as pre-medications and what they do
Diazepam, lorazepam
Anxiolytic/sedative, amnesia, little respiratory and cardaic depression
