General Anesthesia Flashcards
What is pain?
Pain is an unpleasant sensory or emotional experience associated with actual or potential tissue damage
What is General Anesthesia?
Medically induced coma with loss of protective reflexes resulting from the administration of one or more general anaesthetic agents
What are the 5 aims of General Anaesthesia?
- Unconsciousness
- Amnesia
- Analgesia
- Relaxation of skeletal muscles
- Loss of autonomic nervous system reflexes
What are the 3 ideal factors of a general anaesthetic?
- Rapid Onset - Induce smooth and rapid loss of consciousness
- Rapid Offset - Allow for prompt recovery of consciousness after discontinuation
- Wide Safety Margin and no adverse effects
What is Balance techniques?
Combining general anaesthetics to achieve all the desirable effects and favourable properties and minimise risks
Should GA be used for minor procedures?
No. Use LA or monitored anaesthesia instead
What is Monitored Anaesthesia?
Patient maintains a patent airway and responds to commands
What are the 4 stages of Anaesthesia?
- Analgesia - initially analgesia without amnesia, then both are achieved quickly
- Excitement - Amnestic but appear delirious, irregular respiration, may retch or vomit if stimulated
- Surgical Anaesthesia - Regular respiration recurs (stops) followed by Apnea. Loss of eye movements, eye reflexes
- Medullary Depression - Severe depression of brain stem and medullary function
What is the most reliable sign of surgical anesthesia?
Loss of motor and autonomic response to noxious stimuli
What other drugs are used in conjunction with anesthesia?
Opioid Analgesics
Muscle Relaxants
Cardiovascular Drugs
Ventilators
What are examples of inhaled gaseous analgesics?
Nitrous Oxide
Cyclopropane
What are examples of inhaled liquid analgesics?
Ether Halothane Enflurane Desflurane Isoflurane Sevoflurane Methoxyflurane
What type of Anesthetic is commonly used now?
Inhaled Liquid Analgesics
What are the 5 factors that affect the rate at which a therapeutic brain concentration of inhaled analgesic is achieved?
- Solubility - low solubility in blood –> high arterial tension rapidly –> rapid equilibration with the brain –> fast onset of action eg. NO
- Anesthetic concentration in inspired air - increase = increase rate of transfer into blood/brain
- Rate and depth of pulmonary ventilation - increase –> increase anesthetics with moderate to high solubility (eg. haloethane) ; decrease (opioid analgesics) –> reduce onset
- Pulmonary blood flow - increase –> decrease in rate of rise of anesthetic (moderate to high solubility) tension in the blood and brain
- Arteriovenous Concentration Gradient - depends on uptake of anesthetic by tissues (lipophilic) = high, high conc gradient increases the time to achieve equilibrium with brain
What are the 6 factors that affect the rate of recovery from inhaled analgesia?
- Solubility in blood - insolube –> eliminated faster ; soluble –> accumulated in muscle, skin and fat
- Duration of exposure –> increase
- Pulmonary blood flow
- Rate of ventilation
- Hepatic metabolism (eg. haloethane)
- Bacteria in GI tract (eg. NO)
What is the mechanism of action of Inhaled Anaesthetics?
Modifies ion currents by direct interactions with multiple members of the ligand gated ion channel family
- GABA - direct activation –> indirectly increase Cl flux
- Glycine
- Nicotonic
Acts at multiple levels of the CNS –> affects sensitivity of specific neurons and pathways esp for:
- Spino-thalamic tract (pain)
- Reticular activating system (consciousness)
What are the effects that inhaled anaesthetics has on our organ systems?
- Cardiovascular:
a. Decreased mean arterial pressure –> reduction in cardiac output (haloethane, enflurane), decrease in systemic resistant (isoflurane, sevoflurane)
b. Variable effect on heart rate - Depression of myocardial function
- Respiratory:
a. Decrease in minute ventilation (Apnea)
b. Reduced response to hypercapnia (Respiratory Depressant)
c. Increase apnoic threshold - important to support ventilation in recovery
d. Depression of muco-ciliary function
e. Bronchodilation (haloethane, sevolflurane) - Brain: Increase cerebral blood flow by decreasing cerebral vascular resistance
- Renal: Impairs renal autoregulation by reducing renal blood flow
- Liver: Decreased hepatic blood flow
- Uterus: Halogenated anesthetics are potent uterine muscle relaxants
Why are inhaled anesthetics bad for patients with increased intracranial pressure? What inhaled drug should be used?
Because it also increases cerebral blood flow.
Nitrous Oxide - least likely to increase cerebral blood flow
Which inhaled drug can cause liver damage?
Haloethane - Hepatitis, sudden and severe liver necrosis (several days after anaesthesia)
Reactive metabolites may directly damage the liver or initiate immune mediated responses
Which inhaled drug can cause Renal Dysfunction?
Methoxyflurane
Releases fluoride during metabolism
What toxic side effect can inhaled GA drugs have?
Malignant Hyperthermia
Autosomal dominant skeletal muscle disorder
GA triggers hypertension, tachycardia, severe muscle rigidity, hyperthermia and acidosis
Increase in muscle cell calcium
Treatment: Dantrolene to reduce calcium and supportive measures
What is haloethane usually used for?
Adults: maintain anesthesia - medium onset and recovery
Children: Induction
What is Nitrous Oxide usually used for?
Adjunct, Labour pain Lacks potency 20%: significant analgesia 30-80%: sedation 80% (max): cannot give complete unconsciousness or surgical anaesthesia
What are IV anesthetics used for?
Induction: Onset of action is faster as it goes directly into bloodstream
Short outpatient procedures: Recovery is sufficiently rapid
Combined with inhaled or LA: Lack analgesic properties
What is the mechanism of action of Barbiturates (IV GA)?
- Binds to GABA receptors and facilitates actions of GABA by increasing duration of GABA gated chlorine channel opening
- Acts on AMPA receptors to depress glutamate mediated excitation
- Non-synaptic membrane effects
Which IV GA drug is desirable for patients with raised intracranial pressure? Why?
Thiopental
Decreases cerebral metabolism, O2 consumption and blood flow.
What is an inducing GA drug?
Barbiturate - Thiopental
What is the mechanism of action of Benzodiazepines (IV GA)?
Potentiates GABAergic inhibition by increasing the efficiency of GABA without directly activating GABA receptors, through an increase of the frequency of GABA gated chloride channel openings.
What are examples of Benzodiazepines?
Diazepam
Lorazepam
Midazolam
What is the Benzodiazepines used for?
Pre-anesthetic Medication and Adjuvants during procedures
Properties: Sedative, Anxiolytic and Amnestic
What is are 2 negative effects of Benzodiazepines?
High dosages are required for deep sedation –> prolongs anesthetic recovery
Treatment: Flumazenil (benzodiazepine antagonist - short duration of action of 90 mins hence may need multiple doses)
May cause anterograde amnesia
What is the mechanism of action of Propofol (IV GA)?
- Potentiation of GABA receptor activity –> slows the channel closing time
- Sodium channel blocker
- Endocannabinoid system activity - endogenous lipid based retrograde neurotransmittor
What is the most popular IV GA?
Propofol! Rate of onset as rapid as barbiturates, but rate of recovery is even faster
What is Propofol used for?
Induction Maintenance Sedation Monitored Anesthesia Procedural Sedation
Which GA drug induces Dissociative Anesthesia?
Ketamine
What is the mechanism of action of Ketamine (IV GA)?
NMDA receptor antagonist
What is Dissociative Anesthesia?
Catatonia (cannot move), amnesia, analgesic WITHOUT lost of consciousness
What is special about Ketamine?
Only IV anesthetic with both analgesic and anesthetic properties.
Who should Ketamine be used on?
Poor risk elderly patients
Those in cardiogenic or septic shock
Stimulates the CVS through stimulation of Sympathetic CNS and inhibiting uptake of noradrenaline
Who should Ketamine not be used on
Neurosurgical Patients
Increases Intracranial Pressure
What are the negative side effects of Ketamine?
Post-operative disorientation, illusions and dreams
What is Balanced Anaesthesia?
- IV for induction
- Inhaled for maintenance
- Muscle relaxants to facilitate tracheal intubation and to optimise surgical procedures
- LA to provide pre- or peri-operative analgesia
- Cardiovascular Drugs to control transient autonomic responses to noxious surgical stimuli
What are some alternatives to GA?
Monitored Anesthesia Care
Conscious Sedation
What is Monitored Anesthesia Care?
LA + IV GA
The patient is sedated, analgesic and amnestic BUT allowed to continue breathing by themselves.
- Midazolam (pre-mediation)
- Titrated, variable rate propofol infusion (moderate to deep sedation)
- Opioid Analgesics/Ketamine (analgesia and amnesia)
What is Conscious Sedation?
Sedated but conscious (NO DEEP SEDATION)
- Midazolam, Diazepam (reversible by Flumanezil)
- Propofol (sedation)
- Opioid Analgesics (reversible using Nalaxone)
What is Opioid Analgesics reversible with?
Nalaxone
Does GA reduce risk of Tranurethral resection of the prostate (TURP) syndrome?
Yes!
Positive pressure ventilation increases venous pressure and reduces absorption of irrigation fluid.
What are IV GAs?
Barbiturates - Thiopental
Benzodiazepines - Diazepam, Lorazepam, Midazolam
Propofol
Ketamine
