General Anesthetics Flashcards
What is general anesthesia?
A medically induced coma and loss of protective reflexes
What are the overall aims of general anesthesia?
Unconsciousness
Amnesia
Analgesia
Relaxation of skeletal muscles
Loss of autonomic nervous system reflexes
Ideally, should have rapid and smooth loss of consciousness, fast and easy recovery, and a high margin of safety with no adverse effects
General anesthesia vs monitored anesthesia?
General anesthesia is best used for extensive procedures
Monitored anesthesia is best used in minor procedures where the patient can maintain a patent airway and respond to commands
What are the stages of anesthesia?
Analgesia (initially without amnesia, then with amnesia)
Excitement (amnestic but delirious, with irregular respiration and can easily retch or vomit if stimulated)
Surgical anesthesia (regular respiration recurs with apnea, loss of eye movements and eye reflexes)
Medullary depression
What stage of anesthesia is avoided?
Excitement
What is the most reliable sign of surgical anesthesia?
Loss of motor and autonomic response to noxious stimuli
What are the broad categories of inhalational general anesthetics?
Gas
Liquids
What are the broad categories of intravenous general anesthetics?
Inducing agents
Dissociative anesthesia
Neurolept analgesia
How does PK affect inhaled anesthetics?
Absorption and distribution influence onset of anesthesia, where brain concentration depends on transfer of anesthetic from alveolar air to blood then from blood to brain
Absorption and distribution also affect recovery time, where if the drug is more widespread in other tissues you take longer to recover as the drug has to escape back out from the lungs
Metabolism and excretion affect rate of recovery
How does solubility affect inhaled anesthetics?
E.g. nitrous oxide: Low solubility in blood = transfer to brain quickly = reaches arterial tension quickly and rapidly equilibrates with brain to induce fast onset
How does anesthetic concentration in the inspired air affect inhaled anesthetics?
Increased rate of transfer into blood and therefore brain
Anesthetics with moderate solubility can be given at a higher concentration initially to induce fast onset, then lowered to maintain optimal depth of anesthesia
How do rate and depth of pulmonary ventilation affect inhaled anesthetics?
Increase in ventilation can significantly increase anesthetics with moderate to high solubility like halothane
Depression in respiration by opioid analgesics reduces onset of anesthesia
How does pulmonary blood flow affect inhaled anesthetics?
Increase in blood flow decreases the rate of rise of anesthetic tension in blood as blood flows too fast, decreasing rate of rise of anesthetic concentration in the brain for anesthetics with moderate to high solubility
How does the arteriovenous concentration gradient affect inhaled anesthetics?
Concentration gradient depends on uptake of anesthetic by tissues
A high concentration gradient increases the time taken to achieve equilibrium with the brain, increasing onset
How do metabolism and excretion affect rate of recovery of inhaled anesthetics?
Relatively soluble anesthetics are eliminated faster. More insoluble anesthetics accumulate in muscle, skin and fat and are more slowly eliminated
More anesthetic in the body = longer recovery time
Pulmonary blood flow (to a point) and rate of ventilation increase = increase rate of recovery via respiratory excretion
Hepatic metabolism contributes to clearance, e.g. halothane
Bacteria in the GI tract break down nitrous oxide
How do inhaled anesthetics work?
Interact with ligand gated ion channels to reduce function (GABA, nicotinic and glycine)
Act at multiple levels of the CNS depending on the sensitivity of specific neurons and pathways (spinothalamic tract and reticular activating system)
What are the effects of inhaled anesthetics on the CVS?
Reduces cardiac output and systemic resistance to reduce mean arterial pressure
Variable effect on heart rate
Depression of myocardial function
What are the effects of inhaled anesthetics on the respiratory system?
Decrease in minute ventilation
Reduced response to hypercapnia
Increase apneic threshold (means you need to support ventilation in recovery)
Depression of mucociliary function
Halothane and sevoflurane cause bronchodilation
What are the effects of inhaled anesthetics on the brain?
Increases cerebral blood flow by decreasing cerebral vascular resistance
Can be bad for patients with increased intracranial pressure
Nitrous oxide is least likely to increase cerebral blood flow, good for neurosurgery
What are the effects of inhaled anesthetics on the kidneys?
Impairs renal autoregulation by reducing renal blood flow
Methoxyflurane releases fluoride during metabolism, may cause renal dysfunction
What are the effects of inhaled anesthetics on the liver?
Decreased hepatic blood flow
Repeated exposure to halothane may cause liver damage
Reactive metabolites may directly damage the liver or initiate immune mediated responses
What are the effects of inhaled anesthetics on the uterus?
Halogenated anesthetics are potent uterine muscle relaxants
What is malignant hyperthermia?
An autosomal dominant skeletal muscle disorder, where general anesthesia triggers hypertension, tachycardia, severe muscle rigidity, hyperthermia and acidosis, can be fatal
Caused by an increase in muscle cell calcium levels
Treated with dantrolene to reduce calcium release
What is halothane?
An inhaled general anesthetic, mostly used to maintain anesthesia but sometimes used for induction esp in children
Relaxes skeletal muscles, potentiating skeletal muscle relaxants
Can cause dose-dependent respiratory depression
Decreases blood pressure as it depresses cardiac output with little change in systemic resistance
Can rarely cause hepatitis
What is nitrous oxide?
An odourless inhaled general anesthetic
Rapid onset and recovery but lacks potency, where it can produce significant analgesia and sedation but cannot produce complete unconsciousness or surgical anesthesia
Used as an adjunct for other inhaled anesthetics, and used as an analgesic agent for labour pain
Pros and cons of using intravenous general anesthetics?
Fast onset and recovery, commonly used for induction or short outpatient procedures
Do not require specialized vaporiser equipment or disposal equipment
Most lack analgesic properties, have to be combined with inhaled or local anesthetics
What are barbiturates?
An intravenous general anesthetic
Binds to GABA receptors, increasing duration of chloride channel opening, and also acts on AMPA receptors to depress glutamate mediated excitation
Thiopental used for induction as it rapidly crosses BBB
High doses or continuous infusion decreases arterial blood pressure, stroke volume and cardiac output
Potent respiratory depressants
Decreases cerebral metabolism, oxygen consumption and blood flow, good for decreasing intracranial pressure
What are benzodiazepines?
An intravenous general anesthetic
Potentiates GABA without directly activating GABA receptors, increasing frequency of chloride channel opening
Diazepam, lorazepam and midazolam are used as pre-anesthetic medication and during procedures performed under LA due to sedative, anxiolytic and amnestic properties
Alone do not act very quickly and do not induce surgical anesthesia
High doses may prolong post anesthetic recovery and cause anterograde amnesia
Can have flumazenil to accelerate recovery
What are benzodiazepines?
An intravenous general anesthetic
Potentiates GABA without directly activating GABA receptors, increasing frequency of chloride channel opening
Diazepam, lorazepam and midazolam are used as pre-anesthetic medication and during procedures performed under LA due to sedative, anxiolytic and amnestic properties
Alone do not act very quickly and do not induce surgical anesthesia
High doses may prolong post anesthetic recovery and cause anterograde amnesia
Can have flumazenil to accelerate recovery
What is propofol?
An intravenous general anesthetic
Potentiates GABA receptor activity, slowing channel closing time, blocks sodium channels and has some endocannabinoid system activity
Rapid onset, recovery faster than barbiturates, patients feel subjectively better with less nausea
Used for induction, maintenance, sedation and monitored anesthesia
Rapidly metabolized by the liver and excreted by the kidneys
Decreases blood pressure and respiration
What is ketamine?
An intravenous general anesthetic
NMDA receptor antagonist producing dissociative anesthesia, associated with catatonia, amnesia and analgesia without a loss of consciousness
Highly lipophilic, rapidly distributes to the brain
Metabolised by the liver and excreted by the kidneys and in bile
Has analgesic and anesthetic properties!
Stimulates CVS, useful in elderly patients or patients with cardiogenic/septic shock
Increases intracranial pressure, decreases respiratory rate
Associated with post-op disorientation, illusions and dreams
What is balanced anesthesia?
IV for induction
Inhaled for maintenance
Muscle relaxants to facilitate tracheal intubation and to optimize surgical procedures
LA to provide pre- or peri-operative analgesia
CV drugs to control transient autonomic responses to noxious surgical stimuli
What is monitored anesthesia?
Use regional or local anesthesia supplemented by IV anesthetics
Midazolam for pre-medicative anxiolysis, amnesia and mild sedation
Propofol for moderate to deep sedation
Opioid analgesics or ketamine
Patients might still require respiratory support!
What is conscious sedation?
Patient retains the ability to maintain a patent airway and respond to commands
Midazolam, diazepam for anxiolysis, amnesia and mild sedation
Propofol for sedation
Opioid analgesics reversible via nalaxone
