General Anesthetics DSA Flashcards
List the general inhaled anesthetics
Desflurane Enflurane Halothane Isoflurane Nitrous oxide Sevoflurane
List the general intravenous anesthetics
Dexmedetomidine Diazepam (Valium) Etomidate Fentanyl Fospropofol Ketamine Lorazepam (Ativan) Methohexital Midazolam (Versed) Propofol (Diprivan) Thiopental
The anesthetic state produced by general anesthetics is a collection of component changes in behavior or perception that include?
Which agent can be used alone to achieve all five?
Unconsciousness
Amnesia
Analgesia
Attenuation of autonomic reflexes to noxious stimulation
Immobility in response to noxious stimulation (skeletal muscle relaxation)
None
Describe the modern practice of anesthesiology
Relies on the use of combinations of intravenous and inhaled drugs (balanced anesthesia) to take advantage of favorable properties of each agent while minimizing their adverse effects
Utilizes sedatives, neuromuscular blocking agents, local anesthetics, and analgesics in addition to general anesthetics to induce anesthesia
Describe monitored anesthesia care
Sedation-based anesthetic techniques
Utilized for diagnostic and/or minor therapeutic surgical procedures
Performed without general anesthesia
Typically involves the use of midazolam for a premedication (to provide anxiolysis, amnesia, and mild sedation) followed by a titrated propofol infusion (to provide moderate to deep levels of sedation)
Potent opioid analgesic or ketamine may be added to minimize the discomfort associated with injection of local anesthesia and surgical manipulations
Describe conscious sedation
Used primarily by nonanesthesiologists (dentists)
The patient retains ability to maintain a patent airway and is responsive to verbal commands
Benzodiazepines and opioid analgesics (fentanyl) in conscious sedation protocols have the advantage of being reversible by specific receptor antagonist drugs (flumazenil and naloxine)
Describe deep sedation
Similar to light state of general (intravenous) anesthesia involving decreased consciousness from which the patient is not easily aroused
Transition from deep sedation to general anesthesia is fluid and sometimes difficult to clearly determine where transition is
Accompanied by a loss of protective reflexes, an inability to maintain a patent airway, and lack of verbal responsiveness to surgical stimuli
Intravenous agents used in deep sedation protocols mainly include sedative-hypnotics propofol and midazolam, sometimes in combination with potent opioid analgesics or ketamine, depending on level of pain associated with surgery or procedure
Dsecribe intensive care unit (ICU) sedation
Patients who require mechanical ventilation for prolonged periods
Sedative-hypnotic drugs and low doses of IV anesthetics
Describe mechanism of general anesthetic action
Anesthetics affect neurons at various cellular locations, but the primary focus has been on synapse Presynaptic action may alter release of neurotransmitters, whereas a postsynaptic effect may change frequency or amplitude of impulses exiting synapse At organ level, effect of anesthetics may result from strengthening inhibition or from diminishing excitation within CNS (studies on isolated spinal cord tissue have shown that excitatory transmission is impaired more strongly by anesthetics than inhibitor effects are potentiated) Chloride channels (gamma-aminobutyric acid-A (GABAa) and glycine receptors) and potassium channels (K2p, possibly Kv, and Katp channels) remain primary inhibitory ion channels considered legitimate candidates of anesthetic action Excitatory ion channel targets include those activated by acetylcholine (nAChRs and mAChRs) by excitatory amino acids (amino-3-hydroxy-5-methyl-4-isoxazol-proprionic acid (AMPA), kainite, and N-methyl-D-aspartate (NMDA) receptors), or by serotonin (5-HT2 and 5-HT3 receptors)
Describe volatile anesthetics
Halothane, enflurane, isoflurane, desflurane, sevoflurane
Have low vapor pressures and thus high boiling points so that they are liquids at room temperature and sea-level ambient pressure
Describe gaseous anesthetics
Nitric oxide
Have high vapor pressures and low boiling points and are in gas form at room temperature
Describe pharmacokinetics of inhaled anesthetics
Volatile and gaseous inhaled anesthetics are taken up through gas exchange in alveoli
Uptake from alveoli into blood and distribution and partitioning into the effect
Describe uptake and distribution of inhaled anesthetics
Driving force for uptake of an inhaled anesthetic is alveolar concentration
Two factors that determine how quickly alveolar concentration changes:
- Inspired concentration or partial pressure
- Alveolar ventilation
Increases in either inspired partial pressure or in ventilation will increase rate of rise in alveoli and will accelerate induction
Partial pressure in alveoli is expressed as ratio of alveolar concentration (Fa) over inspired concentration (Fi); the faster Fa/Fi approaches 1, the faster anesthesia will occur during an inhaled induction
What coefficients affect solubility?
Blood:gas partition coefficient
Brain:blood partition coefficient
Describe the blood:gas coefficient
Defines the relative affinity of an anesthetic for blood compared with that of inspired gas (blood solubility)
There is an inverse relationship between blood:gas partition coefficient values and the rate of anesthesia onset
Agents with low solubility (nitrous oxide, desflurane) reach high arterial pressure rapidly, which in turn results in rapid equilibration with brain and fast onset of action
Agents with high solubility (halothane) reach high arterial pressure slowly, which results in slow equilibration with brain and slow onset of action
Describe the brain:blood partition coefficient
Values for inhaled anesthetics are relatively similar and indicate that all agents are more soluble in brain than in blood
Describe blood:gas coefficient, brain:blood coefficient, MAC (vol%), metabolism (%), and comments about nitrous oxide
0.47
1.1
>100
None
Incomplete anesthetic; rapid onset and recover
Describe blood:gas coefficient, brain:blood coefficient, MAC (vol%), metabolism (%), and comments about desflurane
0.42
1.3
6-7
Describe blood:gas coefficient, brain:blood coefficient, MAC (vol%), metabolism (%), and comments about sevoflurane
0.69
1.7
2
2-5
Rapid onset and recovery
Describe blood:gas coefficient, brain:blood coefficient, MAC (vol%), metabolism (%), and comments about isoflurane
- 4
- 6
- 4
Describe blood:gas coefficient, brain:blood coefficient, MAC (vol%), metabolism (%), and comments about enflurane
1.8
1.4
1.7
8
Medium rate of onset and recovery
Describe blood:gas coefficient, brain:blood coefficient, MAC (vol%), metabolism (%), and comments about halothane
2.3
2.9
0.75
>40
Medium rate of onset and recovery
Why is induction of anesthesia slower with more soluble anesthetic gases?
For a given concentration or partial pressure of two anesthetic gases in inspired air, it will take much longer for blood partial pressure of more soluble gas (halothane) to rise to same partial pressure as in alveoli.
Since concentration of anesthetic agent in brain can rise no faster than concentration in blood, onset of anesthesia will be slower with halothane than with nitrous oxide
The alveolar anesthetic concentration (Fa) approaches inspired anesthetic concentration (Fi) fastest for ___
Least soluble agents
Nitrous oxide>desflurane>sevoflurane>isoflurane>halothane
Describe cardiac output effect on uptake of inhaled anesthetics
Increase in pulmonary blood flow (increased cardiac output) will increase uptake of anesthetic and decrease rate by which Fa/Fi rises, which will decrease rate of induction of anesthesia (Fa decreases because of increased pulmonary blood flow, which essentially dilutes drug in alveoli)
An increase in cardiac output and pulmonary blood flow will increase uptake of anesthetic into blood, but the anesthetic taken up will be distributed and diluted into all tissues, not just CNS
This results in slower rise in partial pressure in blood due to greater volume of distribution
Describe alveolar-venous partial pressure difference effect on uptake of inhaled anesthetics
Anesthetic partial pressure difference between alveolar and mixed venous blood is dependent mainly on uptake of anesthetic by tissues, including nonneural tissues
The slower the rate and extent of tissue uptake, the greater the difference in anesthetic gas tensions between arterial and venous blood, the more time it will take to achieve equilibrium with brain tissue
Since anesthetics must be carried from tissues to lungs for primary elimination, larger A-V concentration differences means less drugs are returning for elimination, which may increase time for awakening
Describe distribution of inhaled anesthetics
- Concentrations of anesthetics in blood will increase as rate and depth of ventilation is increased
- Depression of respiration by opioid analgesics slows onset of anesthesia of inhaled anesthetics if ventilation is not manually or mechanically assisted
- Increasing pulmonary blood flow (cardiac output) slows rate of increase in arterial concentration of anesthetic because a larger volume of blood is exposed to anesthetic. Thus, blood capacity increases, and anesthetic concentration rises slowly (Reverse is true: decreasing pulmonary blood flow will increase rate of rise of arterial anesthetic concentration)
- Brain, heart, liver, kidneys, and splanchnic bed are highly perfused and receive over 75% of resting cardiac output and have higher immediate concentrations of anesthetic
- Although muscle and skin constitute about 50% of total body mass, anesthetics accumulate more slowly in these tissues than highly perfused tissues because they receive only one-fifth of resting cardiac output
Describe elimination of inhaled anesthetics
- Many of the processes of anesthetic transfer during recovery are the reverse of those that occur during induction
- Inhaled anesthetics that are relatively insoluble in blood and brain are eliminated at faster rates than more soluble anesthetics
- Clearance of inhaled anesthetics via lungs is major route of elimination from body, although some agents are metabolized by liver to varying degrees
- Duration of exposure to anesthetic can have a significant effect on recovery time, especially in case of more soluble anesthetics
- accumulation of anesthetics in muscle, skin, and fat increases with prolonged exposure (especially in obese patients), and blood tension may decline slowly during recovery as anesthetic is slowly eliminated from these tissues
- although recovery may be rapid with more soluble agents following a short period of exposure, recovery is slow after prolonged administration of halothane or isoflurane
Describe minimal alveolar concentration (MAC) required to prevent response to a surgical incision
- Concentration of inhalation anesthetics that prevents movement in response to surgical stimulation in 50% of subjects (measure of potency, ED50)
- Values are expressed as volume %, the percentage of atmosphere that is anesthetic at MAC
- Dose of 1 MAC of any anesthetic prevents movement in response to surgical incision in 50% of patients (individual patients may require 0.5-1.5 MAC)
Ex: 1 MAC of isoflurane is 1.4 volume %, while 1 MAC of halothane is 0.75 volume % - MAC values greater than 100% indicate that even if 100% of inspired air at barometric pressure is the anesthetic, the MAC value would still be less than 1, and other agents must be supplemented to achieve full surgical anesthesia (nitrous oxide)
- Since nitrous oxide lacks potency to produce surgical anesthesia, it is combined with volatile or intravenous anesthetics to produce a state of balanced general anesthesia (using nitrous oxide to produce 40% MAC in combination with 70% of a volatile agent’s MAC would yield a total of 110% MAC, sufficient for surgical anesthesia in most patients)
Describe the stage of analgesia of CNS depression
Stage 1
Initially patient experiences analgesia without amnesia
Both analgesia and amnesia are produced at end of stage
Describe stage of excitement of CNS depression
Stage II
Patient often appears delirious
Respiration is irregular in both volume and rate
Retching and vomiting may occur if patient is stimulated
Regular breathing is established at end of this stage
Describe stage of surgical anesthesia of CNS depression
Stage III
Begins with regular breathing and extends to complete cessation of spontaneous respiration (apnea)
Reliable indication that this stage has been achieved is loss of responsiveness to painful stimuli (trapezius muscle squeeze) and reestablishment of regular breathing (EEG cerebral indices are monitored as well)
Describe stage of medullary depression of CNS depression
Stage IV, deepest stage
Includes severe depression of vasomotor center in medulla as well as respiratory center
Death will occur without circulatory and respiratory support
Inhaled anesthetics decrease the ___ of the brain and increase ___. Not beneficial in what patients?
Metabolic rate
Cerebral blood flow
Patients with increased intracranial pressure
Describe inhaled anesthetic effects on respiratory system
All volatile anesthetics are respiratory depressants (body has reduced response to increased levels of carbon dioxide)
Depression of mucociliary function in airway can result in pooling of mucus and postoperative respiratory infections
Describe toxicity of inhaled anesthetics
Common side effects include nausea and vomiting
Halothane may cause hepatitis with or without previous exposure (symptoms include anorexia, nausea, myalgias, arthralgias, and rash, eosinophilia, hepatomegaly, and jaundice. Develops 2 days to 3 weeks after exposure)
Agents metabolized to products including fluoride ions may cause renal toxicity (enflurane and sevoflurane)
In combination with succinylcholine, inhaled volatiles anesthetics may cause malignant hyperthermia, which consists of rapid onset tachycardia and hypertension, severe muscle rigidity, rhabdomyolysis, hyperthermia, hyperkalemia, and acid-base imbalance with acidosis (dantrolene is antidote)
Describe intravenous anesthetics
Nonopioid IV anesthetics are widely used to facilitate rapid induction of anesthesia and have replaced inhalation as preferred method of anesthesia induction in most settings (except for pediatric anesthesia)
Similar to inhaled anesthetics, IV agents do not produce all and only five desired changes in behavior or perception (unconsciousness, amnesia, analgesia, inhibition of autonomic reflexes, and skeletal muscle relaxation)
Therefore, balanced anesthesia with multiple drugs (inhaled anesthetics, sedative-hypnotics, opioids, neuromuscular blocking drugs) is used to minimize unwanted side effects
IV anesthetics are highly lipophilic and preferentially partition into highly perfused lipophilic tissues (brain, spinal cord), which accounts for their quick onset of action
Describe induction, recovery, and comments of etomidate
Rapid onset and moderately fast recovery
Provides CV stability, causes decreased steroidogenesis, and involuntary muscle movements
Describe induction, recovery, and comments of ketamine
moderately rapid onset and recovery
CV stimulation, increased cerebral blood flow, and emergence reactions that impair recovery
Describe induction, recovery, and comments of methohexital
Rapid onset and rapid recovery
Preferred over thiopental for short ambulatory procedures
Describe induction, recovery, and comments of midazolam
Slow onset and recovery. Flumazenil reversal available
Used in balanced anesthesia and conscious sedation. Provides CV stability and marked amensia
Describe induction, recovery, and comments of propofol
Rapid onset and rapid recovery
Used in induction and for maintenance, can cause hypotension, has useful antiemetic action
Describe induction, recovery, and comments of thiopental
Rapid onset and rapid recovery (bolus dose). Slow recovery following infusion
Standard induction agent, causes CV depression, avoid in porphyrias
Describe induction, recovery, and comments of fentanyl
Slow onset and recovery. Naloxone reversal available
Opioid used in balanced anesthesia and conscious sedation, produces marked analgesia
Describe propofol
Most commonly used parenteral anesthetic in US
PK properties allow for continuous infusions and maintenance of anesthesia, sedation in ICU, conscious sedation and short-duration general anesthesia in locations outside operating room (interventional radiology suites, emergency department, dental offices, etc)
Poor solubility in water and is formulated as emulsion containing soybean oil, glycerol, and lecithin (major component of egg yolk phosphatide fraction), making allergic reactions possible in susceptible patients (solution appears milky white and slightly viscous)
Describe MOA of propofol
Most likely targets GABAa receptors as agonist and potentiates chloride current (other receptors likely involved)
Describe pharmacokinetics of propofol
Rapidly metabolized in liver (phase I and II reactions (glucuronide and sulfate conjugates)) with extensive extrahepatic metabolism (lung tissue may account for elimination of up to 30% of bolus dose)
Low “hangover” effect likely due to high plasma clearance
Rapid rate of onset, rapid recovery, and patients are able to ambulate quickly after use
Time of onset (as determined by time to unconsciousness) is 15-30 seconds
Describe context-sensitive half-life of propofol
Describes elimination half-time after a continuous infusion as a function of duration of infusion
Key factor in suitability of a drug for use as maintenance anesthetic
Context sensitive half-life for propofol is brief, even after a prolonged infusion, and recovery remains relatively prompt
Describe context-sensitive half-life of common intravenous anesthetics
Duration of action of single intravenous doses of anesthetic/hypnotic drugs is similarly short for all and is determined by redistribution of drugs away from their active sites
However, after prolonged infusions, drug half-lives and durations of actions are dependent on factors such as rate of redistribution of drug, amount of drug accumulated in fat, and drug’s metabolic rate.
Half-lives of some drugs such as etomidate, propofol, and ketamine increase only modestly with prolonged infusions.
Others (diazepam and thiopental) increase dramatically
CNS effects of propofol
General suppression of CNS activity even though excitatory effect such as twitching or spontaneous movement are occasionally observed during induction
No analgesic properties
Decreases cerebral blood flow and cerebral metabolic rate for oxygen (CMRO2), which decreases intracranial pressure (ICP) and intraocular pressure
Produces burst suppression in EEG when administered in large doses, which conveys a neuroprotective effect during neurosurgical procedures (as does decreased CMRO2)
Describe cardiovascular effects of propofol
Compared with other induction agents produces most pronounced decrease in systemic blood pressure due to profound vasodilation in both arterial and venous circulation leading to reductions in preload and afterload
Hypotensive effects are augmented by inhibition of normal baroreflex response
Describe respiratory effects of propofol
Potent respiratory depressant, generally produces apnea after an induction dose
Causes a greater reduction in upper airway reflexes than thiopental does, which makes it well suited for instrumentation of airway, such as placement of a laryngeal mask airway
Pain on injection of propofol is common, so premedicate with ___
Opioid or coadminister lidocaine
Describe fospropofol
Water-soluble prodrug of propofol that is rapidly metabolized by alkaline phosphatase, producing propofol, phosphate, and formaldehyde (metabolized by aldehyde dehydrogenase in liver and in erythrocytes)
Effects are similar to propofol, but onset and recovery are prolonged because prodrug must first be converted into active form
Less pain on administration than propofol
Common adverse effect is experience of paresthesias (including perineal discomfort or burning sensation) and pruritus (including genital, perineal, and generalized pruiritus) are mostly limited to first 5 minutes of administration and usually described as mild-moderate in intensity (mech unknown. No pretreatment helpful)
Describe etomidate
Hypnotic but not analgesic effects
MOA: enhances actions of GABA on GABAa receptors
Causes minimal cardiovascular and respiratory depression, useful in patients with impaired CV and respiratory systems
Produces rapid loss of consciousness and less rapid recovery rate compared to propofol
Extensively metabolized by liver and in plasma
CNS effects of etomidate
Potent cerebral vasoconstrictor, decreases cerebral blood flow and ICP
CV and respiratory effects of etomidate
CV system stability is maintained even after bolus injection
Minimal change in heart rate and cardiac output
Respiratory depressant effects are less pronounced compared to barbiturates
Describe endocrine effects of etomidate
Causes adrenocortical suppression by producing a dose dependent inhibition of 11beta-hydroxylase (necessary for conversion of cholesterol to cortisol)
Suppression lasts 4-8 hours after induction dose
Endocrine adverse effects limit use for continuous infusion
Describe ketamine
MOA: NMDA receptor antagonist
Produces a dissociative anesthetic state characterized by catatonia amnesia, analgesia with or without loss of consciousness (pt’s eyes remain open with a slow nystagmic gaze)
Similar in structure to phencyclidine (PCP)
Lacrimation and salivation are increased upon administration, and premedication with anticholinergic may be indicated
Only IV anesthetic to produce profound analgesia, stimulation of sympathetic nervous system, bronchodilation, and minimal respiratory depression
Describe CNS effects of ketamine
Increases cerebral blood flow as well as CMRO2, not recommended for use in patients with intracranial pathology, especially increased ICP
Unpleasant emergence reactions after administration are main factor limiting ketamine’s use (vivid colorful dreams, hallucinations, out-of-body experiences, increased and distorted visual, tactile, and auditory sensitivity)
May induce a euphoric state, which explains potential abuse
Describe CV effects of ketamine
Can increase systemic blood pressure, heart rate, and cardiac output, presumably by centrally mediated sympathetic stimulation
Ketamine is considered to be a direct myocardial depressant, a property masked by stimulation of sympathetic nervous system (depressant actions of ketamine may be more apparent in critically ill patients with limited ability to increase their sympathetic nervous system activity)
Describe dexmedetomidine
MOA: alpha-2 adrenergic agonist that produces hypnosis presumably from stimulation of alpha-2 receptors in locus ceruleus and analgesic effects at level of spinal cord
Sedative effect more completely resembles physiologic sleep state through activation of endogenous sleep pathways
Infusion results in moderate decreases in heart rate and systemic vasucular resistance and systemic blood pressure, bradycardia may require treatment
Principally used for the short-term sedation of intubated and ventilated patients in an ICU setting or as adjunct to general anesthesia
What are anesthetic adjuncts used for?
To augment specific components of anesthesia, permitting lower doses of general anesthetics with fewer side effects
Describe opioid analgesics
Have been used in combination with large doses of benzodiazepines to achieve a general anesthetic state
Common agents: IV fentanyl, sufetanil, remifentanil, morphine
MOA: agonists at opiate receptors
Due to adverse effects (impaired ventilation, tolerance after surgery, awareness during anesthesia) opiates are often used as premedication and as an adjunct to both IV and inhaled anesthetics to provide perioperative analgesia
Describe barbiturates
Thiopental (prototype) and methohexital
Highly lipophilic and quick plasma: brain equilibrium
Cause dose-dependent CNS depression ranging from sedation to general anesthesia (no analgesia)
Cause respiratory depresssion
MOA: acts on GABAa receptor to increase duration of channel opening (agonist) and enhances inhibitory neurotransmission
Methohexital may be preferred over thiopental for short ambulatory procedures due to its rapid elimination
Induce cytochrome P450 enzymes
Largely replaced as induction agents by propofol
Describe benzodiazepines
Common agents: diazepam, lorazepam, midazolam
MOA: acts on GABAa receptor to increase receptor sensitivity to GABA (agonist) and enhances inhibitory neurotransmission
Commonly used in perioperative period because of their anxiolytic properties and ability to produce anterograde amnesia. Actions can be terminated by antagonist flumazenil
Midazolam is water-soluble (diazepam and lorazepam are not) and is considered drug of choice for parenteral administration
Midazolam is frequently administered intravenously before patients enter operating room because it has a more rapid onset, shorter elimination half-life (2-4hrs), and steeper dose-response curve than other benzodiazepines
Potent anticonvulsant properties, used in treatment of status epilepticus, alcohol withdrawal, and local anesthetic-induced seizures
