Inhalation Agents (Dewan Part 5) Flashcards
Describe the difference between the immobilization function of inhaled agents versus the sedating effects
The immobilizing effect of inhaled anesthetics involves a site of action in the spinal cord, whereas sedation/hypnosis and amnesia involve supraspinal mechanisms
The potency of general anesthetics correlates with ____.
their solubility in oil, indicating the importance of their interaction with hydrophobic targets and cell membrane
General anesthetics act by ___
binding directly to amphiphilic cavities in proteins.
Volatile inhaled anesthetics enhance ___.
(Mechanism of Action)
inhibitory synaptic transmission in the following ways:
- postsynaptically by potentiating ligand-gated ion channels activated by γ-aminobutyric acid (GABA) and glycine
- extrasynaptically by enhancing GABA receptors and leak currents
- presynaptically by enhancing basal GABA release.
What is the difference between volatile and non-volatile inhalational agents?
Their chemical properties
Non-volatile anesthetics have high vapor pressures and low boiling points, meaning they are in gas form at room temperature.
Whereas volatile anesthetics have low vapor pressures and high boiling points, meaning they are liquids at room temperature and so require vaporizers during administration.
Example of non-volatile gases
nitrous oxide (N2O)
Example of Volatile Gases
halothane, isoflurane, desflurane, sevoflurane
Inhaled anesthetics suppress ____.
excitatory synaptic transmission
- presynaptically by reducing glutamate release (volatile agents)
- postsynaptically by inhibiting excitatory ionotropic receptors activated by glutamate (gaseous agents).
General anesthetics cause a reduction in ___.
nerve transmission at synapses.
Volatile anesthetics primarily affect the function of ___.
ion channel and neurotransmitter receptor proteins in the membranes of nerve cells, which are lipid environments.
Inorganic agents (N2O) inhibit ___.
NMDA receptor (N methyl , N aspartate)
How does the spinal cord mediate immobility?
Inhibition of glutamate channels.
GABAa stimulation produces __.
supraspinal inhibition of transmission.
Inhalation Agents cause what effects?
dose dependent and reversible effects.
- Loss of consciousness/amnesia/hynosis (hypocampus)
- Analgesia (Spinal Thalamic tract)
- Muscle relaxation
- Immobility with stimulation (ventral horn)
Unconsciousness effect occurs from which area?
cerebral cortex, thalamus
Amnesia occurs from which area?
amygdala , hippocampus
Analgesia occurs from which area?
spinothalamic tract
Immobility occurs from which area?
spinal cord receptors
Guedel’s Stages of Anesthesia
I: Awake: amnesia, analgesia
II: Excitation * (stage two)
III: Surgical Anesthesia
IV: Overdose, lose life sustaining reflexes.
Describe Guedel’s Stage 1
Stage of Analgesia or disorientation
From the beginning of induction of anesthesia to loss of consciousness.
Describe Guedel’s Stage 2
Stage of Excitement or stage of delirium
- From loss of consciousness to onset of automatic breathing.
- Eyelash reflex disappears but other reflexes remain intact.
- Coughing, vomitting, and struggling may occur.
- Respirations can be irregular with breath-holding.
Describe Guedel’s Stage 3
Stage of Surgical Anesthesia: from onset of automatic respiration to respiratory paralysis. It is divided into four planes:
- Plane 1- Eye reflex is lost, swallowing reflex disappears
- Plane 2- Beginning of paralysis of intercostal muscles. Laryngeal reflex is lost although inflammation of the upper respiratory tract increases reflex irritability. Corneal reflex disappears. Secretion of tears increases (a useful sign of “light” anesthesia)
- Plane 3- From beginning to completion of intercostal muscle paralysis. Diaphragmatic respiration persists but there is progressive intercostal paralysis.
- Plane 4- Apnea
Describe Guedel’s Stage 4
From stoppage of respiration until death. Anesthetic overdose cause medullary paralysis with respiratory arrest and vasomotor collapse.
Anesthetic Neurotoxicity
Ongoing concern that early exposure to anesthetics can promote cognitive impairment later in life. Concern that exposure affects the development and elimination of synapses in the infant brain.
Volatile anesthetics have been shown to promote apoptosis by altering cellular calcium homeostatic mechanisms.
Anesthetic agents have also been suggested to contribute to tau protein hyperphosphorylation (associated with Alzheimer’s Disease).
Inhalational agents have been associated with ischemic pre-conditioning.
What is ischemic pre-conditioning?
Although inhalational agents have been suggested as contributing to neurotoxicity, they have also been shown to provide both neurological and cardiac protective effects against ischemia-reperfusion injury.
Ischemic preconditioning implies that a brief ischemic episode protects a cell from future, more pronounced ischemic events.
Who defined the concept of MAC?
Dr. Ted Eger
MAC is simply a measure of relative potency
What is MAC?
Minimum alveolar concentration
The MAC of an inhaled anesthetic is the alveolar concentration that prevents movement in 50% of patients in response to a standardized painful stimulus (surgical incision).
1.3 MAC of any volatile anesthetic has been found to ___
prevent movement in about 95% of patients
0.3-0.4 MAC is associated with ____
awakening from anesthesia (MAC awake) when the inhaled drug is the only agents maintaining anesthetic (a rare situation)
MAC is decreased by 6% for ___
every decade of age, regardless of volatile anesthetic
MAC is realtively unaffected by __
sex, length of surgery, thyroid function, or potassium level (but uptake increases over time)
What factors decrease MAC?
- Age (Elderly)
- Anemia
- Hypothermia
- Drugs (sedatives, narcotics, alpha agonists, lithium, local anesthetics)
- Hypoxemia
- Hyponatremia/hypercalcemia
- Pregnancy
- Acute use of alcohol/drugs except cocaine
- Extreme hypercarbia
What factors increase MAC?
- Youth
- Hyperthermia
- Hypernatremia
- CNS Stimulants (Cocaine and Amphetamines)
- Red hair (pheomelanin)
- Chronic Alcohol
- Ephedrine
Characteristics of Nitrous Oxide and class
NMDA receptor antagonist
Colorless, odorless, nonexplosive, nonflammable (but equally as capable as oxygen of combustion)
It can be kept at room temperature and ambient pressure. It can be kept as a liquid under pressure because its critical temperature lies above room temperature.
Cardiovascular Effects of Nitrous Oxide
stimulates the sympathetic nervous system
Slightly depresses myocardial contracility in vitro
arterial BP, CO and HR are relatively unchanged or slightly elevated due to stimulation of chatecholamines
Cerebral Effects of Nitrous Oxide
By increasing CBF and cerebral blood volume, NO produces a mild elevation of intracranial pressure.
Also increases cerebral oxygen consumption (CMRO2)
Concentrations of Nitrous Oxide below MAC may provide ___.
analgesia in dental durgery, labor, traumatic injury, and minor surgical procedures.
Neuromuscular and GI effects of Nitrous Oxide
Does not provide significant muscle relaxation. In fact, at high concentrations in hyperbaric chambers, NO causes skeletal muscle rigidity.
GI: Increases the risk of post-op nausea/vomiting as a result of activation of chemoreceptor trigger zone in the medulla.
Renal and Hepatic Effects of Nitrous Oxide
Renal: Decrease kidney blood flow by increasing renal vascular resistance which leads to a drop in GFR and urine output.
Liver: Hepatic blood flow probably falls but to a lesser extent than with the volatile agents.