Test 3: General Principles of Inhaled Anesthetics Flashcards
The concentration/partial pressure of anesthetic in the lungs is assumed to be the same as in the _____.
Brain
Why can you assume that the concentration/partial pressure of anesthetic in the lungs is assumed to be the same as in the brain?
The concentration or partial pressure of anesthetic in the lungs is assumed to be the same as in the brain because the drugs are highly lipid soluble and diffusible, and they quickly and easily reach equilibrium among the highly perfused body compartments (Central Compartment/Vessel Rich Group).
What is MAC?
The minimum alveolar concentration required to produce anesthesia (lack of movement) in 50% of the population upon surgical stimulation. It is age dependent in that the required dose peaks at approximately 6 months of age and then decreases with increasing age.
What are two factors that may affect uptake early in anesthetic administration?
1) drug solubility in the rubber and plastic machine parts
2) total machine liter flow of the gases chosen
What parts of the machine can retain small quantities of anesthetic gases?
The rubber and plastic components of the machine, in addition to the ventilator and absorbent.
Why does it matter if parts of the machine retain small quantities of gases?
Theoretically, this could slow administration to the patient at the start of anesthetic delivery.
-The effect on uptake is minimal in actual clinical practice and essentially ceases after approximately 15 minutes of administration.
-Risk occurs when anesthetizing patients with malignant hyperthermia.
Which gases are triggering agents for malignant hyperthermia?
All gases except N2O.
How can you avoid exposure to anesthetic gases in a MH prone patient?
-Flush Anesthesia Machine with 100% O2 at 10 L/min for at least 20 minutes
-Replace breathing circuits and the carbon dioxide canister
-Inactivate or remove vaporizers
-Ideal: INH free machine
How does flow of the gases chosen effect uptake early in anesthetic administration?
-Low liter flows of oxygen and nitrous oxide carrier gas, although economical, deliver the anesthetic more slowly at the start of induction.
-Increasing liter flows for the first few minutes of the anesthetic minimizes this effect without unduly adding to cost. (temporary - don’t forget to turn flows back down!)
-Almost everything is going out to scavenging (kinda wasteful), but can be used for quicker onset of action.
What are the symptoms of Malignant Hyperthermia?
Following exposure to one or more triggering agents:
-muscle rigidity
-hyperthermia
-rapid onset of tachycardia and hypercapnia
-hyperkalemia
-metabolic acidosis
Inhalation Agents are ___ at room temperature.
Liquids
What is Vapor Pressure?
The pressure exerted by a vapor in equilibrium.
-Molecules escape the liquid phase (vaporize) until equilibrium is achieved
-Pressure is created by the vapor molecules bombarding the walls of the container
-Temperature dependent
What is LeChatelier’s Principle?
When a system at equilibrium is subjected to change in concentration, temp, volume, or pressure, then the system readjusts itself to partially counteract the effect of the applied change and a new equilibrium is established.
-Inc in temp = inc in Vapor Pressure
Which principles are temperature dependent?
-Vapor Pressure
-Blood:Gas Solubility
-Oil:Gas Solubility
What is the Blood:Gas Solubility Coefficient indicative of?
The speed of Uptake & Elimination (Onset and recovery)
How do you determine the Blood:Gas Solubility Coefficient?
The amount of agent that is blood soluble vs the amount of agent that is lipid soluble.
It reflects the proportion of the anesthetic that will be soluble in the blood, “bind” to blood components, and not readily enter the tissues (blood phase) versus the fraction of the drug that will leave the blood and quickly diffuse into tissues (gas phase).
-Ratio of the INH concentration in blood to concentration in alveoli when the two are in equilibrium (partial pressures are equal)
The higher the blood:gas solubility coefficient (the more soluble the drug), the ____ the brain/spinal cord uptake (the rate anesthesia is achieved).
The higher the blood:gas solubility coefficient (the more soluble the drug), the slower the brain/spinal cord uptake (the slower rate anesthesia is achieved).
-Slower induction & Slower emergence
Soluble drugs remain in the _____ in greater proportion than less soluble drugs, therefore less of the agent is released to the tissues during the early, rapid-uptake phase of induction. (Less of the drug is released for uptake)
Soluble drugs remain in the blood in greater proportion than less soluble drugs, therefore less of the agent is released to the tissues during the early, rapid-uptake phase of induction. (Less of the drug is released for uptake)
How does blood:gas solubility affect CNS uptake?
Increased amount blood soluble = decreased CNS uptake. Takes longer to get to target tissues.
-Slower induction - slower emergence (ex: Isoflurane). Takes longer to set up.
What is the Partition Coefficient? (B:G Solubility coefficient)
-Reflects the solubility of the anesthetic
-Distribution ratio describing how the INH agent distributes itself between two phases at equilibrium (Partial pressures are equal in both phases)
-Description of the ability/capacity of the blood phase or tissue phase to accept the anesthetic.
How does temperature affect solubility?
As temp of a liquid decreases, the solubility of a gas in that liquid increases.
