Lecture 10: Maintenance of Anesthesia (Exam 2) Flashcards
What are some currently used inhalants
- Isoflurane
- Sevoflurane
- Desflurane
- N2O
What are the characteristics of a desirable inhalant
- Less reactive
- more potent
- nonflammable (halogenated by adding Fl, Cl, or Br)
What properties determine the method of admin
- Boiling point
- Liquid density (specific gravity)
- Vapor pressure
What properties help to determine kinetics in the px
- Blood/gas partition coefficient
- Oil/gas partition coefficient
Both are used to determine the solubility of the gas
Define a gas
Agent that exists in gaseous form @ room temp & sea level pressure
What is an example of a gas
N2O
Define a vapor
Gaseous state of a substance that @ ambient temp & pressure is a liquid
What are some examples of a vapor
- Isoflurane
- Sevoflurane
- Halothane
- Desflurane
What gases laws are used to predict the behavior of a gas
- Boyle’s law
- Charle’s law
- Gay-Lussac’s law
- Dalton’s law of partial pressure
Define Boyle’s law
For a fixed amount of gas @ a constant temperature, the pressure exerted by the gas inversely proportional to its volume
Define Charle’s law
The volume of a fixed amount of gas is directly proportional to its absolute temperature when the pressure remains constant
Define Gay-Lussac’s law
The pressure of a gas increases as the temperature increases if the volume of the gas remains constnat
define Dalton’s law of partial pressure
The total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of each individual gas in the mixture
What is vaporization
Change in state from a liquid to a gas
At what point is the gas saturated
When the equilibrium is reached & no further loss of molecules to the gas phase occurs
Define the vapor pressure of an anesthetic
The measure of the ability to evaporate (enter the gas phase)
Saturated vapor pressure = what
Max concentration of molecules in vapor state
What is the saturated vapor pressure (SVP) dependent on
The temperature which is unique for each anesthetic agent
What happens when the temp of a liquid increases
- More molecules escape liquid phase & enter the gas phase
- Higher vapor pressure
What happens if the temp of a liquid decreases
Lower vapor pressure/concentration
T/F: The SVP of most anesthetics is safe for clinical use
False; it is not safe
What are the two streams that gases are diverted into
- By pass
- Vaporizing chamber
Describe modern vaporizers
- Variable-bypass
- Concentration-calibrated
- Agent-specific
- Temperature compensated
What affects the speed of induction & recovery
The rate of uptake & distribution in the body that is effected by the gas in tissues and “in the blood” (AKA the blood/gas coefficient)
Is a lower or higher blood/gas partition coefficient more desirable
Lower number
What is the oil/gas partition coefficient
The solubility of the gas in a lipid (oil) correlates w/ the anesthetic potency
(more/less) gas dissolves in solvent as temperature increases
Less
What increases alveolar delivery (Pa)
- Increased inspired anesthetic concentration
- Increased alveolar ventilation
What increases inspired anesthetic concentration
- Increased vaporization of agent
- Increased vaporizer dial setting
- Increased fresh gas flow
- Decreased gas volume of patient breathing circuit
What increases alveolar ventilation
- Increased minute ventilation
- Decreased dead space ventilation
What factors decrease removal from alveoli
- Decreased blood solubility of anesthetic
- Decreased cardiac output
- Decreased alveolar-venous anethetic gradient
Define minimum alveolar concentration (MAC)
- The min alveolar concentration of inhaled anesthetic @ 1 atmosphere that produces immobility in 50% of subjects exposed to a supramaximal noxious stimulus
- Corresponds to ED50
What is the relationship between potency & MAC
Inverse relationship (the higher the potency the lower the MAC)
At what MAC is ED95 achieved (95% of px are anesthetized)
1.2 to 1.4
What MAC is surgical anesthesia
1.5 MAC
Fill out the MAC Values for these species:
What factors can increase MAC
- Hyperthermia
- Drugs that cause CNS stimulation
- Increased metabolic rate &/or stress (like hyperthyroidism)
What Factors decrease MAC
- Blood pressure < 50 mmHg
- Hypothermia
- Drugs that cause CNS depression
- Smaller body weight/size
- Age of animal
- Hyponatremia
- PaO2 < 40 mmHg or PaCO2 > 95 mmHg
- Pregnancy
- Disease state
What factors do not effect MAC
- Blood pressure > 50 mmHg
- Anticholinergics
- Duration of anesthesia
- Gender
- Abnorm potassium
- Metabolic alkalosis or acidosis (paO2 > 40 mmHg or PaCO2 of 10-95 mmHg)
Describe Isoflurane
- Stable in storage (no preservative needed)
- Low blood-gas solubility
- More potent than sevoflurane
- Fairly rapid induction & recovery (good muscle relaxation)
- < 1% metabolized in body (mostly in the lungs)
- Reasonable cost
- Mask induction (noxious odor may lead to breath holding & bronchoconstriction)
- Produces carbon monoxide when exposed to desiccated CO2 absorbent
- Hypoventilation is common
What causes hypotension when using Isoflurane
Due to vasodilation & decreased myocardial contractility
Describe sevoflurane
- Lower-blood gas partition coefficient than isoflurane (more rapid induction & recovery)
- Lower potency than isoflurane
- Similar cardio-respiratory depression to isoflurane
- Good muscle relaxation (could trigger malignant hyperthermia
- Mask induction has less odor & smoother induction
- ~ 3% metabolized in the body (rest is eliminated via lungs)
- Can be degraded by CO2 absorbents to produce compound A
What are the pharmacodynamics of inhalant anesthetics
- MOA: multiple cell receptors & ion channels; spinal cord & brain involved
- Reversible immobilization results in all species even plants & protozoa
What are the effect of inhalant anesthetics on the CNS, Resp, & CV system
- CNS - decrease cerebral metabolic rate (oxygen consumption); either no change or an increase in cerebral blood flow; decrease in cerebral BF; decrease in cerebral perfusion pressure; increase in ICP
- Resp - drug & species specific depression of ventilation causes increased CO2; bronchodilation; desflurane irritates the airway
- CV - can decrease cardiac output & BP; arrhythmias associated w/ certain drugs may be exaggerated
What is TIVA & PIVA
Anesthetic techniques utilizing IV infusion of one or more drugs to produce a suitable anesthetic state
Why should TIVA be limited to 1 hour
B/c of prolonged recovery times associated w/ longer TIVA time
T/F: Supplemental O2 is still provided during TIVA & PIVA
True
What is an example of TIVA
“Triple drip” infusion of ketamine, xylazine, & guaifenesin to produce general anesthesia in a horse having a castration
Give an example of PIVA
“MILK” + reduced isoflurane concentration for a dog having a TPLO
Describe Guaifenesin (GG)
- Centrally acting muscle relaxant w/ sedative props
- Co-admin w/ other anesthetic agents for the IV induction &/or maintenance of anesthesia
- Used in horses & ruminants
- MOA is unclear but likely effects sites in the brain & spinal cord
- No analgesic props
- Wide therapeutic margin but OD can result in cardio-resp depression
- Hepatic metabolism & renal excretion (more rapidly eliminated in female ponies)
- Precipitates out of solution when stored below room temp & should be rewarmed before us
- Prepared as a 5 to 15% solution in .9% NaCl or 5% dextrose
