Pharm of Inhatation Agents
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| What is the partial pressure of a gas? | The force exerted by the gas on the liquid it is dissolved in.
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| Partial Pressure of gas | A force that attempts to drive molecules out of a solution and into the gas phase.
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| PA | Alveolar Partial Pressure
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| Pb | Brain Partial Pressure
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| Pa | Arterial Blood
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| Why is the PA used as an index of the depth of anesthesia? | It is an indirect measure of Pb, therefore is used to index depth of anesthesia
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| Is a factor that will effect the partial pressure gradients necessary for anesthesia? | Transfer of inhaled anesthetic from machine to alveoli (anesthetic input)
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| Is a factor that will effect the partial pressure gradients necessary for anesthesia? | Transfer of inhaled anesthetic from alveoli to arterial blood
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| Is a factor that will effect the partial pressure gradients necessary for anesthesia? | Transfer of inhaled anesthetic from arterial blood to brain (anesthetic loss)
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| What factor will effect alveolar partial? | Input into the alveoli minus the loss into the arterial blood.
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| Input factor | Inspired partial pressure
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| Input factor | Alveoli ventilation
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| Input factor | Characteristics of breathing
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| Input factor | Functional residual capacity
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| loss factor | blood gas partition coefficient
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| loss factor | cardiac output
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| loss factor | alveolar to venous partial pressure difference
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| How does the inspired concentration of inhaled anesthetic effect the rate of increase of the alveolar concentration? | A high PI (Inhaled partial pressure) from the anesthetic machine is needed during initial administration to offset the impact of uptake. Time helps accelerate induction of anesthesia.
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| Second gas effect | ability of high volume uptake of one gas to accelerate the rate of increase on PA of a concurrently administered companion gas
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| Second gas effect | simultaneous adm. of slower agent (halothane) with a faster drug (nitrous oxide) in high concentrations can speed the onset of the slower agent.
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| How does the inspired concentration of inhaled anesthetic effect the rate of increase of the alveolar concentration? | A high PI (inhaled partial pressure) from the anesthesia machine is needed during initial adm. to offset the impact of uptake. This helps accelerate induction of anesthesia.
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| How does alveolar ventilation effect the rate of the increase in PA? | Increased ventilation promotes input of inhaled anesthetic to offset uptake into blood.
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| Increased ventilation | net effect is more rapid rate of increase in the PA and thus an increase in induction
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| Controlled ventilation as hyperventilation and decreased venous return | accelerates rate of increase in PA by increased input (increased VA) and decreased uptake (decreased cardiac output)
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| Spontaneous ventilation of inhaled anesthesia | produces a dose dependent depressant effect on alveolar ventilation
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| Spontaneous ventilation | has a protective negative feedback mechanism that prevents excessive depth of anesthesia when ventilation is decreased
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| Spontaneous ventilation | when concentration of partial pressure in the brain decreases to a certain threshold then ventilation increases to deliver more anesthesia
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| Mechanical ventilation | the protective mechanism against development of an excessive depth of anesthesia is lost
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| Mechanical ventilation | it may be appropriate to decrease the PI of volatile anesthesia to maintain PA similar to spontaneous
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| Volume of Anesthetic Breathing System | acts as a buffer to slow the attainment of PA
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| Solubility of inhaled anesthetics in the rubber or plastics components of the Breathing System | initially slow the rate at which PA rises, and at conclusion of anesthesia may slow the rate at which PA decreases.
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| Gas inflow from anesthetic machine | helps to negate the buffering effect
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| Subsequent reuse of same breathing system | can lead to malignant hyperthermia
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| What is FRC? | Functional residual capacity
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| The greater the alveolar ventilation to FRC ratio | the more rapid is the rate of increase in PA
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| The greater the alveolar to FRC ratio | the quicker the induction
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| Arterial tension | is directly dependent on both the rate and depth of ventilation
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| An increase in ventilation | can increase arterial tension of anesthetics of moderate to high blood solubility (halothane) but only slightly increase that of one with low blood solubility (nitrous oxide)
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| What factors of pulmonary ventilation effect the rate of PA | hyperventilation or hypoventilation
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| hyperventilation by mechanical control | increases the speed of induction with inhaled anesthetics that would normally have a slow onset
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| depression of ventilation | may slow onset of anesthesia of SOME inhaled agents
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| increase in pulmonary blood flow | decreases the induction rate
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| in shock patients | decreased CO and increased ventilation may accelerate the induction process
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| slows induction rate | more soluble gases
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| low blood solubility | few molecules are needed to raise its partial pressure and the arterial tension rises quickly
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| Intermediate soluble | Isoflurane
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| Poorly Soluble | Nitrous Oxide
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| Poorly Soluble | Desflurane
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| Poorly Soluble | Sevoflurane
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| blood:gas PC Isoflurane | 1.46
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| brain:blood PC Isoflurane | 1.6
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| muscle:blood PC Isoflurane | 2.9
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| fat:blood PC Isoflurane | 44.9
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| oil:gas PC Isoflurane | 98
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| blood:gas PC Nitrous Oxide | 0.46
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| brain:blood PC Nitrous Oxide | 1.1
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| muscle:blood PC Nitrous Oxide | 1.2
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| fat:blood PC Nitrous Oxide | 2.3
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| oil:gas PC Nitrous Oxide | 1.4
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| blood:gas PC Desflurane | 0.42
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| brain:blood PC Desflurane | 1.3
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| muscle:blood PC Desflurane | 2
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| fat:blood PC Desflurane | 27.2
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| oil:gas PC Desflurane | 18.7
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| blood:gas PC Sevoflurane | 0.69
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| brain:blood PC Sevoflurane | 1.7
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| muscle:blood PC Sevoflurane | 3.1
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| fat:blood PC Sevoflurane | 47.5
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| oil:gas PC Sevoflurane | 55
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| oil gas PC | parallels anesthetic requirements
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| MAC | 150/oil:gas PC
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| 150 constant | average value of the oil:gas solubility and MAC for several inhaled anesthetics with widely divergent lipid solubilities
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| high cardiac output | results in rapid uptake which slows PA
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| low cardiac output | speeds up PA since there is less uptake into the blood
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| Alveolar to venous partial pressure differences | tissue uptake of the inhaled anesthetics
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| highly perfused tissue | brain, heart, kidneys
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| equilibrate rapidly with PA | vessel rich group (high perfused)
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| poorly perfused tissues | skeletal muscle
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| how inhalant agents are eliminated | once inhalant agents of gas ceases the alveoli-blood-brain gradients are reversed and the pulmonary epithelium becomes the channel of elimination
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| drug characteristic has greatest effect on elimination | lower the solubility the faster the rate of elimination
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| drug characteristic that effects elimination | higher the solubility, the higher the extent of accumulation over the same exposure of time
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| how inhalant agents are metabolized | liver metabolism is significant for halothane (15%) but only 2-5% for enflurane and 0.2% for isoflurane
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| MAC | that concentration at 1 atmosphere pressure which causes immobility in 50% of patients when exposed to a noxious stimuli
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| 1.3 MAC | effective in 99% of patients
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| MAC bar | the dose that blocks adrenergic and cardiovascular response in 50% of individuals
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| increases MAC | hyperthermia
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| increases MAC | chronic alcohol use
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| increases MAC | drug induced increases in CNS catecholamine levels (MAOI, cocaine, ephedrine, levodopa)
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| increases MAC | hypernatremia
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| increases MAC | hyperthyroid
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| decreases MAC | hypothermia
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| decreases MAC | increasing age
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| decreases MAC | pre-op meds
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| decreases MAC | drug induced decreases in CNS cathecolamine levels
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| decreases MAC | alpha 2 agonist
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| decreases MAC | acute alcohol ingestion
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| decreases MAC | post-partum (returns to normal in 24-72 hrs)
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| decreases MAC | pregnancy
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| decreases MAC | lithium
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| decreases MAC | neuraxial opioids
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| decreases MAC | lidocaine
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| decreases MAC | hypoxia
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| decreases MAC | blood pressure < 40mmHG
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| decreases MAC | cardiopulmonary bypass
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| decreases MAC | hyponatremia
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| decreases MAC | anemia
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| decreases MAC | metabolic acidosis
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| decreases MAC | opioids
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| decreases MAC | opiod agonist-antagonist
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| decreases MAC | diazepam
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| decreases MAC | methyldopa
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| decrease MAC | reserpine
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| decreases MAC | chronic dextroamphetamine
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| decreases MAC | alpha-2 agonist
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| decreases MAC | lithium
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| decreases MAC | ketamine
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| decreases MAC | pancuronium
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| decreases MAC | physostigmine (10x clinical dose)
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| decreases MAC | lidocaine
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| decreases MAC | chlorpromazine
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| decreases MAC | verapamil
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| decreases MAC | hydroxyzine
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| decreases MAC | ^9 tetrahydrocanabinol
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| Isoflurane vapor pressure | 240
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| Desflurane vapor pressure | 681
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| Sevoflurane | 160
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| Isoflurane MAC | 1.2
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| Desflurane MAC | 6
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| Sevoflurane MAC | 2
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| highly perfused tissues | equilibrate rapidly with the Pa.
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| after 3 time constants | 95% of the returning venous blood is at the same partial pressure as the Pa
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| uptake of gas is decrease | after 3 time constants
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| continued uptake of gases after saturation of vessel rich groups | leads to anesthesia of the skeletal and fat tissues
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| sustained tissue uptake of the anesthestic into the large tissue mass (skeletal/fat) | causes the returning venous blood to be at a lower partial pressure than the Pa
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| when the returning venous blood is at a lower partial pressure than the Pa | the (alveolar to venous) Av-D difference is maintained and the uptake from the lungs continues for several hours.
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