Week 2 - MAC, Uptake, & Distribution of Inhalation Agents Flashcards
What is Minimum Alveolar Concentration (MAC)?
Fraction of expired agent or alveolar concentration at which 50% of patients move upon noxious stimulation (incision)
*Alveolar concentration is the closest we can get to measuring the brain concentration
What are the general MAC Values?
MAC Awake = 1/3 to 1/4 MAC
MAC Intubation = ~1.2 MAC (>> MAC because it is more stimulating than incision)
MAC Incision = MAC (1)
MAC BAR (Blunt Autonomic Responses) = 30-40% greater than MAC
What factors decrease MAC?
Hypothermia
Severe HoTN
Advance age
Opioids, Ketamine
Chronic Admin of Amphetamine
Reserpine, alpha-methyldopa
Cholinesterase Inhibitors
IV local anesthetics
Pregnancy
Hypoxemia (PaO2 <40)
Anemia
Alpha2 agonists
What factors increase MAC?
Hyperthermia
Hyperthyroidism
Alcoholism
Acute admin of dextroamphetamine
Young Age
What factors have no effect on MAC?
Duration of anesthesia
Sex
Metabolic acid-base status
Hypercapnia and hypocapnia
Isovolemic anemia
Hypertension
What is the MAC and MAC Awake for Nitrous Oxide?
MAC = 105 Vol%
MAC Awake = 68 Vol%
MAC Awake % of MAC = 64%
What is the MAC, MAC Awake, and MAC BAR for Isoflurane?
MAC = 1.15 Vol %
MAC Awake = 0.49 Vol%
MAC Awake % of MAc = 42%
MAC BAR = 1.5
What is the MAC, MAC Awake, MAC Intubation, and MAC BAR for Desflurane?
MAC = 6 Vol%
MAC Awake = 2.5 Vol%
MAC Awake % of MAC = 37%
MAC Intubation = 7.7
MAC BAR = 7.8
What is the MAC, MAC Awake, MAC Intubation, and MAC BAR for Sevoflurane?
MAC = 1.6 Vol%
MAC Awake = 0.62 Vol%
MAC Awake % of MAC = 37%
MAC Intubation = 3 MAC
BAR = 3.2
What is the MAC and MAC Awake for Halothane?
MAC = 0.75 Vol%
MAC Awake = 0.41 Vol%
MAC Awake % of MAC = 55%
What is the MAC and MAC Awake for Propofol?
MAC = 14 mcg/mL
MAC Awake = 2.69 mcg/mL
MAC Awake % of MAC = 19%
How does the solubility affect anesthetic action?
Low Blood:Gas solubility means fast in/fast out
- N2O, Des, Sevo have low solubility
- Low Solubility means more molecules for anesthesia (goes to brain faster)
High solubility means slow on/slow off
- Hal and Iso have high solubility
- High solubility means more anesthetic in blood, muscle, fat, and less for the brain
*Once equilibration is reached, all anesthetic left goes to the brain
How long does it take for the vessel rich group coefficients, muslce:blood, and fat:blood to become saturated/reach equilibrium?
- Alveolar = 1-5 min
- Vessel Rich Group (Brain, Heart, Spleen, Liver, Kidney, Endocrine) = 5-20 min — 10% body weight, 75% CO, 70mL blood/100mL tissue/min (N2O 50% equilibrated in 2 min)
- Muscle:Blood = starts 20min to 2-3 hours — 50% body weight, 19% CO, 3mL blood/100mL tissue (N2O 50% equilibrated in 20-25 min)
- Fat:Blood = starts 2-3 hours till ___ (vaporizer off) — 20% body weight, 6% CO, total perfusion 400mL/min (N2O 50% in 75 min)
What are the components of the Uptake formula? (4)
- Cardiac Output (pushes blood into lungs to soak up anesthetic molecules)
- Blood:Gas solubility (depends on anesthetic dissolved in blood and tissue)
- Alveolar to Venous partial pressure gradient
- Barometric Pressure (not dependent on the agent itself, just the solubility of that agent)
How is uptake related to each component of the formula? (directly or inversely)
- Uptake from lungs is DIRECTLY proportional to CO (decreased CO = less blood through lungs = less uptake)
- Uptake from lungs is DIRECTLY proportional to alveolar ventilation (more alveolar ventilation = more anesthetic in alveoli = greater uptake)
- Uptake from lungs is INVERSELY proportional to venous partial pressure (high venous partial pressure = less gradient = less uptake)
— No CO = No Uptake — No Tissue Gradient = No Uptake —