Volatiles Flashcards
Factors affecting Fi
1) Fresh gas flow –> higher flow = higher FiO2
2) Volume of system –> lower volume = actual FiO2
3) Absorption of circuit –> lower absorption = actual FiO2
Factors affecting FA
1) Uptake of gases
2) Alveolar Ventilation
3) Concentration of gas
Gas uptake
greater the uptake = slower rise in FA/Fi ratio -> slower induction
Factors affecting uptake
1) Blood solubility
2) Alveolar blood flow
3) Partial pressure difference
Blood solubility of gas
affinity of a gas to be taken up in the blood
- more soluble gas => greater uptake => slower induction
Blood Gas Coefficient = high value means more soluble
Alveolar Blood Flow
should equal cardiac output in absence of shunting
- as CO increases = more anesthetic uptake = slower induction
Partial pressure difference
depends on difference between gas and venous blood
- vessel rich organs reach steady state first
- fat group has a tremendous solubility
Ventilation
alveolar ventilation is the most important factors affecting FA
- constantly replacing anesthetic gas with more gas due to high ventilation will result in a higher FA => faster induction and faster equilibrium
Gas Concentration
increasing the concentration of inspired gas => quicker rise in alveolar concentration = faster induction
Second Gas Effect
nitrous is more soluble in the blood than other gases, if it is used at same time as other gases, those gases will be relatively more insoluble and reach FA/Fi quicker => faster induction
Factors affecting Fa
V/Q mismatch => increased difference between alveolar/arterial
Factors affecting Elimination
1) Exhalation - most important
2) High gas flow
3) Low anesthetic volume
4) High cerebral blood flow
5) Increased ventilation
Volatile Pharmacodynamics
no single MOA identified - appears to be related to GABA function
Minimum Alveolar Concentration
MAC is the alveolar [ ] that prevents movement in 50% of patients when surgical stimulus applied
- additive among agents
*lower the value, the more potent the volatile
MAC of anesthetic gases
Nitrous = 105 Halothane = 0.75 Isoflurane = 1.2 Desflurane = 6.0 Sevoflurane = 2.0
Factors that increase MAC
Young Age Chronic EtOH Hypernatremia Hyperthermia MAOI, amphetamines, cocaine
Factors that decrease MAC
Old Age Acute EtOH Hypothermia Anemia Hyponatremia Hypercarbia Hypoxia Pregnancy
General characteristics of volatiles
CV: decrease SVR Resp: increase RR but decrease TV, blunt response to hypoxia and hypercarbia CNS: increase CBF, ICP and CMRO2 NMJ: malignant hyperthermia Renal: decrease RBF and GFR
Nitrous Oxide
colorless and odorless
*NMDA receptor antagonist
- gas at room temperature, liquid under pressure
CV: stimulates sympathetics
Contraindication: air containing cavities => expanding space from being more soluble
Inhibits Vit B12 synthesis - aplastic anemia
Halothane
halogenated alkane
CV: dose dependent reduction in MAP from myocardial suppression
Contraindications: don’t use with intracranial lesions due to increasing CBF and ICP
*Liver Failure = centrilobular necrosis
Isoflurane
nonflammable and pungent
CV: preserves the baroreceptors, reflexes still in tact, dilation of coronaries (STEAL)
*Metabolized to trifluoroacetate -> fluoride
*Contraindications: just MH
Desflurane
substitution of fluorine makes it different -> RAPID induction and emergence
CV: rapid increase in DES = stimulates catecholamines
Resp: pungent - laryngo/bronchospasm
*minimal transformation in humans, desflurane is degraded by dessication in the CO2 absorbent
Sevoflurane
halogenated fluorine, non-pungent and non-flammable
- rapid increase in alveolar [ ] = rapid induction
- blood-fat solubility is close to iso (long case = long time to come off)
CV: reduction in SVR, possible QTc prolongation
Biotransformation by CYP450
