Inhaled Agents Flashcards
Isoflurane - blood-gas coefficient
1.46
Isoflurane - MAC value
1.15
Isoflurane - vapor pressure at 20°C
240 mmHg
Isoflurane - cassette color & airway compatibility
Purple, pungent/irritating
Isoflurane - brand name
Forane
Enflurane - blood-gas coefficient
1.9
Enflurane - MAC value
1.63
Enflurane - vapor pressure at 20°C
172 mmHg
Enflurane - cassette color & airway compatibility
Orange, pungent/irritating
Enflurane - brand name
Ethrane
Halothane - blood-gas coefficient
2.54
Halothane - MAC value
0.76
Halothane - vapor pressure at 20°C
244 mmHg
Halothane - cassette color & airway compatibility
Red, nonpungent
Halothane - brand name
Fluothane
Desflurane - blood-gas coefficient
0.45
Desflurane - MAC value
6.0
Desflurane - vapor pressure at 20°C
669 mmHg
Desflurane - cassette color and airway compatibility
Blue, pungent/irritating
Desflurane - brand name
Suprane
Sevoflurane - blood-gas coefficient
0.65
Sevoflurane - MAC value
1.85
Sevoflurane - vapor pressure at 20°C
160 mmHg
Sevoflurane - cassette color and airway compatibility
Yellow, nonpungent
Sevoflurane - brand name
Ultane
Nitrous oxide - blood-gas coefficient
0.46
Nitrous oxide - MAC value
104
Nitrous oxide - vapor pressure at 20°C
38k mmHg
Nitrous oxide - tank color and airway compatibility
Blue tank, nonpungent
Solubility - lower partition coefficients
- Lower partition coefficients imply decreased solubility, faster equilibration of partial pressure (alveolus ⇔ blood ⇔ brain),
-
Rapid induction
- Ex: desflurane
Solubility - higher partition coefficients
- Higher partition coefficients imply increased solubility, slower equilibration as more molecules are dissolved in the blood
-
Prolonged induction
- Ex: halothane
Solubility - partition coefficients
- Partition coefficients express relative solubility of anesthetic gas at equilibrium
- Tissue:Blood coefficient = time for equilibrium of tissue with arterial blood
Cardiac output and induction
- Increased CO results in faster uptake but decreased alveolar concentration (Fa)
-
Prolonged induction
- More blood passing through the lungs = anesthetic is getting carried away faster
Alveolar-venous concentration gradient and induction
- Depends on uptake by desired (brain) and undesired (fat, muscle) tissues
- Tissue uptake is determined by partition coefficients and regional blood flow
-
Less tissue uptake means blood returns to alveolus with higher partial pressure
- Fa increases faster
Factors that speed rate of induction (increases Fa/Fi)
- Use of agents with low solubility (low partition coefficients)
- Low CO w/ minimal R→L shunting and preserved CBF
-
Increased alvolar MV, increased conc. of agent, increased FGF rate
- Replaces anesthetic taken up by the bloodstream)
- Ex: Pediatric patients
- Faster induction due to increased alveolar ventilation, decreased FRC, increased % of blood flow to brain
Content-sensitive elimination time
- Longer duration of anesthetic is associated with longer time to recovery
- Over longer time, more anesthetic is deposited in undesired tissues and must be “washed out”
- Effect is more pronounced with increased solubility of agent
Diffusion hypoxia
- High concentrations of relatively insoluble gases (N2O) diffuse out of the blood and enter the alveolus, displacing and replacing alveolar concentration of O2 and CO2
- Dilution of alveolar O2 can lead to hypoxia
- Dilution of CO2 can decrease ventilatory drive and worsen hypoxia
- Administer high flow 100% O2 for 5-10 minutes after discontinuation of N2O
MAC
- Reference point (1 MAC) = alveolar conc. at which 50% of patients will not move in response to a standardized surgical stimulus
- Analogous to ED50
- MAC is greatest at 1 yr and is reduced by 6% per decade
What is MACBAR?
- 1.5 - 2 MAC
- Concentration which _b_locks adrenergic response to nociceptive stimuli
What is MACAware?
- ≈ 0.4 - 0.5 MAC
- Concentration at which 50% of patients will not be forming long term memory
What is MACAwake?
- 0.15 - 0.5 MAC
- Concentration at which 50% of patients open eyes on command
Factors that decrease MAC (increase potency)
- Acidosis
- Acute alcohol use
- Advanced age
- Benzodiazepines
- Increased altitude
- Intravenous anethestics
- Hypotension (severe)
- Hypoxia
- Opiates
- Pregnancy
Factors that increase MAC (decrease potency)
- Chronic alcohol use
- Very young age (closer to 1 y of age)
- Increased temperature ( >42°C)
- Decreased altitude
- Drugs (MAOIs, TCAs, cocaine, acute amphetamine use)
Systemic effects of inhaled agents - CV
- All volatile agents are dose-dependent CV depressants, though mechanism of decreased BP differs
- Heart rate effects vary with MAC and inspired concentration rate of change
Systemic effects of inhaled agents - pulmonary
- All agents increase RR with decrease in TV
- Overall volatile agents cause decrease in MV and increase in PaCO2
- All agents blunt ventilatory response to hypoxemia, volatile agents decrease response to hypercarbia
- Volatile agents are potent bronchodilators
- Minimal inhibition of hypoxic pulmonary vasoconstriction (HPV)
Systemic effects of inhaled agents - neurological
- All agents increase** CBF causing **increased ICP and impair autoregulation of vascular tone
- Volatile agents decrease CMR (N2O may increase)
- All agents decrease SSEP / MEP signals
Systemic effects of inhaled agents - hepatic
-
Halothane** causes **both hepatic artery vasoconstriction and decreases portal vein flow
- Potential for hypoxic hepatic injury
- Other volatile agents preserve vascular supply better with increase** in hepatic artery flow **compensating** for **decreased portal vein flow
Systemic effects of inhaled agents - renal
- All agents decrease RBF, GRF, UO
- Untreated hypotension can cause acute kidney injury
Differential physiological effects of halothane
- HR: ⇔ or slight decrease
- SVR: ⇔
- CO: significantly decrease
- Contractility: significant decrease
- HBF: significant decrease
Differential physiological effects of isoflurane
- HR: slight increase
- SVR: slight decrease
- CO: ⇔
- Contractility: slight decrease
- HBF: slight decrease
Differential physiological effects of sevoflurane
- HR: ⇔
- SVR: slight decrease
- CO: ⇔
- Contractility: slight decrease
- HBF: slight decrease
Differential physiological effects of desflurane
- HR: slight increase
- SVR: slight decrease
- CO: ⇔
- Contractility: slight decrease
- HBF: slight decrease
Differential physiological effects of N2O
- HR: ⇔ or increase
- SVR: ⇔ or increase
- CO: ⇔ or increase
- Contractility: ⇔ or decrease
- HBF: slight decrease
Inhalational anesthetics - N2O
-
MAC of 104% precludes use as solo agent for surgical anesthesia
- Used at 30 - 70% conc. as adjuvant to IV or potent inhaled anesthetics
- Low solubility = rapid onset / offset of action
- Nonpungent, has analgesic properties
- Disadvantages
- Rapidly diffuses into and expands air-containing cavities → avoid in air embolism, PTX, bowel obstruction, pneumocephalus, middle ear and retinal procedures
- Prolonged exposure → inhibits B12-dependent enzymes for myelin and nucleic acid synesthes
- Megaloblastic bone marrow change possible > 12 - 24 h use
- Nonflammable but does support combustion
- Increases PONV
- Sympathomimetic CV effects
Inhalational anesthetics - isoflurane
- Inexpensive, slower onset / offset of action
- Pungent
-
Coronary vasodilator
- Possibility for coronary “steal” effect → flow diverted away from vessels with fixed lesions
Inhalation anesthestics - desflurane
- Most rapid onset / offset among volatiles
- Very pungent → may be irritant in patients prone to bronchospasm
- High vapor pressure requires an electrically heated vaporizer → eliminates variations in delivery owing to changes in ambient temp.
- Rapid increase or high conc. may cause transient but significant sympathetic stimulation
Inhalational anesthestics - sevoflurane
- Least pungent (best choice for inhalational induction)
- Fast onset / offset of action
- Controversial potential for nephrotoxicity due to metabolic production of fluoride ion and degradation to Compound A
- Compound A production increases with low FGF, high conc. sevoflurane, desiccated barium lime absorbent
Inhalation anesthetics - halothane
- Low pungency
- Inexpensive
- Esp. potent bronchodilator
- Rare but fulminant postop autoimmune hepatitis
- CV depression and myocardial desensitization to catecholamines (increases vent. dysrhythmias)
Inhalational anesthetics - Heliox
- Nonanesthetic gas mixture
- Commonly 70 - 79% helium and 21 - 30% O2
-
Lower density of gases promotes laminar flow, reduces turbulence in upper airway obstruction
- Helps decrease the work of SV