Inhalational Agents Flashcards
4 phases of pharmacokinetics
Uptake, distribution, metabolism (minimal), elimination
The ___ of gas in the alveolus determines the effect, NOT the ___
Partial pressure
Concentration
At higher altitude, how will the same concentration of gas affect the anesthesia
Higher altitude = lower pressures = less anesthesia
How does gas flows affect the inspired concentration of gas compared to the delivered concentration from the machine?
Higher gas flows -> closer inspired Fi to machine Fi
Things that increase alveolar concentration of gas
INPUT minus UPTAKE
- Decreased uptake into bloodstream (low solubility, low cardiac output, saturated bloodstream)
- Higher ventilation of fresh gas into the alveoli (higher flows, higher concentration of inspired gas)
- Concentration effect
- Second gas effect
What is the concentration effect? Why?
Higher inspired concentrations -> F(A) approaches F(I) faster
Higher inspired concentration means there is more to buffer the uptake into blood vessels, so F(A) won’t drop as quickly
What is the second gas effect?
As nitrous oxide is taken up into blood vessels faster than volatile agents, the relative concentration of the volatile agent increases, leading to quicker anesthesia
Right-to-left shunt…how is volatile agent rate of anesthesia affected?
How will it affect IV anesthetics?
Higher P(A) but lower P(a), so dilution with non-ventilated blood leads to slower onset
Faster onset, since bypassing some of lungs
Left-to-right shunt…how is volatile agent rate of anesthesia affected?
Little effect for inhaled OR IV drugs
Sevo, iso, des, N2O, halo…put in order of solubility
How does this affect F(A)/F(I) over time?
Low -> High…
N2O > Des > Sevo > Iso > Halo
More soluble = longer to reach F(A)/F(I) of 1
N2O…
- Blood:Gas Partition Coefficient
- Partial pressure at 20ºC
- MAC
- 0.47
- 39,000
- 104%
Desflurane…
- Blood:Gas Partition Coefficient
- Partial pressure at 20ºC
- MAC
- 0.42
- 681
- 6%
Sevoflurane…
- Blood:Gas Partition Coefficient
- Partial pressure at 20ºC
- MAC
- 0.69
- 160
- 2.15%
Isoflurane…
- Blood:Gas Partition Coefficient
- Partial pressure at 20ºC
- MAC
- 1.4
- 240
- 1.2%
Halothane…
- Blood:Gas Partition Coefficient
- Partial pressure at 20ºC
- MAC
- 2.3
- 243
- 0.75%
Enflurane…
- Blood:Gas Partition Coefficient
- Partial pressure at 20ºC
- MAC
- 1.8
- 175
- 1.68%
What does blood:gas partition coefficient mean?
At steady state, 1mL of blood contains ___ as much gas as 1mL alveolar gas
At 50% N2O in inspired gas, how many mL N2O will be in 1mL of blood?
0.5mL x 0.47 = 0.235mL N2O in 1mL blood
How does increased fat in blood affect induction via gases?
Fat:blood partition coefficient is >1, so the gas wants to be in fat more than in blood, so it will SLOW induction
How do volatile gases affect the brain?
- Decreased O2 use
- Decreased vascular resistance -> increased CBF and ICP
At 0.5 MAC, is CBF higher or lower than normal?
NEITHER…at 0.5 MAC, decreased O2 use counteracts vasodilation, so CBF stays constant
At 1 MAC, is CBF higher or lower than normal?
HIGHER…plateau’d decrease in O2 use, so vasodilation is more prominent -> increased CBF
How do gases affect CV?
Vasodilation -> decreased MAP, BUT C.O. maintained
Which gas has negative effect on cardiac contractility?
Halothane
How do gases affect lungs?
- Decreased Vt but increased RR, so MV maintained
- Dose-dependent decreased response to high CO2 and low O2
- Bronchodilation
How do gases affect kidneys?
How do gases affect muscles?
Decreased RBF -> decreased GFR
Muscle relaxation (NOT N2O)
N2O…
- Potency
- Solubility
- Particularly contraindicated when?
- Effect on CV?
- Effect on pulm?
- Effect on analgesia?
- Prolonged exposure can lead to what?
- Effect on muscles?
- Should periodically do what?
- Low potency (high MAC)
- Low solubility - rapid uptake and elimination
- Diffuses into air-filled spaces (pneumothorax, middle ear, bowel, blebs, ETT cuff) and causes expansion
- Unmasks myocardial depression in CAD or bad hypoT
- pHTN after prolonged
- NMDA antagonist -> MAYBE analgesia
- Prolonged -> bone marrow suppression, peripheral neuropathies
- NO affect on muscles (no MH, no relaxation)
- Periodically remove air from ETT cuff
Isoflurane…
- Potency
- Solubility
- Why do patients not like it?
- Effect on CV?
- Effect on brain?
- At 2 MAC, produces _____
- High potent (MAC 1.2%)
- High solubility -> longer onset and elimination
- VERY pungent
- Vasodilation -> less perfusion to stenotic coronaries that can’t dilate (coronary steal)
- Minimal increased CBF (vasodilation) and ICP
- At 2 MAC -> silent EEG
Sevoflurane…
- Potency
- Solubility
- Desiccated CO2 absorbant?
- Normal CO2 absorbant? So?
- Lower potent (MAC 2.1%)
- Lower solubility -> pretty quick on and off
- Desiccated absorbant –> CO –> FIRE
- CO2 absorbant -> Compound A (nephrotoxic in rats)
- SO, keep flows above 2 L/min to prevent rebreathing
Desflurane…
- Potency?
- Vapor pressure? So?
- Effects if given while awake?
- Desiccated CO2 absorbant?
- What to know about changing concentration?
- LOW potency (MAC 6.6%)
- HIGH vapor pressure, so boils at sea level, so must be kept in heated/pressurized container to regulate concentration
- Awake –> PUNGENT -> bronchospasm, coughing, coughing, laryngospasm, breath-holding, salivation
- Desiccated absorbant -> CO -> FIRE
- Rapidly increased concentration -> increased sympathetics (tachy, hypertension)
At equilibrium, ___ partial pressure = ___ partial pressure
Alveolar = brain
1 MAC = ED__
So, ED95 = __ MAC
What does that mean?
50
1.3
At 1.3 MAC, 95% of people will not move to surgical stimulus
What determines potency?
What determines solubility?
Rank least to most potent
Oil:Gas partition coefficient
Blood:Gas partition coefficient
N2O < Des < Sevo < Iso < En < Halo
MAC (aware)
MAC (movement)
MAC (intubation)
MAC (blunt autonomic response)
- 4 (verbal/tactile stimulation)
- 0 (surgical incision)
- 3 (intubation, LMA); also ED95 for incision
- 6 (no tachy/hyperT to noxious stimuli)
How does MAC change with age?
Greatest at 6mo old, then declines
6% per decade after 40y/o
Things that decrease MAC (meds, physiology, pathology, substances)
Meds: Depressants, ketamine, alpha-2 agonists, verapamil, local anesthesia
Physio: Increasing age after 1 y/o
Path: hypothermia, hypoxia, hypercarbia, anemia, sepsis, hyponatremia
Substances: acute EtOH, chronic meth
Things that increase MAC (meds, physiology, pathology, substances, genetic)
Meds: amphetamines, L-dopa, ephedrine, TCAs Physio: Increasing age < 1 y/o Path: hyperthermia, hypernatremia Substances: chronic EtOH Genetic: red hair
Things that increase risk of intra-op awareness
What are you aware of?
When during the surgery?
How to manage? (after)
Paralytics, children, chronic drugs/EtOH, surgeries requiring light anesthesia
Hearing voices
Induction and emergence
Counseling
Signs of light anesthesia
Tearing, bucking, coughing, dilated pupils, sweating, movement, tachy/hyperT
How to prevent intraop awareness?
Premedicate, gas > TIVA, MAC > 0.5-0.7, avoid paralytics, redose IV meds before very stimulating things, increase BP with meds rather than decreasing the gas
