Volatile Anesthetics Flashcards
How do volatile agents effect respiratory rate and tidal volume?
Increase respiratory rate, decrease tidal volume
Which inhaled agent tends to increase CO?
N2O via mildly increased sympathetic tone
How do volatile agents effect blood pressure? What is the mechanism?
All decrease BP
Isoflurane, Sevoflurane, and Desflurane cause decreases in SVR
Enflurane and Halothane cause decreased BP via direct myocardial depression
What is the equation for volatile anesthetic uptake?
Uptake = [solubility x CO x (Pa - Pv)]/barometric pressure
How does a transpulmonary shunt effect uptake of volatile anesthetics?
In pts with transpulmonary shunt, blood from the unventilated alveoli contains no volatile agent. This volatile-deficient blood mixes with blood from the ventilated alveoli, producing an arterial partial pressure much less than expected.
Since uptake into pulmonary blood will be less than normal, the rate of rise of the FA/FI ratio will be accelerated. However, the increase in blood partial pressure will be slowed.
For very soluble agents, induction is not delayed. For insoluble agents, induction is delayed.
How does increasing alveolar ventilation affect the rate of rise of the FA/FI ratio?
Increased alveolar ventilation will accelerate the rate of increase in FA/FI ratio for all volatile anesthetics.
However, the effect is much greater in magnitude for agents with high blood solubility.
Why is it recommended to run gas flows of at least 2L/min when using Sevoflurane?
It prevents rebreathing (not formation) of Compound A
What is the concentration effect?
The greater the inhaled anesthetic concentration, the faster the increase in FA/FI
Explain diffusion hypoxia
After finishing a N2O anesthetic, large volumes of N2O are released into the lungs (due to it being so insoluble) and dilute all gases including O2 and CO2.
Reduction in O2 leads to hypoxia, while decrease in CO2 reduces the respiratory drive.
Which volatile agent could theoretically cause coronary steal syndrome by preferentially dilating small coronary arterial resistance vessels?
Isoflurane
What is the main reason for the difference between the dial on a given vaporizer and the gases actual end-tidal volume?
Rebreathing in a circle system
To overcome this, either flows must be turned up much higher or the dial must be set above actual desired concentration.
List the determinants of FI (inspired gas concentration)
Fresh gas flow
Volume of the breathing circuit
Any absorption by the machine or breathing circuit
List the determinants of FA (alveolar gas concentration)
Uptake
Ventilation
Volatile concentration
List the MAC values of volatile agents
Halothane - 0.8% Isoflurane - 1.2% Sevoflurane - 2% Desflurane - 6% Nitrous oxide - 105%
List the blood:gas coefficients of volatile agents
Desflurane - 0.42 Nitrous oxide - 0.47 Sevoflurane - 0.65 Isoflurane - 1.4 Halothane - 2.4
Why must the desflurane vaporizer be heated?
The vapor pressure of desflurane (660) is very close to atmospheric pressure (760) at room temperature, meaning that very small changes in volatile temperature or barometric pressure can greatly impact vaporizer output.
Heating the vaporizer raises the volatile’s vapor pressure, allowing it to be metered much more accurately.
1 mL of liquid volatile will generate about how many mL of volatile vapor?
200 mL
Why does nitrous oxide expand in air spaces? Which air space will there be the most rapid expansion?
Since nitrous oxide is 34 times more soluble than nitrogen, it can enter air containing space more rapidly than air, causing expansion.
Nitrous oxide encounters less barriers to diffusion in the blood than in crossing other membranes and therefore expands faster in blood (air embolism) than in other air spaces of the body.
How does nitrous oxide affect bone marrow?
It causes inhibition of methionine synthetase
Define the second gas effect
Uptake of large volumes of a first gas accelerates the alveolar rate of rise of a second gas
Why is induction accelerated in neonates and pregnant patients?
They have a increased ratio of minute ventilation to FRC
How do amphetamines effect MAC?
Acute use increases MAC
Chronic use decreases MAC
How do a2-agonists effect MAC?
Decrease MAC
How does thyroid disease effect MAC?
No change in MAC
