Lecture 11 (Exam 3 Inhaled Anesthetics Part 1) Flashcards
Pharmacokinetics describes these four things in inhaled anesthetics.
- Uptake from the alveoli into the pulmonary capillary blood.
- Distribution
- Metabolism
- Elimination via Lungs
The pharmacokinetics of inhaled anesthetics are influenced by ________.
Aging
What factors does aging contribute to the pharmacokinetics of inhaled anesthetics?
Decreased body mass
Increase fat mass, increase Vd for lipid-soluble drugs
Decrease clearance if the pulmonary exchange is impaired (COPD, atelectasis)
Increase time constraints d/t lower cardiac output
Boyle’s Law states that at a constant temperature, pressure and volume of a gas are __________ ___________.
Inversely proportional.
Application:
As positive pressure ventilation begins, the bellows contract. This will cause pressure to increase within the ventilator and circuit resulting in anesthetic gases flowing from high pressure to low pressure (lungs).
What is Fick’s law of diffusion?
It describes the rate at which molecules move from an area of high concentration to an area of low concentration.
Application:
Once the molecules get to the alveoli, they move around randomly and begin to diffuse into the pulmonary capillary.
What three things does diffusion depend on?
- Partial pressure gradient of the gas (high to low)
- Solubility of the gas (diffusion)
- Thickness of the membrane
According to Graham’s Law of Effusion, smaller molecules effuse faster dependent on _________.
Solubility (Not just based on size alone)
Example:
Carbon dioxide, molecular wt 44 g
Oxygen, molecular wt 32 g
Carbon dioxide is 20x more diffusible because it is more soluble than O2.
An inhaled anesthetic will travel from the alveolus to the pulmonary artery and to the ___________ in one or two circulation times to exert its effect.
brain
What is alveolar pressure an indicator of?
How is alveolar pressure measured?
Depth of anesthesia (“Is patient asleep enough?”)
Recovery of anesthesia (“Is the patient waking up?”)
Pulmonary alveolar pressure can be measured through end-tidal gas.
If the amount of anesthetic gas is greater in the alveolus (PA) than in the brain (PBr) what does that mean?
If the amount of anesthetic gas is greater in the PBr than in PA, what does that mean?
The patient is still awake, there is still room in the brain for anesthetic gas.
Gas is leaving from the brain (vessel-rich) to the alveolus where the patient will eventually wake up.
What are the 3 partial pressure gradients that affect the induction process?
- Anesthetic machine to alveoli
- Alveoli to blood
- Arterial blood to the brain
What is a partition coefficient?
The partition coefficient refers to the relative solubility of the anesthetic gas between two compartments.
What four factors will affect the input of the volatile anesthetic from the anesthetic machine to the alveoli?
- Inspired partial pressure
- Alveolar ventilation
- FRC
- Anesthetic breathing system (Is there a lot of breathing?)
What 3 factors will affect the uptake of volatile anesthetics from the alveoli to the blood?
- Blood: gas partition coefficient
- Cardiac output
- Alveolar-to-Venous partial pressure differences
What 3 factors will affect the uptake of volatile anesthetics from the arterial blood to the brain?
- Brain: blood partition coefficient
- Cerebral blood flow (dependent on CO)
- Aterial-to-Venous partial pressure difference
What factor contributes to how rapidly PA (pressure of the alveolus) approaches PI (Pressure of Inspired Volatile)?
Concentration Effect (concentration gradient)
6% desflurane will get a patient sedated faster than 1% desflurane.
How would you offset a slow induction from highly soluble volatiles?
Over pressurization, a large increase in inspired pressure (PI).
Get the patient asleep in a couple of breaths, but sustained delivery will result in an overdose.
1 _________ breath of high concentration of Sevoflurane (7%) will result in loss of eyelash reflex (stage 2).
vital capacity
The second gas effect is the use of high-volume gas (________) concurrently with a volatile (sevoflurane, isoflurane, desflurane).
Nitrous (N2O)
Why is Nitrous always used in the second gas effect?
Nitrous creates a concentration effect.
The high volume of N2O uptake into the pulmonary capillary (very diffusable) will increase the concentration of the 2nd gas (ie: desflurane). This will increase the uptake of 2nd gas due to the gradient.
Nitrous oxide likes to diffuse into ___________ cavities.
air-filed cavities (lungs, bowels, ear canals)
What are some procedures where you want to avoid using nitrous oxide?
Open belly (compliant space)
Ear Cases (non-compliant space)
Eye Cases (non-compliant space)
What happens if you give N2O to patients with a pneumothorax?
It will increase the size of the pneumothorax by 250% when compared to giving supplemental oxygen.
If nitrous oxide was administered during intraocular retinal repair, what will happen?
The patient will have retinal artery vision loss within one hour.
By ________ alveolar ventilation, you can increase the speed of induction.
increasing
(Increasing respiration rate from 10 bpm to 20 bpm).
This will work to an extent..
Hyperventilation will cause a decrease in __________ which will decrease cerebral blood flow (vasoconstriction) and limit the speed of induction.
PaCO2
Differentiate between spontaneous ventilation and mechanical ventilation.
Spontaneous ventilation has a dose-dependent depressant effect on alveolar ventilation (negative feedback loop). As input decreases d/t decreased ventilation the volatile redistributes from tissue w/ high concentration (brain) to tissues w/ low concentration (fat). As brain concentration decreases, ventilation increases.
When mechanical ventilation, the body is not able to provide a negative feedback loop. The ventilator will continue to administer molecules at a set rate.
