Lec 15 - Inhalant Anesthetics (Pt. 2) Study Guide

Dr. Johnson - Exam 2

What is the formula for a time constant?

time constant (min) = volume (L) / flow (L/min)

Given flow and volume information, determine the rate of change (how long to achieve a desired % of change).

volume = FGF (fresh gas flow) = TC

Rank the order of distribution of inhalants to the 4 different tissue groups (1st saturated to last saturated).

vessel rich → muscle → fat → vessel poor (bone, cartilage)

What is the undesired effect caused by distribution of inhalant to tissues other than the vessel rich group?

delayed uptake by other groups delays full equilibrium in the brain (conc. pt inhalaning will never be that in the brain)

Clinical effect is always determined by what?

partial pressure of oxygen in the brain (NOT conc. administered)

What are the 2 factors that affect the delivery of inhaled anesthestics to the alveoli.

1. inspired concentration (P_i)

2. alveolar ventilation (V_A)

How do changes in P_i and V_A change the clinical effect of anesthesia?

higher the concentration of iso (P_i) = faster pt goes to sleep

larger/deeper breaths (V_A) = faster speed of induction

What 3 factors affect removal of inhaled anesthetics from the alveoli?

1. blood:gas PC

2. cardiac output (CO)

3. alveolar-venous concentration difference (P_A - P_V)

How do changes to the blood:gas PC change the clinical effect of anesthesia?

more soluble in blood = washed away faster = slower induction

want soluble inhalants!!

How do changes to cardiac output change the clinical effect of anesthesia? In which patients is this a concern?

more blood washing to lungs = slower induction

very fit patients

How do changes to the alveolar-venous concentration difference change the clinical effect of anesthesia?

higher difference = higher concentration gradient = more washed away = slower induction

What measurable variable reflects the partial pressue of an inhalant present in the brain?

P_A (exhaled)

T/F: The mechanims of action of inhalants is well understood.

FALSE — remains undefined

What does evidence support as the MOA for inhalants?

non-specific interactions with protein channels

probably interacting with multiple receptors/channels in different locations

Which receptors are likely involved in the MOA of inhalants?

GABA_A

2-pore K⁺ channels

glycine receptors

Which locations does the MOA of inhalants likely occur?

thalamus

cortex (cortico-thalamic projections)

both

Example of predicting rate of change (wash in/wash out) based on time constants

5L bucket, 0% NaCl — add 10% NaCl at 1 L/min — TC = 5 mins → conc. = 6.3% NaCl @ 5 mins

after 4 TC (20 mins), change in conc. is 98% complete