Inhalation Agents -2 Flashcards
What is general anesthesia?
State in which the body is rendered insensible to pain or other stimuli
What is the goal of general anesthesia?
Produce and maintain a constant partial pressure of inhalation anesthetic in the brain.
How to we accomplish a constant partial pressure in the brain?
Produce a partial pressure in the alveoli ( PA) which produces in a partial pressure in the blood (Pa) which in turn produces a partial pressure in the brain (Pbr)
What are the four phases of uptake and distribution?
1- Developing an inspired anesthetic concentration
2- Development of an alveolar anesthetic concentration
3-Development of blood anesthetic concentration
4- Distribution of the anesthetic agent from the blood to the tissue
How do we develop an inspired anesthetic concentration?
1- Begins with introduction of an anesthetic agent into the delivery system of an anesthetic machine and circuitry.
2- VENTILATION introduces the gas into the lungs “inspired gas” or “FI”
2- Use “ HIGH FLOWS” of delivery of gases (O2, N2O/ O2 ) in the 5-10L/min range, can precisely control the partial pressure of an anesthetic agent inspired and accomplish what is called the “WASH IN”
What is FA ( alveolar gas concentration) determined by?
1- uptake = (co E blood:gas x C(A-V) x Q 2- Ventilation 3- Concentration effect and Second gas effect a)concentration effect b) augmented inflow effect **p.78
What is Fa?
Arterial gas concentration and is affected by V/Q mismatch
T/F: High concentrations of inspired gases are rapidly removed from the lungs by the blood. This tends to encourage increased inspired volume volumes of fresh gases at a high concentration, increasing minute ventilation as a result?
True
What is the second step of the concentration effect?
Increased inspiration volumes promotes an increase alveolar PP and helps to offset the decrease in partial pressure of the gases brought on by pulmonary capillary uptake, which in turn promotes the rapid induction of anesthesia
High concentrations mean greater uptake and the greater the uptake the great the ______ _____ is augmented
Inspired volume
- Spontaneous breathing patient, turn gas on, concentration effect, body sense the gas and breathes faster.
- Breathing faster to keep the alveoli full, speed up the induction
- -> b/c they are keeping the gas in the alveoli d/t increase RR = higher concentrations and greater uptake
Developing an alveolar anesthetic concentration
T/F: Involves the uptake of the inspired anesthetic from the delivery system into the lungs at the alveolar level
True
The rate at which alveolar PP of the anesthetic rises is determined by what 2 factors?
- Inspired concentration ( increase uptake Dial conc. and increase gas flows)
2- Alveolar Ventilation ( wash in, quicker respiration)
T/F: When alveolar ventilation is high, the partial pressure of the anesthetic in the alveoli decrease rapidly?
False- increase rapidly.
When there are two anesthetic gases present i the lung, what phenomenon can occur?
Second gas effect
What is the second gas effect?
When the first gas ( N2O) is used, it is picked up rapidly from the alveoli by the blood. This rapid crossing of N2O into the blood tends to pull the second gas with it, so that the arterial PP of the second has rises more rapidly than it would if it were done alone in the alveoli.
*** alveoli is smaller after N2O leaves, increased the PP of the second gas allowing for quicker arterial pulling
Developing blood- anesthetic concentration: Three factors that determine how rapidly anesthetics pass from the inspired gases to the blood.
1- Solubility of the agent
2- Rate of blood flow through the lungs (CO)
3-Partial pressure of the agents in the arterial/venous blood (Pa)
What is the solubility of agent in blood?
Expressed as blood:gas partition coefficient.
-Represents ratio of anesthetic concentration in the blood to the anesthetic
concentration in a gas (alveolar) when the two are in a state of equilibrium
solubility of agent in blood =
anesthetic blood concentration/ anesthetic alveolar concentration
T/F: The more soluble the agent is, less of it must be dissolved in the blood in order to raise the PP?
False- More of it must be dissolved in the blood in oder to raise PP.
- *These agents require a longer induction time because of the amount required to develop PP in the blood.
- *Insoluble gases, very little needs to be dissolved before the PP needed is reached = quick induction time ( N20, DES, little bus )
- **The higher the number the longer it takes to anesthetize the patient
Rate of pulmonary blood flow
T/F: The rate of blood passing through the pulmonary tissue influences how fast the anesthetic agent will be picked up from the alveoli.
True
T/F: The higher the blood flow, less blood is exposed to the agent, and the slower the agent is picked up from the alveoli and delivered to the tissues?.
False- The more blood that is exposed to the agent, the faster the agent is picked up
PP of arterial/mixed venous blood- As arterial blood leaves the lungs it circulates through the tissue where the anesthetic agent is transferred. When this initial venous blood is brought back to the lungs, what is its partial pressure?
-The PP is very low, as most was delivered to the tissue which also had very low to no PP
A/V mixed blood- with each circulation time more anesthetic is delivered to the tissue and their PP rises, the returning venous blood will also begin to have higher PP as it returns to the lungs. How does this affect are uptake?
As the venous PP rises, there is less picked up from the alveoli and uptake decrease.
Distribution of the anesthetic agent from the blood to the brain and other tissues:
As the agent is delivered to the tissues by the arterial blood, the PP is the tissues begins to rise and approach the PP of the blood. The rate at which this occurs depends on what two things?
1- Solubility of the gas in the tissues
–expressed as the tissue:blood co ef.
–Most agents are equally solute in the lean tissue and blood so that their PP are very similar at equilibrium
2- Tissue Blood Flow
–Higher the blood flow to a particular tissue–> faster the anesthetic is delivered and the faster the PP and concentration in that area will rise
What are the four categories or tissue groups?
Vessel Rich: Brain, heart , liver, kidney, endocrine
- Muscle: ( also skin)
- Fat: adipose tissue
- Vessel poor: Bone ligaments, teeth, hair cartilage
T/F:Partial pressure in arterial blood/tissues:
-As tissues take up the agent, the PP of the agent increase toward that of blood and uptake will begin to slow. Uptake decreases as you reach equilibrium.
True
T/F: The rise in alveolar ( FA) anesthetic concentrations towards the inspired ( FI) concentration is the most rapid with the least soluble agents?
True
-Faster with N2O- DES- SEVO
Stages of Anesthesia
Review Slide 16
What happens in Stage one in anesthesia?
Brain gas tension is very low
- Dorsal horn activity decreases and there is decreased synaptic transmission in the spinothalamic tract
- It BEGINS with the administration of anesthesia and ENDS with LOC
Stag II- Delirium or Excitement
Describe what happens
The PP of the brain rises and there is a BLOCKADE of INHIBITORY NEURONS which enhances and facilitates synaptic transmission ( excitement phase)
- Will see increased in muscle tone, irregular breathing, jaw clenching, involuntary activity, pupils dilate, Bp and HR are elevated
- This stage extends from the LOC to the beginning of surgical anesthesia
What occurs in stage 3 of anesthesia ? Stage of Anesthesia
PP of the brain ( Pbr) further increases giving rise to progressive depression of the ascending ( sensory) pathways of the reticular activating system, producing a suppression of spinal reflex activity or skeletal muscle relaxation.
-Movement into this stage is characterized by the return of regular respiration, excitement subsides, pupils become centered, cough, gag and eyelid reflex are absent.
