Inhalation Induction Flashcards
Fi
refers to the concentration (partial pressure) of volatile agent in the anesthesia machine/inspiratory tubing of the anesthesia circuit
On your machine, it would the the inspired gas you are using
Fa
refers to the concentration (partial pressure) of volatile agent in the alveoli
“Fa” can also refer to the concentration of agent in the expiratory tubing of the anesthesia circuit
On your machine, it would be the end tidal gas that you are using
Fi/Fa ratio
The Fa/Fi ratio tells us how concentrated the alveoli are (Fa) compared to how concentrated the machine is (Fi)
Fa/Fi ratio will always be less than 1 when the volatile agent is turned on
Fa will eventually get close to Fi, meaning that the Fa/Fi ratio will eventually approach 1
What does the Fa/Fi curve tell us?
This Fa/Fi curve shows us how fast the Fa/Fi ratio of each inhalational agent approaches 1
In other words, it’s telling us how fast each inhalational agent builds up in the alveoli.
If the inhalational agent has a steep slope on the Fa/Fi curve, it means that the partial pressure of agent (Fa) in the alveoli is rapidly increasing, which will lead to faster diffusion into the blood, which will lead to a faster inhalational induction.
Low blood solubility
If an inhalational agent has low blood solubility (a low blood gas coefficient), there is a slower initial diffusion of agent into the blood (because the agent is not soluble in blood)
This leads to a higher initial Desflurane concentration in the lungs (more rapid increase in Fa)
High blood solubility
If an inhalational agent has high blood solubility (a high blood gas coefficient), there is a faster initial diffusion of agent into the blood (because the agent is soluble in blood)
This leads to a lower initial Isoflurane concentration in the lungs (slower increase in Fa)
If Desflurane has a lower blood gas coefficient than nitrous oxide, why does nitrous oxide have a steeper Fa/Fi curve?
Because N20 is typically used in much higher concentrations
Fa/Fi curve summary
- A steeper curve means that the agent builds up quickly in the lungs
- Agents with low blood solubility will quickly increase in the lungs and thus have steeper curves
- Agents with steeper curves have cause faster inhalational inductions
How would you speed up inhalation induction?
- Select a higher percentage on the vaporizer dial
- Use higher fresh gas flow rates
- Decrease the circuit volume (The shorter the circuit, the faster the concentration in the circuit will increase)
- Select a volatile agent with the lowest blood solubility
How much oxygen does a patient need?
Oxygen consumption in an awake, normothermic, 70kg male is ≈250mL/min
“Oxygen consumption is reduced 15-20% under general anesthesia in most patients”
What are advantages to using low fresh gas flow?
- Cost effective (Less total agent will travel through the machine and less will end up being wasted through scavenging)
- Preserves tracheal heat & moisture
- Slows the drying process of soda lime CO2 granules
- Better preserves the patient’s body temperature
What are disadvantages to low fresh gas flow?
- Slower inhalational induction (Takes longer for the agent to be carried to the patient)
- Slower emergence from anesthesia (Takes longer to “wash” the agent out of the machine)
What are disadvantages to high fresh gas flow?
- Expensive (Any gas delivered in excess of what the patient needs will be lost to scavenging) (Higher FGF = more wasted inhalational agent)
- Dries out the patient’s airway, which leads to moisture and heat loss
- Accelerates the drying out of the soda lime CO2 granules
What are advantages of high fresh gas flow?
- Faster inhalational induction (More quickly carries the agent to the patient)
- Faster emergence from anesthesia (“Washes” the agent out of the machine at a faster rate)
What are patient factors that can speed up inhalation induction?
- When a patient has low cardiac output (However, even though each mL of blood picks up quite a bit of agent, less total agent is being picked up, because the total mL’s per minute passing through the lungs is less)
- Higher minute ventilation = faster inhalational induction
- Deeper breaths
- Faster breaths
- Low FRC
Inhalation induction summary
How much cardiac output does vessel rich organs receive? What are the clinical implications?
The “vessel rich” organs receive ≈75% of cardiac output, and include the brain, heart, liver, kidney, and endocrine organs
- These organs are quickly saturated with agent after the vaporizer is turned on
- Agent quickly leaves the organs after the vaporizer is turned off
What is the wakeup mechanism?
- The vaporizer is turned off (and the fresh gas flow is eventually turned up)
- The concentration of agent in the machine and lungs decreases (sped up by fast flow rate)
- Agent starts diffusing from the blood to the lungs (faster if agent has low blood solubility)
- The concentration of agent in the blood decreases
- Once the blood concentration decreases, agent starts to diffuse out of the tissues (brain, fat) and into the blood
The more saturated the fat gets with agent, the longer wakeup will take
- As the concentration of agent in the fat decreases, the concentration of agent in the blood further decreases
- When the blood (and brain) concentration decreases enough, the patient wakes up
Summary Of Factors That Delay Emergence From Inhalational Anesthetics
