Inhalation Anesthetics Flashcards
The pressure that a vapor exerts above the liquid of a closed container
Vapor pressure
Which agents have vapor pressures?
Isoflurane, Sevoflurane, Desflurane, Nitrous Oxide
Rank the vapor pressures from highest to lowest
- Nitrous Oxide (38,770)
- Desflurane (669)
- Isoflurane (238)
- Sevoflurane (157)
Why is vapor pressure important?
- You can overdose a patient with a higher vapor pressure than what you needed
- You can underdose a patient with a lower vapor pressure than what you needed
What are the safety features of volatile agents?
- They are color coded (sevo is yellow, iso is purple, des is blue)
- Vaporizers have key adapters
- Vaporizers have a safety interlock system to prevent more than 1 vaporizer being on at a time
Refers to a liquid getting cold as it evaporates
Latent heat of vaporization
How do you keep Desflurane from getting cold?
It is stored in a specialized vaporizer that is heated and pressurized
Condensing a gas is easier when..
- The liquid is made cooler
2. The liquid is pressurized
The temperature at which a gas can be liquefied under pressure
Critical temperature
If a gas has a low critical temperature it is (harder or easier) to liquefy
Harder to liquefy
If a gas has a high critical temperature it is (harder or easier) to liquefy
Easier to liquefy
Properties of oxygen
Ideal gas
Critical temperature of -119 C
Ideal gas law equation
P/V =P/V
Properties of nitrous oxide
NOT an ideal gas
Critical temperature of 36.5 C
Properties of air
Ideal gas
Critical temperature of -140.6 C
21% O2 79% N2
Full tank of oxygen
2,000 psi
660 L
An E cylinder has 1500 psi and the anesthetist is delivering 2L/min to the patient. How long can oxygen be delivered?**
P/V = P/V 2000psi/660L = 1500psi/V V=495L 495L x (1 min/2L) = 247.5 min
Full tank of nitrous oxide
750 psi
1590L
Tank contains liquid until 75% of that volume is depleted
The pressure begins to drop when there is <400L
An E cylinder of N2O is being used. The volume has dropped from 1590L to 600L. What is the pressure in the cylinder?
750 psi
What does variable bypass mean?
Some fresh gas flow bypasses the vaporizer
Occurs in isoflurane and sevoflurane
How is desflurane different from iso and sevo?
Desflurane is not a variable bypass vaporizer. It all bypasses the vaporizer
The highest blood CO2 level at which at patient can remain apneic
Apneic threshold
Less of a drive to breathe
High apneic threshold
More of a drive to breathe (cannot hold breath as long)
Low apneic threshold
How do anesthetics raise the apneic threshold?
They suppress the patient’s drive to breathe, meaning it will take a much higher CO2 than normal to stimulate breathing
How does pain affect the apneic threshold?
It lowers it. Patients in pain cannot hold their breath
Assuming a patient is on a ventilator, you must get the patient spontaneously ventilating again prior to wake up. How would you do this?
- Reverse muscle paralysis if applicable
- Allow CO2 level to increase by decreasing RR or making the patient go apneic
- The patient will begin to breathe when the apneic threshold is reached (when CO2 is high enough)
The lowest oxygen level at which the patient can no longer remain apneic
Hypoxic drive
When does hypercarbia have a higher effect than hypoxia in stimulating breathing?
In healthy patients
When does hypoxia have a greater effect than hypercarbia in stimulating breathing?
In patients with lung disease
Affect ventilation by responding to changes in the hydrogen ion concentration of CSF, determined by PaCO2
Central chemoreceptors
Affect ventilation by responding primarily to changes in PaO2
Peripheral chemoreceptors (in carotid body)
Definition of MAC
Minimal alveolar concentration of an exhaled gas that will prevent movement in 50% of patients (non-paralyzed) during surgical incision
Patients are less likely to move when…
breathing a higher percentage [MAC value] of agent. MAC reflects potency
How do we know if a patient is aware or not when they’re paralyzed?
Vital signs (Inc HR and Inc BP)
If a patient has been given narcotics and you want to prevent awareness, what MAC would you give?
> 0.8 MAC
If a patient has been given narcotics and you want to prevent movement, what MAC would you give?
> 1.0 MAC
If a patient has NOT been given narcotics and you want to prevent awareness, what MAC would you give?
> 1.0 MAC
True/false: It is easier to prevent awareness than movement
True
Factors that decrease MAC
- IV anesthetics
- Old age
- Acute alcohol intoxication
- Pregnancy
- Temperature related (hypothermia, mild hyperthermia)
- Blood related (hypoxia, hypercarbia, hypotension, anemia)
Factors that increase MAC
Young
Hyperthermia
Alcohol (chronic)
Hypernatremia
What does it mean if an agent is less soluble in the blood? (has a low blood:gas partition coefficient)
There is less agent in the blood than in the lungs at equilibrium
What does it mean if an agent is more soluble in the blood? (has a high blood:gas partition coefficient)
There is more agent in the blood than in the lungs at equilibrium
Neurological effects of volatile agents
- Dilates cerebral vasculature, increases CBF, increases ICP
- Can cause emergence delirium in kids
- Decreases possibility of seizures (increases seizure threshold)
- Decreases cerebral metabolic rate of oxygen
Cardiac effects of volatile agents
- Decreases cardiac contractility
- Causes systemic vasodilation
3 .Causes coronary vasodilation
Respiratory effects of volatile agents
- Rapid, shallow breathing causing CO2 to increase
- Lower tidal volumes
- Increases apneic threshold
- Decreases hypoxic drive
- Causes bronchodilation
Other effects of volatile agents
- Decreases renal blood flow
- Decreases hepatic blood flow
- Prolongs muscle relaxants
- PONV
- Malignant hyperthermia
- Inhibits “non-shivering thermogenesis” in pediatric patients
Effects of nitrous oxide
- Analgesic properties
- PONV
- Stimulates sympathetic nervous system (inc PVR)
- Dilates cerebral vasculature
- Raises seizure threshold
- Supports combustion as much as O2
- Can cause post op diffusion hypoxia
- Expands closed air-containing cavities
- Increases cerebral metabolic rate of oxygen
- Harmful effects: bone marrow depression, inhibit DNA synthesis enzymes, neuro deficits
Contraindications of nitrous oxide
- Laryngeal, pharyngeal, facial surgery with laser or bovie
- Ophthalmic surgery
- Severe COPD pts
- Cardiopulmonary bypass surgeries
- Abdominal surgeries (prolonged intestinal, laps, bowel obstruction)
- Inner ear surgeries
- 1st trimester of pregnancy
- Neurosurgery (inc ICP, recent craniotomy, recent dural closure)
Concentration (partial pressure) of volatile agents in inspiratory tubing
Fi
Concentration (partial pressure) of volatile agent in the alveoli
Fa
Induction mechanism
- Volatile agent is turned on
- Concentration in the machine increases (Fi)
- Concentration in alveoli increases (Fa)
- Agent from alveoli diffuses into the blood
- Agent diffuses from blood into brain
- Patient falls asleep once the agent in the blood is sufficient
If an agent is not soluble in blood it has a (high/low) blood:gas partition coefficient
Low (Desflurane) -leads to higher initial desflurane concentration in the lungs, more rapid increase in Fa
Characteristics/effects of low blood solubility
Slower initial diffusion of agent into the blood
Higher initial concentration in the lungs
Rapidly diffuses into the brain
Large overall concentration gradient, faster diffusion into the blood
Characteristics of high solubility
Faster initial diffusion of agent into the blood
Lower initial concentration in the lungs (slower increase in Fa)
Slower to diffuse into the brain
Smaller overall concentration gradient, slower overall diffusion into the blood
True/false: The Fa/Fi will never reach 1
True
If Desflurane has a lower blood:gas partition coefficient than Nitrous Oxide, why does Nitrous Oxide have a steeper Fa/Fi curve?
Nitrous Oxide is used in higher concentrations than Desflurane
How do you speed up inhalation induction?
- Select a higher percentage on the vaporizer dial
- Use higher fresh gas flow
- Decrease the circuit volume
- Use a low blood soluble solvent (Des)
How much oxygen does a patient need?
250mL in an awake, normothermic 70kg male
O2 consumption is reduced 15-20% under general anesthesia
What is the minimum fresh gas flow we need to ensure the circuit has adequate pressure?
0.5 L/min
Advantages of low fresh gas flow
- Cost effective
- Preserves tracheal heat and moisture
- Slows the drying process of soda lime CO2 granules
- Better preserves the patient’s body temperature
Disadvantages of low fresh gas flow
- Slower inhalation induction
2. Slower emergence from anesthesia
Advantages of high fresh gas flow
- Faster inhalation induction
2. Faster emergence from anesthesia
Disadvantages of high fresh gas flow
- Expensive
- Dries out the patient’s airway
- Accelerates the drying out of soda lime granules
Patient factors that increase speed of inhalation induction
- Low cardiac output
- Higher minute ventilation
- Low FRC
How does low cardiac output speed up inhalation induction?
The blood that passes the lungs is exposed to the alveoli longer, making the blood more concentrated with agent, allowing it to diffuse into the brain faster. Because the blood is moving slower, there will be more agent building up in the lungs, it will move into the blood faster
How does higher minute ventilation lead to faster inhalation induction?
Deeper breaths lead to more surface area for gas exchange
Faster breaths constantly replenish the alveoli with higher concentrations of volatile agent
How does low FRC speed up inhalation induction?
Less space in the alveoli
HIgher concentration of agent in the lungs
True/False: pediatrics have a faster inhalation induction due to having a lower FRC?**
True
Why do children have a faster wakeup/emergence?
Blood flow to vessel rich organs is faster than in adult
Cardiac output % of muscles
20%
Cardiac output % of fat
5%
Clinical implications of the vessel rich group
Brain, kidneys, heart, liver, endocrine organs
quickly saturated with agent when the vaporizer is turned on, and agent quickly leaves when the vaporizer is turned off
Clinical implications of the fat group
Agent slowly builds up in fat and is slow to leave the fat
Volatile agents are more soluble in fat than in other groups
Wakeup mechanism
- Vaporizer turned off
- Concentration of agent in machine and lung decreases
- Agents start to diffuse from the blood to the lungs
- Concentration of agent in the blood decreases
- Once the blood concentration starts to decrease, agent diffuses out of the brain, fat, etc into the blood
- As the concentration in fat decreases, concentration of agent in the blood decreases
- When the blood and brain decreases enough, the patient wakes up
Factors that speed up wakeup/emergence
- Low blood solubility
- High fresh gas flow
- High minute ventilation
- High cardiac output
- Open APL
- Low FRC
Factors that decreases the speed of emergence
- Age
- More agent dissolved in fat (obese, long surgery, higher concentration of agent)
- Low FGF
- Higher blood solubility
- Higher amount of narcotics
- Low body temperature
- Low cardiac output
- Lung disease
- Closed APL
