Lecture 5 - Volatile Anesthetics Flashcards Preview

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Flashcards in Lecture 5 - Volatile Anesthetics Deck (17):
1

what are the inhaled anesthetics?

which is a gas at room temp? which are liquids?

What can they reliably be used for?

what desired effect is not achieved ?

liquids at room temperature -- Sevoflurane, Desflurane, Isoflurane

Gas at room temperature -- N2O

Reliable for: Unconsciousness, Amnesia, Immobility

Disadvantage:
They are not analgesics
Patients body will still respond to the pain (eg tachycardia and HTN)

2

what is monitored when the patient is under anesthesia ?

which levels allow for monitoring concentrations in the brain?

Vital Signs: ECG, HR, BP, O2 Sats,
Spontaneous Respiration -- if the patient is breathing on their own
(tachypnic would indicate pain)
Movement in response to surgery
Levels of Exhaled gases --
O2, CO2, N2O, volatile anesthetics
Correlate with levels in the brain

3

what are the mechanisms behind:
- Immobility
- Amnesia
-CNS depression -

Poorly understood overall
Immobility -- acting on the spinal cord
Amnesia - hippocampus, amygdala, cerebral cortex
CNS Depression:
Enhance inhibitory NTs -- GABA and Glycine
Block Excitatory -- NMDA

4

what is Minimum Alveolar Concentration (MAC)


what is the goal MAC for surgical anesthesia ?

which of the inhaled anesthetic agents cannot achieve this goal on its own?

Definition: Concentration of anesthetic required to suppress movement to surgical stimulus in 50% of patients



Goal MAC = 1 - surgical anesthesia

N2O Max MAC = .7; must be used in combination with other drugs

5

what are 5 important characteristics to know about inhaled anesthetics

The dose needed to achieve MAC = 1
(lowest to highest -- Isoflurane < Sevoflurane < Desflurane < N2O)

Blood Gas Partition Coefficient (solubility of the gas in the blood) (most soluble to least -- Iso > Sevo > N20 > Des)

Blood brain coefficienct (measure of brain solubility)
(Sevo, Iso, Des, N2O

Oil Gas partition coefficient -- lipid solubility
(Iso> Sevo> Des> N20)

Vapor pressure


% Metabolized -- clinically insignificant

6

Isoflurane -- charactersitic profile


whats the significance of its solubility?

the most lipid and blood soluble --- Longer emergence;
second most brain soluble ---
Least dose needed for MAC = 1 --- most potent
○ Not very expensive --- used a lot

7

Sevoflurane characteristic profile

• Medium solubility- good for longer cases and obese patients
• Medium potency
• Not very expensive

Used for inhaled induction in pediatrics - (for kids woh hate needles; can knock them out first)

8

Desflurane characteristic profile

• Desflurane:
• Least soluble- good for longer cases and obese patients
• Least potent- highest dose needed to achieve 1 MAC, so have to use a lot of it

Most expensive compound --- therefore not used that often

9

N2O Characteristic profile

• N2O: Not used much
○ Increases n/v
○ Not good for surgeries on spaces/organs that contain air (bowel, lungs ear) → will cause distension because diffuses faster into air filled space than nitrogen can leave
○ Cannot achieve 1 MAC alone, so have to use in combination with another agent
○ In pediatrics, can use for inhalation induction along with sevoflurane

Not very expensive

10

Pharmacokinetics --

equilibriation between what three partial pressures?

how long does it take P(brain) to equilibriate wiht PA

how is Pbr measured?

• PA ↔ Pa ↔ Pbr
• Alveolar arterial brain partial pressure
• Uptake from alveoli into systemic circulation
• Uptake from circulation into brain- site of action of anesthetics
• Redistribution of anesthetic throughout body

6-12 minutes for Pbr to equilibriate with PA

• P alveolar mirrors Pbr -- can't measure the Pbr directly
§ P-alveolar -- also measure of rate of induction and recovery from anesthesia; also a measure of potency

11

Factors Determining PA:

Solubility -- want to be less solubule in blood than it is in lipid and brain (N2O
Inspired anesthetic partial pressure (PI)
Alveolar Ventilation -- Increased ventilation accerlates induction by more rapidly increasing PA

Cardiac Output -- Low CO = faster induction; high CO = harder to load up quickly

12

Emergence From Anesthesia ---
what happens when the gas is turned off?
at what MAC does a patient awake?

PA=zero
Partial pressure gradient is reversed
• Stored anesthetic in tissues diffuses down its concentration gradient into the blood and is exhaled

wake up; MAC = .2-.3

13

Factors for Emergence --

• Solubility of agent
• Less soluble agents will allow faster emergence
• Depends on alveolar ventilation
• More ventilation allows faster emergence
• Depends on cardiac output
• Lower CO (slower moving train) allows faster emergence; can offload all of the gas into the Alveoli, which then gets exhaled and the patient emerges faster; but this plays a very small role

Tissue Concentration:

14

Inhaled anesthetics --
effects on the Circulatory system (MAP, HR, other)
which can prolong QT interval?

MAP --
VA -- the volatile anesthetics cause vasodilation and decrease MAP and SVR
N20 = no change

HR -- Small increase (Iso>des); Small Decreased (N2O and sevo)

VAs are cardioprotective (ischemic preconditioning)

VA(esp Sevo) may prolong QT

15

Inhaled anesthetics --
Respiratory Effects:

which drugs are best for inhalation induction? why?

why must patients be intubated on these drugs?

Increase respiratory rate and decrease TV
Decrease in FRC --
Increase in dead space
Bronchodilation

Inhalation induction using sevo and N2O because are nonpungent

Depression of pharyngeal and laryngeal reflexes ---- and must intubate


16

Inhaled anesthetics: CNS effects

which has some mild analgesic properties?

All cause cerebral vasodilation
Increased cerebral blood flow (CBF)
Increased intracranial pressure (ICP)

Uncoupling effect -- paradoxically increased CBF but decrased Cerebral metabolic rate
therefore neuroprotective


analgesic: N2O has some mild analgesic properties

17

Inhaled anesthetics
effects on Neuromuscular system; Renal system; hepatic system

Neuromuscular effects:
• VA cause dose-related skeletal muscle relaxation -- decreased tone
• VA enhance activity of all paralytics --

Decreased renal blood flow:
• Decreased GFR, decreased urine output; usually clinically insignificant mild transient decreases

Decreased hepatic blood flow --- most are metabolized in the liver, but still insignificant