Lecture 5 - Volatile Anesthetics Flashcards
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)
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
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
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
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
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
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)
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
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
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
Factors Determining PA:
Solubility – want to be less solubule in blood than it is in lipid and brain (N2O<iso)
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
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
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:
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
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
