Inhalants Flashcards
Why use inhalants?
They cause unconsciousness, anti-nociception, and immobility
Disadvantages of inhalants
- Requires specialized equipment
- Requires airway access
- Adversely affects mult organ systems
- Occupational & environmental hazards
- Horses: assoc w/ incr rate of complications
What are some unique properties of inhalants?
- Administered & eliminated via lungs
- Predictable & rapid titration
- State of anesthesia = reversible
What considerations should be made when choosing an inhalant?
- Know what equipment is available (each requires own vaporizer)
- Desired speed of induction & recovery: diff solubilities
- Overall costs (Desflurane $$$ > Sevo > Iso $)
Why do inhalants require vaporizers?
Isoflurane, sevoflurane, and desflurane are VAPORS!
Define saturated vapor pressure
Pressure molecules push against walls of a container when in equilibrium at standard temperature; dictates the max # of vapor molecules available for delivery to a patient
What is the saturated vapor pressure of sevoflurane? Isoflurane? Desflurane?
- 20.7%
- 31.3%
- 88.0% - most volatile
*these concentrations will cause ANESTHETIC OVERDOSE! - why we need vaporizers
Why require a vaporizer?
Dilutes inhalants with a carrier gas to deliver a clinically safe concentration of inhalant to the patient
What to do if mixing occurs?
DON’T USE ON PATIENTS - send out vaporizer for servicing or drain and run a high flow of O2 through for several hours then check for trace vapor levels with a gas analyzer
What is the major goal of inhalant delivery?
Achieve an adequate partial pressure in the brain to create unconsciousness
The partial pressure of the brain should be the same as that of what other body part?
Alveolar
Why might it be difficult to keep patients anesthetized at high altitudes using equal concentration?
B/c there’s a lower atmospheric pressure at higher altitudes, so the partial pressure will be lower (% of inhalant x atm pressure)
A rapid increase in alveolar partial pressure leads to ________
A rapid increase in anesthetic depth
In order to rapidly increase anesthetic depth, you must do what 2 things?
Increase alveolar delivery and minimize uptake from alveoli
To increase alveolar delivery, you need to do what 2 things?
Increase fresh gas flow and minimize breathing circuit volume
Define time constant
Time it takes to make 50% change in a closed system (exponential equation)
Time constant = volume/flow
When the time constant decreases, what 2 things happen?
Faster change in inhalant concentration in the system and faster anesthetic induction
How many time constants are needed to make 98.5% change in the system?
Five (exponential increase)
T or F: agents with low solubility in blood cause faster induction
True
Higher solubility agent definition and example
Gas rapidly moves into blood, but concentration that reaches brain increases more SLOWLY; e.g. isoflurane
What is an example of a low solubility agent?
Desflurane
What are ways to minimize uptake from alveoli?
Using agents with low solubility and slowing cardiac output
What are the steps for rapid induction or quickly making changes in anesthetic depth?
- Increase inspired concentration of inhalant
- increase fresh gas flow
- ventilate patient
- minimize dead space within circuit
- premed to minimize pain/anxiety
- utilize agents with low blood:gas partition coefficient
What are the 6 steps to facilitate rapid recovery from inhalant anesthesia?
1) Minimize delivery of inhalant (turn off vape, remove excess inhalant from reservoir, remove P from machine)
2) Utilize agents w/ low blood solubility
3) Increase fresh gas flow
4) Assist ventilating the patient
5) Shorten duration of anesthesia
6) Increase cardiac output
Define minimum alveolar concentration?
Minimum alveolar concentration of an anesthetic that prevents gross purposeful movement in 50% of patients exposed to a noxious stimulus
- surgical procedures induce patients at 2-3 x MAC
- anesthetic maintenance 1.2-1.5 x MAC
Potency is inversely related to what?
Minimum alveolar concentration
Rank these according to potency: Sevoflurane Desflurane Isoflurane
Isoflurane > Sevo > Desflurane
What factors increase MAC?
- Hyperthermia
- Hypernatremia
- Drugs that cause CNS stim
- Incr levels of excitatory NTs
What factors decrease MAC?
- Other anesthetics
- Hyponatremia
- Hypotension (MAP<50 mmHg)
- Hypothermia
- PaO2 <40 mmHg
- PaCO2 > 90 mmHg
- Pregnancy
- Incr age
What factors have no effect on MAC?
- Gender
- Normal resp gas concentrations
- Duration of anesthesia
- Metabolic acidosis/alkalosis
- Mild to moderate anemia
What are the CV effects of inhalants?
- CV depression - decr myocardial contraction/CO
- Hypotension
- decr CO, widespread vasodilation
- Minimal direct effect on HR & Arrhythmogenicity
- Dose dependent
Tx: fluid tx + inotrope/vasopressors
What are the effects of inhalants on the respiratory system?
- Resp depression: depressed alveolar ventilation —> hypoventilation
- apnea possible
- tolerance to elevated arterial CO2
- Bronchodilation
- Dose dependent
Tx: support ventilation
What are the neurological effects of inhalants?
- Cause unconsciousness and immobility
- Elevations in ICP: doses > 1 MAC
- cerebral vasodilation–> incr cerebral perfusion pressure
- Decr cerebral metabolic rate
What are the effects of inhalants on MS system?
Malignant hyperthermia - genetic mutation of ryanodine receptor
- Inhalants cause massive Ca release from SR in mm —> elevated arterial CO2, tachycardia, hyperthermia, e- imbalances, acidosis—> circ shock & death
Tx: STOP inhalants! Actively cool patient, Dantrolene (impossible to find)
Ways to limit occupational hazards of inhalants
- Utilize scavenging systems
- Limit use of mask & chamber inductions
- Avoid unnecessary spillage
- Turn off vaporizer & fresh gas flow when disconnecting patients
- Pregnant women can wear charcoal facemask
- Well-ventilated rooms
What are the advantages of NO?
- Speeds induction & Reduces inhalant dose
- Minimal CV effects
What are the disadvantages of NO?
- Avoid use in animals with closed gas spaces (colic, GDV, pneumothorax, etc)
- Diffusion hypoxia at end of anesthesia
