Inhalant Anesthetics Flashcards
purpose of inhalant anesthetics
- produce general anesthesia by:
- rendering patient unconscious
- providing some degree of muscle relaxation
- do not have any inherent analgesic properties
- suitable for wide variety of species
main difference with inhalant anesthetics
administered and in large part removed from the body via the lungs
do not rely on hepatic metabolism and renal elimination
how inhalants are administered
liquid anesthetics are first vaporized and then administered in an enriched concentration of oxygen via a breathing circuit to the patient
requires specialty equipment (heavy, bulky)
equipment needed for inhalants
- vaporizer
- source of oxygen
- anesthetic machine
- breathing circuits
- scavenging equipment
scavenging equipment is essential to:
help minimize occupational hazards involving waste gases
potency of inhalants
is an expression of the relationship between the administered dose of an inhalant and the anesthetic effect that is obtained
MAC is the most commonly used expression of potency of inhalants
MAC
minimum alveolar concentration of anesthetic which prevents gross, purposeful movement in 50% of patients exposed to a noxious stimulus
similar to ED50
determined in young healthy animals without use of additional CNS depressant drugs
mirrors the brain partial pressure of the inhalant
MAC of isoflurane
Dog: 1.14-1.5%
Cat: 1.28-1.6%
Horse 1.3-1.6%
MAC of sevoflurane
Dog: 2.1-2.4%
Cat: 2.6-3.1%
Horse: 2.3-2.8%
MAC of desflurane
Dog: 7.2-10.3%
Cat: 9.8-10.3%
Horse: 7.0-8.0%
general guideline for MAC during surgery
anesthesia is begun at 2-3 times MAC for the particular agent and anesthesia can be maintained at 1.5-2 times agent’s MAC value
will depend on individual patient, other drugs used and type of surgery
factors that can increase MAC
- hyperthermia
- hypernatremia
- drugs that cause CNS stimulation (ephedrine)
variables that decrease MAC
- drugs such as premedications and induction agents
- use of local anesthetics
- mean BP below 50 mmHg
- hyponatremia
- hypothermia
- substantial alterations in respiratory gases
- severe anemia
- age of patient
- pregnancy
factors that do not alter MAC
- gender
- normal respiratory gas concentrations
- duration of anesthesia
- metabolic acidosis or alkalosis
- moderate anemia
vapors
administered as a vapor mixed in a gas (usually oxygen) but at ambient temperature and pressure exist as liquids
saturated vapor pressure of a liquid
escaping gas molecules from a liquid exert pressure on the sides of the container
highest anesthetic concentration that can be achieved is determined by:
saturated vapor pressure of the agent
equation for maximum percentage of inhalant that can be achieved
anesthetic vapor pressure
————————————– x 100
atmospheric pressure
atmopsheric pressure at sea level = 760 mmHg
delivered concentration of anesthetic
percent setting that is on the vaporizer dial
inspired or inspiratory concentration (FI) of an anesthetic
concentration of inhalant that the patient inspires
What should Fi be if patient is apneic?
inspired concentration of anesthetic is 0
What is the Fi if the patient is on a non-rebreather?
the inspired concentration of anesthetic should be equal to that set on the vaporizer
What is the FI if the patient is on a circle rebreathing system?
the inspired concentration of anesthetic will be less than that set on the vaporizer as the concentration in the circle breathing system is diluted out by expired gases
expired anesthetic concentration
the concentration of inhalant anesthetic that is contained in the expiratory gases of the patient
reflection of the alveolar concentration of inhalant
alveolar concentration (Fa) of anesthetic
concentration of anesthetic that is in the arterial blood and delivered to the brain
the anesthetic concentration in the brain and spinal cord produce unconsiousness and immobility during inhalant general anesthesia
factors that affect getting the inhalant administered from the vaporizer to the brain:
- inspired anesthetic concentration
- solubility of the anesthetic in the blood
- patient’s CO
- ventilation
- the alveolar to venous partial pressure difference of anesthetic
factors that affect inspired concentration of inhalant delivered to patient:
- rate at which fresh gas is flowing into the system
- volume of breathing circuit
- if inhalant is absorbed by the anesthetic machine or breathing circuit (rubber)
- not very common anymore with newer inhalants
higher the carrier gas flow rate is through the vaporizer =
the more rapid the circuit concentration will approach the concentration exiting the vaporizer
therefore, rate at which the circuit in a circle rebreathing system and inspired anesthetic concentration can be increased is directly proportional to rate of fresh gas flow going through vaporizer and entering breathing circuit
effect of the volume of the breathing circuit
a circuit with a larger volume will take longer to equilibrate and reach the desired concentration of anesthetic being delivered to patient
time constant
amount of time it takes to fill a ‘container’ with desired substance
time constant (T) (min) = volume (L) / flow (L/min)
3 time constants to reach 95% of desired concentration
container = lungs and anesthetic machine
factors that affect concentration of inhalant in alveoli
- solubility of anesthetic in the blood
- patient’s CO
- alveolar ventilation
- alveolar to venous PP difference of anesthetic
Fa reflects amount of anesthetic in brain
anesthetic solubility
- inherent property of gas and is influenced by ambient temperature
- major factor in uptake and distribution of anesthetic agents
- expressed as partition coefficient
partition coefficient of an anesthetic gas
measure of its solubility in a specific solvent at a specific temperature
defined as the ratio of gas concentrations in the two phases at equilibrium
blood:gas partition coefficient
blood and inspired gas
provides information about speed of onset, recovery and change in anesthetic depth
the higher the blood:gas partition coefficient, the more inhalant agent will favor being in the blood (greater solubility)
an inhalant with a higher blood:gas partition coefficient:
will have greater uptake by the tissues and will have a lower Fa/Fi ratio
longer onset of action
greater the cardiac output
=
the greater amount of blood carrying inhalant away from the alveoli and to the tissues
this will decrease the alveolar (Fa) anesthetic concentration and slow down the rate at which the Fa reaches the inspired anesthetic concentration (Fi)
a decrease in cardiac output =
causes less blood to flow through the lungs and therefore less anesthetic is removed from them rendering the onset of general anesthesia more quickly
anesthetic overdose
delivery of agent to alveoli depends on the:
inspired anesthetic concentration
and
alveolar ventilation
higher the alveolar ventilation
=
the faster the alveolar anesthetic concentration will reach the inspired anesthetic concentration
anesthetics that decrease ventilation will decrease rate of rise of alveolar concentration of inhalant anesthetics –>support ventilation
(Pa-Pv)
partial pressure difference between alveolar and venous blood
venous blood will retain some inhalant when it returns to the lungs for re-oxygenation
highly perfused tissues (brain…) equilibrate rapidly with Pa of anesthetic compared to less well perfused tissues
role of metabolism in inhalant removal
minimal
but toxic metabolites can still be produced!
inhalants and cardiovascular system
inhalants have a large impact on CV
all reduce CO in a dose dependent fasion
type, dose, and concurrent drug administration during anesthesia all affect CV system
detrimental due to effects on oxygen delivery (perfusion)
oxygen delivery
product of oxygen content of blood and CO
equations:
CO (Q) =
BP =
SV x HR
Q x systemic vascular resistance (SVR)
inhalants have a positive or negative inotropic effect?
negative
results in a decreased stroke volume
inhalants cause an increase or decrease in peripheral vascular resistance?
decrease
which in combo with reduction in CO will causes a decrease in arterial BP
what factors enhance CV compromise due to inhalants?
- mechanical ventilation
- changes in PaCO2
- surgical stimulation
- length of exposure to the inhalant
- other drugs administered with inhalant
ventilation
arterial concentration of CO2
tidal volume and respiratory frequency play an important part
inhalants cause a dose related decrease in ventilation
an increase in PaCO2 will __________________ in a conscious animal?
what do inhalants do to this response?
stimulate respiration
blunt the response
“safety” mechanism
mechanical ventilation’s effect on the “safety mechanism”
as inhalant concentration increases in the brain, ventilation becomes reduced or absent, and thus inhalant uptake will be reduce
mechanical ventilation causes the safety mechanism to be lost
= respiratory arrest, then cardiac arrest can occur
use of a balanced anesthetic technique
will allow for a suitable plane of anesthesia with decreased cardiorespiratory compromise as long as the drugs themselves do not further exacerbate unwanted side effects
MAC sparing drugs: benzos, sedatives (Ace and alpha2s), opioids, and local anesthetics
nitrous oxide use
can be used to supplement inhalants
gas at room temp, not a volatile anesthetic agent
MAC in animals is ~188% (can’t be used alone)
mild analgesic and anesthetic effects, few CV side effects and minimal respiratory depression
detrimental consequences of nitrous oxide
if administered at greater than 70% of total gas flow, severe hypoxemia can occur
very insoluble gas which readily diffuses out of blood and into closed gas spaces-will cause rapid expansion of closed gas spaces (pneumothroax, sinuses) causing volume and pressure in cavity to increase
can cause bone marrow supppression and anemia
fairly high abuse potential which can be fatal
