Clinical use of inhalant anesthetics (Granone) Flashcards
inhaled anesthetics produce anesthesia by
- unconsciousness
- immobility
- muscle relaxation
- no inherent analgesia properties
inhalent anesthetics don’t depend on
- hepatic or renal function
inhaled anesthetics allow
- rapid and precise adjustment of anesthetic depth
- rapid and complete recovery
Inhaled anesthetics exist as
- liquids, administered as vapors
- requires device for accurate conversion
Potency
- dose expressed as minimum alveolar concentration (MAC)
MAC
- minimum alveolar concentration of anesthetic which prevents gross, purposeful movement in 50% of patients exposed to a noxious stimulus
- birds don’t have alveoli so it’s ‘minimal anesthetic concentration’
relationship between MAC and potency
- inversely related
- higher the MAC number, the less potent a drug is
- MAC is additive
What we put in the alveoli mirrors the
partial pressure in the brain
MAC values
ISO/SEVO/DES
Dogs and cats
- Dogs: 1.14-1.15 / 2.1-2.4 / 7.2-10.3
- Cats: 1.28-1.6 / 2.6-3.1 / 9.8-10.3
two sites of action of inhalents
- brain (amnesia)
- Spinal cord (immobility)
MAC at induction
MAC during a nesthesia maintenance
- need 2-3 times agent MAC to induce
- need 1.5-2 times agent MAC
*Take into consideration simultaneous drug administration and procedure being performed
Factors that inc MAC
- Hyperthermia
- Hypernatremia
- Drugs that cause CNS stimulation
- Increased levels of excitatory NTs
Factors that dec MAC
- Other anesthetics
- Hyponatremia
- Hypotension
- MAP < 50 mmHg)
- PaO2 below 40 mmHg
- PaCO2 above 90 mmHg
- Pregnancy
- inc Age
Factors that don’t affect MAC
- Gender
- Normal resp gas concentrations
- Duration of anesthesia
- Metabolic acidosis/alkalosis
- Mild to moderate anemia
Atropine is a CNS …..
stimulant
All inhalants are administered as vapors except
N2O
At ambient temp and pressure, inhalant anesthetics are
liquids
Saturated vapor pressure
pressure exerted by a vapor when it exists in equilibrium with its liquid
Inhalant concentration
(Vapor pressure/ barometric pressure) X 100 = vol %
Inspired concentration of anesthetic (FI)
- Patient inspired concentration
- Not the same with RB vs. NRB systems
Expired anesthetic concentration
- Anesthetic concentration expired gases
- Reflects alveolar concentration
- Measured by gas analyzer
Avleolar concentration of anesthetic (FA)
- reflects arterial anesthetic concentration delivered to brain
- concentration in brain and spinal cord produce general anesthesia
Vaporizer to Brain
- Inspired anesthetic concentration
- Solubility of anesthetic in blood
- Cardiac output of patient
- Ventilation
- Alveolar to venous partial pressure difference of anesthetic
Factors Affecting FI
- Fresh gas flow rate
- inc flow rate through vaporizer; faster concentration reached
- anesthetic concentration change is proportional to fresh gas flow rate
- Circuit volume
- larger volume, longer equilibration time
- machine absorption of inhalant
- older inhalants; rubber parts
Flow meter on anesthetic machine is like…..
- the gas pedal in the car
Time Constant
- Amount of time to fill container with desired substance
- Container: lungs, machine
- Substance: inhalant
- 3T = 95% concentration change to be achieved
Solubility
- total amount of solute dissolved within a solvent at equilibrium
- inherent property of vapor/gas
- influenced by ambient temperature
- Expressed as partition coefficient
- Blood:gas partition coefficient
- how much something wants to stay in blood = bad
- we want inhalent in alveoli, not blood
- relative affinity of an anesthetic vapor for blood compared to alveolar gas
- Blood:gas partition coefficient
Blood:gas partition coefficient
- speed of onset
- speed of recovery
- change in anesthetic depth
Higher the blood:gas partition coefficient
- inhalant favor being in blood (more soluble)
- greater uptake by tissues
- lower FA/FI ratio
- longer onset of action, recovery
High partition coefficient means drug wants to stay in
blood
low partition coefficient means drug wants to stay in
alveoli
Inc CO and uptake
- Inc CO = greater amount of blood carrying inhalant away from alveolit to tissue
- slows anesthetic onset
- slows rate of FA
Dec CO and uptake
- dec CO = less blood flow through lungs and less anesthetic removed
- quicker onset of anesthesia
- anesthetic overdose
Inc alveolar ventilation …
- Faster FA reaches FI
Partial pressure in alveoli is a balance between
- input to alveoli => delivery
- loss from alveoli => uptake
Alveolar ventilation is more important with….
highly soluble agents
apnea at induction equals….
awakening
PA - PV
- Venous blood returning to lungs for re-oxygenation will retain some inhalant
- PA - PV gradient must exist for uptake to occur
- Difference related to amount of anesthetic taken up by tissues
- highly perfused tissues equilibrate faster
- As gradient dec, uptake dec
Distribution depends on …..
- blood flow and tissue capacity
- inc flow and capacity; inc uptake
Inhalant Metabolism
amount of metabolism from most to least
- minimal role in removal of inhalant from body
- most
- methoxyflurane: 50-75%
- Sevoflurane: 2-5%
- Isoflurane: 0.2%
- Desflurane: 0.02%
Inhalent metabolism and toxic metabolites
- sevoflurane => compound A
- Isoflurane, desflurane, enflurane => carbon monoxide
Cardiovascular effects
- largely impacted by inhalant anesthetics
- All inhalants reduce cardiac output
- negative inotropic effect: decrease in stroke volume
- Decrease peripheral vascular resistance: negative effect on BP
- Dose dependent
- Dysrhythmias => halothane, less with iso and sevo
Things that enhance cardiovascular compromise (5)
- mechanical ventilation
- PaCO2 changes
- Surgical stimulation
- Length of inhalant administration
- Concomitantly administered drugs
Pulmonary system effects
- Dose related decrease in ventilation
- blunt the response to increased CO2
- can act as a safety mechanism
*inc PBRAIN => dec ventilation => dec uptake => dec PBRAIN
*inhalant overdose => respiratory arrest => cardiac arrest
Inhalent anesthetics not often administered….
alone
- premeds
- induction agents
- CRIs
When using inhalant anesthetics strive to use
lowerst possible concentration
- decrease adverse effects
- MAC sparing drugs
When using inhalant anesthetics you should use a
balanced anesthetic technique
Nirous Oxide
- gas at room temp
- MAC 188% (can only put something at 100%)
- mild analgesia and anesthetic effects
- minimal cardiopulmonary effects
- admin at 2:1 ratio with O2
- Diffuses rapidly into closed gas spaces
- GDV, Colics, middle ear probs, pneumothorax
- high abuse potential => death
Summary of inhaled anesthetics
- unique administration and elimination
- allow for precise control and adjustment
- clinical properties are determined by
- physical and chemical characteristics
- solubility of agent
- MOA not fully known
- All possess desirable and undesirable effects
*Maybe don’t use with sick patients
