Test 1- Inhalant Anesthesia Flashcards
Vapor
Vapor = Gaseous state of substance that is liquid at ambient temp and pressure
Halothane, Isoflurane, Sevoflurane, Desflurane
Gas
Gas = exists in gaseous state at ambient T and P
N2O, Xenon
Dalton’s law of partial pressure
Total pressure of a gas mixture is equal to the sum of the partial pressure of the individual gases
Vapor pressure
Vapor pressure = Pressure exerted by vapor molecules when liquid and vapor phases are in equilibrium
Depends on temperature
Increases with increasing temperature
Inversely related to boiling poi
What’s a problem with Desflurane?
It’s boiling point is close to room temperature
saturated vapor pressure
Vapors have a maximum administration percentage = saturated vapor pressure
Vapor pressure/Barometric pressure
Ex. Iso 32%
Vaporizers needed to reduce this to clinically useful doses
Solubility
Expressed as a partition coefficient Concentration ratio of an anesthetic
in the solvent and gas phases
Describes capacity of a given solvent to dissolve the anesthetic gas
Blood-gas partition coefficient
Most clinically useful number
Describes amount of an anesthetic in the blood vs. alveolar gas at equal partial pressure
The anesthetic in the alveolar gas represents brain concentration
This is the location of effect
Anesthetic dissolved in blood is pharmacologically INACTIVE
What is the order of gases from most soluble to least?
Halothane, Isoflurane, Sevoflurane, Desflurane
Blood-gas partition coefficient
Low blood-gas PC
Less anesthetic dissolved in blood at
equal partial pressure (more in alveoli)
Shorter time required to attain a partial pressure in the brain
Short induction and recovery
Ex. Iso, Sevo, Des
Clinically more useful
High blood-gas PC
More anesthetic dissolved in blood at
equal partial pressure (less in alveoli)
Longer time required to attain a partial pressure in the brain
Long induction and recovery
Ex. Halothane
Uptake of inhalants
Inhaled anesthetics move down pressure gradients until equilibrium achieved
Vaporizerbreathing circuit alveoliarterial bloodbrain
Partial pressure in the brain (Pbrain) is roughly equal to that in the alveoli (P )A
P = gas delivery to alveoli – removal by A
blood from lungs
Ways to increase Pa
- INCREASE anesthetic delivery to alveoli
- DECREASE removal from alveoli
Increase alveolar delivery
Increase inspired anesthetic concentration (PI)
Increase vaporizer setting
Increase fresh gas flow
Decrease breathing circuit volume
Increase alveolar ventilation Increase minute ventilation
Decrease dead space ventilation
Decrease removal from alveoli
Decrease blood solubility of anesthetic
Decrease cardiac output
Patients with low CO will have a faster
rise of P A
Decrease alveolar-venous anesthetic gradient
Reflects tissue uptake of anesthetic
Concentration effect
The higher the P , the more rapidly P
IA
approaches PI
A high PI is required at the beginning of gas anesthesia to quickly increase P
Offsets impact of uptake (removal of anesthetic by pulmonary circulation)
As uptake into blood decreases, PI can be decreased
Anesthetic elimination
Requires decrease in P A
Same variables that affect a rise in P
Especially agent solubility (blood- gas PC) and alveolar ventilation
So how would you quickly
decrease Pa ?
Turn off vaporizer
Disconnect patient and flush O2
Turn up O2 flow
Dilute anesthetic in circuit as it is
exhaled from patient
Increase ventilation (IPPV)
Increase fresh gas to alveoli
Minimum Alveolar Concentration (MAC)
Minimum alveolar concentration of an anesthetic that prevents movement in 50% of patients exposed to a noxious stimulus
Allows comparison of potency between agents
Inverse relationship (high MAC = low potency)
What’s the MAC for iso and Sevo?
Iso- 1.3
Sevo- 2.3
MAC increases and decreases?
Mac is…
MAC
MAC is additive, therefore:
(0.5 x MACA) + (0.5 x MACB) = 1 MACAB
Important if:
Changing gases in the middle of a case
Using N2O
Using partial intravenous anesthesia (PIVA)
Effects of the volatile anesthetics
Cardiovascular
Respiratory
Neurologic
Renal
Hepatic Other
CV effects of volatile anesthesics
Respiratory effects of inhalant anesthetics
Respiratory
Decrease ventilation
Depress central and peripheral
chemoreceptors
Decreased responsiveness to CO2
Respiratory arrest at 1.5-3 MAC
Bronchodilation
Desflurane and isoflurane – irritating odor
Sevoflurane – least irritating
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https://basicsofpediatricanesthesia.files .wordpress.com/2013/07/mask- induction.jpg
Neurologic effects of inhalanat anesthetics
Increase intracranial pressure at > 1 MAC
Increase cerebral blood flow
Decrease cerebral metabolic rate
Act on brain and spinal cord to produce immobility (not analgesia)
Suppress seizure activity (except Enflurane)
Renal effects of inhalant anesthetic
Decrease glomerular filtration rate and renal blood flow due to decreased CO
Renal failure (methoxyflurane)
Compound A
Produced from sevoflurane breakdown in CO2 absorbent (baralyme > soda lime)
Higher concentrations formed during: Prolonged anesthesia
Low fresh gas flows
Desiccated absorbent
Nephrotoxic in rats
Compound A
Produced from sevoflurane breakdown in CO2 absorbent (baralyme > soda lime)
Higher concentrations formed during: Prolonged anesthesia
Low fresh gas flows
Desiccated absorbent
Nephrotoxic in rats
Hepatic effects of inhalant anesthesia
Reduce liver blood flow and O2 delivery (related to decrease in CO)
Halothane can cause hepatotoxicity (2 types)
- Increased liver enzymes – mild, self- limiting,
- “Halothane hepatitis” – immune- mediated, often fatal
Malignant hyperthermia
Myopathy occurring in genetically predisposed pigs, dogs, cats, horses, (people)
Exposure to inhalant anesthetic
Esp. halothane, but also iso, sevo, des
Uncontrolled muscle contraction severe hyperthermiadeath
First sign is often a rapid increase in EtCO2
Malignant hyperthermia Treatment
Treatment
Discontinue volatile anesthetic, flush with
O2, switch to new circuit if possible
Provide 100% O2
Administer dantrolene – muscle relaxant
Fluids, active cooling
Death likely despite treatment EXTREMELY rare
Maybe once per CAREER (>30 yrs) for an anesthesiologist
Nitrous Oxide
Used more frequently in people (lower MAC) than animals
Max administration 75% (need > 25% O2)
Low solubility (blood-gas PC 0.47)
Minimal CV and resp depression
Mild analgesic
Transfer to closed gas spaces
Equilibration leads to N2O accumulating rapidly (more soluble in blood) while Nitrogen leaves slowly (less soluble)
GI tract, sinuses, middle ear, pneumothorax, GDV, cuff of ET tube
Avoid in disease states causing increased closed gas space
Diffusion hypoxia
When N2O administration is stopped, it diffuses quickly out of the blood into alveoli (down concentration gradient)
Displaces O2 from alveoli
If breathing room airhypoxia
When discontinuing N2O, provide 100% O2 for 5-10 minutes to prevent
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Common complications of inhalant anesthesia
Common complications of inhalant anesthesia
Anesthetic-related
Hypotension
Hypoventilation
Hypothermia
Machine-related
Closed pop-off
Stuck inspiratory- expiratory valves
Exhausted soda lime
Inadequate O2 flow in non-rebreathing system
Human error
Intubation mishaps
Laryngeal damage
Stuck tube
Aspirated tube
Tracheal tears
Anesthetic overdose
Hypotension
Hypotension
MAP<60 mmHg (small animal) or <70 mmHg (large animal)
Roughly corresponds to Doppler BP of 80 or 90
FIRST, evaluate patient and TURN DOWN THE VAPORIZER if the patient is too deep for the current level of stimulation
This is the most appropriate and effective treatment for hypotension during inhalant anesthesia
If patient is light, consider adding a MAC-sparing drug and THEN turn down the vaporizer
Opioid, benzodiazepine, lidocaine, ketamine, etc.
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If still hypotensive, evaluate the underlying cause and treat appropriately
Decreased vascular volume (hypovolemia)? Give crystalloid and/or colloid bolus
Vasodilation?
Give vasopressor
Decreased contractility? Give inotrope
Hypoventilation
Definition: PaCO2>40 mmHg OR EtCO2>45 mmHg
However, EtCO2 up to 50-55 mmHg may be tolerated in certain patients (slightly complicated and controversial topic)
FIRST, evaluate patient and TURN DOWN THE VAPORIZER if the patient is too deep for the current level of stimulation (sound familiar?)
Perform IPPV Manual
Mechanical
Hypothermia
Inhalant anesthesia abolishes the normal vascular compensatory mechanisms to conserve heat
Causes peripheral vasodilation increased heat LOSS
Prevention is more effective than treatment
Warm patient before induction (especially small ones)
Bubble wrap feet
Keep patient covered, minimize scrub time and
exposure to water/alcohol
Increase room temp
Forced warm air heating (BAIR hugger)
Closed pop-off
What happens?
Bag fills, breathing system pressure increases, pressure transmitted to patient lungs and thoracic cavity
Decreased venous return (decreased preload)compressed great vessels (increased afterload)DECREASED CARDIAC OUTPUT
Clinical signs = apnea, bradycardia, fading Doppler signalcardiopulmonary arrest
Treatment
Pull rebreathing bag off (usually
faster than unscrewing pop-off)
Start CPR if patient has arrested
Evaluate for pulmonary injury (auscultation, chest radiographs)
Revisit habits and safety checklists
Use quick-release or safety pop-offs
Don’t take hand off closed pop-off
until open again (for leak check)
Potential for pneumothorax
Stuck inspiratory-expiratory valves
What happens?
Rebreathing system becomes bidirectional
Causes rebreathing of expired gas hypercarbia
Signs?
Easy to see on a capnograph (rebreathing waveform)
If no capnograph, need to be more vigilant
Should be obvious visually if valves are sticking open
If concerned, check that valves are functional by inspiring and expiring through *flushed* system
What are signs of hypercarbia in your patient? Treatment
Dry and clean valves frequently, especially after long cases
Replace as needed
Exhausted soda lime
What happens?
Soda lime no longer removes CO2 from
expired gases
Patient rebreathes CO2hypercarbia
Will look THE SAME as stuck inspiratory-expiratory valves on capnograph (rebreathing waveform)
What else can cause this waveform?
**Inadequate O2 flow in a non-rebreathing system**
Note that the waveform does NOT return to baseline (0) between breaths
Possibilities:
Stuck insp/exp valves
Exhausted soda lime
Inadequate O2 flow in a non-rebreathing
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system
Intubation mishaps
Laryngeal damage
From laryngoscope or stylet most
commonly
Be gentle! Tissues are sensitive
Swelling can lead to post-op airway obstruction
If you use a stylet for cat intubation, it SHOULD NOT protrude past the end of the ET tube
Place the laryngoscope on the base of the tongue (don’t touch the epiglottis)
Intubation mishaps
Stuck tube
Do not force an ET tube that is too big
for your patient
It may go in, but not come out!
What would you do?
Re-anesthetize quickly (propofol or
alfaxalone)
Cut tube to allow compression
Intubation mishaps
Aspirated tube
Aspirated tube
If patient bites through tube and aspirates the intra-
tracheal portion
What would you do?
Re-anesthetize quickly (propofol or alfaxalone)
Provide O2
Retrieve tube
Long forceps
Re-intubate with smaller tube, inflate cuff, then pull both tubes out together
Tracheoscopy or bronchoscopy may be required
Ideally, avoid this complication by providing proper
patient monitoring
Don’t wait too long to extubate!
Tracheal tears
Not uncommon in cats
Associated with overfilling of the
tube cuff
Only fill until there is no leak at 15- 20 cm H2O, no more
Do not add air unless there is a leak
Associated with traction on tube (weight of breathing tubes) and turning/twisting patient
ALWAYS disconnect patient from breathing system before moving
Anesthetic overdose
Inhalant anesthetics have a very low therapeutic index (difference between a therapeutic and fatal dose)
Overdose can happen very quickly
Especially at high flow rates and vaporizer settings
If in doubt about the status of your patient – turn the inhalant down or OFF while you evaluate the situation
Remember that patient movement does not mean a conscious patient
A very low blood pressure (MAP < 50) indicates inadequate cerebral blood flow for consciousness
(The patient doesn’t need inhalant anesthesia to stay asleep)
TURN THE INHALANT ANESTHETIC OFF UNTIL BP HAS IMPROVED!
Sick patients often need VERY LITTLE inhalant
Utilize your MAC-sparing drugs – opioids, benzos, lidocaine, ketamine, etc.
A moving patient is tolerable in most cases and preferred over a dead patient
