Inhalant Anesthesia Flashcards
Anesthesia
Obtained with the absorption of a drug by the respiratory system, reaching the systemic circulation and the CNS
Clinical effects
Amnesia
Unconsciousness (narcosis)
Immobility (muscle relaxation)
Not analgesia (few exceptions)- so we need to include a separate pain drug
Advantages
Rapid adjustment of anesthetic depth Minimal metabolism Elimination by respiration Rapid recovery Economical
Mechanism of Action
Not completely known
Activation of inhibition of different molecular targets- GABA, glycine, glutamate, serotonin, ACh, K+ channels
Amnesia- GABAa- hippocampus
Narcosis- GABAa- cortex
Immobility- dec glutamate receptors and K+ channels/ inc glycine receptor activity- spinal cord
Vapor
Gaseous state of a substance that is liquid at ambient temperature and pressure
Halothane, isoflurane, sevoflurane, desflurane
Gas
Exists in gaseous state at ambient temperature and pressure
Nitrous oxide, xenon
Nitrous Ocide
Laughing gas
low blood gas (0.47)
Mild analgesic
Accumulates in closed gas spaces
Xenon
Very expensive
mostly experimental
Minimal cardiovascular depression
Boyle’s law
Volume is inverse to pressure
pressure inc = volume dec
Charles’s law
Volume and Temperature are proportional
inc volume= inc temp
Gay-Lussac’s law
Pressure proportional to temperature
inc pressure = inc temperature
Dawton’s law
Total pressure of gas mixture is equal to the sum of the partial pressure of the individual gases
Vapor pressure
Pressure exerted by vapor molecules when liquid and vapor phases are in equilibrium
only changes with temperature
Boiling point
temperature at which the vapor pressure equal to the atmospheric pressure- inversely related to vapor pressure
Desflurane
Boiling point (23.5C) is close to room temperature
Electric heated vaporizer required
-Desflurane maintained in gaseous form (high pressure 2ATM)
-blends with fresh oxygen to achieve vaporizer settings
More used in human medicine
Expensive
Horses
Vapors
Maximum administration percentage
Vaporization at ambient
Vapor pressure/Barometric pressure
vaporization chamber
isoflurane is 32%- this is way higher than the clinical dose so we need a way to reduce this. in a vaporizing chamber the o2 will pass on the surface of the liquid anesthetic and vaporize the anesthetic and they will mix with o2 from the bypass chamber which dilutes it
Solubility
Anesthetic vapors dissolve in liquids and solids
Equilibrium is reached when the partial pressure of the anesthetic is the same in each phase
-partial pressures are equal
-number of anesthetic molecules are not equal
expressed as partition coefficient
Concentration ratio of an anesthetic in the solvent and gas phase
Describes the capacity of a given solvent to dissolve the anesthetic gas
Blood-gas partition coefficient
Most clinically useful number
Describes the amount of anesthetic in blood vs alveolar gas at equal partial pressures
The alveolar partial pressure represents the brain concentration after equilibrium (although usually measured as %)
anesthetic dissolved in the blood is pharmacologically inactive
Most to least soluble
Halothane, isoflurane, Sevoflurane, Desflurane
Low Blood-gas partition coefficient
Less anesthetic dissolved in the blood at equal partial pressures (more in alveoli)
Shorter induction and recovery times (shorter time required to achieve steady state in the brain)
Clinically more useful (iso, sevo, des) (bc faster recovery time)
Hypothermia increases anesthetic solubility
Halothane: high BGPC- longer induction and recovery times
Uptake of inhalants
Inhalants move down pressure gradients until equilibrium is achieves
Vaporizer-breathing circuit- alveoli- arterial blood- brain
Partial pressure of the brain is roughly equal to that in alveoli
Pa: gas delivered to alveoli is removed from the lungs by blood
Ways to increase Pa
Increase the anesthetic delivery to alveoli
Decrease anesthetic removal from alveoli
Increase speed of induction and change the anesthetic plane
Increase alveolar delivery
Increase inspired anesthetic concentration -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
-different agents
decrease cardiac output
-low CO - lower extraction of anesthetics from the lungs- faster rise of Pa
Decrease alveolar-venous anesthetic gradient
-tissue uptake from anesthetic
Concentration effect
The higher the Pi the more rapid Pa approached Pi
A high Pi is required at the beginning of gas anesthesia to quickly increase Pa
Offsets impact of uptake (removal of anesthetic by pulmonary circulation)
As uptake into blood decreases, Pi should be decreased
Anesthetic elimination
requires decrease in Pa
Same variable that affect rise in Pa
Especially agent solubility and alveolar ventilation
Anesthetic time and decreased body temperature
How would you quickly decrease Pa
turn off vaporizer
disconnect patient and flush the breathing system with O2
Turn up O2 flow (dilute the anesthetic in the circuit)
Increase ventilation (increase fresh gas to alveoli)
Minimum Alveolar Concentration (MAC)
MAC of an anesthetic necessary to prevent movement in 50% of patients exposed to a noxious stimulus at sea level
MAC between species is consistent
Dose of the inhalant anesthetic- allow comparison between agents
-high mac- low potency
Alveolar concentration (%)-way to measure the dose given Measures in % and not partial pressure MAC colorado= MAC sea level/Barr col/barsealevel
Gas analyzer
gold standard to measure anesthetic dose
end tidal gas concentration
Factors that change MAC- increase
Hyperthermia Hypernatremia Drugs causing stimulation of CNS decreased age Red hair in people
Factors that change MAC- decrease
Hypothermia hyponatremia drugs causing depression of CNS increased age severe hypotension hypoxemia metabolioc acidosis/hypercapnia pregnancy anemia
MAC multiples
Used to describe dose
1 MAC- immobility in 50% of the patients
1.3 MAC- immobility in 95% of the patients
MAC is additive: 0.5MACa+ 0.5 MACb= 1MACab
changing the anesthetic agents during the procedure, using N2O, using partial intravenous anesthesia (PIVA)
Systemic effects of volatile anesthetic
Cardiovascular Respiratory Neurologic Renal Hepatic Other
Cardiovascular effects
dec CO dec BP dec Systemic vascular resistance dec contractility (inotropy) no change to inc HR (chronotropy)
Respiratory system
Dec ventilation
-inhibit central CO2 and peripheral O2 chemoreceptors
Apnea at 1.5-3 MAC
bronchodilation
Desflurane and isoflurane: irritating odor
Sevoflurane: less irritating- used for mask inductions (usually bird)
Ventilation
arterial partial pressure of CO2
monitor ventilatory status
Normal PaCO2: 35-45 mmHg
Hypoventilation PaCO2 > 45mmHg
Neurology system
Inc intracranial pressure (ICP) at > 1 MAC
-due to cerebral vasodilation
-sevoflurane increases less the ICP-so use for MRI for neuro patient
Decrease cerebral metabolic rate
immobility, hypnosis, amnesia-spinal cord and brain
Suppress seizure activity (except for enflurane)
Renal system
Decrease glomerular filtration rate and renal blood flow
Due to decrease in CO (dec blood flow to like every organ)
Renal failure (methoxyflurane)- dont have anymore
Compound A
Produced by secoflurane breakdown in CO2 absorbent -Baralyme > soda lime Higher concentrations formed during -prolonged anesthesia -desiccated absorbent -low fresh gas flow Nephrotoxic in rats
Hepatic system
Reduced liver blood flow and O2 delivery
-decrease CO
Halothane hepatotoxicity
-increased liver enzymes- mild, self limiting
-halothan hepatitis- immune-mediated, often fatal
Minimal hepatic metabolism (modrn agents)
Malignant hyperthemia
Myopathy occurring in genetically predisposed animals and humans
Exposure ot inhalant anesthetics (esp halothane, but also others)
Increase in intracellular calcium-uncontrolled muscle contractions
Severe hyperthermia- death
Extremely rare
First sign can be rapid increase in ET CO2
Tx: administer dantrolene- calcium channel antagonist- muscle relaxation
-discontinue volatile anesthetic, change the machine and flush with O2
-Provide 100% O2
-fluids, active cooling
Death is likely despite treatment
Nitrous oxide
used more in people and some universities
maximum administration is 74% (need >25% O2)
low solubility (BGPC: 0.47)
Minimal cardiorespiratory depression
Analgesic effect (NMDA antagonist)
Transfer to closed spaces
-some organs contain air (stomach, intestines, middle ear)
-other (GDV, colic horse, pneumothorax, cuff of ET tube)
N2O is more soluble in blood than Nitrogen
N2O accumulate rapid while Nitrogen leaves slowly
Avoid in disease states causing increased closed gas space
Diffusion hypoxia
when N2O is stopped- diffuses quickly out of the blood to alveoli
Displaces O2 and other gases from alveoli
If breathing room air- hypoxemia
Provide 100% O2 when discontinuing for at least 10 min
Environmental safety
Trace levels of inhalant anesthetics can cause adverse health effects
Fetal development and health
Reduce occupational exposure
Greenhouse effect/ozone layer
Reducing Occupational exposure
Use scavenging system
check and minimize leaks
avoid mask and chamber induction
keep patients attaches to the circuit after anesthetics are turned off
Minimize exposure to exhaled gas from patient (specially horses)
monitor waste gas concentration
Ventilate operation rooms
Common complications of inhalant anesthesia-anesthesia related
hypotension
hypoventilation
hypothermia
Common complications of inhalant anesthesia-machine related
Closed pop-off
stuck inspiratory expiratory valves
exhausted soda lime
inadequate O2 flow in non-rebreathing circuits
Common complications of inhalant anesthesia- human error
Anesthetic overdose Intubation mishaps -laryngeal damage -stuck tube -aspirated tube -tracheal tears -obstructed ET tubes
Hypotension
MAP <60 mmHg (SA) and <70 LA- doppler below 80-90 mmHg
Maintain cerebral, renal, and striated muscle blood flow
50% of cases can be treated turning the vaporizer down
-patient is deep- achieve adequate anesthetic depth
-patient is light- consider administering a MAC sparing drug before turning vaporizer down- benzo, opioid, ketamine
If still hypo, evaluate underlying cause
dec vascular tone (dehydration, hypovolemia)- crystalloids and or colloids
Vasodilation- vasopressor
Dec contractility- inotrope
Hypoventilation
ETCO2 >45 mmHg
-mild hypercapnia can be tolerated in certain patients
IPPV- manual, mechanical
Check patient depth- turn vaporizer down if possible
Hypothermia
Inhalant anesthesia abolish normal thermoregulation -vasoconstriction, shivering impaired -vasodilation occurs as an effect of the drug Prevention more effective than tx: warm patient before induction keep covered minimize scrub time inc room temp forced air heating warm water blankets radiation lamps/devices
Closed pop off valve
should be open
increase in the breathing system pressure transmitted to pts lungs and thoracic cavity
dec CO
-decreases venous return (preload)
-compress great vessels (afterload)
CS: apnea, bradycardia, fading doppler signal, cardiopulmonary arrest
pneumothorax in some cases
safety checklist
use quick release valve to ventilate
dont take hand off closes pop off until open again
Closes pop off valve- tx
pull rebreathing bad off/unscrewing pop-off
start CPR if patient arrested
Evaluate pulmonary injury (auscultation, chest radiographs)
Stuck inspiratory/expiratory valves
Rebreathing system becomes bidirectional
-causes rebreathing of expired gases- hyper capnia
Signs: capnograph- rebreathing wave form
-no capnograph: observe movements of valves
-respiratory acidosis
Tx: dry and clean valves frequently
replace valves
Exhausted soda lime
CO2 is no longer removed from the expiratory gas
Patient rebreaths CO2
Signs: capnograph: same as stuck inspiratory/expiratory valves
-waves never return to baseline during inspiration
Differentials for rebreathing wave: inadequate O2 flow in non rebreathing systmes
damage of inner tube of bain circuits
Intubation mishaps-Laryngeal damage
from laryngoscope or stylet usually- be gental
swelling can lead to post operatory airway obstruction
Stylet should not protrude past the end of the tube
bougie
place laryngoscope at base of tongue-dont touch epiglottis
Intubation mishaps-stuck tube
dont force large ET tubes on your patient
it may go in and not come out
re anesthetize the patient and cut the tube to decompress
Intubation mishaps-obstructed ET tubes
plugs (secretions), blood, FB
loss of capnograph waves/high EtCO2
aspirate the lumen of tube
replace the tube
Intubation mishaps-aspirated tube
Usually if patient bites
reanesthetize patient quickly- provide O2
Retrieve tube- long forceps, reintubate with smaller tube, inflate cuff, then pull both tubes out together
traceoscopy
Provide proper patient monitoring- aggressive dogs, parrots
tracheal tears
not uncommon in cats
overfilling of the ET tube
fill until there is no leak at 20 cmH20
do not add more air unless there is a leak
Movement of the patient connected to breathing systme
always disconnect patient before moving
CS: subq emphysems; pneumothorax and pneumoperitoneum
Tx: supportive care- provide time for trachea to form a fibrin seal; surgical repair
Birds- complete tracheal rings- uncuffed tubes
Anesthetic overdose
Inhalant anesthetics- low therapeutic index- fatal dose/therapeutic dose
Overdose can happen fast- esp at high flow rates and vaporizer settings
If in doubt about status of pt, turn inhalants off while you evaluate situation- movement doesnt mean you pt is conscious; its preferable that your patient is light than deep in anesthesia
Severe hypotension )MAP< 40-50 mmHG)
-MAC sparing, if tx of hypotension not effective, turn inhalants off
Sick pts: MAC sparing-metabolic acidosis; need very little inhalant; use MAC sparing drugs/TIVA
