Anesthesiology: Exam 1 Principles of Inhalant Anesthesia & Monitoring 1 & 2

Vapor

Substance that is liquid at ambient temperature & pressure

Ex: Halothane, isoflurane, sevoflurane, desflurane

Vapor pressure

Pressure exerted by vapor molecules when liquid & vapor phases are in equilibrium

*Directly affected by temperature (increased temp = increased pressure)

Boiling point

Temperature at which vapor pressure = atmospheric pressure

*Inversely related to vapor pressure

Vapor pressure/ Barometric pressure

What is the formula for vaporization at ambient pressure?

Vaporizers

Vapors have a maximum administration percentage. What machine reduces this to a clinically useful dose?

*Mixes 100% oxygen with vapor's max %

-Adjusted by a knob that offers 1-5% of vapor)

Gas

Exists in a gaseous state at ambient temperature & pressure

-Ex: Nitrous oxide, xenon

N2O (Nitrous oxide)

AKA Laughing gas, provides mild analgesia & accumulates in closed gas spaces

Fast induction and recovery

Nitrous oxide has a low blood gas partition coefficient (0.47). What does this mean in terms of induction & recovery?

Xenon

Very expensive, mostly experimental gas with minimal cardiovascular depression (CO & BP don't change)

-Good for patients with heart disease but not commonly used

Boyle's Law

States that the volume of a gas is inversely proportional to the pressure

Charles's Law

States that volume of a gas is directly proportional to the temperature

Gay-Lussac's Law

States that the pressure of a gas is directly proportional to the temperature

Dawton's Law

States that the total pressure of a gas mixture is = to the sum of the partial pressure of the individual gases

Inhalant equilibrium

Reached when the partial pressure of the anesthetic is the same in each phase (CNS, blood, & alveoli)

Solubility

Describes the capacity of a given solvent (blood) to dissolve the anesthetic gas

-Expressed as partition coefficient (PC)

Partition coefficient (PC)

Concentration ratio of an anesthetic in the solvent (blood) & gas phase (alveoli)

Blood-gas partition coefficient

Describes the amount of anesthetic in the blood vs. alveolar gas at equal partial pressures

-Most clinically useful number

Brain concentration

When calculating the blood gas partition coefficient, the alveolar partial pressure represents the ____ ____ after equilibrium (although usually measured as %).

Higher

Does a higher or lower Blood-gas partition coefficient indicate greater gas solubility in the blood?

Inactive, brain (CNS)

Anesthetic dissolved in the blood is pharmacologically _____ and does not produce unconsciousness until it travels to & reaches sufficient partial pressure in the _____.

Halothane, isoflurane, sevoflurane, desflurane (HISD - he isn't super dissolvable)

What the 4 most to least soluble vapors discussed?

Lower

Does a higher or lower blood-gas partition coefficient indicate less anesthetic dissolved in the blood at equal partial pressures (more in alveoli)?

Lower

Do higher or lower blood-gas partition coefficients (BG PC) provide shorter induction & recovery times?

*Since it takes less time to achieve equilibrium

Lower

Are higher or lower blood-gas partition coefficients (BG PC) more clinically useful?

Higher

Do higher or lower blood-gas partition coefficients (BG PC) cause longer induction & recovery times?

-Ex: Halothane

Hypothermia (Patient becomes deeply anesthetized bc more drug is diluted in the blood = longer recovery)

Does hypothermia or hyperthermia increase anesthetic solubility?

Pressure gradients

Anesthetic inhalants move down ____ ____ until equilibrium is achieved

*Anesthetic leaves Vaporizer → breathing circuit → alveoli → arterial blood → CNS in brain

Alveoli

Partial pressure of the brain (Pbrain) is roughly equal to the partial pressure in the ____.

PA

Partial Pressure of the inhalant anesthetic in the alveoli

*Pressure of the gas that's delivered to alveoli & removed from the lungs via the blood

Increase

During induction, do you increase or decrease the PA to reach equilibrium quicker?

Decrease

Do you increase or decrease the PA to recover the patient quicker?

*Remember inhalants are reversed/eliminated via respiration & move from high to low pressure gradients

Delivery, removal, induction

Ways to increase Partial Pressure (PA):

1) Increase anesthetic ____ to the alveoli

2) Decrease anesthetic ____ from the alveoli

3) Increase speed of ____ & change the anesthetic plane

Inspired anesthetic concentration (PI)

What concentration do these changes increase:

-Increase vaporizer setting

-Increase fresh gas flow (flowmeter)

-Decrease breathing circuit volume (Large circuit = small anesthetic concentration = takes longer for anes. concen. to increase)

Minute, dead space

To increase anesthetic delivery to the alveoli (& PA), alveolar ventilation can be increased by:

1) Increasing ___ ventilation via mechanical/manual ventilation

2) Decreasing ___ ____ ventilation (Ex: ET tube too long)

Decrease anesthetic removal from the alveoli (which increases PA)

How do the following changes affect anesthetics in the alveoli:

-Decrease blood solubility of anesthetic via different agents

-Decrease cardiac output

-Decrease alveolar-venous anesthetic gradient (Tissue uptake from anesthetic)

PA, induction

A low cardiac output causes slower extraction of anesthetics from the lungs which creates a faster rise in ____ & therefore faster _____.

PI (Inspired anesthetic concentration)

A high ___ is required at the beginning of gas anesthesia to quickly increase the PA .

-As the uptake into the blood decreases, this pressure should be decreased too

Agent solubility, alveolar ventilation

What 2 factors affect anesthetic elimination & decreases PA ?

*Anesthetic time & decreased body temp both affect this too

Hyperthermia

Does hypo or hyperthermia decrease the solubility of inhaled anesthetics (leads to a shorter anesthetic elimination, shorter recovery, & increased MAC)?

Vaporizer, O2, ventilation

Ways to Decrease PA:

-Turn off the ______

-Disconnect patient & flush the breathing system with O2

-Turn up ____ flow = dilutes the anesthetic in the circuit

-Increase ____ = increases fresh gas to alveoli

Minimum alveolar concentration (MAC)

Lowest dose of inhalant anesthetic at which 50% of patients show no response/movement to noxious stimuli

*Consistent between species

*Allows comparison between inhalant agents

Low

An inhalant with a High MAC indicates ____ potency.

Alveolar concentration (%)

What measurement describes how much (dose) of an inhalant was given?

Gas analyzer

Instrument that's the gold standard for measuring anesthetic dose

*Looks at end-tidal gas concentration

*Tells you the actual concentration of anesthetics & not a target concentration (what the vaporizer does)

1.3-1.4%

Isoflurane MAC Range in Dogs, Cats, & Horses

2.3-2.4%

Sevoflurane MAC Range in Dogs, Cats, & Horses

7.0-9.0%

Desflurane MAC Range in Dogs, Cats, & Horses

200%

General MAC of Nitrous oxide in dogs, cats, & horses

Iso, sevo, des, NO

Rank the inhalant anesthetics from lowest to highest MAC (ie most to least potent)

Increase

Do the following factors increase or decrease MAC:

-Hyperthermia (decreases anesthetic solubility)

-Hypernatremia

-Drugs causing CNS stimulation

-Decreased age

Decrease

Do the following factors increase or decrease MAC:

-Hypothermia (increases anesthetic solubility)

-Hyponatremia

-Drugs causing CNS depression

-Increased age

-Hypozemia

-Severe hypotension

-Pregnancy

-Anemia

-Metabolic acidosis/hypercapnia

50%, 95%

1 MAC describes immobility in ___% of the patients, while 1.3 MAC describes immobility in ___% of the patients.

True

True or False: Patients recover faster from inhalants than injectables since they just have to breathe out the drug versus full metabolism of injectables.

Decrease (= decreased BP due to vasodilation/less SVR & decreased CO)

Do inhalants increase or decrease cardiac contractility & systemic vascular resistance?

False (HR is not changed or minimally increased due to compensation for vasodilation)

True or False: Heart rate is heavily increased by inhalants due to decreased cardiac contractility.

Respiratory depression

Inhalants decrease ventilation by causing dose dependent ____ ____ via inhibition of central CO2 & peripheral O2 chemoreceptors.

*Apnea at 1.5-3.0 MAC

Bronchodilation

Do inhalant anesthetics promote bronchodilation or bronchoconstriction?

PaCO2 (Arterial partial pressure of CO2)

Ventilation is monitored by _____.

-Normal range = 35-45 mmHG

45

A PaCO2 > ___ mmHg indicates Respiratory depression & Hypoventilation.

Increasing (patient will be deeper anesthetized)

Does increasing or decreasing the inhalants cause an increased MAC, higher PaCO2, & subsequent respiratory depression?

Cerebral vasodilation

Inhalants can increase intracranial pressure (especially at MAC > 1) by causing ____ ____

*Sevoflurane increases the ICP less than other inhalants but propofol is the best drug to use in animals with a high ICP

Cerebral metabolic rate

Inhalants decrease ____ _____ ____, meaning the brain cells require less oxygen to survive.

*Can also suppress seizure activity

GFR (glomerular filtration rate), renal blood flow

What 2 factors in the renal system are decreased by inhalants & subsequent decreased CO?

-Older inhalants (methoxyflurane) caused Compound A formation but these inhalants are not used in modern practice

Blood flow

Inhalants reduce liver ___ ____ & O2 delivery due to decreased CO

-Modern agents have minimal hepatic metabolism compared to Halothane (hepatotoxic)

Malignant hyperthermia

Extremely rare myopathy occurring in genetically predisposed animals

-Mechanism: Genetic failure of a receptor causes uncontrollable release of Ca (Increased intracellular C) when exposed to inhalants → uncontrolled muscle contractions → increases temp of animal → Hyperthermia

EtCO2 (End-tidal carbon dioxide)

Peak concentration of carbon dioxide exhaled at the end of a breath

*Breath sample; normal range is 35-45 mmHg

Rapid increase in EtCO2

First sign of Malignant hyperthermia

-Other signs: increase in temp, red skin

*NOT A GOOD PROGNOSIS with this condition

Inhalants, stress

2 main triggers of Malignant hyperthermia

-Can both be present

Pigs

What animals are more prone to Malignant hyperthermia?

Dantrolene

Calcium channel antagonist for muscle relaxation used to treat Malignant hyperthermia

*Also discontinue volatile anesthetic, flush machine with O2 to remove residue, & provide 100% O2 to patient if this condition occurs

Ozone depletion

All inhalants cause the Greenhouse Effect, but nitrous oxide also causes _____ ____.

Guedel's stages of anesthesia

Anesthesia staging system created by places humans under anesthesia using Ether

*Has 4 stages & 4 planes within stage 3

Stage 1

Stage of voluntary excitement +/- mild sedation

-Beginning of induction to loss of consciousness

-Increased HR & RR from epinephrine release

*Disorientation (excitement) with normo-reflexia

-Salivation

Stage 2

Stage of involuntary excitement

*Period of early loss of consciousness

-Mydriasis with central eye position +/- nystagmus

-All reflexes present

- Strong jaw tone & response to surgical stimulation

Stage 3

Stage of surgical anesthesia that includes 4 Planes

-No movement in response to surgical stimulus with progressive loss of motor & autonomic reflex until cessation of breathing

Plane 1

Stage 3 with Response to noxious (pain) stimulus still present with normal breathing & pupil size

-Central eye position

-Weakening palpebral/pedal reflexes

-No swallow reflex & weak jaw tone

*Intubation is possible

Plane 2

Medium plane in Stage 3 with mydriasis & ventro-medially rotated eyes

-Surgical procedures can be performed

-Palpebral & pedal reflexes weak to absent

-Cardiovascular & respiratory function minimally affected

-Adequate muscle relaxation

Plane 3

Medium to deep plane in Stage 3 with mydriasis & central located eyes

-Palpebral & pedal reflexes absent

-Weak corneal flex & decreased lacrimation

-Cardiovascular and respiratory functions depressed

-Profound muscle relaxation

Plane 4

Dangerously deep plane in Stage 3 in which the patient is barely breathing or apnea

-Centrally located eyes with strong mydriasis

-Absent palpebral & corneal reflex

-Uncoordinated (jerky) breathing movements

-HR & arterial BP drop significantly

-Cardiopulmonary failure imminent

Corneal reflex

What reflex do you ALWAYS want to be present during anesthesia?

-Absence = dead or too deeply anesthetized

Stage 4

Stage of anesthetic overdose

-Respiratory arrest followed by circulatory failure

-Corneal reflex absent

-Anus is dilated, feces/urine may be voided

-Death from anesthetic overdose ensues shortly without immediate resuscitation

Palpebral, pedal (may be absent)

In an Ideal anesthetic depth, what 2 reflexes are weak?

Ventro-medially

In an Ideal anesthetic depth, what direction are the eyes rotated?

*In ruminants, dogs (some), cats (some)

Autonomic

In an ideal anesthetic depth, do the motor or autonomic reflexes remain stable?

Too light

Based on these signs, is the patient too light or too deeply anesthetized:

-Movement of limbs / neck in response to surgical stimulus

-Blinking

-Increased tear production (lacrimation)--> horses

-Nystagmus (horses)

Too light

Are Tachypnea & tachycardia signs that your patient is too light or too deeply anesthetized?

Too deep

Are bradycardia, bradypenia/apnea, & hypotension signs that your patient is too light or too deeply anesthetized?

Too deep

Based on these signs, is the patient too light or too deeply anesthetized:

-Absent palpebral reflex

-Absent pedal reflex

-Absent corneal reflex

-Dry wrinkled cornea (fishy eye)

Motor (somatic) reflexes

What type of reflexes are these:

-Palpebral reflex

-Corneal reflex

-Pupillary light response (not very practical)

-Pedal (withdrawal) reflex: small animals

-Anal sphincter tone / reflex: large animals

-Auricular reflex (Ear flick): cats

Lacrimation, nystagmus

What 2 eye related activities are important for evaluating anesthetic depth in HORSES?

Autonomic reflexes

What type of reflexes are these:

-Respiratory rate

-Heart rate

-Arterial Blood pressure

-Sweating (difficult to observe in thick-haired animals)

Stage 3 Plane 2-3

Ideal anesthetic stage/plane

Premedications, injectable induction

What 2 types of medications can be given to quicken/skip Stages 1 & 2 of anesthesia?

Ketamine

Dissociative anesthetic & induction agent that immobilizes patients completely but eye reflexes & ability to swallow remain

False (normal with ketamine for these reflexes to remain)

True or False: A dog that was induced with ketamine still has spontaneous blinking & a palpebral reflex. This dog is likely too light and more ketamine should be given.

Minute ventilation (VE)

Volume of air inspired or expired per minute

Tidal Vol (VT) x Resp rate (f)

Formula for minute ventilation

Tidal volume

Amount of air that moves in or out of the lungs with each normal breath

*Normal: 10 -15 mL /kg

ETCO2

End tidal CO2 measured via capnography after the end of each exhaled breath

PaCO2

Partial pressure of CO2 in arterial blood

*Measured via arterial sample