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AS 421 part II


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Breathing Circuits Functions
A. Deliver oxygen to patient B. Deliver anesthetic gas to patient C. Remove carbon dioxide produced by patient D. Provide a method to assist or control ventilation
Rebreathing Circuit (Circle Breathing System)
A. All or part of the exhaled gases are retained within the system to be re-inhaled by the patient B. Exhaled gases consist of inhalant anesthetic, oxygen and carbon dioxide. C. Carbon dioxide is removed chemically.
Fresh gas inlet
. entry on inspiratory side of circle.
Inhalation and exhalation breathing tubes
usually corrugated to reduce likelihood of obstructions if tubes bent
Inhalation and exhalation one way valves
direct gas flow away from the patient on expiration and toward the patient on inspiration
can contribute to mechanical dead space
Carbon dioxide absorbing canister
a. calcium hydroxide combines with CO2 in a chemical reaction that yields water, heat and calcium carbonate (limestone) b. change granules ~ q 8 hrs of anesthesia c. evidence of exhaustion of granules is color change (white to purple/blue-but then turns back to white), soft and crushable granules become hard and non-crushable, no palpable heat production, increased inspired CO2 on capnography.
Rebreathing bag (reservoir bag)
a. located on the absorber side of the circle, upstream or downstream from the canister b. ideal minimum bag size is 6 times the tidal volume (10 ml/kg); therefore 60 ml/kg c. allows controlled ventilation d. allows assessment of respiratory rate and tidal volume e. if bag is too large it impairs monitoring and slows changes in anesthetic concentration f. if the bag is too small it increases pressure in the circuit and makes it difficult for the patient to exhale. There is some risk that excessive pressure could rupture alveoli. g. It is recommended that the anesthetist “bag” or “sigh” the animal once every 5-10 min. during anesthesia to help prevent atalectasis at 20 cm H2O pressure.
Pressure relief valve (pop-off valve)
a. vents gas to the scavenger system. b. located between expiratory one-way valve and the canister c. prevent inadvertent buildup of pressure
Advantages of rebreating system
1. Economical-recirculate and reuse expired oxygen and anesthetic vapor 2. Minimal fresh gas flow and anesthetic agent utilization compared with non-rebreathing system. 3. Humidifying inspired gas, preserves heat and moisture of the patient.
Non-rebreathing System Reasons for using
1. Less resistance to breathing (in the circle) 2. Smaller mechanical dead space 3. Can more rapidly control changes in anesthetic concentration 4. Simple devices, less potential for malfunction 5. Light weight, less bulk 6. Easier to clean and maintain 7. More portable
Non-rebreathing System disadvantages
1. Requires high flow of dry, cool gas to patients which causes significant heat and humidity loss. 2. Significant higher waste of carrier gas and anesthetic results in increased cost.
Non-rebreathing system is recommended for animals under____.
Common Systems of non-rebreathers:
1. Bain’s Circuit 2. Norman(mask) elbow 3. Ayre’s T Piece 4. Mapleson A (Magill) system 5. Kuhn circuit
Anesthetic chamber or Mask
A. Requires high O2 flow rates-5 L/min. B. Increased risk to personnel from waste gases
Scavenger Systems
A. Purpose: to eliminate excess anesthetic gases from the work area to minimize breathing by personnel B. Flow into scavenger system is dictated by the pop-off valve in a rebreathing circuit and by valves in the reservoir bag of a non-rebreathing circuit.
Active scavenger system
a. Suction created by a vacuum pump or fan draws gas into the scavenger and outside of the building. b. Advantage: The most efficient type of scavenging system. c. Disadvantages: Costly, requires more maintenance, needs to be turned on daily.
Passive scavenger system
a. Uses the positive pressure of the gas in the anesthetic machine to push gas into the scavenger. b. Anesthetic vapors heavier than air, so transfer hoses should travel a downward course toward the exhaust. c. Types i. Elimination through an outside wall, distance not greater than 20 feet. ii. Elimination to room ventilation exhaust, distance not greater than 10 feet. iii. Activated charcoal canister- F’Air which is replaced after 8-12 hours of use, or a weight gain of 50 g. These units are unable to absorb N2O and are relatively inefficient at flow rates greater than 2L/min
standard for maximum personnel exposure levels
1. 2 ppm halogenated anesthetic (iso) 2. 25 ppm N2O
Room air is
21% O2
Metabolic oxygen needs for small and large animals
small animals-4-10 ml/kg/min b. large animals-2-3 ml/kg/min
Color code for oxygen and nitrous oxide
Oxygen-green nitrous oxide- blue
4. When O2 is compressed it remains in gas form. O2 tank pressure drops proportionally as used. To determine volume remaining in an E tank, multiply the remaining pressure x 0.3. To determine volume remaining in an H tank, multiply the remaining pressu
1. Dry, well ventilated area away from sparks, heat, flame. 2. Avoid storage in temperature extremes. 3. Empty tanks-label clearly, store separately if possible. 4. Store cylinders upright, chained to wall or otherwise secured. Potential “missile”! Handle pressurized cylinders with respect.
DISS-Diameter Index Safety System
a. Oxygen regulator and pressure hose has different diameter than nitrous regulator and pressure hose.
PISS-Pin Index Safety System
a. Present on E tank and machine yoke-cylinder has holes that accept configuration of pins on the anesthetic machine. b. If pins are broken, this safety measure can be defeated.
Tank sizes
cylinders sizes A (smallest)-H(largest)
O2 cylinders
E tank: empty wt 13lb, volume full 660L, filling pressure 2,200 psi. H tank: empty wt 130lb, volume full 6,600 L, filling pressure 2,200 psi.
Pressure Regulator
A. Provide safe operating pressures of 45-50 psi. B. Prevent flowmeter fluctuation as cylinders empty. C. Can be either part of anesthetic machine (E tank) or separate device (H tank)
A. Allows measured flow of gas to pass into vaporizer. B. Enter bottom, pushes up indicator, exits top. 1. All indicators read from top except the ball indicator, which is read from the middle. C. Calibrated for each gas’s density and viscosity. If using nitrous and oxygen you will have 2 flowmeters. D. Turn knobs on and off gently-excessive force can damage the valve seat
A. Allows volatile liquid anesthetic to become vapor, adds a controlled amount of vapor to the flow of oxygen, and regulates the amount of vapor entering the breathing circuit. B. Classification-Non-precision vs. Precision
which vaporizer is used today
oxygen flush valve
1. Allows oxygen to bypass the flowmeters and vaporizers and enter the system at 40-70 L/min. 2. Designed to allow the anesthesia circuit to be filled with anesthetic-free oxygen very quickly. 3. Don’t use with a non-rebreathing circuit as this high pressure has a potential for trauma to the patient’s lungs. 4. DO NOT use to fill the system or rebreathing bag when one is trying to induce anesthesia. This will only dilute the anesthetic concentration in the system. Use the flowmeter instead.
common gas outlet
The exit port where gases leave the anesthetic machine and enter the breathing circuit.
Pressure gauge
used to measure pressure within the gas cylinders in psi.
vital sign CRT
>2 sec may mean hypotension <80 mmHg-consider excessive anesthetic depth or circulatory shock ii.hypothermia, vasodilation and cardiac failure can also cause ↑ CRT
Vital sign MM color
i. Gingiva, not tongue ii. pale-blood loss, anemia, poor perfusion iii. blue or purple-cyanosis-decreased tissue oxygenation. Think respiratory failure or upper airway obstruction.
Vital sign Pulse strength
i. femoral, carotid, dorsal pedal, lingual arteries ii. poor strength may indicate hypotension excessive anesthetic depth excessive vasodilation (pre anesthetic drugs, dehydration) cardiac insufficiency excessive blood loss
Blood loss
i. A 3x3 gauze sponge holds 5-6 ml of blood ii. healthy animals can tolerate blood loss up to 15% of blood volume without serious circulatory effects (13 ml/kg in dog and cat)
Respiration rate and Depth (look at thorax and bag)
i. 8 /min. is minimum acceptable, 8-20 normal rate ii. With ↑ anesthetic depth, see ↓ in rate and tidal volume tidal volume-volume of air inspired and expired in one breath. hypoventilation-decreased resp. rate and tidal volume. iii. Tidal volume ↓~ 25 % during anesthesia and may lead to collapse of alveoli-atelectasis iv. Bagging-careful and controlled squeezing of reservoir bag can help to reverse atalectasis. Perform q. 5 min. v. Tachypnea (rapid rate) or hyperventilation (↑ depth) may be seen. Causes include ↑ CO2 in blood (CO2 absorber not adequate) underlying pulmonary disease surgical stimulation ↓ anesthetic depth vi. should be smooth and regular respiration-if difficult or labored consider obstruction vii. Timing-Inspiration ½-1 1/2 sec, expiration sec. May see apneustic pattern (inspiration followed by a long pause before expiration) w/ ketamine
Hypothermia causes
shaving, washing no muscle activity to generate heat ↓ metabolic rate open body cavity vasodilation-drugs pediatric and geriatric patients less able to thermoregulate.
Hypothermia changes greatest in first __min
Hypotherthemia can prolong recovery due to...
decresased hepatic metabolization of drugs.
Prevention of hypothermia
warm IV fluids circulating warm water heating pad towels on surgery table
Reflexes of anesthestic depth are:
Reflex-involuntary response to a stimulus to protect from injury. Progressively depressed as anesthetic deepens. Used to assess anesthetic depth, but doesn’t give information on homeostatic mechanisms
Palpebral (blink)reflex
lightly tap on medial or lateral canthus, observe for blink. i. Most retain at stages I and II, and initial III ii. variations- individual, specie, drugs iii. surgical plane-usually present in barbiturate, occas. In methoxyflurane, seldom with halothane iv. loss of reflex indicates increased depth, return indicates imminent arousal
Swallowing reflex
occurs spontaneously when awake and lightly anesthetized. Lost at medium anesthetic depth. Regained just before recovers. i. observe ventral cervical region ii. Indicates it is safe to remove endotracheal tube-if animal vomits at this time it should swallow instead of aspirate.
Pedal reflex
or pinch digit or pad and observe for signs of flexion. Usually lost during induction.
Ear flick (pinna reflex)
in cats and cows. Stroke hairs of inner surface of pinna. i. usually present well into stage III, esp. w/ ketamine. ii. May be lost if tested too frequently w/in a short period.
Corneal reflex
touch cornea w/ sterile object or drop of saline. Observe for blink and withdrawal of eye into orbital fossa i. Not usually tested in dog and cat except to check if too deep. ii. Present until stage III, plane 4
Laryngeal reflex
when larynx touched by object-protects from aspiration. i. May see if passing endotracheal tube and not deep enough-esp. cats.
Muscle tone (anesthetic depth/reflex)
Muscle Tone-relaxation promoted by diazepam and xylazine. Increased tone seen w/ ketamine, tiletamine
Jaw tone (muscle tone)
passive resistance i. variations in breed and specie ii. use 1 hand to assess.
Forelimb extension (muscle tone)
flexion and extension to assess relaxation
anal tone (muscle tone)
very relaxed
Eye position
i. Central stage I and II ii. Ventromedial stage III, plane 2 (may not see w/ ketamine) iii. Central stage III, plane 3 and 4, stage IV
Pupil Size
i. miotic-light anesthetic(small) ii. dilated as depth increases (BUT-consider atropine causes dilated pupils)
Salivary/Lacrimal secretions
a. Decrease w/ increasing anesthetic depth b. Use ophthalmic ointment to protect from corneal drying.
Heart rate and Respiration
tend to ↓ with ↑ anesthetic depth and visa versa a. BUT: heart rate can also decrease w/ vagal stimulation (such as occurs w/ endotracheal intubation). b. Heart rate can also increase in response to hypotension, pain, certain drugs (ketamine, atropine) c. resp. rate can increase or decrease independent of anesthetic depth-depended on O2 and CO2 concentration and drugs.
Response to surgical stimulation
a. Manipulation of viscera, suspensory ligament pulling, etc. can cause increased heart rate and blood pressure, but not nec. too light unless the change is dramatic. If no response, animal may be too deep. b. Increased resp. rate and movement does indicate too light of anesthetic.
Protect airway
a. disconnect ET tube when turning-kinking, trauma b. support hoses of anesthetic machine c. no interference with excursion of thorax d. tilting table can compromise cardiac and respiratory function
Protect from injury related to position or restraint
a. natural position if possible b. ropes/restraints can cause circulatory or nerve compromise
Patient positioning and comfort. Protect from injuring self:
1.Induction 2. transport; support back and hips. 3. Recovery; protect from thrashing around in kennel.
Recovery; the period between-
discontinuation of anesthtic adminstration and the time animal can maintain sternal recumbency with out support.
Stages of Recovery-Usually progresses back through the same anesthetic stages experienced during induction.
1. Heart rate, respiratory rate and volume increase. 2. Pupil rotates back to central (may rotate ventrally again and return to central before arousal). 3. Reflex responses come stronger. 4. May shiver 5. Swallow, chew, attempt to lick 6. Signs of consciousness return, including voluntary movement.
Length of Recovery-dependent on many factors
1. length of anesthetic 2. patient’s condition prior to anesthetic 3. type and route of anesthetic 4. hypothermia decreases anesthetic drug excretion 5. breed of patient and response to certain drugs
During Recovery(anesthetic turned off until sternal recumbency)
1. Monitor vital signs q 5 minutes- MM color, CRT, breathing 2. Ensure airway remains open-esp. brachycephalic breeds-observe until fully conscious. 3. Remove IV catheter or butterfly only when patient appears fully recovered 4. Administer O2 at high flow rate (ex.200 ml/kg/min.) for 5 min. or until swallows . a. helps to flush anesthetic out of the animal’s system for faster recovery b. less expired waste gas c. periodic bagging helps reinflate collapsed alveoli and increases rate of anesthetic gas removal.
a. place in sternal or lateral recumbency with neck extended to help maintain patent airway. b. swallow reflex indicates time for removal—unless voluntary movement of head or limbs, spastic movement of tongue, or attempts to chew-remove tube. c. Exception: Brachycephalic –extubate when animal able to lift head and prepare for possible respiratory distress d. do not delay extubation in cats-laryngospasm e. deflate cuff and untie gauze before signs of arousal i. if blood or fluid present in oral cavity, can leave cuff partially inflated.
Stimulate patient
a. gentle stimulation-talking, pinching toes, opening mouth, moving limbs, rubbing chest. b. increases stimulation of reticular activation center (RAC) to assist in faster return to consciousness. c. turn patient q. 10-15 min. to prevent pooling of blood in dependent parts of body-hypostatic congestion.
Reassure patient-quiet calm handling
a. remove restraints and monitoring devices b. administer post-op analgesics.
Keep warm, apply external heat as needed
a. warm towels b. hot water bottles c. infrared heat lamps 3 ft from patient d. circulating warm water heat pads e. check temp to make sure not over heating
Laryngospasm is heard...
On inspiration, sounds like animal can not breath.
the pressure gauge of a 1/2 full oxygen tank which was originally filled at 2200 psi would read ___ psi
What is the function of the pressure regulator?
provide safe operating pressures prevent fluctuatrons of flow meter as clylinder empties.
why should you not use the oxygen flush valve in a non-rebreathing circuit?
don't us w/ a non rebreathing system as this increase pressure has a potential for trauma to the patients lungs.
what are two things that would cause you to believe the granules in a carbon dioxide absorber are exhausted
color change from white to purple. increased inspired CO2 on capnography
state 1 function of the rebreating (reservoir) bag and the ideal minimum bag size.
ideal minimum bag size is 6 times the tidal volume. Allows assessment of RR and volume of respiration.
State 1 advantage of a non-rebreathing system
smaller mechanical dead space
State 1 disadvantage of a non-rebreathing system
requires high flow of dry, cool gas to patients which causes patinet to lose heat and humidity.
State 1 advantage of a rebreathing system
economical- recirculate and reuse of expired oxygen and anesthetic vapor.
what are the 2 main types of scavenger systems
Active and passive
Tidal volume
volume of air insipired and expired in on breath.
difference between a vital sign and reflex
vital sign: is used to measure the animals life signs. Like HR, RR, CRT...etc... let's you know the patient is alive. Reflex can be used to measure depth of anesthesia.
at what stages/planes of anesthesia is the eye position central?
i. Central stage I and II ii. Ventromedial stage III, plane 2 (may not see w/ ketamine) iii. Central stage III, plane 3 and 4, stage IV

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