Anesthesia Free guide

The Anesthesia Machine and Breathing Systems: Components, Circuits, Leak Test and Scavenging

Anesthesia (D7 C, D; K45) 15 min read, 3,287 words Last verified

How oxygen and inhalant travel from the tank to the patient and on to the scavenger, how to pick a circuit, flow rate and bag by body weight, and how to run the leak test and spot exhausted absorbent.

Introduction

The anesthesia machine takes oxygen from a high-pressure tank, lowers its pressure, meters its flow, adds a measured amount of inhalant anesthetic and sends the mixture to the patient through a breathing system (also called a breathing circuit). AAHA describes this equipment as "life-critical": once the patient is connected, the machine and circuit become part of the patient's respiratory system and can either support or impair breathing.1

On most teams the veterinary technician sets up, checks, runs and troubleshoots this equipment. This guide follows the gas from the tank to the patient and on to the scavenger, then covers choosing a circuit, setting oxygen flow, sizing the reservoir bag, the leak test, CO2 absorbent and waste gas control.

Key facts

  • A full E oxygen cylinder holds about 660 L at about 2,200 psi. Gauge reading (psi) times 0.3 estimates the liters left.3 AAHA: start only if the tank reads above 200 psi.1
  • Non-rebreathing circuits (NRCs) are commonly used below about 3 to 5 kg; rebreathing (circle) circuits above that weight.1
  • NRC oxygen flow: about 200 to 400 mL/kg/min. Circle maintenance flow: about 20 to 40 mL/kg/min, with a minimum of 500 mL/min.1
  • Leak test before every patient: close the pop-off, occlude the patient end, fill to 20 to 30 cm H2O, turn the oxygen off and watch that the pressure holds.1
  • Change the CO2 absorbent after 8 hours of use. A rise in inspired CO2 on the capnograph is a sign of exhausted absorbent.12
  • The oxygen flush valve delivers about 35 to 75 L/min and bypasses the vaporizer. Never use it while a patient is on an NRC.31
  • Every machine should have an in-circuit manometer and a safety pop-off valve.1
  • Active scavenging systems are far more effective than passive ones, such as activated charcoal canisters.1

How the VTNE tests this

  • Identify a component and its job: flowmeter, vaporizer, unidirectional valves, pop-off valve, reservoir bag, absorbent. Domain 7 Task C, "Prepare anesthetic equipment and related supplies to ensure operator and patient safety".6
  • Choose the circuit, flow rate or bag size for a patient of a given weight.
  • Run the machine check: the order of the leak test steps and what a falling pressure means. Domain 7 Task D, "Maintain anesthetic equipment and related materials to ensure reliable operation"; K45 covers care, maintenance and use of anesthetic equipment.6
  • Troubleshoot: a bag that is overfilling, a manometer that keeps rising, or a capnograph that shows inspired CO2.
  • Calculate: minutes of oxygen left in a tank, or the NRC flow for a small patient.

Core concepts

The four parts of the system

Lee divides an anesthesia machine into four parts: a high-pressure system where the cylinders or pipeline attach; a low-pressure system where oxygen and inhalant are mixed; the breathing system that carries the mixture to the patient; and the scavenging system that collects excess gas and sends it to waste.3 AAHA suggests learning the parts by following the oxygen from its source, through the machine and circuit to the patient, then from the patient to the bag and finally to the scavenger.1 Figure 1 does exactly that.

Gas flow through an anesthesia machine and circle rebreathing system, from the oxygen cylinder to the patient and on to the scavenger 1. Oxygen supply Full E tank: 2,200 psi 2. Pressure regulator drops to about 45 psi 3. Flowmeter read float at widest part 4. Precision vaporizer adds inhalant to fresh gas 5. Common gas outlet fresh gas leaves machine Oxygen flush valve 35 to 75 L/min, no inhalant 6. Fresh gas inlet inspiratory side of circle 7. Inspiratory valve one way, toward patient 8. Y-piece and ETT to and from patient 9. Expiratory valve one way, away from patient 11. CO2 absorbent removes CO2, then reuse 10. Reservoir bag and manometer Pop-off valve excess gas to scavenger bypass
Figure 1. Gas flow through an anesthesia machine and circle system. Oxygen pressure is reduced, the flow is set, inhalant is added, and the fresh gas enters the circle on the inspiratory side. Exhaled gas passes the expiratory valve; CO2 is removed by the absorbent and the gas is reused, while excess gas leaves through the pop-off valve to the scavenger. The dashed path is the oxygen flush, which bypasses the vaporizer.

High-pressure side: oxygen supply and gauges

In the US, oxygen cylinders are green. Size E is the cylinder most often hung on the machine. A full E tank holds about 660 L of oxygen at about 2,200 psi (pounds per square inch); a full H tank holds about 6,600 L.3 Because oxygen is stored as a gas, the gauge falls in step with the amount used, so the gauge can be used to estimate what is left. Cylinders attach to a hanger yoke using a pin-indexed safety system, which stops the wrong gas from being connected. A small washer (Bodok seal) is needed at the yoke inlet for a tight fit.3 A dropped cylinder can explode because of its high pressure.3

The pressure regulator (pressure-reducing valve) lowers cylinder pressure to a working pressure of about 45 psi and keeps the flow steady as the tank empties. Pipeline gas arrives at about 50 psi, so the machine uses the pipeline first when both are attached.3

Low-pressure side: flowmeter, vaporizer and common gas outlet

The flowmeter controls how much gas reaches the patient. Most are tubes with a ball or bobbin float that rises with the flow; read the flow at the widest part of the float.3 Each flowmeter is calibrated for one gas.

The precision vaporizer turns liquid inhalant into vapor and adds it to the fresh gas at the percentage set on the dial. Part of the gas passes through the vaporizing chamber and part bypasses it; the two streams rejoin at the outlet. Precision vaporizers are agent specific, temperature compensated and sit out of the circuit, on the machine.3 Their output becomes unreliable at very low flow, which is why AAHA sets a minimum total flow of 500 mL/min with a circle.1 AAFP recommends a full service, including recalibration, every 3 years.2

The common gas outlet is where the oxygen and inhalant mixture leaves the machine. It is a frequent source of leaks.3

The oxygen flush valve sends pure oxygen straight to the common gas outlet at about 35 to 75 L/min, bypassing the vaporizer. It dilutes the inhalant in the circuit and can over-pressurize small lungs.3

The circle (rebreathing) system

In a rebreathing circuit, the patient breathes the same gases again after CO2 is removed by the absorbent. A circle has a CO2 absorbent canister, a Y-piece, inspiratory and expiratory breathing tubes, inspiratory and expiratory one-way (unidirectional) valves, a fresh gas inlet, a pressure manometer, a pop-off valve and a reservoir bag.4 The one-way valves keep gas moving in one direction so exhaled gas passes through the absorbent before it is breathed again. A sticking valve lets CO2 build up in the circuit.4 Fresh gas enters on the inspiratory side.4

The pop-off valve, also called the adjustable pressure-limiting (APL) valve, lets excess gas leave the circuit to the scavenger. It stays open during spontaneous breathing. If it is left closed, pressure builds up in the patient's lungs.4 A safety pop-off valve or a high-pressure alarm guards against this.1

The reservoir (rebreathing) bag holds gas for the next breath, shows the breathing pattern and lets the anesthetist give breaths by hand. A bag that is too small cannot supply a large breath; one that is too large is hard to read as a breathing monitor. Lee gives 0.5 L for cats and small dogs and 2 L for a 20 kg dog.4

Circle systems save oxygen and inhalant, cause less pollution and keep gases warm and moist. The trade-offs are more resistance to breathing from the absorbent and valves, and a slower change in depth after the vaporizer is changed.24

Non-rebreathing systems: Bain and Jackson-Rees

In a non-rebreathing circuit there is no absorbent. A high fresh gas flow washes exhaled CO2 away from the patient.2 NRCs have less resistance and less equipment dead space, which matters in small patients.1 Because fresh gas enters next to the endotracheal tube (ETT), a vaporizer change reaches the patient quickly. The cost is more oxygen and inhalant use, more pollution and cold, dry gas that can add to hypothermia.4

The Mapleson system groups these circuits by layout. The Bain is a Mapleson D circuit, the Ayre's T-piece is Mapleson E and the Jackson-Rees is Mapleson F.4

Dead space

Equipment dead space is the part of the circuit where inspired and expired gases mix, mostly at the patient end. Too much dead space makes the patient rebreathe CO2. AAHA advises keeping it at or below 2 to 3 mL/kg, about 20% of the tidal volume, and using pediatric circuits for small patients.1

Absorbent exhaustion

Soda lime is mostly calcium hydroxide with smaller amounts of sodium hydroxide. It absorbs CO2 in a reaction that gives off heat. As it is used up, the soft, crushable granules turn hard.4 A dye changes color as the absorbent is used, but Lee warns not to trust it alone: most dyes change a little late, and the color can revert after the absorbent rests.4 The most useful sign is inspired CO2 above zero on the capnograph.24 AAFP notes that how fast absorbent is used depends on canister size, flow rate and humidity, so log hours and change single canisters after 8 hours of use.2

Waste anesthetic gas and scavenging

Waste anesthetic gases (WAGs) leak from machines, vaporizers, circuits, uncuffed ETTs and faulty scavengers, and are breathed out by recovering patients. Short-term exposure can cause fatigue, headache, dizziness and irritability.5 NIOSH recommends exposure limits of 2 ppm for halogenated anesthetics used alone and 25 ppm for nitrous oxide.5

  • Active scavenging uses a vacuum or fan to pull gas away from the pop-off valve. It is the more effective method and adds little resistance to breathing, but it needs an interface that draws in room air so suction does not empty the circuit.14
  • Passive scavenging sends gas from the pop-off valve through a hose to the outside or into an activated charcoal canister. Charcoal canisters have high resistance, must be weighed and changed when full, and do not absorb nitrous oxide.4
Table 1. Machine and circuit components: function and common fault
ComponentFunctionCommon fault the technician finds
Oxygen cylinder and gaugeStores oxygen; gauge shows pressureTank below 200 psi at setup1
Pressure regulatorLowers tank pressure to about 45 psiMissing yoke washer gives a poor seal at the yoke3
FlowmeterSets fresh gas flowRead at the top of the float instead of its widest part3
Precision vaporizerAdds a set percentage of one inhalantEmpty or overdue for service; flow below 500 mL/min12
Common gas outletFresh gas leaves the machineLoose connection leaks3
Oxygen flush valveRapid pure oxygen, no inhalantUsed with an NRC, causing barotrauma1
Unidirectional valvesKeep gas moving one way round the circleSticking valve lets CO2 build up4
CO2 absorbentRemoves exhaled CO2Exhausted granules; inspired CO2 rises24
Pop-off (APL) valveReleases excess gas to the scavengerLeft closed; airway pressure climbs4
Reservoir bagHolds gas; shows breathing; manual breathsWrong size for the patient4
ManometerShows circuit pressure in cm H2OIgnored while pressure rises1
ScavengerRemoves waste gas from the roomDisconnected, kinked or turned off5

Source: AAHA 2020 anesthesia guidelines; AAFP 2018 feline anesthesia guidelines; Lee, Anesthetic Machines and Anesthetic Equipments; Merck Veterinary Manual.

Procedure or technique

Pre-use machine check

  1. Supply. Confirm the oxygen tank reads above 200 psi or the oxygen generator works. Fill the vaporizer with liquid inhalant.1 Merck advises filling vaporizers in a well-ventilated area to cut staff exposure.5
  2. Absorbent. For a circle, check the canister's logged hours and change the absorbent after 8 hours of use. AAFP recommends a tag on the canister that records hours and dates.12
  3. Circuit and bag. Connect the right circuit for the patient's weight and choose a bag of suitable size.12
  4. Leak test (Figure 2). Close the pop-off. Occlude the patient end. Raise circuit pressure to 20 to 30 cm H2O with the oxygen flowmeter, then turn the oxygen off. The manometer should stay steady. If it does, open the pop-off, then release the occlusion. If it falls, take the machine out of use and troubleshoot.1 Lee advises filling the circuit with the flowmeter rather than the flush valve.3
  5. Valves. For a circle, breathe through the circuit and watch the correct valve move on inspiration and on expiration.1
  6. Scavenging. Connect the scavenger to the pop-off outlet and turn on the active system.1
  7. Repeat the leak test after changing any circuit part.1

During anesthesia

  • Keep the pop-off open during spontaneous breathing. To give a manual breath, close it or press the safety pop-off button, squeeze the bag while watching the manometer, then reopen it.1
  • Watch the manometer. Lee notes that pressure above 20 cm H2O in small animals or 30 cm H2O in large animals is unsafe.4
  • Use the capnograph to check the flow. On an NRC, adjust the flow to keep inspired CO2 below 5 mm Hg.1
  • Merck advises turning on the vaporizer only after the circuit is connected to the patient.5
Flowchart: pre-use machine check and low-pressure leak test of the breathing circuit Supply check O2 above 200 psi; vaporizer filled Circuit and absorbent Circuit fits weight; absorbent hours logged Scavenger connected and on Bag size chosen; manometer reads zero Close pop-off, occlude patient end Fill to 20 to 30 cm H2O, then turn O2 off Does the pressure hold steady? Yes No Pass Open pop-off FIRST, then release the occlusion. Check valves move. Ready for the patient. Leak: do not use Check canister seal, hoses, bag, gas outlet connection, valve domes. Tighten or replace parts. Repeat the leak test Still leaking: pull machine from use and report
Figure 2. Pre-use machine check and leak test. The order matters at the end: open the pop-off before releasing the occlusion, so the circuit does not stay pressurized. A circuit that will not hold pressure is not used until the leak is found.

Choosing the circuit, oxygen flow and bag

Table 2. Choosing a breathing system by body weight
ItemNon-rebreathing (Bain, Jackson-Rees)Rebreathing (circle)
Typical patientBelow about 3 to 5 kg; AAFP: NRC for cats under 3 kg12Above about 3 to 5 kg1
How CO2 is removedHigh fresh gas flow2Absorbent plus one-way valves2
Oxygen flow200 to 400 mL/kg/min120 to 40 mL/kg/min in maintenance, at least 500 mL/min1
Resistance to breathingMinimal2From absorbent and valves2
Heat lossGreater, from high flows2Less; rebreathing conserves heat2
Speed of depth changeShort2Longer2
Flush valveNever while connected1Use with care; dilutes inhalant3

Source: AAHA 2020 anesthesia guidelines, Step 2b; AAFP 2018 feline anesthesia guidelines, Table 3.

Older teaching gives a cutoff of 6 kg for moving a patient to a circle.4 Current AAHA guidance uses about 3 to 5 kg, and pediatric circles can be used below that if an NRC is not available.1 For cats on a circle, AAFP lists 2 to 3 L/min for the first 15 minutes to change the inhalant concentration faster, then a maintenance minimum of 0.5 L/min, with a 1 L bag from 3 to 10 kg.2

Flowchart: pressure rising in the circuit

A climbing manometer or a bag that keeps swelling is an emergency. AAHA warns that a blocked expiratory limb on an NRC, with its high flow, pressurizes the whole system and can cause airway damage or circulatory collapse within minutes.1 AAFP adds that in cats this damage can happen in less than 30 seconds.2 Relieve the pressure first, then find the cause.

Decision tree: what the technician does when circuit pressure is rising or the reservoir bag is overfilling Manometer rising, bag overfilling, or high-pressure alarm sounds Act now: open the pop-off valve or press safety pop-off; tell the veterinarian Was the pop-off left closed? Yes No Keep it open Close only to give a breath, then reopen Scavenger hose kinked, blocked, off? Yes Clear or fix it Check the interface and vacuum setting; passive hose must not be pinched Flush valve used or flow too high? Yes Stop flush, reset flowmeter to the planned rate No No Check the circuit itself Kinked or blocked expiratory limb, stuck expiratory valve, wrong hose connection. If still rising, disconnect circuit at the ETT and follow the veterinarian
Figure 3. Pressure rising in the circuit. The first action is always to relieve pressure at the pop-off valve and tell the veterinarian. Then work from the most common cause (a closed pop-off) through the scavenger and fresh gas flow to the circuit itself.

Species differences

  • Cats. An average cat has about 400 mL of lung capacity, so high NRC flows and the flush valve put cats at real risk of barotrauma (lung injury from pressure).1 AAFP recommends an NRC for cats under 3 kg; above 3 kg either type can be used. A modern circle with lightweight valves and low dead space can be used safely in cats over 3 kg.2
  • Small dogs. Under about 3 to 5 kg, use an NRC, or a pediatric circle if no NRC is available.1
  • Medium and large dogs. Use a circle. Lee lists bags of 0.5, 1, 2, 3 and 5 L for small animals, with a 2 L bag typical for a 20 kg dog.4
  • Horses and large animals. Large-animal circles use bags of 15, 20 or 30 L.4 Their larger airways tolerate somewhat higher pressures: Lee puts the unsafe level at about 30 cm H2O, against 20 cm H2O for small animals.4

Calculation

Worked example 1: how long will this E tank last?

The E-tank gauge reads 1,000 psi and the flowmeter is set at 3 L/min. Lee's factor for an E tank is 0.3 L per psi.3

Oxygen left = 1,000 psi x 0.3 L/psi = 300 L

Time left = 300 L / 3 L/min = 100 min

Plan to change tanks well before this, because the estimate is approximate and the patient should never run short.

Worked example 2: NRC flow for a 4 kg cat

Low end: 4 kg x 200 mL/kg/min = 800 mL/min (0.8 L/min)

High end: 4 kg x 400 mL/kg/min = 1,600 mL/min (1.6 L/min)

AAFP's table lists 0.8 L/min as the minimum for a 4 kg cat on an NRC, which matches the low end.12

Worked example 3: circle flow for a 10 kg dog

10 kg x 30 mL/kg/min = 300 mL/min. This is below the 500 mL/min minimum, so set the flowmeter to 500 mL/min.1

  • Staff safety. A sealed (leak-free) machine and a working scavenger limit staff exposure to anesthetic gases.1 Merck lists leak tests, routine vaporizer servicing, good room ventilation and careful vaporizer filling as the core controls.5
  • Regulation. OSHA provides advisory guidelines on waste anesthetic gases, and some states have their own rules.1
  • Pregnancy. Merck reports that with proper scavenging and ventilation, current evidence does not show a higher risk of reproductive problems in exposed female veterinarians.5 Staff who are pregnant should still follow the practice's own safety policy.
  • Cylinders. Secure cylinders so they cannot fall.3
  • Checklists and maintenance logs. AAHA recommends equipment checklists, and AAFP recommends yearly replacement of O-rings and tubing and a vaporizer service every 3 years.12
  • Client education. Owners can be told that the machine is checked before every patient and that the team uses circuits sized to their pet.

Common mistakes and exam traps

  • Leaving the pop-off closed after a leak test or a manual breath. This is the classic cause of a swelling bag and rising pressure.4
  • Releasing the occlusion before opening the pop-off at the end of a leak test. AAHA's order is pop-off open first, then release.1
  • Using the flush valve on an NRC. It sends high-pressure flow straight into the airway.1
  • Trusting the absorbent color. Granules can look fresh after resting. Use hours of use and inspired CO2 on the capnograph.24
  • Running a circle below 500 mL/min for a small dog. The vaporizer output may be inaccurate.1
  • Confusing the two flow rules. 200 to 400 mL/kg/min is for an NRC; 20 to 40 mL/kg/min is for a circle in maintenance.1

Quick review

  1. Gas path: tank, regulator, flowmeter, vaporizer, common gas outlet, circuit, patient, pop-off, scavenger.
  2. Full E oxygen tank: about 660 L at about 2,200 psi; psi x 0.3 = liters left.
  3. Start only with more than 200 psi in the tank.
  4. Read the flowmeter at the widest part of the float.
  5. NRC below about 3 to 5 kg; circle above.
  6. NRC flow 200 to 400 mL/kg/min; circle maintenance 20 to 40 mL/kg/min, minimum 500 mL/min.
  7. Bain is Mapleson D; Jackson-Rees is Mapleson F.
  8. Leak test: pop-off closed, patient end occluded, 20 to 30 cm H2O, oxygen off, pressure holds.
  9. Pop-off open during spontaneous breathing.
  10. Change absorbent after 8 hours of use; rising inspired CO2 means exhaustion.
  11. Never use the flush valve on a patient connected to an NRC.
  12. Active scavenging is far more effective than passive; charcoal does not absorb nitrous oxide.

Test yourself

5 questions written for this guide. Pick an answer, then check it.

Question 1During the pre-use check of a circle system, a technician closes the pop-off valve, occludes the Y-piece, fills the circuit to 25 cm H2O and turns the oxygen off. The manometer needle slowly falls toward zero. What should the technician do?

Show answer

Answer: B. Keep the machine out of use and look for the leak, then retest

B is right: the pressure should stay steady once the oxygen is off. A falling needle means a leak, and AAHA says to pull the machine from use and troubleshoot (canister seal, hoses, bag, connections), then repeat the test. A is wrong: a leaking circuit risks poor delivery to the patient and staff exposure to waste gas. C is wrong: masking a leak with more flow increases waste gas and does not fix the fault. D is wrong: the flush valve does not change the result and Lee advises filling the circuit with the flowmeter.

Source 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats, Step 2c, AAHA

Question 2An oxygen E tank on the anesthesia machine reads 600 psi. The flowmeter is set at 2 L/min. Using a factor of 0.3 L per psi, about how long will the oxygen last?

Show answer

Answer: D. 90 minutes

D is right: 600 psi x 0.3 L/psi = 180 L, and 180 L / 2 L/min = 90 minutes. A is wrong: it divides 60 by 2, dropping a zero. B is wrong: it uses 0.2 instead of 0.3 as the factor. C is wrong: 180 is the number of liters left, not minutes.

Source Anesthetic Machines, Carrier gases, Western University of Health Sciences (Lee L)

Question 3A 3 kg cat will be maintained on isoflurane through a Bain non-rebreathing circuit. Using the AAHA range of 200 to 400 mL/kg/min, which oxygen flow range fits this cat?

Show answer

Answer: A. 600 to 1,200 mL/min

A is right: 3 kg x 200 mL/kg/min = 600 mL/min and 3 kg x 400 mL/kg/min = 1,200 mL/min. B is wrong: it is the circle maintenance range (20 to 40 mL/kg/min) and would let the cat rebreathe CO2 on an NRC. C is wrong: it ignores body weight. D is wrong: it is ten times too high and would waste gas and cool the cat.

Source 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats, Step 2b, AAHA

Question 4A 25 kg dog is breathing on its own on a circle system. The reservoir bag is getting bigger with each breath and the manometer reads 15 cm H2O and rising. What should the technician check first?

Show answer

Answer: C. Whether the pop-off valve has been left closed

C is right: a bag that keeps filling and a rising manometer in a spontaneously breathing patient most often mean the pop-off valve is closed. Open it at once and tell the veterinarian. A is wrong: exhausted absorbent raises inspired CO2, not circuit pressure. B is wrong: vaporizer level affects inhalant delivery, not pressure. D is wrong: a bag that is too small would empty, not overfill, and bag size does not cause pressure to build.

Source Anesthetic Equipments: Breathing circuits and scavenging system, Adjustable pressure limiting valve, Western University of Health Sciences (Lee L)

Question 5Two hours into a procedure on a circle system, the capnograph shows inspired CO2 above zero and climbing. The absorbent dye looks unchanged. What is the most likely equipment cause?

Show answer

Answer: A. Exhausted CO2 absorbent, even though the color looks fresh

A is right: a rise in inspired CO2 is a sign of exhausted absorbent, and Lee warns that indicator dyes change late and can revert, so the color is not reliable. A sticking unidirectional valve is the other equipment cause to check. B is wrong: higher fresh gas flow lowers rebreathing, it does not cause it. C is wrong: a slightly large bag makes breathing harder to read but does not add CO2. D is wrong: active scavenging is the preferred method and does not cause rebreathing.

Source AAFP Feline Anesthesia Guidelines, Table 2 Routine maintenance for anesthesia equipment safety, J Feline Med Surg

0 of 5 answered

Practise this topic

Two questions from our VTNE practice bank on this topic.

Question 1During spontaneous breathing in an anesthetized patient on a rebreathing circuit, what is the recommended position of the pop-off (APL) valve?

Show answer

Answer: C. Open, so excess gas vents to the scavenger

The pop-off valve is a pressure relief valve that protects the patient from high airway pressure. When the patient is breathing on its own, the valve is left open so that excess gas flows out to the scavenging system and circuit pressure stays low. It is only partly closed for a moment when the anesthetist squeezes the bag to give a breath, and then it is opened again.

Keeping the valve fully closed lets gas build up in the circuit, which can impede venous return and cause barotrauma, so it does not help tidal volume. Anesthetists do not cycle the valve open and closed with every breath of a spontaneously breathing patient; it simply stays open. Removing the valve would take away the safety outlet that sends waste gas to the scavenger and prevents pressure buildup.

Source Overview of anesthetic machines and circuits (Weil A, 2011), dvm360
The ins and outs of the anesthesia machine (Cooley K, 2009), dvm360
2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats (Grubb T et al., 2020), American Animal Hospital Association

Practise all 17 questions on this topic Free account. Or try the free practice quiz.

Figure credits

  • Figure 1 Original diagram, VTNE Exam Prep
  • Figure 2 Original diagram, VTNE Exam Prep
  • Figure 3 Original diagram, VTNE Exam Prep

References

  1. Grubb T, Sager J, Gaynor JS, Montgomery E, Parker JA, Shafford H, Tearney C. 2020 AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats. J Am Anim Hosp Assoc. 2020;56(2):59-82. doi:10.5326/JAAHA-MS-7055 aaha.org/wp-content/uploads/globalassets/02-guidelines/2020-anesthesia/anesthesia_and_monitoring_guidelines_final.pdf
  2. Robertson SA, Gogolski SM, Pascoe P, Shafford HL, Sager J, Griffenhagen GM. AAFP Feline Anesthesia Guidelines. J Feline Med Surg. 2018;20(7):602-634. PMC10816483 pmc.ncbi.nlm.nih.gov/articles/PMC10816483/
  3. Lee L. Anesthetic Machines. Western University of Health Sciences, College of Veterinary Medicine, veterinary anesthesia and analgesia teaching notes. westernu.edu (accessed 2026-10-31) westernu.edu/mediafiles/veterinary/vet-anesthesia-analgesia/anesthetic-machines.pdf
  4. Lee L. Anesthetic Equipments: Breathing circuits and scavenging system. Western University of Health Sciences, College of Veterinary Medicine, veterinary anesthesia and analgesia teaching notes. westernu.edu (accessed 2026-10-31) westernu.edu/mediafiles/veterinary/vet-anesthesia-analgesia/anesthetic-equipments.pdf
  5. Dorman D. Waste Anesthetic Gases as a Veterinary Workplace Hazard. Merck Veterinary Manual. merckvetmanual.com (accessed 2026-10-31) merckvetmanual.com/public-health/toxicological-hazards-in-the-veterinary-workplace/waste-anesthetic-gases-as-a-veterinary-workplace-hazard
  6. American Association of Veterinary State Boards. 2023 Veterinary Technician National Examination Content Outline. aavsb.org (accessed 2026-10-30) aavsb.org/wp-content/uploads/2025/04/AAVSB-2023-Veterinary-Technician-National-Examination-Content-Outline-2.pdf

Spotted an error? Email support@vtneexam.com and we will check it against the sources.