SCV 202: Lecture 6 Anesthetic Machine Parts

4 basic parts of the anesthetic machine

Compressed gas supply; anesthetic vaporizer; breathing circuit; scavenging system (blue tube and collection system)

O2 tank sizes

E tank = 660 L; H tank = 6900 L

Liters left in an E tank

PSI × 0.3

Liters left in an H tank

PSI × 3

Minutes of anesthesia left in a tank

Liters left ÷ O2 flow rate (L/min)

Compressed gas colors

O2 is green in the US (white in Canada); nitrous oxide is blue; CO2 is grey

Pin index safety system

On E tanks; prevents the wrong gas from being attached to the machine

Opening and closing a tank

Open: turn the valve counter-clockwise. Close: turn clockwise

O2 cylinder hazards

O2 is combustible (fire hazard); cylinders are under pressure and can cause corneal lacerations and skin injury

Minimizing O2 cylinder risks

No ignition sources near cylinders; shield during electrocautery or laser procedures; use a wrench to open or change tanks; strap cylinders vertically to the wall; use valve caps and hand carts; wear eye protection

Tank gauge

Shows the pressure in the compressed gas tank (PSI)

Pressure-reducing valve

Reduces tank pressure to 40–50 PSI; works passively; a line pressure gauge may sit between it and the flowmeter

O2 flowmeter

Measures O2 in L/min. O2 enters at the bottom and exits at the top; further reduces pressure to 12–15 PSI. Handle the valve gently. If it is off, the patient is not getting O2

O2 flush valve

Delivers pure O2 straight to the breathing circuit, bypassing the flowmeter and vaporizer. High pressure (50 PSI) and high flow (35–75 L/min), so use with care

Order O2 flows through the machine

Compressed gas tank → tank gauge → pressure-reducing valve → O2 flowmeter → vaporizer

Vaporizer color coding

Each vaporizer is calibrated to one agent: isoflurane = purple, sevoflurane = yellow, halothane = red

Filling the vaporizer

Use pour-spouts or nozzles; wear gloves and a mask; fill between the upper and lower lines of the indicator window

Range of vaporizer settings

MAC × 1 = light Stage III; MAC × 1.5 = moderate; MAC × 2 = deep

VOC vaporizer

Vaporizer-out-of-circuit: not inside the breathing circuit. O2 from the flowmeter goes through the vaporizer before entering the circuit. These are precision vaporizers

VIC vaporizer

Vaporizer-in-circuit: O2 goes from the flowmeter straight into the circuit, and exhaled gases pass through the vaporizer. These are non-precision vaporizers

WAGs — sources

The anesthetic machine, gas exhaled by the patient, vaporizer filling, liquid anesthetic spills

WAGs — hazards

Eye and skin irritation, headaches and dizziness, decreased fertility, cardiovascular and respiratory effects, nausea and loss of appetite

Minimizing WAG exposure

Use a scavenging system; minimize leaks; avoid mask and chamber delivery when possible; avoid spills; wear PPE when filling; cuff the ET tube; use a dosimeter to monitor

Breathing circuit — functions

Delivers anesthetic mixed with O2 from the fresh gas outlet to the patient and carries expired gases away. Can be rebreathing (circle) or non-rebreathing

Rebreathing circuit — how it works

The CO2 absorber canister (soda lime) removes CO2 from exhaled gas, so the gas can be recycled and inhaled again with added fresh gas. This allows lower O2 flow rates

Non-rebreathing circuit — how it works

No CO2 absorber; gas cannot be recycled. High flow rates push waste gas away from the patient

Choosing a breathing circuit

Based on patient size, cost, control of anesthetic depth, conservation of heat and moisture, and waste gas production. The choice sets the equipment needed and the O2 flow rate

Circuit by patient size

Rebreathing: over 7 kg (15.4 lb); patients 3–7 kg need pediatric breathing tubes. Non-rebreathing: under 3 kg (6.6 lb)

O2 flow rates by circuit

Rebreathing: 50–100 mL/kg/min. Non-rebreathing: 200–300 mL/kg/min

Parts of the rebreathing system

Breathing tubes (Y tubing or F tube), unidirectional valves, CO2 absorber canister, pressure manometer, reservoir bag, pop-off valve, negative pressure relief valve

Breathing tube styles

Y-tubing: corrugated tubes with a Y connector. Coaxial F-tube: inspiratory (inner) and expiratory (outer) tubes combined

Unidirectional valves

Keep gas flowing one way; they open and close as the patient breathes

Negative pressure relief valve

Also called the air intake valve; lets room air into the circuit if negative pressure is detected

CO2 absorber canister

Contains calcium hydroxide granules that react with CO2 to form calcium carbonate. Replace when depleted: color changes from white to violet, after 6–8 hours of anesthetic time, or when granules become brittle

Pressure manometer

Shows the pressure inside the breathing circuit (cm H2O, mm Hg, or kPa). Watch it when bagging the patient; it should never go over 20 cm H2O

Pop-off valve (open)

Lets excess gas leave the circuit and go to the scavenger; prevents too much pressure or volume in the circuit

Pop-off valve (closed)

Builds pressure in the circuit so you can manually ventilate or do a machine pressure check. Reopen it immediately afterward

Scavenge system

Carries waste anesthetic gas away from the breathing circuit (controlled by the pop-off valve). Types: active and passive. Parts: scavenge tubing and, for passive, a charcoal canister

Reservoir bag — functions

Flexible gas storage; shows respiratory rate and depth; confirms ET tube placement; lets you deliver anesthetic gas or pure O2 by manual ventilation. Sizes range from 0.5 L to 30 L

Reservoir bag size

Tidal volume is about 10 mL/kg, and the bag should hold about 6 times that: 60 mL/kg of body weight, rounded up to the next liter

Why manually ventilate (bag) a patient

Minimize atelectasis (bag every 5–10 minutes during a procedure); force fresh gas into the alveoli for gas delivery and exchange; help remove CO2

How to give a breath

Close the pop-off valve, squeeze the reservoir bag 1–2 times while watching the manometer (under 20 cm H2O), then reopen the pop-off valve

Pressure (leak) check — steps

1) Open the O2 tank. 2) Close the pop-off valve. 3) Block the patient end of the Y tubing. 4) Set the flowmeter to 2 L/min. 5) Let pressure rise to 20 cm H2O. 6) Turn flow down to 0.2 L/min. 7) Hold for 20 seconds. 8) Reopen the pop-off valve

Common places for leaks

Reservoir bag, soda lime canister, breathing tubing, connectors

Example: reservoir bag for a 40 lb dog

40 lb ÷ 2.2 = 18.2 kg. 18.2 × 60 mL = about 1,091 mL (1.09 L), rounded up to the next liter = 2 L bag

Example: rebreather O2 flow for a 40 lb dog

18.2 kg × 50–100 mL/kg/min = about 910–1,820 mL/min = 0.9–1.8 L/min

Example: tank time on an E tank at 1234 PSI

1234 × 0.3 = 370 L. At a flow of 1 L/min, that is about 370 minutes (about 6 hours)

Example: vaporizer settings for isoflurane in a dog (MAC 1.3)

1.3% (× 1), 1.95% (× 1.5), 2.6% (× 2). For a cat (MAC 1.63): 1.63%, about 2.45%, 3.26%