Tests to Check Breathing Circuits in Anaesthesia

Why This Matters

Understanding the Two Separate Systems to Test

Critical concept (Miller's, p. 2408): If the machine has an outlet check valve, positive pressure applied to the breathing circuit will NOT pass upstream into the low-pressure system. Therefore you cannot use a circuit pressure test to check for machine leaks — you need the negative-pressure suction bulb test instead.

Test 1 — Low-Pressure System Leak Test (Negative-Pressure / "Universal" Test)

Procedure (MM, Table 4–3, Step 5 / MA, Item 8):

1. Verify the machine master switch and all flow control valves are off
2. Disconnect the breathing circuit from the common gas outlet
3. Attach the specially configured suction bulb to the common (fresh) gas outlet
4. Squeeze the bulb repeatedly until fully collapsed (this creates subatmospheric pressure in the low-pressure circuit, which opens any check valve present)
5. Verify the bulb stays fully collapsed for at least 10 seconds
6. Open one vaporizer at a time and repeat the squeeze-and-hold — this is critical because most vaporizer leaks are not detected unless the vaporizer is on
7. Remove the suction bulb and reconnect the fresh gas hose

Why "universal"? This test works on machines both with and without a check valve, because it creates subatmospheric (negative) pressure, which naturally opens check valves. A positive-pressure test only works on machines without a check valve. — MA, p. 2409

Machines without accessible common gas outlet (many contemporary workstations): Negative-pressure manual testing cannot be performed. Instead, manual positive-pressure testing of vaporizers is done, or low-pressure testing is part of the automated checkout. — MA, p. 2409

Test 2 — Breathing Circuit (High-Pressure) Leak Test

Procedure (MM, Table 4–3, Step 11):

1. Set all gas flows to zero (or minimum)
2. Close the APL (pop-off) valve
3. Occlude the Y-piece (the patient connection)
4. Pressurize the breathing system to ~30 cm H₂O using the O₂ flush
5. Ensure pressure remains fixed for at least 10 seconds
6. Open the APL valve — verify that pressure decreases promptly

MM (p. 150): "A gradual decline in circuit pressure indicates a leak within the breathing circuit."

Where are leaks most commonly found? (MM, p. 150)

• Most common single site: Base plate of the CO₂ absorber
• Most common disconnection: Between the right-angle connector and the tracheal tube
• Other sites: open APL valve, cracked O₂ analyser adaptor, loose tubing, improperly adjusted scavenging unit, or leaks around the ETT cuff in an intubated patient

Test 3 — Oxygen Monitor Calibration Check

Procedure (MM, Table 4–3, Step 9):

1. Remove the O₂ sensor and expose to room air → verify it reads 21%
2. Verify the low-O₂ alarm is enabled and functioning
3. Reinstall the sensor in the circuit
4. Flush the breathing system with O₂ → verify the monitor reads >90%

Test 4 — Scavenging System Check

Procedure (MM, Table 4–3, Step 8 / MA, Item 9):

1. Ensure proper connections between scavenging system and both the APL valve and ventilator relief valve
2. Adjust the waste-gas vacuum (active systems: set to evacuate 10–15 L/min)
3. Fully open the APL valve and occlude the Y-piece
4. With minimum O₂ flow, allow scavenger reservoir bag to collapse completely → verify absorber pressure gauge reads ~0
5. Activate the O₂ flush, allow reservoir bag to distend fully → verify absorber pressure gauge reads <10 cm H₂O

For active closed scavenging systems, also test the negative-pressure relief valve: turn off all flow control valves, occlude the Y-piece, then verify that the breathing pressure gauge does not go significantly negative. — MA, p. 2411

Test 5 — Unidirectional Valve Function Test

Procedure (MM, Table 4–3, Step 12):

1. Place a second breathing bag on the Y-piece (simulates patient lungs)
2. Set appropriate ventilator parameters for the next patient
3. Switch to automatic ventilation (ventilator) mode
4. Turn ventilator on; fill bellows and breathing bag with O₂ flush
5. Set O₂ flow to minimum
6. Verify during inspiration: bellows delivers appropriate tidal volume
7. Verify during expiration: bellows fills completely
8. Set fresh gas flow to ~5 L/min; verify bellows and simulated lungs fill and empty appropriately without sustained pressure at end-expiration
9. Check for proper action of unidirectional valves (watch that inspiratory valve opens on inhalation, expiratory valve opens on exhalation — neither should move in reverse)
10. Switch back to manual (Bag/APL) mode and manually ventilate — ensure appropriate feel of resistance and compliance
11. Remove the second breathing bag

Test 6 — Flowmeter Test (Hypoxic Mixture Prevention)

1. Adjust the flow of all gases through their full range — check for smooth float operation and undamaged flow tubes
2. Attempt to create a hypoxic O₂/N₂O mixture → verify the machine produces correct changes in flow or triggers an alarm (proportioning safety system)

Miller's also lists this as an additional check that some institutions include: "Testing the proportioning system by attempting to create a hypoxic oxygen–nitrous oxide mixture." — MA, p. 2419

Test 7 — Quick Intraoperative Leak Test (Back-Pressure Test)

"Pinching the tubing that connects the machine's fresh gas outlet to the circuit's fresh gas inlet creates a back pressure that obstructs the forward flow of fresh gas from the anesthesia machine. This is indicated by a drop in the height of the flowmeter floats. When released, the floats should briskly rebound and settle at their original height."

• If floats drop when pinched → system upstream is intact (leak is in the circuit, not the machine)
• If floats do not drop → substantial leak exists within the machine upstream of the outlet

Summary: The FDA/Morgan Checkout Sequence in Full (Table 4–3)

MS MAIDS — Final Pre-Induction Mnemonic (Miller's, p. 2420)

Key Teaching Points

1. Low-pressure leak test ≠ breathing circuit leak test — they test different zones with different techniques
2. The negative-pressure (suction bulb) test is "universal" — it works regardless of whether a check valve is present; positive-pressure tests fail on machines with a check valve
3. Each vaporizer must be tested individually during the low-pressure leak test
4. Intraoperatively, a falling bellows is a sign of a leak exceeding fresh gas inflow — estimate leak size by increasing FGF until bellows height stabilises
5. Automated machine self-tests do not replace all manual checks — no machine automatically checks everything; providers must understand exactly which items their machine's self-test covers — MA, p. 2309