3.3 Pre-Use Anesthesia Machine Checkout & Low-Pressure Leak Testing

3.3 Pre-Use Anesthesia Machine Checkout & Low-Pressure Leak Testing

The pre-use anesthesia machine checkout is the most critical preventative safety routine performed in perioperative medicine. Technical malfunctions, unrecognized circuit disconnections, depleted gas reserves, and vaporizer leaks account for significant preventable morbidity and mortality. For the Certified Anesthesia Technologist (Cer.A.T.T.), mastering the regulatory standards, mechanical engineering principles, and step-by-step physical testing of high-, intermediate-, and low-pressure systems is a core professional competency.

---

Regulatory Evolution: 1993 FDA Protocol vs. 2008 ASA Guidelines

Pre-use checkout standards have evolved alongside advancements in anesthesia machine technology, transitioning from purely manual procedures on mechanical devices to hybrid manual-electronic protocols on microprocessor workstations:

The FDA Anesthesia Apparatus Checkout Recommendations (1986, Revised 1993)

• The U.S. Food and Drug Administration (FDA) published a 24-step checkout in 1986 and revised it in 1993 to a shorter 14-step list that served as the national reference for more than a decade.
• Designed for older, purely pneumatic and mechanical workstations (such as the Ohmeda Modulus and Dräger Narkomed series).
• Relied entirely on manual operator manipulation, including physical negative-pressure bulb tests, mechanical fail-safe checks, and manual oxygen flush verification.
• Limitations: As microprocessors, electronic gas mixers, internal leak transducers, and automated self-tests spread in the 2000s, one fixed list no longer fit every machine: some steps (such as the negative-pressure leak test) did not apply to all designs, and many workstations automated parts of the check.

The 2008 ASA Pre-Anesthesia Checkout Recommendations

• Developed by the American Society of Anesthesiologists (ASA) in 2008, this modern framework updated checkout procedures for computerized workstations.
• Rather than one fixed list, the 2008 ASA recommendations describe 15 items, specify which are done daily and which are repeated before each procedure, and encourage departments to write machine-specific checklists:
  1. Full Daily Checkout: A comprehensive inspection performed every 24 hours (typically in the morning prior to the first surgical procedure).
  2. Abbreviated Turnover Checkout: A targeted, rapid inspection performed between sequential surgical cases throughout the day using the same anesthesia machine.

---

High-Pressure System Checkout (Cylinders & Yokes)

The high-pressure circuit consists of all components exposed to cylinder gas pressures (up to 2200 psig for oxygen): the cylinder hanger yokes, Pin Index Safety System (PISS) pins, Bodok seals, yoke check valves, Bourdon cylinder pressure gauges, and first-stage pressure regulators.

Step-by-Step High-Pressure Testing Protocol

1. Physical Inspection: Inspect the rear hanger yokes. Verify that each cylinder is mounted correctly with its specific PISS pins engaged and that only ONE clean Bodok washer is present on the yoke nipple.
2. Open Cylinder Valve: Using an approved cylinder wrench, rotate the cylinder spindle counterclockwise until fully open. Observe the corresponding Bourdon pressure gauge on the front machine panel:
   • Oxygen E-Cylinder: Must indicate a minimum of 1000 psig (approximately 300 L of reserve gas). The 2008 ASA recommendations call for confirming that the auxiliary oxygen cylinder is at least half full (about 1000 psig).
   • Medical Air & Nitrous Oxide Cylinders: Verify adequate pressures if installed.
3. Perform High-Pressure Leak Decay Test:
   • Close the cylinder valve tightly using the wrench.
   • Observe the cylinder pressure gauge for at least 1 full minute.
   • Acceptable Standard: The gauge should show no meaningful drop. A rapid pressure drop indicates a leak at the Bodok seal, a cracked gauge Bourdon tube, or a leaking first-stage regulator seat.
4. CRITICAL STEP: Turn the Cylinder OFF!
   • After verifying cylinder contents and leak integrity, the technologist must close the cylinder valve.
   • The Silent Depletion Hazard: Why must cylinder valves remain closed during routine surgery? Anesthesia machine first-stage regulators reduce cylinder pressure to 40 to 48 psig (nominally 45 psig), while the hospital central pipeline operates at 50 to 55 psig. Under normal conditions, the higher pipeline pressure holds the cylinder check valve seated and closed. However, if hospital pipeline pressure experiences a transient drop or fluctuates below 45 psig, the machine will automatically and silently draw oxygen from the open cylinder! Because the pipeline gauge still shows partial pressure, the clinical team will not notice. When a catastrophic central pipeline outage subsequently occurs, the backup cylinder will already be completely depleted!

---

Intermediate-Pressure System Checkout (Pipelines & Fail-Safes)

The intermediate-pressure circuit receives gas stepped down from cylinders (45 psig) or directly from central pipelines (50–55 psig). It includes pipeline hoses, Diameter Index Safety System (DISS) fittings, the master on/off switch, oxygen fail-safe valves, second-stage regulators, the oxygen flush valve, and the ventilator drive gas supply.

Testing Pipeline Connections & Pressure

1. Connect the pipeline hoses (Oxygen, Medical Air, Nitrous Oxide) to their respective wall terminal quick-connects or DISS outlets.
2. Verify that all pipeline pressure gauges on the anesthesia workstation display 50 to 55 psig (345 to 380 kPa).

Oxygen Flush Valve Functional Test

• The oxygen flush valve delivers unmetered, pure oxygen directly from the intermediate circuit to the common gas outlet, completely bypassing flowmeters and vaporizers.
• Operational Specifications: Delivers a high, continuous flow rate of 35 to 75 L/min at line pressure (50 psig).
• Testing Procedure: Depress the oxygen flush button firmly. Confirm a high-velocity rush of gas at the common gas outlet and a brisk rise on the circuit pressure manometer. Release the button and verify that it springs back immediately without sticking or continuing to flow.
• Failure Mode (Barotrauma Hazard): A sticking oxygen flush button will deliver 75 L/min into a closed breathing circuit, causing catastrophic pulmonary barotrauma within seconds. Additionally, activating the flush during the inspiratory phase of mechanical ventilation can transmit dangerously high pressure to the patient's lungs on ventilators without fresh gas decoupling.

Oxygen Fail-Safe & Low O2 Pressure Alarm Test

• Mechanism: The oxygen fail-safe valve (Pressure Sensor Shut-Off Valve) shuts off or proportionally decreases the flow of nitrous oxide and other non-life-supporting gases if the oxygen supply pressure falls (about 20 psig in threshold designs such as the Datex-Ohmeda pressure sensor shut-off valve).
• Testing Procedure:
  1. Open both oxygen and nitrous oxide flowmeters to 2 L/min.
  2. Disconnect the oxygen pipeline hose from the wall outlet (with the backup oxygen cylinder closed).
  3. As the residual oxygen in the intermediate piping bleeds down, confirm that the low oxygen supply pressure alarm sounds as pressure falls.
  4. Confirm that the nitrous oxide flowmeter float drops completely to zero before or simultaneously with the total cessation of oxygen flow, verifying that hypoxic gas delivery is mechanically impossible under low-pressure conditions.

---

Low-Pressure System Circuit Leak Testing

The low-pressure circuit extends from the flow control valves downstream to the Common Gas Outlet (CGO). It includes the glass flowmeter Thorpe tubes, the flowmeter manifold block, vaporizer mounting stations (Selectatec manifold), volatile agent vaporizers, and the internal check valve (if present).

Why the Low-Pressure Circuit is Uniquely Vulnerable

1. Fragile Structural Components: Flowmeter tubes, O-rings, and internal vaporizer sumps operate at just slightly above atmospheric pressure (1 to 2 psig) and are prone to mechanical fractures, gasket degradation, and loose connections.
2. Lethal Clinical Consequences of a Leak: A leak in the low-pressure circuit occurs upstream of the patient breathing system. Consequently, fresh gas and anesthetic vapor leak out into the room before ever reaching the circuit, leading to intraoperative patient awareness, delivery of hypoxic gas mixtures, or complete hypoventilation.
3. Masking by an Outlet Check Valve: On machines with a check valve near the common gas outlet, positive pressure applied from the breathing circuit closes the check valve, so a breathing-system pressure test cannot detect leaks upstream of it.

The Negative-Pressure Suction Bulb Test (Universal Low-Pressure Test)

Workstations manufactured with an internal outlet check valve positioned between the vaporizers and the common gas outlet (such as Datex-Ohmeda / GE machines) cannot be leak-tested using positive pressure. Positive pressure applied at the CGO forces the check valve closed, falsely masking internal leaks upstream in the vaporizers. Therefore, the negative-pressure suction bulb test is the leak test the 1993 FDA checkout recommended for these machines; current ASA guidance calls for following each manufacturer's specific low-pressure leak test.

NEGATIVE-PRESSURE SUCTION BULB TEST PROTOCOL:

[ 1. Machine Master Switch OFF, Flowmeters OFF ]
                        |
                        v
[ 2. Attach Rubber Suction Bulb to Common Gas Outlet (CGO) ]
                        |
                        v
[ 3. Squeeze Bulb repeatedly until FULLY COLLAPSED ]
                        |
                        v
[ 4. Baseline Test: All Vaporizers OFF ] ---> Bulb must remain collapsed >=10 seconds
                        |
                        v
[ 5. Sequential Testing: Turn ONE Vaporizer ON (1%) ]
                        |
                        +---> Bulb remains collapsed >=10 sec? ---> Vaporizer PASSED
                        |
                        +---> Bulb RE-EXPANDS within 10 sec?   ---> Vaporizer LEAK DETECTED!
                                                                    (Quarantine & Replace)
                        |
                        v
[ 6. Turn Vaporizer OFF; Repeat Step 5 for each adjacent Vaporizer ]

Step-by-Step Negative-Pressure Test Sequence:

1. Power Down: Turn the machine master switch OFF and ensure all needle flowmeter control valves are turned OFF.
2. Connect Bulb: Attach a compliant, spring-loaded rubber suction bulb (equipped with an internal one-way check valve) to the Common Gas Outlet (CGO).
3. Evacuate the Circuit: Squeeze the suction bulb repeatedly until it is completely flat and collapsed, creating a vacuum in the low-pressure system. The negative pressure pulls the machine's internal check valve open, exposing the low-pressure piping to the vacuum.
4. Baseline Manifold Test: With all vaporizers turned OFF, observe the bulb. The bulb must remain completely collapsed for at least 10 seconds. If the bulb reinflates, a baseline leak exists in the flowmeter manifold or CGO plumbing.
5. Individual Vaporizer Testing:
   • Turn the concentration dial of the first vaporizer (e.g., Isoflurane) ON to a low setting (e.g., 1%). This opens the vaporizer's internal rotary valves and connects its vaporizing chamber to the test vacuum.
   • Squeeze the bulb flat again.
   • The bulb must remain fully collapsed for at least 10 seconds.
   • If the bulb reinflates within 10 seconds, an internal leak is present in that specific vaporizer (e.g., loose liquid filler cap, degraded O-ring seal, or leaking internal chamber). The vaporizer must be turned off, tagged, quarantined, and removed from clinical service.
   • Turn that vaporizer OFF.
6. Test Adjacent Vaporizers: Repeat the identical procedure for every vaporizer mounted on the Selectatec manifold (e.g., Sevoflurane, Desflurane).

---

Automated Electronic Self-Tests on Modern Workstations

Modern microprocessor-controlled workstations (e.g., GE Aisys CS2, Avance, and Dräger Apollo/Perseus) execute sophisticated automated self-tests during morning boot-up. These diagnostic routines last 3 to 8 minutes and utilize internal pressure transducers and closed proportional valves to automatically evaluate:

• Internal electronic gas mixer integrity and proportional valve calibration.
• Internal breathing system compliance factor (C(circuit)).
• Ventilator piston or bellows drive sealing and spill valve seating.
• Airway pressure transducer zeroing and battery backup capacity.

Critical Technologist Responsibilities During Automated Checks

While automated self-tests are highly reliable, the technologist must remember: the computer CANNOT inspect external mechanical assemblies. The Cer.A.T.T. must manually verify the following items that bypass automated self-testing:

1. Vaporizer Mounting & Locking: Physically grasp each vaporizer and verify that the Selectatec locking lever is fully engaged in the locked position, preventing dislodgement during case turnover.
2. Vaporizer Liquid Levels & Filler Caps: Visually inspect sight glasses to confirm adequate liquid agent (between MIN and MAX lines). Ensure screw-cap or Quik-Fil filler ports are tightened securely.
3. Waste Gas Scavenging Interface: Inspect the active scavenging flowmeter float; ensure it is adjusted and bobbing stably within the green operating zone.
4. Suction Regulator & Canister: Occlude the suction tubing and confirm the vacuum regulator produces suction strong enough to clear the airway. Verify canister has a clean hydrophobic filter and liner.
5. Carbon Dioxide Absorbent: Visually inspect the sodalime or calcium hydroxide canister. Verify that absorbent granules are not exhausted (by indicator color and inspired CO2 on capnography), not desiccated or channeled, and that the canister locking lever is fully sealed.
6. Backup Manual Resuscitator (Ambu Bag): Physically confirm that a dedicated, functional self-inflating manual resuscitator bag and mask are mounted at the workstation with access to an independent oxygen supply.

---

Breathing System Pre-Use Tests (Circle System & Valves)

Before connecting any patient to an anesthesia workstation, the circle breathing system must undergo rigorous positive-pressure leak testing and unidirectional valve flutter inspection.

1. Positive-Pressure Circle System Leak Test

This test evaluates the integrity of the patient breathing tubes, Y-piece, bacterial filter, heat and moisture exchanger (HME), reservoir breathing bag, and APL valve.

POSITIVE-PRESSURE CIRCLE LEAK TEST ALGORITHM:

[ 1. Assemble Full Circuit (Tubes, Bag, Y-piece, Filter) ]
                          |
                          v
[ 2. Occlude Y-piece tightly on Machine Test Plug ]
                          |
                          v
[ 3. Rotate APL Valve fully clockwise to CLOSED (70 cmH2O) ]
                          |
                          v
[ 4. Pressurize Circuit to 30 cmH2O using Oxygen Flush Button ]
                          |
                          v
[ 5. Observe Airway Pressure Gauge for 10–15 Seconds ]
        |
        +---> Pressure holds rock-steady at 30 cmH2O? ---> PASSED
        |
        +---> Pressure drops rapidly? ---> LEAK DETECTED!
              (Check Y-piece occlusion, APL tightness, bag mount, hose splits)

• Procedure:
  1. Attach the complete patient circuit and reservoir bag.
  2. Occlude the patient Y-piece tightly using the red rubber leak test port on the machine frame.
  3. Close the Adjustable Pressure Limiting (APL) valve completely by rotating it clockwise to 70 cmH2O.
  4. Depress the oxygen flush button until the airway pressure manometer indicates 30 cmH2O.
  5. Release the flush button and observe the gauge for 10 to 15 seconds.
• Standard: The pressure must hold at 30 cmH2O for at least 10 seconds. If pressure plummets, tighten the reservoir bag connection, inspect corrugated hoses for tears, ensure the water trap is sealed, and check the CO₂ absorber gasket.

2. APL Valve Opening Functional Test

• With the breathing circuit still pressurized to 30 cmH2O from the preceding leak test, rotate the APL valve knob fully counterclockwise to the MIN (open) position.
• Expected Response: The circuit pressure must immediately drop to zero (or baseline PEEP), and gas must vent freely into the scavenging system.
• Failure Mode: If pressure remains locked at 30 cmH2O, the APL valve is mechanically jammed or obstructed in the closed position. Using this machine would expose a spontaneously breathing or manually ventilated patient to lethal pulmonary barotrauma.

3. Unidirectional Flutter Valve Inspection

The circle breathing system relies on two unidirectional flutter valves (inspiratory and expiratory) to enforce one-way circular airflow and prevent rebreathing of exhaled carbon dioxide.

• Visual & Functional Inspection:
  • Remove the Y-piece from the test plug and attach a secondary test lung or reservoir bag to the patient connection.
  • Squeeze the breathing bag to simulate manual ventilation, and cycle the mechanical ventilator.
  • Look directly through the clear plastic domes housing the valve discs.
  • Inspiratory Valve: Must rise briskly off its seat during inspiration and seat completely flat during expiration.
  • Expiratory Valve: Must rise briskly during expiration and seat completely flat during inspiration.
• Clinical Failure Mode (The Incompetent Flutter Valve): If moisture condensation, dried blood, mucus, or warping causes either valve disc to stick in the open position, gas will flow bi-directionally. During inspiration, the patient re-inspires previously exhaled gas from the expiratory limb that has bypassed the carbon dioxide absorber! The multigas monitor will immediately show elevated inspired carbon dioxide (FiCO₂ > 0 mmHg) and progressive patient respiratory acidosis that cannot be corrected by increasing minute ventilation.