Machine Malfunction: Loss of Pipeline Oxygen and Stuck Expiratory Valve

Oxygen Supply Sources

Pipeline Inlets

• Most clinical facilities where anesthesia machines are used have a central oxygen supply that delivers oxygen from the source to the operating room (OR) via a tubing or pipeline system.^2,3
• The hosing used for oxygen delivery can be distinguished from that used for the administration of other gases, e.g., nitrous oxide and air, by distinct, noninterchangeable diameter index safety system (DISS) connections or quick couplers (Figure 1A).^2,3,6
  • Two other pipelines may be seen connected to the back of the anesthesia workstation: a hose for suctioning fluids by the surgical team (medical-surgical vacuum) and a hose for waste anesthetic gas disposal for scavenging.^4,5
• Gases flow to the anesthesia machine through the pipeline inlets at a pressure of 50 to 55 pounds per square inch gauge (psi), 344.75 to 379.225 kilopascals (kPa), or 3.45 to 3.795 bar.^2,3,4

E-Cylinder Inlets

• At least one oxygen E-cylinder must always remain attached to the anesthesia machine as a backup oxygen source in the event of pipeline failure or disruption to the central supply.^1,2,3
• The safety feature of each tank’s noninterchangeable connections by way of the pin index safety system is nearly analogous to the DISS used for pipeline gas supplies, except for a hanger yoke assembly method in lieu of colored tubing (Figure 1B).^2,6
• The pressure inside the full oxygen cylinder ranges from 1900-2200 psi (13,100.5-15,169 kPa or 131.1-151.8 bar), about forty-fold higher than the pipeline oxygen pressure of 50-55 psi. Most machines have a high-pressure regulator, which reduces the cylinder pressure to 45-47 psi (310.275-324.065 kPa or 3.105-3.243 bar). This lower pressure allows preferential flow of oxygen from the pipeline, unless the pipeline pressure falls below 45 psi.^2,3,8
• An oxygen tank contains 600-660 L of oxygen when full. Following Boyle’s law, the amount of oxygen available for delivery to a patient is inversely proportional to the cylinder oxygen pressure. By monitoring the oxygen pressure, the remaining volume available can be calculated. For an oxygen E-cylinder:
  • Volume oxygen (L) = 0.3 x Pressure oxygen (psi)
• As mentioned above, high-pressure regulators allow the anesthesia machine to draw from the pipeline gas supply before utilizing the cylinder supplies unless the pressure in the pipeline drops to less than 45 psi; thus, it is imperative to ensure that the E-cylinders are closed while the machine is in use to prevent escape of backup oxygen and possible inability to oxygenate the patient in an emergency.
• The oxygen tank should be at least half full (at least 1000 psi) prior to use of the anesthesia machine, per the first step of the Federal Drug Administration (FDA) anesthesia machine checklist.⁷

Unidirectional Valves

• Most modern anesthesia machines use the circle breathing system, a closed apparatus that relies on the proper functioning of unidirectional valves.
• In a circle system, anesthetic gases can be safely rebreathed and conserved, given appropriate functioning and positioning of downstream scavenging and elimination elements, e.g., a carbon dioxide absorber and adjustable pressure-limiting valve (APL), respectively.^2,3,4
• During inhalation, gas flows through the inspiratory valve to the patient’s lungs while the expiratory valve remains closed. In pneumatically driven ventilators, gas from the oxygen supply enters the bellows housing, increasing the chamber pressure and compressing the bellows, allowing gas to flow through the circuit to the patient.²
• During exhalation, the expiratory valve opens, allowing gases from the patient’s lungs to be passively exhaled through the circuit and into the bellows, which ascend due to the increased pressure.²

Detection and Management of Loss of Pipeline Oxygen or Stuck Expiratory Valve

• To optimize patient safety and decrease mortality associated with the administration of anesthesia by way of the anesthesia machine, the pre-use checklist, developed by the FDA in 1993, ought to be performed by the anesthesia provider (anesthesiologist, anesthesiologist assistant, or certified registered nurse anesthetist) before its use.⁷
• However, pipeline malfunction (Figure 3) or an expiratory valve that remains “stuck” in the open or closed position (Figures 4 and 5) may occur at any point in time during use of the anesthesia machine and are often accompanied by distinct capnograph patterns.^9,10

Loss of Pipeline Oxygen

• Signs:
  • Low oxygen-supply pressure alarm
  • Low-pressure alarm
  • The fail-safe device will cut off nitrous oxide automatically if it is also being administered
  • The ventilator alarm in most pneumatic ventilators will also sound
  • Gradual or sudden decrease in peripheral oxygen saturation (SpO₂) or arterial oxygen saturation (SaO₂)
• To troubleshoot the cause of the above signs, the anesthesia provider should:
  • Communicate the situation to the OR team and call for assistance
  • Open the backup oxygen cylinder on the machine and hand-ventilate through the circle system
  • Closing the APL valve, reducing gas flows to a minimum, and discontinuing use of the pneumatically driven ventilator will help conserve oxygen.
  • Maintain anesthesia with a volatile agent if appropriate
  • Ensure adequate inspired oxygen and agent concentrations
  • Disconnect the failed pipeline from the wall and do not reuse until formally tested for composition and quality
  • Communicate with the OR manager about oxygen cylinder supplies and request additional backup cylinders
  • Complete surgery as efficiently as possible
• If SpO₂ and/or SaO₂ are decreased despite the above actions, increase the FiO₂ to 100%, assess the patient, and then investigate the following:
  • Is the pipeline tubing securely plugged into the wall outlet?
  • Are there any foreign objects obstructing flow through the pipeline?¹¹
  • Are there equal, bilateral breath sounds on pulmonary auscultation?
  • Is the endotracheal tube (ETT) insertion depth the same as previously documented?
  • Is the E-cylinder mounted and at least half full?
  • Are there kinks in the circuit?
  • Are there foreign objects in the circuit?
  • Does oxygen saturation increase after suctioning the ETT?
• If vital signs do not normalize after troubleshooting and switching to a separate oxygen E-cylinder as the oxygen supply:
  • Call for help.
  • Alert the surgical team of the patient’s change in clinical status and request a temporary pause in the procedure. This step is especially crucial in surgeries utilizing electrocautery to instrument the airway.

Stuck Expiratory Valve

• To troubleshoot the cause of the above signs, the anesthesia provider should first assess the patient and then investigate the following:
  • Is the machine switched to mechanical ventilation mode?
  • Are there equal, bilateral breath sounds on pulmonary auscultation?
  • Is the ETT insertion depth the same as previously documented?
  • Is the pilot balloon overinflated?
  • Are there kinks in the circuit?
  • Are there foreign objects in the circuit?
  • Do airway pressures decrease after suctioning the ETT?
• If vital signs do not normalize after troubleshooting:
  • Communicate the situation to the OR team and call for assistance.
  • Switch to 100% oxygen, open the APL valve, and hand ventilate. An open expiratory valve will result in loss of pressure in the circle system, while a closed expiratory valve will result in high pressure in the circle system despite hand ventilation.
  • Switch to a separate self-inflating, nonrebreathing bag with a separate oxygen source and total intravenous anesthesia while removing, cleaning, drying, and replacing the expiratory valve. A new breathing circuit is often necessary, and on occasion, the entire anesthesia machine may need to be replaced.
  • After the situation has been rectified and ventilation is confirmed to be functioning properly, normal ventilation can be resumed.