Rationale and Discussion for Proposed Algorithm
Itemized Discussion Points
(a) Call for help (the anesthesia technician, other clinicians)
At this point, for the hypoxic "O2" pipeline algorithm to be initiated, there is a low FiO2 alarm and/or a low FiO2 reading that does not match the set FiO2. The clinician should not wait for a low SpO2 alarm to respond because if the patient is pre-oxygenated before a gas mix-up occurs, it may be several minutes before SpO2 drops. The clinician should be taught to set the low FiO2 alarm close to the desired FiO2. Thus, if set FiO2 is 0.5, the low FiO2 alarm should be set at 0.4 or 0.45, not at 0.21. The clinician should trust the oxygen sensor and SpO2 monitor and not waste precious time verifying whether the O2 sensor reads correctly in room air (this should have been already done anyway during step 9a of the 1993 FDA pre-use check). Help should be called because additional hands and brains are helpful in managing a problem.
(b) Disconnect the patient from the circuit at the elbow connector with the gas sampling line still attached to the elbow connector
The primary responsibility of the clinician is to the patient, not debugging the suspected fault with the machine. Therefore, isolate the patient from the machine as soon as a machine malfunction is suspected. If the Y-piece needs to be closed off to debug the machine, use a 3-L reservoir bag as the "patient". The gas sampling line should be left on the circuit when disconnecting from the patient so that in step (f) of the proposed algorithm, FiO2 can be monitored while flushing with O2.
(c) Ventilate (or ask for assistance to manually ventilate) with a self-inflating resuscitation bag (SIRB or "Ambu" bag) using room air. If the patient needs O2-enriched air, use O2 from a stand-alone O2 cylinder.
Manual ventilation with a self inflating resuscitation bag (SIRB or "Ambu" bag) with room air containing 21% O2 is better than continued mechanical or manual ventilation (using the 3 L reservoir bag) with a hypoxic gas mixture when the objective is to re-establish oxygenation and a normal SpO2. If the patient requires O2-enriched air during ventilation with an SIRB or Mapleson system, do not use supplemental O2 from an auxiliary O2 flow meter or common gas outlet installed on the anesthesia machine because these auxiliary O2 outlets will also supply hypoxic gas if the O2 pipeline is still connected to the anesthesia machine.
(d) Disconnect O2 pipeline - audible alarm should sound confirming O2 pipeline disconnection
The O2 pipeline has to be disconnected for O2 to flow from the reserve O2 cylinder. This step is in anticipation of step (e). O2 pipeline disconnection precedes opening the O2 cylinder so that hypoxic gas inflow into the machine is interrupted as soon as possible. Also, disconnecting the O2 pipeline before opening the cylinder will, in most anesthesia machines, generate an audible alarm that provides a confirmatory cue to the clinician that the correct pipeline (O2) has been disconnected. In the heat of the action, it may be possible that a clinician disconnects the N2O pipeline instead of the O2 pipeline such that opening the O2 cylinder in the ensuing step does not help. If the O2 cylinder is opened first, there will be no audible alarm when the O2 pipeline is subsequently disconnected. The clinician may check that the O2 pipeline pressure gauge reads zero to verify that the O2 pipeline is disconnected.
(e) Open the O2 cylinder - audible sound should indicate that O2 cylinder is open and not empty
The O2 cylinder is opened to provide access to the reserve O2 stored in the O2 cylinder. An audible signal will be generated in most anesthesia machines to provide a confirmatory cue to the clinician that the O2 cylinder has been opened and the anesthesia machine is now being supplied with O2 from the O2 cylinder. If the O2 cylinder had been opened before the O2 pipeline was disconnected, there would be no audible confirmatory cue that O2 is flowing from the O2 cylinder. The clinician may also check that the O2 cylinder pressure gauge reads more than zero to verify that the O2 cylinder is not empty.
(f) Press the oxygen flush button until the O2 analyzer displays rapidly increasing FiO2 values
The hypoxic gas in the anesthesia machine piping and breathing circuit needs to be flushed out in anticipation of reconnecting the patient to the anesthesia machine. If the gas in the opened O2 cylinder is O2, then FiO2 will increase as the gas from the O2 cylinder replaces the hypoxic gas. A circuit disconnected at the Y-piece also provides for faster flushing of the hypoxic gas from the circuit. An increase in FiO2 when using gas from the O2 cylinder confirms that that gas supplied by the O2 pipeline is hypoxic. Now that the gas in the O2 cylinder is confirmed to be O2, if the patient requires O2-enriched air during ventilation with an SIRB or Mapleson system and a stand-alone O2 cylinder is not available, supplemental O2 from an auxiliary O2 flow meter or common gas outlet installed on the anesthesia machine may now be used if the O2 pipeline remains disconnected from the anesthesia machine. The plumbing leading up to auxiliary O2 outlets may be flushed of residual hypoxic gas by opening them momentarily prior to connection to the SIRB or Mapleson system. If time allows, a further confirmatory test may be performed by verifying that the measured FiO2 corresponds to the flow meter settings or set FiO2. Fuel (galvanic) cell O2 sensors are common in anesthesia machines and have a slow response time compared to the faster paramagnetic O2 analyzers. The user should be patient when waiting for the FiO2 to increase with these slow O2 sensors.
(g) Alert all other locations supplied by the same central oxygen supply
Simultaneous patient deaths in multiple anesthetizing locations supplied by the same oxygen central supply have been reported. Early detection in one room and prompt communication of the problem to other sites using oxygen from the same O2 central supply may save lives and prevent multiple deaths. This proposed algorithm points out the need to develop procedures for alerting all locations (not limited to anesthetizing locations) in a hospital or institution supplied by a given O2 central supply system as well as immediately informing the O2 manufacturer so that other institutions supplied by the same manufacturer or plant may be warned in a timely manner.
(h) Reconnect breathing circuit to patient, resume ventilation (mechanical or spontaneous) using O2 from the O2 cylinder and order more O2 tanks
Volatile anesthetics cannot be safely delivered while ventilating with an SIRB or Mapleson system. If the surgical case is still in progress, reconnect the patient to the machine supplied with O2 from the O2 cylinder and resume inhalational anesthesia.
(i) If extra O2 cylinders are not readily available and patient was previously being mechanically ventilated, ventilate manually using the reservoir bag to save the oxygen used as ventilator drive gas
As discussed above, O2 makes up all or part of the drive gas for gas-driven anesthesia ventilators. To eliminate the oxygen consumed by gas-driven anesthesia ventilators and make the finite amount of gas in a back-up O2 E-cylinder (660 L when full at 2,000 psig) last longer, the clinician should resort to manual ventilation if the expected case duration is close to or exceeds the calculated time to exhaustion of the O2 E-cylinder. The time to exhaustion is calculated by dividing the residual O2 volume in the cylinder by the rate of consumption of O2. Residual volume in liters (L) in an E-cylinder is calculated by dividing the cylinder pressure in psig by 2000 psig and multiplying by 660 L. For example, 1,000 psig represents (1000/2000) * 660 = 330 L. The rate of consumption of O2 during mechanical ventilation is the sum of the O2 flow meter setting and the minute ventilation (tidal volume in L x respiratory rate in breaths/min). For example, if FGF is 0.5 L/min O2 and 1.0 L/min N2O and VT is 0.7 L and RR is 10 bpm, then the minute volume is 7 L/min and the total O2 consumption is 7.5 L/min. The expected time to exhaustion is thus approximately 330 L/7.5 L/min = 44 minutes, ignoring the gas sampled by the gas analyzer and leaks.
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