Home / Further Study / Algorithms
Last Updated 5/13/02
Observations and Consequences of a Hypoxic O2 Pipeline Condition

Itemized Discussion Points

1. N2O is supplied, instead of O2, via the O2 flow meter

Color coding of cylinders, hoses and gas connectors and Diameter Index Safety System (DISS) and Pin Index Safety System (PISS) are among the safety features designed to prevent gas mix-ups. These safety features help but are not foolproof. Behind the walls, gas piping is generally not color-coded and thread-indexed allowing accidental misconnection of pipes. Furthermore, there has been a reported instance where a bulk O2 tank was misfilled by an overzealous employee who defeated the safety mechanism. In the recent New Haven case, the pins preventing hook-up of an O2 line to an N2O outlet were allegedly missing.

2. N2O is supplied, instead of O2, when pressing the "O2 flush" valve

The O2 flush valve is supplied by gas in the "O2" pipeline if (a) the O2 pipeline is connected and (b) if the O2 pipeline is connected and the O2 cylinder is also open. The O2 flush valve is supplied by the O2 cylinder if the O2 pipeline is disconnected and the O2 cylinder is connected, open and not empty.

3. The "O2 failsafe" device is fooled and allows delivery of a hypoxic gas mixture

The "O2 failsafe" (a misnomer because it does not fail in a safe mode) failed because it relies solely on loss of pressure in the O2 pipeline to activate. If the pressure of non-O2 gas in an O2 pipeline is adequate, the "O2 failsafe" will not activate.

4. The "hypoxic guard" device (O2 proportioning device) does not prevent delivery of a hypoxic gas mixture

The "hypoxic guard device" fails because its design (mechanical or pneumatic) assumes that the gas in the O2 pipeline is oxygen.

5. N2O is used as the drive gas, instead of O2, during mechanical ventilation

The ventilator bellows drive gas is usually pure O2 or pure O2 entraining room air. The O2 making up all or part of the drive gas is supplied from (a) the O2 pipeline if connected and (b) the O2 pipeline if the O2 pipeline is connected and the O2 cylinder is also open. The ventilator drive gas is supplied by the O2 cylinder if the O2 pipeline is disconnected and the O2 cylinder is connected, open and not empty.

6. N2O, that is now the drive gas, is vented directly to the room in most anesthesia ventilators with gas driven bellows

Most gas-driven anesthesia ventilators vent their drive gas directly to ambient air. The Datex-Ohmeda 7900 ventilator is an exception and vents the bellows drive gas to the scavenging system and will therefore not vent the N20 drive gas, during an O2/N2O mix-up, to ambient air.

7. The O2 analyzer, if present, alarms when FiO2 drops below alarm threshold

The use of an O2 analyzer is not guaranteed or mandated at all locations where O2 is delivered. For example, an O2 analyzer may not be used in conscious sedation cases where the patient is generally not intubated but is instead supplied with supplemental O2 via nasal prongs. Such was apparently the case in the New Haven incident. If an O2 analyzer is not used, there will be no low FiO2 alarm to warn of the gas mix-up

8. The pulse oximeter, if present, alarms when SpO2 drops below alarm threshold

If an O2 analyzer is not in use or defective, the patient becomes the "FiO2 monitor" and a low SpO2 alarm will eventually trigger from hemoglobin desaturation. The SpO2 alarm is, in this scenario, a late indicator of a problem with oxygenation and more importantly, a non-specific indicator. The clinician may conceivably look first to the patient's physiology or condition (such as old age or advanced sickness) to diagnose the cause of the low SpO2 alarm, wasting precious minutes. This is allegedly what happened in New Haven where the first fatality on a Friday may have been attributed to old age. A younger fatality on the following Tuesday prompted a thorough investigation that found an O2 pipeline connected to an N2O outlet.

9. Initially, end-tidal CO2 will be normal if ventilation (not oxygenation) is adequate

The washout of CO2 (ventilation) is based on the minute volume (tidal volume times respiratory rate) of gas, not on the species of the gas. Therefore, if minute ventilation is adequate, there will be no ETCO2 alarm to help in early diagnosis. Eventually, there will be no O2 to support production of CO2.

10. Airway pressure will be normal if ventilation is adequate

Airway and peak airway pressures are primarily determined by the tidal volume and the lung characteristics of the patient such as resistance and compliance. The rise in airway pressure during mechanical inspiration is dependent on the tidal volume of gas, not the species of the gas, such that airway pressure will be normal during a gas mix-up.

Further Discussion

A hypoxic O2 pipeline already present at the start of a first case of the day will be detected if the clinician performs a pre-use check according to the 1993 FDA Anesthesia Apparatus Checkout Recommendations. Specifically, step 9c [Reinstall (O2) sensor in circuit and flush breathing system with O2] and step 9d [Verify that monitor now reads greater than 90%] of the 1993 FDA pre-use check recommendations will raise a warning that something is amiss and potentially help to identify the hypoxic O2 pipeline condition.

The footnote at the end of the 1993 FDA pre-use check recommendations states that "If an anesthesia provider uses the same machine in successive cases, these steps (9c and 9d included because there is an asterisk next to step 9 - authors' addition) need not be repeated or may be abbreviated after the initial checkout." Thus, if the gas in an O2 pipeline becomes hypoxic after the first case of the day, an abbreviated pre-use check per the 1993 FDA recommendations for a following case of the day may not detect the low FiO2 associated with a hypoxic O2 pipeline condition.

The 1993 FDA pre-use check also does not guard against gas in the O2 pipeline becoming hypoxic in the middle of a case. While this may appear far-fetched, we have been informed of such a case in a military hospital where the central O2 supply was actually a bank of O2 H-cylinders in the cellar. A medical orderly was tasked with monitoring the pressure gauges on the O2 cylinders and opening a new H-cylinder when the one being used was nearing exhaustion. The new "O2 cylinder" that was opened while cases were proceeding was misfilled with Argon and multiple fatalities occurred at different anesthetizing locations.

Most developed countries have guidelines for testing O2 outlets after construction or renovation and before they are used. This system too has been reported to fail. In one reported instance, the O2 outlets after the addition of 3 new ORs were correctly plumbed and passed when tested. Subsequently, the O2 outlets were repositioned and during repositioning of the outlets, the gas pipes were crossed. The O2 outlets were not tested again after the repositioning and patient fatalities occurred in different anesthetizing locations.