Cause of Device-Related Incident
Device factors; Support system failures

Clinical Specialty or Hospital Department
Anesthesia; Obstetrics and Gynecology; OR / Surgery

Device Factors
Device failure

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Overdose

Support System Failures
Lack or failure of incoming and pre-use inspections

Tampering and/or Sabotage
*Not stated

User Errors
Failure to perform pre-use inspection

UMDNS
Anesthesia Unit Vaporizers [10-144]; Anesthesia Units [10-134]

Pre-Use Anesthesia Check Fails to Find Faults



Hazard [Health Devices Sep 1988;17(9):274-6]

Problem

A patient suffered cardiac arrest during surgery when an excessive amount of anesthetic agent was delivered because of two malfunctions in the anesthesia unit that went undetected during pre-use check.

Discussion

The anesthesia system was equipped with both a flowmeter-controlled vaporizer and a concentration-calibrated vaporizer. When the concentration-calibrated vaporizer did not seem to produce adequate anesthesia, it was shut off and the flowmeter-controlled unit substituted. However, an excessively high concentration of agent was delivered, which was not discovered until later.

In ECRI's investigation, the primary fault was found to be a manifold disconnection (or partial disconnection) at the outlet of the calibrated vaporizer. This disconnection would ordinarily be easy to find during pre-use testing; however, a second fault, a leak of approximately 400 mL/min from the flush valve into the common outlet, masked the first fault, so that the system appeared to be working properly.

The system passed the tests used by the anesthetist because, even with complete disconnection of the manifold, the flush valve leakage was sufficient to produce up to 60 cm H2O pressure at the common outlet or in the breathing circuit during the leak test. (Note that the U.S. Food and Drug Administration [FDA] checklist tests tolerate leakages of a few hundred mL. If the FDA procedure had been used, it would have detected this problem, but only if done carefully and the results thoroughly analyzed to confirm the problem. It is not known what checklist was used by the reporting hospital.) The flow into the breathing system was also sufficient to keep the ventilator bellows filled.

The tests in ECRI's Pre-Use Checklist for Anesthesia Units were devised to locate such faults by separate leak tests for the machine and complete system. During ECRI's investigation, a leak test of the machine (ECRI test 7), excluding the breathing system, disclosed only that there was excessive leakage into the system manifold. This leakage could not be occurring through the flowmeters, which indicated no flow. An alternative source had to be found.

The second confirming test, which also explained what happened during the procedure, was checking vaporizer output. A rough test of vaporizer output using an oxygen analyzer (ECRI test 8) is described in our checklist. Had our test been used, the calibrated vaporizer output (measured at the fresh gas outlet) would have been found to be zero as the vaporizer dial was turned in steps up to 5%. However, the flowmeter-controlled vaporizer output would have been found to be excessively high because the only diluent flow would have been that portion of the flush valve leakage that went to the common outlet and breathing circuit.

Recommendations

  1. Photocopy and use the ECRI Pre-Use Checklist for Anesthesia Units.
  2. If the hospital uses the FDA checklist or similar procedures, perform the tests or their equivalents completely and carefully. Be sure that you interpret the results accurately. Add the tests for leakage in the anesthesia machine alone and for both N2O percent and vaporizer output using Tests 7 and 8 from ECRI's checklist or their equivalents.

UMDNS Terms

  • Anesthesia Unit Vaporizers [10-144]
  • Anesthesia Units [10-134]

Cause of Device-Related Incident

Device factor: Device failure

Support system failure: Lack or failure of incoming and pre-use inspections

Mechanism of Injury or Death

Overdose


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