Cause of Device-Related Incident
Device factors; External factors; Support system failures; User errors

Clinical Specialty or Hospital Department
Clinical/Biomedical Engineering; Ophthalmology; OR / Surgery

Device Factors
Device interaction

Document Type
User Experience Network (UEN) reports

External Factors
Medical gas and vacuum supplies

Mechanism of Injury or Death
Failure to deliver therapy

Support System Failures
Lack of competent accident investigation

Tampering and/or Sabotage
*Not stated

User Errors
Failure to perform pre-use inspection

UMDNS
Cataract Extraction Units, Phacoemulsification [17-596]; Regulators, High-Pressure Gas [13-323]; Vitrectomy Units [14-386]

Faulty Gauge on Nitrogen Regulator Causes Decreased Aspiration during a Vitrectomy Procedure



User Experience Network™ [Health Devices Jul 1998;27(7):269-70]

Hospital

During ophthalmic surgery to perform a posterior vitrectomy, our surgical staff noticed reduced aspiration while using the vitrectomy feature of an ophthalmic surgery unit. Other than this decreased performance, the unit provided no indication of a problem. After the procedure, biomedical engineering personnel tested the unit. No problems were found, and the unit was returned to service. Reduced aspiration was encountered during a subsequent procedure and again could not be duplicated in testing.

When contacted, the supplier's technical support personnel indicated that low nitrogen supply pressure could cause performance degradation without an indication (e.g., visual display) or alarm. Subsequent inspection of the nitrogen regulator in the operating room (OR) revealed that its gauge was inaccurate and that the pressure being delivered was actually significantly lower than the pressure indicated on the gauge. The faulty regulator gauge was replaced, and the unit is now working properly.

ECRI

Some phacoemulsification and vitrectomy units use nitrogen or another compressed gas to drive the vibrating cutting element of the handpiece and to regulate aspiration levels. Each such unit, regardless of whether it is receiving wall- or tank-supplied gas, requires pressure within a specified range (e.g., 80 to 100 psi for nitrogen) to ensure proper operation. Pressure outside this range could adversely affect the cutting speed of the handpiece and/or the level of aspiration.

While some units monitor compressed gas pressure, certain of these units—such as the one involved in the reported incident—are designed to signal only an abrupt pressure drop or the absence of a compressed gas source. The alarm thresholds of such units may thus be set well below the pressures that will affect performance of the unit. This was the case in the reported incident: the pressure provided by the defective nitrogen regulator was low enough to hamper the ophthalmic surgery unit's performance, but not low enough to trigger the unit's pressure alarm. Therefore, the surgeon observed a decrease in aspiration level but received no indication or alarm identifying the cause of the problem.

Complicating the investigation of this problem was the fact that postincident testing did not use the regulator involved in the incident, but rather a different—accurate—nitrogen regulator, with which the unit operated properly. Since no regulator problem was identified in this initial testing, the unit was reconnected to the faulty regulator in the OR, and the problem reoccurred. As a general rule, if a problem cannot initially be reproduced during testing in the biomed lab, a system approach to testing should be used—ideally at the site of the incident—and should include all auxiliary devices and power sources present during the incident.

The low-pressure alarm design described above is not unique to the reported model; other units also will alarm only when a pressure loss or significant pressure drop is detected. In general, ECRI believes that alarms should provide a warning to the user before any degradation in performance occurs (although they should allow the surgeon to continue the procedure, provided that the ophthalmic surgical unit remains safe to operate).

Users should ensure that gas supply levels remain within the specified ranges by periodically monitoring pressure before and during surgical procedures. If aspiration levels appear compromised (e.g., below the set levels), the settings and nitrogen pressure levels should be examined. While pre-use checks will not identify a faulty gauge such as the one described in this report, they will likely detect most pressure changes.

Recommendations

  1. Inform users that performance degradation can occur with some ophthalmic surgical units without an alarm or indication and that this degradation may be caused by a change in the gas supply pressure levels.
  2. Review the operator's manual of the ophthalmic surgical unit for the recommended nitrogen pressure, and instruct OR staff to ensure that the pressure delivered to the unit remains within the specified range. (Note: This will not identify a faulty gauge such as the one described in this report.)
  3. Add pressure regulators located in critical care locations (e.g., ORs) to your inspection and preventive maintenance (IPM) program, if they are not already included. Confirm that appropriate pressures are available during scheduled inspections of the medical gas system and when using regulators with compressed gas cylinders. For additional information, see Suction Regulators (Procedure/Checklist 459-0595) in ECRI's Health Devices IPM System. Also refer to the Medical Gas/Vacuum Systems procedure (440-0595) in that same publication for recommended pressures and flows.

UMDNS Terms

  • Cataract Extraction Units, Phacoemulsification [17-596]
  • Regulators, High-Pressure Gas [13-323]
  • Vitrectomy Units [14-386]
  • Cause of Device-Related Incident

    Device factor: Device interaction

    User error: Failure to perform pre-use inspection

    External factor: Medical gas and vacuum supplies

    Support system failure: Lack of competent accident investigation

    Mechanism of Injury or Death

    Failure to deliver therapy


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