Cause of Device-Related Incident
User errors

Clinical Specialty or Hospital Department
Anesthesia; CCU / ICU / NICU; Nursing; Pulmonary / Respiratory Therapy

Device Factors
Design / labeling error

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death
Failure to deliver therapy; Monitoring failure; Suffocation

Support System Failures
*Not stated

Tampering and/or Sabotage
*Not stated

User Errors
Inappropriate reliance on an automated feature; Incorrect control settings

UMDNS
Breathing Circuits, Ventilator [15-613]; Pressure Alarms, Airway [14-351]; Ventilators [15-613]

Low-Pressure Alarms for Sensing Ventilator Disconnects



Hazard [Health Devices Jul-Aug 1983;12(9-10):260-1]

Problem

Low-pressure alarms are often used to detect the drop in peak inspiratory pressure (PIP) that occurs when a patient becomes disconnected from a ventilator. Reports from several hospitals indicate that users are unaware of the ineffectiveness of these alarms if the low-pressure alarm point is adjusted too low by the user or preset too low by the manufacturer. Failure to detect such a disconnect can be fatal to the patient.

Discussion

During a disconnect, the drop in PIP may be only a few cm H2O. High flow resistance can be produced in the patient circuit (by the circuit and artificial airways themselves or by their partial occlusion), resulting in appreciable pressure. If this flow resistance generated pressure is above the low-pressure alarm setting, the disconnect will not be sensed.

Because the devices are called low-pressure alarms, users have mistakenly set the pressure-alarm point (if adjustable) at the lowest possible setting. With some units, a single alarm limit is preset by the manufacturer at too low a point.

We recommend that users set the pressure-alarm point just below the peak inspiratory pressure (PIP). On units with multiple alarm set points, we recommend selection of the point closest to the PIP. Also, connect the unit's pressure-sensing line at the junction of the patient circuit and artificial airway. This will allow pressure to be measured as close to the lungs as possible, and the alarm unit will be less likely to be misled by the system pressure.

The low-pressure alarm level may be selectable by a continuously adjustable control or by a control that selects one of several discrete points set by the manufacturer; or the level may be a single discrete point set by the manufacturer.

By design, low-pressure alarms with a single discrete fixed pressure-alarm point are more susceptible to missing a disconnect, especially if the alarm point is preset below 10 cm H2O. One model has a low-pressure disconnect alarm that is preset at 2.5 cm H2O. Because of the high flow resistance of tracheal tube connectors used for infants, a disconnect of the tracheal tube from its connector may go undetected. Also, an infant's artificial airways may develop sufficient back pressure during a partial extubation (which could occur during transport) to prevent detection of the disconnection from the ventilator. Therefore, we recommend that a separate low-pressure alarm unit with an adjustable pressure-alarm point be used whenever the alarm limit cannot be set to just below the PIP.

Users should also note that patient-exhaled moisture can build up in the pressure-sensing line and/or the alarm unit, preventing proper operation. We suggest placing the alarm unit approximately 30 cm above the sensing site and watching for moisture buildup in the sensing line throughout ventilation. Other suggestions for ensuring proper operation are listed in the Recommendations and on the poster "Stand-Alone Low-Pressure (Disconnect) Alarms Can Be Fooled: Proper Set-Up Is Essential."

Given the inability of low-pressure alarms to alarm on all disconnects, mechanically ventilated patients should never be left unattended.

Recommendations

1.  Alert all users of low-pressure disconnect alarms to perform the following when setting up a low-pressure disconnect alarm:

  • Check that all batteries are sufficiently charged.
  • To help prevent moisture entry into the alarm unit during operation, place the unit approximately 30 cm above the pressure-sensing site and watch for moisture buildup in the sensing line during ventilation.
  • Connect the pressure-sensing line at the patient circuit connection at the artificial airway.
  • Use an alarm unit that allows selection of a pressure-alarm point just below the peak inspiratory pressure.
  • With the alarm time delay set to zero, disconnect the patient circuit from the artificial airway and confirm that an alarm occurs.

2.  Never leave mechanically ventilated patients unattended.

UMDNS Terms

  • Breathing Circuits, Ventilator [15-613]
  • Pressure Alarms, Airway [14-351]
  • Ventilators [15-613]

Cause of Device-Related Incident

User errors: Inappropriate reliance on an automated feature; Incorrect control settings

Mechanism of Injury or Death

Failure to deliver therapy; Monitoring failure; Suffocation


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