Low-Pressure Alarms for Sensing Ventilator Disconnects
Hazard [Health Devices Jul-Aug 1983;12(9-10):260-1]
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.
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.
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
- 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.
- 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