Disconnect monitors

Published on 07/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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Disconnect monitors

Glenn E. Woodworth, MD

A review of studies looking at closed-claims databases indicates that the percentage of total claims related to the gas-delivery system has been steadily decreasing over the past few decades—from 3% in the 1970s to 2% in the 1980s, and representing only 1% of total claims in the 1990s. In the most recent closed claims update on patient injuries from anesthesia gas delivery equipment from 2000 through 2011, there were no claims for breathing circuit disconnects. In addition, the morbidity rate from these claims appears to be decreasing as well. However, major anesthesia-related morbidity and mortality risk is still often related to problems with the patient’s airway and ventilation, including problems with the breathing circuit and, in particular, disconnections of the circuit. Therefore, anesthesia providers must be ever vigilant to prevent and detect breathing-circuit problems. One closed-claims analysis indicated that 78% of breathing-circuit problems were deemed preventable by appropriate monitoring.

One of the most common critical incidents with gas-delivery systems is disconnection of the breathing circuit during mechanical ventilation—the most common disconnection sites are between the breathing circuit and the tracheal tube connection and between the breathing circuit and the heat-moisture exchanger, if one is used. Of note, cost-containment measures often advocate reusing breathing-circuit components; however, sterilization procedures may degrade conical plastic fittings, making them more likely to disconnect.

Risk-reduction measures have focused on three general areas: (1) secure locking of mated components (several devices are available; however, their use increases cost, may inhibit quick disconnection if an emergency disconnection is warranted, and may be undesirable if their use leads to an increased risk of an accidental extubation or barotrauma), (2) education, and (3) the use of disconnect monitors and alarms for detection of disconnects.

Disconnects cannot be completely prevented; therefore, monitoring for such an event is essential. Disconnect alarms can be classified into four categories (Box 12-1).

Box 12-1   Types of Disconnect Alarms

Pressure monitors

Respiratory volume monitors

CO2 monitors

Miscellaneous monitors

Low airway pressure alarms

The American Society of Anesthesiologists and the American Association of Nurse Anesthetists recommend that patients being mechanically ventilated have a monitor in the expiratory line of their breathing circuit that is activated if the airway pressure falls below a set value. Called a low-pressure (or disconnect) alarm, the device triggers if the maximum inspiratory pressure does not exceed a set threshold within a predetermined time (usually 15 seconds). This aspect of the pressure monitor is not enabled during spontaneous respiration.

Certain conditions can cause a disconnection, yet can fail to trigger the low-pressure monitor. One potential circumstance that is especially likely is a disconnection of the tracheal tube itself from the 15-mm connector (a site of high resistance). A similar situation occurs if the disconnection is partially obstructed and generates a resistance, for example, if the circuit is disconnected at the Y-piece and is obstructed by bed sheets. The high resistance in these situations creates upstream pressure that may fall above the alarm threshold.

Factors that influence pressure-alarm effectiveness include the disconnection site, pressure-sensor site, threshold alarm limits, inspiratory flow rate, and resistance within the disconnected circuit. Once the ventilator has been activated, the airway pressures should be verified to be within expected limits. The low-pressure alarm should be activated (most modern ventilators activate the low-pressure monitor when the ventilator is turned on and deactivate the monitor when the ventilator is turned off, whereas the high-pressure alarm is always on).

Most modern ventilators set the low-pressure threshold to a factory default or to the last setting entered by the user. It is absolutely essential that the low-threshold pressure and high-pressure alarm be set approximately 5 cm of H2O below and 5 cm of H2O above the peak inspiratory pressure, respectively. If the low-pressure alarm threshold is set too low, the alarm may fail to detect a disconnection. Finally, the anesthesia provider must make sure that the audible alarm can be heard over ambient room noise.

In addition to disconnections of the breathing circuit, a low-pressure alarm may signify other problems in the breathing circuit or anesthesia machine that can lead to problems with ventilation (e.g., subatmospheric, high, or continuing pressure conditions). The optimal location of the pressure monitor is at the Y-piece; however, condensation of water vapor may interfere with the monitor, and many systems place the sensor just proximal to the inspiratory valve to minimize moisture within the sensing line and to provide ready access.

Respiratory volume monitors

The respiratory volume monitor is normally located in the expiratory limb of the breathing circuit. Its function is to measure the exhaled tidal volume, respiratory rate, minute volume, and flow direction. Several different types of volume meters are available. Many include electronic analysis and alarms under conditions of low rate, low volume, or reverse flow and provide a graphic display of flow and volumes during the respiratory cycle. These monitors are useful during spontaneous respiration as well as mechanical ventilation.

Like pressure alarms, volume alarms can fail to signal during a disconnect. Failure to signal is most likely to occur when using a ventilator with bellows that descend during exhalation (e.g., descending or hanging bellows). During expiration, in the presence of a disconnect, descent of the bellows due to gravity may entrain room air and generate a flow within the circuit. A false-negative alarm may also occur even if the breathing circuit is totally obstructed. The compliance of the breathing circuit and compression of gas during an inspiratory cycle may yield an “exhaled volume” that can exceed alarm limits. In addition, during spontaneous respiration, a disconnect on the machine side of either directional valve will go undetected, depending on the location of the volume meter. A frequent criticism of these monitors is that flow in the expiratory limb does not guarantee gas exchange.

Miscellaneous monitors

The esophageal stethoscope, with which breath sounds should be monitored continuously, is an excellent monitor for detecting disconnects. A disconnect can be picked up immediately. Observation of chest wall excursion may also be helpful.

Ascending bellows will often fail to refill completely when there is a disconnect because they utilize expired gas to help refill the bellows (this would usually trigger the low-pressure or volume alarm because ensuing breaths with an inadequately filled bellows would not deliver a complete tidal volume).

Pulse oximeters offer a late warning of disconnections that lead to hypoxemia. Blood pressure and heart rate changes may indicate a disconnection. Hypoxemia typically produces increases in heart rate and blood pressure as hypoxemia sets in. When the hypoxemia becomes severe, the initial “pressor response” is followed by bradycardia and hypotension. Occasionally, an oxygen analyzer will detect oxygen concentrations within the breathing circuit that are lower than expected in the presence of a disconnect.

In summary, alarm monitors with properly set threshold limits should be enabled and functioning. Many experts would testify that it is essential to do this. Finally, even with all of the currently available monitors in use, under certain circumstances, a disconnect or partial disconnect may be missed.

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