Heated Wires Can Melt Disposable Breathing Circuits
Hazard [Health Devices May 1989;18(5):174-5]
ECRI has learned of several incidents in which the heated wires used with
heated-wire humidifiers have melted and sometimes charred disposable breathing circuits.
Melting of the plastic tubing can occlude the breathing circuit, resulting in
hypoventilation; if charring occurs, fumes may enter the patient's lungs. Melting poses
another hazard during general anesthesia: leaking of oxygen-enriched gas creates an
oxygen-enriched atmosphere (OEA) in or near the surgical field, which increases the risk
of fire during laser surgery, electrosurgery, or electrocautery. (Melting can also occur
with heated wire controllers, although we have not received any reports of such
Use of Heated Wires
Many heated humidifiers are designed to accept an electrically insulated
heated wire, which is often used in breathing circuits to prevent condensation of the
humidified gases and enhance humidification. During heated humidification, condensation,
or "rainout," occurs because the walls of the breathing circuit are cooler than
the gas within. To prevent rainout, the heated wire is inserted into the breathing circuit
and connected to the humidifier to maintain the temperature of the walls above the dew
point. Heating of the wire occurs as electric current from the humidifier flows through
it. The resulting heat is conducted through the wire's insulation to the breathing
Disposable breathing circuits are available with the heated wire already
inserted into the circuit; these can be used with compatible heated humidifiers or
heated-wire controllers. Reusable heated wires must be manually inserted (with a draw
wire) into a reusable breathing circuit. (See our evaluation of heated humidifiers in Health
Devices 16(7):222-50, July 1987.)
Causes of Excessive Heating
Many humidifiers and controllers deliver enough power to the heated-wire
circuit to melt the breathing circuit under certain circumstances. For example, excessive
heating can occur if the heated wire has inadequate resistance (which increases the
current through the wire, given a voltage source) or inadequate gas flow across it (which
decreases the transfer of heat from the wire). Factors that increase the heat flux to
specific areas of the breathing circuit include local bunching of the heated wire within
the breathing circuit (i.e., because the heated wire is much longer than the breathing
circuit) or an excessive number of coils wound too close together within the wire's
insulation. Melting can also occur if the heated-wire circuit is covered by surgical
drapes, towels, or bed linens, which prevents room air from cooling the tubing.
Combinations of these factors increase the risk. Poor-quality breathing circuits with thin
walls are especially susceptible to melting.
Preventing Excessive Heating and Gas Leaks
The operator's manual of the heated humidifier should specify the gas flow
necessary to maintain sufficient heat transfer from the wire. However, because many of the
factors that can cause melting are difficult for the user to control, proper monitoring is
critical for detecting loss of breathing circuit integrity.
Use of an electronic exhaled-volume monitor is the best way to detect
breathing circuit leaks or occlusion. The monitor should be set to alarm if a specified
tidal or minute volume is not achieved. However, exhaled-volume monitoring may not be
feasible during neonatal and pediatric ventilation (e.g., time-cycled continuous-flow
pressure-limited ventilation, continuous-flow CPAP, or pressure-limited ventilation). In
these cases, proper use of a low-pressure alarm is critical to alert users to an occlusion
or leak in the breathing circuit. Remember, however, that low-pressure alarms are not
foolproof—pressures generated during partial occlusion or leaks may not violate the
- Alert critical care nursing, anesthesia, and
respiratory care staff to the potential for melting or charring of the
breathing circuit during heated-wire humidification.
- Consult the manufacturer or the operator's manual of
the heated humidifier or heated-wire controller to determine the minimum
flow requirement and any other precautions that will decrease the likelihood
of melting the breathing circuit.
- Ensure that the heated wire is not bunched and is
strung evenly along the length of the breathing circuit.
- When feasible, use an electronic exhaled-volume
monitor to warn of a leaking or occluded breathing circuit during anesthesia
- Set a low-pressure alarm to warn of a leaking or occluded breathing
circuit. As a general guideline, set the low-pressure alarm 5 cm H2O below the peak inspiratory
pressure. Because low-pressure alarms are not foolproof, do not let them
substitute for vigilant monitoring of breathing circuit integrity.
- Never cover the heated-wire circuit with towels,
drapes, or linens.
- Never use reusable wires with disposable breathing circuits.
- Breathing Circuits, Anesthesia [10-139]
- Breathing Circuits, Ventilator [15-003]
- Humidifiers, Heated [12-050]
Cause of Device-Related Incident
Device factors: Design/labeling error; Random component failure
User errors: Abuse of device; Inappropriate reliance on an automated feature
Mechanism of Injury or Death
Failure to deliver therapy; Fire; Suffocation