Hazard [Health Devices May 1989;18(5):174-5]

Problem

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 occurrences.)

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 circuit.

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.)

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.

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 alarm limit.

1. Alert critical care nursing, anesthesia, and respiratory care staff to the potential for melting or charring of the breathing circuit during heated-wire humidification.
2. 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.
3. Ensure that the heated wire is not bunched and is strung evenly along the length of the breathing circuit.
4. When feasible, use an electronic exhaled-volume monitor to warn of a leaking or occluded breathing circuit during anesthesia or ventilation.
5. 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.
6. Never cover the heated-wire circuit with towels, drapes, or linens.
7. Never use reusable wires with disposable breathing circuits.

UMDNS Terms

• Breathing Circuits, Anesthesia [10-139]
• Breathing Circuits, Ventilator [15-003]
• Humidifiers, Heated [12-050]

Device factors: Design/labeling error; Random component failure

User errors: Abuse of device; Inappropriate reliance on an automated feature

Failure to deliver therapy; Fire; Suffocation