Magnetron-based inline microwave fluid warmer.

OBJECTIVE

To develop an in-line microwave fluid warming system that eliminates the difficulties of uneven heating that are characteristic of batch-mode microwave fluid warmers.

METHODS

Using a commercial microwave oven, we developed a method for warming fluid as it flowed through tubing along a defined path in the oven's cavity. Algorithms utilizing either proportional or adaptive control were used to control microwave heating cycles by varying the heating pulse-width during 3-second epochs. Methods of fluid entry and exit were devised to minimize microwave leakage. Heating performance was tested using icewater at multiple flow rates from 18 mL/min to 105 mL/min.

RESULTS

In all warming tests, the system achieved temperature control without exceeding the maximum temperature allowable based on American Association of Blood Banks requirements. The adaptive control maintained the set temperature, with peak-to-peak oscillations of 2 degrees C or less. Microwave leakage was below the commercially required limit for home microwave appliances.

CONCLUSIONS

The combination of proportional and adaptive control is successful in controlling the permanent magnet magnetron microwave energy to heat the icewater tested. The in-line microwave warmer has the potential to become a successful medical fluid warmer. More study is needed to determine the stability of the control system under clinical conditions, and to evaluate its utility for warming blood.