Measurement of ventilatory frequency in unrestrained rodents using microwave radiation

A novel technique for remote determination of breathing frequency in unrestrained rodents using microwave radiation is described. Single mice were placed inside a rectangular waveguide operating at 2450 MHz. Because mice efficiently absorb radio frequency energy at 2450 MHz, any change in their absorption, as occurs rhythmically with breathing, can be detected by monitoring the changes in power transmitted through the waveguide. When the volume of the mouse increases during inspiration, transmitted power through the waveguide momentarily decreases — during expiration the reverse takes place. By differential amplification of the analog output of the power meter used to monitor transmitted power, breathing frequency can be easily recorded on conventional recording equipment. The microwave technique has a definite advantage over other methods in that breathing frequency can be remotely monitored without the need to attach wire leads or surgically implant telemetry probes. This greatly reduces unnecessary stress to the animal. By use of larger waveguides the principle of this technique may be applied to larger species.