Eliminating ultrasonic interference from respiratory muscle EMG

Fine wire recordings of the respiratory muscle electromyogram are often employed to represent muscle activity, and recently ultrasound-sonomicrometry has become a common method of measuring length of respiratory muscles in both acute and chronic preparations. Although recording both EMG and sonomicrometry simultaneously has become standard practice, there has not been any consideration of the potential confounding influence of ultrasound noise upon the recorded EMG spectrum. Activation of the sonomicrometry-ultrasound tranducer introduces a high frequency, high amplitude voltage pulse plus harmonics, which can contaminate the EMG spectrum directly, as well as through aliasing when EMG is sampled directly digitally. We describe the use of a new, combined, wing stabilized sonomicrometry- and EMG measurement transducer to characterize exactly the influence of ultrasound upon the crural diaphragm EMG spectrum, and the development of digital filtering techniques which effectively eliminate the ultrasound interference. Two alternative methods of avoiding ultrasound-EMG interference are also considered. The isolation and elimination of ultrasound-sonomicrometry signal interference may be important in studies where EMG and length are measured together.