The measurement of oesophageal pressure (Poes) allows for the calculation of several important parameters of respiratory mechanics, such as: the work of breathing, respiratory muscle fatigue, airways resistance, etc. To ensure that these parameters are quantified with adequate precision during exercise, it is recommended that Poes catheters display a “flat” frequency response up to 15 Hz. In our experience, however, we have observed that some commercially-available systems display comparatively poor frequency response characteristics (i.e., < 8 Hz).
PURPOSE: We explored whether the poor frequency response of a commercially-available Poes catheter may be adequately compensated via two numerical methods of digital signal compensation.
METHODS: The commercial balloon-catheter used in this report was that manufactured by Akrad Laboratories (CooperSurgical, Trumbull, CT). The dynamic response of the commercial Poes catheter was obtained via pressure “step” testing. A total of 10 step responses were recorded and ensemble-averaged. The numerical correction methods used to compensate the dynamic response of the commercial balloon-catheter were: 1) a double-exponential model method; and 2) a Fourier-based method called Wiener deconvolution. The frequency responses of the uncorrected, and corrected balloon-catheter systems were considered “flat” up until the discrete frequency beyond which more than 5% amplitude and/or phase distortion was observed.
RESULTS: The frequency response of the uncorrected Poes catheter was “flat” up to only 7 Hz. Double-exponential correction notably extended this flat-region to 20 Hz. The greatest improvement in the catheter’s frequency response was observed using Wiener deconvolution - this correction method extended the “flat” region of the catheter’s frequency response to 58 Hz.
CONCLUSIONS: The present report indicates that, if not corrected, the dynamic response of the commercial balloon-catheter is inadequate for recording Poes during exercise. Importantly, however, the frequency response of this balloon-catheter may be extended beyond that recommended (i.e., 15 Hz) using either the double-exponential correction method or Wiener deconvolution - whereby superior results are obtained with the latter method.