Tidal volume (VT) calculation

To smooth data, an exponential moving average filter was used to reduce random noise. Respiratory parameters are calculated based on the monitored respiratory signal. This signal is obtained by analyzing the morphological changes in the chest wall. The 1D signal is obtained by calculating the differences between the spatial average of the depth values in the image at a given time and the spatial average of the depth values of a reference image in given ROI defined on the torso of the patient. The volume-time curve computed by estimating the volume of the ROI as a function of time is obtained as:

wherein D(k) is the measure of average depth variation estimated for k, the image of the acquisition sample, and S is a parameter representing the surface of the ROI quantified in mm², and the multiplicative factor 10³ is the conversion from a volume in mm³ to a volume quantified in mL.

The volume time curve presents a peak for each inhalation and a valley for each exhalation of the patient. Therefore, the tidal volume which is the volume of air passing in or out the lungs in a cycle, is calculated by subtracting the lung volume of the corresponding extrema points. To improve accuracy and eliminate aberrant values, the recorded tidal volume value is considered as the average of all detected inspiratory peak, over a one minute recording window. The calculations were automatically and continuously performed on clinical data over a stable period of at least 10 to 20 min. acquisition per patient (i.e. 10 to 20 recorded values per patient).