Calibration for the Hexoskin ventilation measurements

It is important to understand that Hexoskin respiration loops calculate the movement of the rib cage during the inspiration and expiration events. Therefore, the raw respiration data are measured in arbitrary units.

The minute ventilation adjusted, for example, is based on this formula (that convert arbitrary units in volume):
volume difference x 13.28 x (weight / 65)
The units are in mL.

For example, if your volume difference is 100 (i.e. 59800-59700= 100) when you exhale and for a person of 50 kg,

100 x 13.28 x (50/65) = 1021.54 mL = 1,02 L is the volume.

 

Please note that the minute ventilation measure by Hexoskin is calculated over the last 7 respiration cycles (refer to this documentation under minuteventilation) as for the tidal volume, it corresponds to the volume of air inspired in the last inspiration (refer to this documentation under tidalvolume).

However, it is important to know that ventilation measurements with Hexoskin are precise for the same person but accuracy varies from one person to the other. Thus, the algorithm needs calibration to provide accuracy and a calibration step needs to be done for every Hexoskin user as the chest circumference can be the same for 2 users, but their lung capacity might be different.

 

To do the calibration step, you need a spirometer. We suggest you using a spirometer to measure a forced expiration that will be measured simultaneously with the Hexoskin, then calculate the amplitude of the expiration curve from the Hexoskin raw data. The volume measured with the spirometer will correspond to the amplitude of the expiratory curve. You will then be able to calculate the factor of conversion to change in volume measured by Hexoskin into an absolute value. Please note that if you are comparing the absolute value of lung volume over different recording sessions, it is important to perform the calibration step every time that the user is putting on their Hexoskin shirt.

 

Keywords: ventilatory measurements, minuteventilation, tidalvolume, RIP, AU, respiratory curves, respiratory inductance plethysmography, vol, AU