Calibration for the Hexoskin Ventilation Measurements

Background information

 

The Hexoskin breathing sensors measure the variation of a small current circulating through the respiration loops during inspiration and expiration. This variation is expressed as a value with arbitrary units (a.u.) that will increase (expiration) or decrease (inspiration) proportionally to the volume for each breath. It's important to note that our algorithms weigh the inputs of each sensor (75% thoracic, 25% abdominal) then use a conversion factor to relate the change in current to an actual volume. 

 

Here is an example of the equation used to measure Tidal Volume (TV): 

 

TV =  respiration amplitude (a.u.) * 13.28 (mL/a.u.)

The units are un mL

 

Furthermore, an adjustment is made to provide "adjusted" respiratory metrics. This is achieved using the user's weight:

 

TV 'adjusted' =  respiration amplitude (a.u.) * 13.28 (mL/a.u.) * user weight (kg) / 65 (kg) 
The units are in mL.

 

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

100 a.u. x 13.28 mL/a.u. x 50kg/65kg = 1021.54 mL = 1,02 L is the volume.

 

Please refer to this documentation for the details on how we calculate the breathing metrics. 

 

Calibration

Ventilation measurements with Hexoskin are precise for the same person but accuracy varies from one person to the other. The chest circumference can be the same for 2 users, but their lung capacity might be different. Performing calibration steps will allow you to obtain very precise breathing parameters which would need to be computed manually. 

 

To do the calibration, you will need a spirometer and your Hexoskin shirt + recording device. Here are the steps:

 

- Have the subject don the Hexoskin shirt

- Setup the spirometer on the subject's mouth. 

- Launch a recording by plugging the Hexoskin device to the shirt. 

- Have the subject take a few forced expirations. 

- Disconnect the Hexoskin device from the shirt

- Plug the device to your PC to sync the data. 

 

In the Hexoskin Binary File export, the respiration.wav file contains the combined (weighed) respiration signal, and the spirometer will provide the actual volumetric value of the breath. The custom conversion factor can now be determined to relate the arbitrary units. Our HxConvertSourceFile tool can also be used to obtain the raw breathing data as a .csv format.  

 

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