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) and 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 in 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.
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.
100 a.u. x 13.28 mL/a.u. x 50kg/65kg = 1021.54 mL = 1.02 L is the volume.
Please note that the measurement of minute ventilation by Hexoskin is calculated on the last 7 breath cycles (refer to this documentation under minuteventilation) as for the tidal volume, it corresponds to the volume of inspired air during the last inspiration (refer to this documentation under tidalvolume).
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 + Hexoskin Device. Here are the steps:
- Have the subject don the Hexoskin Shirt
- Launch a recording by plugging the Hexoskin Device into the Shirt.
- Set up the spirometer on the subject's mouth.
- Have the subject take a few forced expirations.
- Disconnect the Hexoskin Device from the Shirt
- Plug the Device to your computer 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 to the arbitrary units. Our HxConvertSourceFile tool can also be used to obtain the raw breathing data in 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 the user puts on their Hexoskin Shirt.