Can I use Arithmetic to calculate Tidal Volume or Minute Ventilation from a flow signal?

We do offer a Spirometry add-on that can be installed to Chart 5 or LabChart 6 and higher for free. After installation the following channel calculations will be available:

  • Spirometry Flow – defines the flow data input source and allows flow head calibration.

  • Spirometry Volume – calculates the volume of air that is inhaled and exhaled. The volume V is obtained by integrating the flow signal F, so that V = ? F dt.

  • Spirometry Minute Ventilation – calculates expired flow in a specified interval of time. The calculation requires a source channel containing flow data, and reports flow (VE) in the expiratory direction only, over a time interval, in units of L/min.

  • Spirometry Tidal Volume – calculates expired volume of a breath, on a breath-by-breath basis. The tidal volume VT is obtained by identifying zero-crossings in the signal, and then integrating the flow signal F for when flow is negative.


Based on the above description, and after setting up the flow channel (here Ch1) to L/s, you can use the following arithmetic formulas to obtain the same result (see also attached data file):


Volume:

Integrate(smoothsec(Ch1,0.3),“cycle“)

Please note, here the flow channel has been smoothed by smoothsec(Ch1, 0.3) before being integrated to remove noise. Especially noise around the zero flow crossing may lead to wrong results. Choose the unit L.


Minute Ventilation: 

EventInterp(CMHeight(IntegrateNeg(Ch1,“timed“,20),IntegrateNeg(Ch1,“timed“,20),0.1))*3

Plesae note, the 20 defines the time interval (20 seconds) for which the Minute Volume is calculated and displayed. Multiplying by 3 leads to the correct value for L/min. Choose the unit L/min.


Tidal Volume:

CMHeight(IntegrateNeg(Ch1,“cycle“),Ch1,0.1).

Choose the unit L.


Please use these calculations with the usual caution. Threshold or other calculations might not work out especially if the flow signal contains artifacts like coughing or hick ups etc. If unsure, please contact your nearest support specialist.