- Thermal drift
- Initial offset drift
- Breath dependant drift
- Volume Correction
- Volume Reset
A great improvement results from a simple modification to the integration of flow, so as to reduce the expired volume:
V = ? F dt (inspiring)
V = ? F/k dt (expiring)
where k (>1.0) is a volume correction ratio. This method, however,
shallow panting breathing, in which most of the air breathed is dead-space air.
The Spirometry Extension accounts separately for dead-space air while
integrating, and uses a more elaborate correction:
V = ? F dt (inspiring, or expiring dead-space air)
V = ? F/k dt (expiring air in excess of dead-space)
Non-ideal distribution of air flow across the flow head’s mesh screen also
appears to contribute to breath-dependent drift. The required volume correction factor is
therefore usually larger than that predicted by the Gas Laws. This component of
the drift is minimized by use of the tubing and droplet filter.
In order to do a Volume correction, it would be best to have the Spirometry extension installed. Here you can do either a manual correction or an automatic correction.
You can manually set the value of the volume ratio k in the Spirometry Volume Dialog after selecting Apply volume correction. The value of the dead-space is not critical, and the default value (400 mL) is suitable in almost all cases. Other dead-space values can be entered by typing directly into the text field.
The extension can calculate a value of k automatically from a data selection. The selection should include a full expiration, a period (20–60 s) of normal breathing (which may include panting), and then another full expiration. When you display the Spirometry Volume dialog, the extension finds the minimum value of volume in the first and last thirds of the selection, and calculates a value of k that makes the values equal. It is important to breathe out fully once in the first third and once in the last third of the selection: these expirations provide reference points at which it can be assumed that the lung volumes are equal. Click Apply to apply the calculated value of k.