Spirometry Volume is an integral calculation of the raw Spirometry Flow data. On occasion you may see drifting in this channel either in a positive or negative deflection. This could be caused by 3 things:
- Thermal drift
- Initial offset drift
- Breath dependent 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, over-corrects shallow panting breathing, in which most of the air breathed is dead-space air. LabChart's 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 clean bore tubing and droplet filter.
In order to accomplish a volume correction for the above, it would be best to have the Spirometry Extension installed. In this LabChart extension you can perform a manual correction, an automatic correction, or an event based baseline reset.
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 in two ways; however, we will discuss the full expiration method here. For the full expiration method, 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.