effect of temperature and humidity in whole-body plethysmography

In whole-body plethysmography, a differential pressure transducer measures the difference in the pressures between the subject chamber and a reference chamber. The signal is the sum of two flows that vary with the subject's respiration:

• nasal flow, generated by flow of air into and out of the nose 
• thoracic flow, generated by compression/expansion of air when the thorax rises/falls.

The thoracic flow is greater than the nasal flow because air is warmed and humidified when it enters the body. Given that the two flows are in phase but of opposite direction, the sum of the two flows is directly proportional to the tidal volume (it is about 1/10th the amplitude).

If the temperature and humidity at different points (subject chamber and subject's body) are known, the effect of these parameters can be quantified and a more accurate estimation of tidal volume obtained. The formula of Drorbaugh and Fenn (1955) can be used for this purpose.

Further reading:
Drorbaugh and Fenn (1955) A barometric method for measuring ventilation in newborn infants. Pediatrics 16: 81-86