Development of an air displacement method for whole body volume measurement of infants.

An infant enclosed in a rigid-walled chamber displaces a volume of air equal to its own volume. The volume of air displaced can be estimated by the change in pressure produced by a standard reduction in the chamber's volume according to Boyle's law. An instrument embodying this principle has been developed in which the differential pressure between two identical chambers is measured during equal sinusoidally imposed volume changes in the two. Problems arising from variable departure of conditions of pressure cycling, from isothermal towards adiabatic, have been dealt with by empirically derived corrections. Data are presented on the use of the method for low birth-weight infants.