Open-flow plethysmography with pressure-decay compensation.

Whole-body plethysmography is widely used to measure ventilation in awake, unrestrained animals. However, the explicit solution for volumetric analysis of the plethysmograph signal depends upon a closed system, which limits experimental design. Although often used, open-flow plethysmography is complicated by the time-decay of pressure signals generated in the open chamber (e.g. equivalent volume displacements will yield different pressure pulse magnitudes depending upon the rate of application, dP/dt). This problem may be alleviated by first characterizing the time rate of pressure-decay, dP(k)/dt, as a function of pressure magnitude, P, in the plethysmograph, dP(k(P))/dt. Then for each point P(t) in the original signal, subtract the corresponding dP(k(P))(t)/dt from each dP(t)/dt of the original signal to determine the decay-compensated derivative for that point, dP*(t)/dt, and then numerically integrate dP*(t)/dt to generate a pressure-decay compensated signal. The result is a 'virtual closed plethysmograph' trace that enables confident quantitative determination of ventilatory events and volumes with the full advantage of an open-flow plethysmograph.