Analysis of Plethysmographic Estimation of Alveolar Pressure

Relationships between the change in a representative alveolar pressure in a pulmonary system which acts as a single mechanical compartment and changes in measurable variables are derived for several plethysmographic systems. Derivations considering air as a one-component gas are presented for the pressure and flow-displacement plethysmographs in which the subject exchanges respired gas with air in the box, and for a plethysmograph from which the subject breathes gas from outside the box using a tube through the box wall. A set of assumptions and approximations which can be invoked to develop the standard differential plethysmographic equations is explicitly stated. The analysis is extended to include multicomponent gas mixtures and mass exchange between alveolar gas and blood for a generalized plethysmographic configuration. Some practical requirements on experimental conditions arising from the derived relationships and the assumptions and approximations used in the derivations are considered.