Static and dynamic pulmonary compliance during upright immersion.

Elastic properties of the lung are typically assessed using the static pressure-volume relaxation manoeuvre. Although dynamic methods have produced equivalent results in normal healthy subjects, their use under obstructive pulmonary disease states and during certain conditions has been questioned. Static (Cst(1)) and dynamic (Cdyn(1)) pulmonary compliance were measured in eight subjects in air (control) and during immersion. Cdyn(1) decreased from 2.94 to 1.61 l kPa-1 following immersion, during which subjects breathed air supplied at mouth pressure (P < 0.05), while Cst(1) remained approximately equivalent (3.16 and 4.07 l kPa-1 respectively, P > 0.05). When immersed subjects were supplied with breathing gas at lung centroid pressure (PLC), Cdyn(1) returned to control status (2.89 l kPa-1; P > 0.05), while Cst(1) remained stable at 3.60 l kPa-1 (P > 0.05). The difference between Cst(1) and Cdynm(1) may imply that flow-resistive properties are included within the latter measurement. It is recommended that Cdyn(1), under these conditions, be interpreted as an index of peripheral airway patency, rather than as an assessment of lung elasticity.