Dynamic elastic pressure-volume loops in healthy pigs recorded with inspiratory and expiratory sinusoidal flow modulation. Relationship to static pressure-volume loops.

Objective: The objective was to analyse relationships between inspiratory and expiratory static and dynamic elastic pressure-volume (P(el)/V) curves in healthy pigs.

Design: The modulated low flow method was developed to allow studies also of the expiratory P(el)/V curves. Static P(el)/V (P(el,st)/V) and dynamic P(el)/V (P(el,dyn)/V) loops were studied in healthy pigs.

Setting: Animal research laboratory in a university hospital.

Material: Ten healthy anaesthetised and paralysed pigs.

Interventions And Measurements: A computer controlled a Servo Ventilator 900C with respect to respiratory rate, inspiratory flow and expiratory pressure to achieve a sinusoidal modulation of inspiration and expiration for determination of P(el,dyn)/V loops from zero end-expiratory pressure (ZEEP) and from a positive end-expiratory pressure (PEEP) of 6 cmH(2)O to 20, 35 and 50 cmH(2)O. The same system was used for studies of P(el,st)/V loops with the flow-interruption method from ZEEP and PEEP to 35 cmH(2)O. Recordings were analysed with an iterative technique.

Results: The feasibility of automated determination of P(el,dyn)/V loops was demonstrated. Differences between P(el,dyn)/V and P(el,st)/V loops were explained by viscoelastic behaviour. P(el,st)/V loops recorded from PEEP to 35 cmH(2)O showed no significant hysteresis, indicating a non-significant surface tension hysteresis. P(el,dyn)/V loops from PEEP and both P(el,st)/V and P(el,dyn)/V loops from ZEEP to 35 cmH(2)O showed hysteresis. This indicates that lung collapse/re-expansion caused P(el)/V loop hysteresis which, in P(el,dyn)/V loops, was augmented by viscoelastic behaviour.

Conclusions: Viscoelasticity influences P(el,dyn)/V curves. Hysteresis caused by surface tension merits re-evaluation. Lung collapse and re-expansion may be indicated by hysteresis of P(el)/V loops.