Objective: To investigate the viscoelastic properties of lungs and thoracic wall in piglets.
Study design: Prospective experimental study.
Animals: Six piglets weighting 30 kg.
Methods: Animals were tracheotomized, anesthetized and mechanically ventilated under controlled conditions. After control measurements of the mechanical properties of the lung of the pigs had been taken, acute lung injury (ALI) was induced by saline lavage. Lung and thoracic wall tissue resistance (DeltaR), which reflects viscoelastic properties and/or time constant inequalities, were determined by using a rapid airway occlusion technique during constant flow inflation (V), at constant tidal volume. was varied from 0.1-0.2 to 1.2 L second(-1) on a single breath. Multiple data sets of DeltaR of lung (DeltaR(L)) and thoracic wall (DeltaR(w)) to inspiratory time (T(I) = V(T)/V) were fitted to a model whose prediction equation was DeltaR = R(2)[1 -exp(-T(I)/tau(2))], where R(2) and tau(2) are the 'viscoelastic' resistance and time constant, respectively. Subscripts (L) and (W) are used to represent lung and thoracic wall, respectively (R(2L), R(2W), tau(2L), tau(2W)). Two more sets of physiological measurements were then taken--the first under zero end-expiratory pressure (ZEEP) and the second under a positive end-expiratory pressure (PEEP) of 10 cmH(2)O.
Results: Data of DeltaR adequately fitted to the prediction equation in all instances. In control, R(2,L) was 15.3 (10.7-22.6) cmH(2)O L(-1) second(-1) (median, interquartile range), tau(2,L) 3.3 (1.9-5.5) seconds, R(2,w) 6.5 (2.2-10.3) cmH(2)O L(-1) second(-1) and tau(2,w) 2.9 (1.1-4.3) seconds. In ALI, R(2,L) significantly increased to 129.6 (105.9-171.3) cmH(2)O L(-1) second(-1) on ZEEP but not significantly decreased to 48.9 (17.8-109.6) cmH(2)O L(-1) second(-1) with PEEP. The corresponding values of tau(2,L) were 7.1 (5.1-11.6) and 4.4 (3.1-5.5) seconds. The values pertaining to thoracic wall did not change significantly among conditions.
Conclusions and clinical relevance: Viscoelastic properties of the lung and thoracic wall in piglets can be described by a viscoelastic model. Values of parameters of this model were markedly increased in ALI and decreased with PEEP.