Respiratory mechanics and morphometry after progressive intraperitoneal effusion.

Respiratory mechanics and thoraco-abdominal morphometry were evaluated in anesthetized, paralyzed, mechanically ventilated rats before and after controlled intraperitoneal injection of warm (37 degrees C) saline. Respiratory system resistances and static elastance were determined in 9 animals using the end inflation occlusion method. Chest wall configuration at both functional residual capacity (FRC) and end inspiration (FRC + VT) was evaluated in: (a) 6 rats by measurements of lateral and anteroposterior diameters, and circumferences at four levels: 3rd intercostal space, xiphoid, subcostal plane and crista iliaca; and (b) 8 rats by measurements of thoracic cephalo-caudal diameter. In addition, FRC changes were measured in 6 rats. Resistances were not altered but static elastance increased progressively. Morphometric changes were similar at both FRC and FRC + VT: cephalo-caudal diameter diminished whereas all other diameters augmented; FRC decreased. In conclusion, intraperitoneal infusion of saline in rats augments elastance, and this is related to a cephalad deviation of the diaphragm plus an increase of the circumferences and diameters of the lower thorax.