The main result of this study is that in healthy dogs under general anesthesia, abdominal surgery interferes with pulmonary aeration, promoting the formation of atelectatic and poorly aerated lung compartments and reducing the normally aerated lung area. We can speculate that the increased intra-abdominal pressure induced by the surgical procedures promotes the cranial displacement of the diaphragm with the consequent reduction of the pulmonary functional residual capacity, and consequently promotes airway closure and alveolar collapse (Duggan and Kavanag 2005; Hedenstierna 2003). Despite the greater impairment of lung aeration, arterial oxygenation was not significantly affected in dogs of the S group compared to dogs of the NS group. The apparently discordant results can be justified by two theories: the use of high FiO₂ compensated for the greater impairment of lung aeration, assuring similar arterial oxygenation in dogs of the S group; we can also speculate that dogs of the S group had a lower cardiac output as a result of the deeper level of anesthesia, abdominal compression, water loss, and bleeding related to the surgical procedures. Several experimental and clinical studies demonstrated that cardiac output might interfere with the percentage of the intrapulmonary shunt. In particular, lower cardiac outputs, within certain limits, reduce the intrapulmonary shunt improving patient oxygenation (Lynch et al. 1979).

Pulmonary atelectasis not only interferes with respiratory mechanics and gas exchange but can also be associated with postoperative pulmonary complications characterized by the permanence of gas-exchange impairment and the development postoperative pulmonary infections (Duggan and Kavanag 2005). Based on the results of this study, abdominal surgery promotes pulmonary atelectasis formation, and thus, the use of PEEP and lung-recruitment maneuvres is recommended to preserve lung aeration during abdominal surgery (Staffieri et al. 2009, 2010).