COMPARISON BETWEEN OXYGEN INDICES AND COMPUTED TOMOGRAPHY (CT) FOR EVALUATION OF ATELECTASIS IN DOGS

Objective - To evaluate arterial blood gas indices in dogs with and without pulmonary atelectasis. Methods - Retrospective study conducted on dogs anesthetized for pulmonary computed tomography (CT) scan; inclusion criteria were the presence of normal arterial blood gas analysis before anesthesia (T0), a blood gas analysis during or immediately after the pulmonary scan (T1), and the absence of cardiopulmonary alterations revealed by CT. On the basis of pulmonary density with the region of interest (ROI), animals were divided into groups with atelectasis (GA), in which at least one evaluated area showed values between −100 and −500 HU, and without atelectasis (GNA) with all evaluated areas lower than −500. PaO2/FiO2, P[A-a]O2 gradient, dead-space/tidal volume ratio (Vd/Vt = [PaCO2 − EtCO2] / PaCO2) and the F-Shunt ([(CcO2 − CaO2) / (CcO2 − CaO2 + 3.5 mL/dL)] × 100 were calculated. A Kolmogorov–Smirnov test was used to evaluate the distribution of data, expressed as mean ± standard deviation and a Student t-test for unpaired values was used to compare results. P-value < 0.05 was considered significant. Results - Thirty-six animals met the inclusion criteria: 21 in GNA and 15 in GA. P[A-a]O2 gradient measured prior to anesthesia was 6.4 ± 4.3 mm Hg in GNA and 9 ± 4 mm Hg in GA. Mean ROI value was −542 ± 72 HU in GA and −666 ± 22 HU in GNA. Vd/Vt at T1 resulted 0.42 ± 0.03 for GNA and 0.45 ± 0.07 for GA. PaO2/FiO2 was significantly higher in GNA at T1 versus T0 (474 ± 27 mm Hg vs. 545 ± 41 mm Hg); no differences were detected in GA [452 ± 77 mm Hg (T0) vs. 491 ± 52 mm Hg (T1)] or between the 2 groups. A significant increase in F-shunt during anesthesia occurred in both GA [−2 ± 4.6% (T0) and 28.2 ± 4.3% (T1)] and GNA [−1 ± 4.4% (T0) and 11.40 ± 3.9% (T1)]. Conclusion: F-shunt, calculated on arterial blood samples, may be a sensitive and useful index to detect pulmonary shunt due to atelectasis. Further studies are needed to verify how this index changes in patients with contextual pulmonary alterations.