Agreement between values for arterial and end-tidal partial pressures of carbon dioxide in spontaneously breathing, critically ill dogs.

OBJECTIVE To determine agreement between arterial partial pressures of carbon dioxide (PaCO(2)) and end-tidal partial pressures of carbon dioxide (PETCO(2)) measured with a nasal catheter in spontaneously breathing, critically ill dogs. DESIGN Validation study. ANIMALS 26 client-owned dogs admitted to an intensive care unit for various conditions. PROCEDURES PaCO(2) was measured with a commercial blood gas analyzer, and PETCO(2) was measured with a sidestream capnograph attached to a nasal catheter. Measurements were obtained twice (ie, with and without supplemental oxygen). Paired values were compared by means of the Pearson correlation method. Level of agreement was assessed by means of the Bland-Altman method. RESULTS Mean difference between PaCO(2) and PETCO(2) when dogs did not receive supplemental oxygen (mean +/- SD, 3.95 +/- 4.92 mm Hg) was significantly lower than mean difference when dogs did receive supplemental oxygen (6.87 +/- 6.42 mm Hg). Mean difference in dogs with a condition affecting the respiratory system (8.55 +/- 5.43 mm Hg) was significantly higher than mean difference in dogs without respiratory tract disease (3.28 +/- 3.23 mm Hg). There was a significant linear correlation and good agreement between measured values of PaCO(2) and PETCO(2). Catheter size, ventilatory status, and outcome were not significantly associated with mean difference between PaCO(2) and PETCO(2). CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that nasal capnography is a clinically relevant method of estimating PaCO(2) in spontaneously breathing, critically ill dogs, but that values should be interpreted with caution in dogs receiving supplemental oxygen and in dogs with conditions affecting the respiratory system.