Arterial pCO2 prediction using saphenous pCO2 in healthy mechanically ventilated dogs

Introduction: Arterial blood gas analysis is the gold standard for the assessment of oxygenation, ventilation, and metabolic status in dogs; however, its execution is difficult and painful. Therefore, venous blood gas analysis is used in its replacement for the assessment of the metabolic status, but it is not clear whether it can be used to assess respiratory function, too. This study aimed at: 1) comparing jugular and saphenous pH and partial pressure of carbon dioxide (pCO(2)) with the correspondent arterial pH and pCO(2) (paCO(2)) in healthy dogs during general anesthesia; 2) clarifying whether the arterial-venous relationship is better expressed in jugular or saphenous blood samples; 3) mathematically transforming venous pCO(2) (pvCO(2)) and evaluating whether the calculated values more accurately agree with paCO(2).Methods: Ninety dogs were included and randomly divided into three groups: Group 1 - arterial vs jugular; Group 2 - arterial vs saphenous; Group 3 - arterial vs jugular vs saphenous blood gases. Each group counted 30 dogs. Pearson correlations were calculated. Bland-Altman plots were generated to describe the agreement between venous and arterial values; clinical limits for pH and pCO(2) set by the authors were, respectively, +/- 0.1 and +/- 2.5 mmHg. Univariate linear regression was applied for predicting paCO(2) from pvCO(2).Results: Saphenous samples showed strong positive correlations with arterial samples for both pCO(2) and pH. Pearson rho values were stronger for pH than for pCO(2). Bland-Altman plots showed good agreement between venous and arterial pH, and poor agreement between pvCO(2) and paCO(2) for both jugular and saphenous samples. Results suggested that saphenous pvCO(2) is preferable with respect to jugular as predictor of paCO(2). The transformation of saphenous pvCO(2) through univariate linear regression produced a model for predicting paCO(2); a Bland-Altman plot assessed the transformed pvCO(2) agreement with paCO(2).Discussion: In healthy, anesthetized, mechanically ventilated dogs, variations of pH between venous and arterial values are clinically acceptable. Venous and arterial blood gases cannot be interchanged for the evaluation of pCO(2). Saphenous pvCO(2) is to be preferable to jugular pvCO(2) as predictor of paCO(2). A formula for the estimation of predicted paCO(2) from saphenous pvCO(2) is proposed.