Role of Pv-aCO(2) gradient and Pv-aCO(2)/Ca-vO(2) ratio during cardiac surgery: a retrospective observational study

INTRODUCTION: Arterial lactate, mixed venous O(2) saturation, venous minus arterial CO(2) partial pressure (P(v-a)CO(2)) and the ratio between this gradient and the arterial minus venous oxygen content (P(v-a)CO(2)/C(a-v)O(2)) were proposed as markers of tissue hypoperfusion and oxygenation. The main goals were to characterize the determinants of P(v-a)CO(2) and P(v-a)CO(2)/C(a-v)O(2), and the interchangeability of the variables calculated from mixed and central venous samples. METHODS: 35 cardiac surgery patients were included. Variables were measured or calculated: after anesthesia induction (T1), end of surgery (T2), and at 6...8.ßhours intervals after ICU admission (T3 and T4). RESULTS: Macrohemodynamics was characterized by increased cardiac index and low systemic vascular resistances after surgery (p.ß<.ß0.05). Hemoglobin, arterial-pH, lactate, and systemic O(2) metabolism showed significant changes during the study (p.ß<.ß0.05). P(v-a)CO(2) remained high and without changes, P(v-a)CO(2)/C(a-v)O(2) was also high and decreased at T4 (p.ß<.ß0.05). A significant correlation was observed globally and at each time interval, between P(v-a)CO(2) or P(v-a)CO(2)/C(a-v)O(2) with factors that may affect the CO(2) hemoglobin dissociation. A multilevel linear regression model with P(v-a)CO(2) and P(v-a)CO(2)/C(a-v)O(2) as outcome variables showed a significant association for P(v-a)CO(2) with S(v)O(2), and BE (p.ß<.ß0.05), while P(v-a)CO(2)/Ca-vO(2) was significantly associated with Hb, S(v)O(2), and BE (p.ß<.ß0.05) but not with cardiac output. Measurements and calculations from mixed and central venous blood were not interchangeable. CONCLUSIONS: P(v-a)CO(2) and P(v-a)CO(2)/C(a-v)O(2) could be influenced by different factors that affect the CO(2) dissociation curve, these variables should be considered with caution in cardiac surgery patients. Finally, central venous and mixed values were not interchangeable.