Role of Pv-aCO2 gradient and Pv-aCO2/Ca-vO2 ratio during cardiac surgery: a retrospective observational study

Arterial lactate, mixed venous O2 saturation, venous minus arterial CO2 partial pressure (Pv-aCO2) and the ratio between this gradient and the arterial minus venous oxygen content (Pv-aCO2/Ca-vO2) were proposed as markers of tissue hypoperfusion and oxygenation. The main goals were to characterize the determinants of Pv-aCO2 and Pv-aCO2/Ca-vO2, and the interchangeability of the variables calculated from mixed and central venous samples. 35 cardiac surgery patients were included. Variables were measured or calculated: after anesthesia induction (T1), end of surgery (T2), and at 6–8 hours intervals after ICU admission (T3 and T4). Macrohemodynamics was characterized by increased cardiac index and low systemic vascular resistances after surgery (p < 0.05). Hemoglobin, arterial-pH, lactate, and systemic O2 metabolism showed significant changes during the study (p < 0.05). Pv-aCO2 remained high and without changes, Pv-aCO2/Ca-vO2 was also high and decreased at T4 (p < 0.05). A significant correlation was observed globally and at each time interval, between Pv-aCO2 or Pv-aCO2/Ca-vO2 with factors that may affect the CO2 hemoglobin dissociation. A multilevel linear regression model with Pv-aCO2 and Pv-aCO2/Ca-vO2 as outcome variables showed a significant association for Pv-aCO2 with SvO2, and BE (p < 0.05), while Pv-aCO2/Ca-vO2 was significantly associated with Hb, SvO2, and BE (p < 0.05) but not with cardiac output. Measurements and calculations from mixed and central venous blood were not interchangeable. Pv-aCO2 and Pv-aCO2/Ca-vO2 could be influenced by different factors that affect the CO2 dissociation curve, these variables should be considered with caution in cardiac surgery patients. Finally, central venous and mixed values were not interchangeable.