Role of Pv-aCO2 gradient and Pv-aCO2/Ca-vO2 ratio during Cardiac Surgery

Background. Arterial lactate, mixed venous O2 saturation and parameters derived from CO2 metabolism as the venous minus arterial CO2 partial pressure (Pv-aCO2) gradient 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 their physiologic determinants of Pv-aCO2 difference, and the Pv-aCO2 /Ca-vO2 ratio and the interchangeability of the variables calculated from mixed and central venous samples. Methods. We made a sub-analysis of 35 cardiac surgery patients included in a previous investigation database. Parameters were measured or calculated: after anesthesia induction (T1), end of cardiac surgery (T2), and at 6-8 hours intervals after ICU admission (T3 and T4). Results. 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 but transient changes during the study (p<0.05). Pv-aCO2 remained high and without changes along the study, and Pv-aCO2/Ca-vO2 was also high and only decreased at T4 (p<0.05). A weak but significant correlation was observed both, globally and at each time interval, between Pv-aCO2 or Pv-aCO2/Ca-vO2 with factors that may affect the CO2 hemoglobin dissociation. Using a similar approach, a multilevel linear regression model with Pv-aCO2 and Pv-aCO2/Ca-vO2 as outcome variables showed a significant association for Pv-aCO2 with mixed venous O2 saturation (SvO2), and base excess (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. Conclusions. Since 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 during the hemodynamic management of cardiac surgery patients. Finally, central venous and mixed values were not interchangeable.