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 (P_v-aCO₂) and the ratio between this gradient and the arterial minus venous oxygen content (P_v-aCO₂/C_a-vO₂) were proposed as markers of tissue hypoperfusion and oxygenation. The main goals were to characterize the determinants of P_v-aCO₂ and P_v-aCO₂/C_a-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.ßhours intervals after ICU admission (T3 and T4).

Macrohemodynamics was characterized by increased cardiac index and low systemic vascular resistances after surgery (p.ß<.ß0.05). Hemoglobin, arterial-pH, lactate, and systemic O₂ metabolism showed significant changes during the study (p.ß<.ß0.05). P_v-aCO₂ remained high and without changes, P_v-aCO₂/C_a-vO₂ 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-aCO₂ or P_v-aCO₂/C_a-vO₂ with factors that may affect the CO₂ hemoglobin dissociation. A multilevel linear regression model with P_v-aCO₂ and P_v-aCO₂/C_a-vO₂ as outcome variables showed a significant association for P_v-aCO₂ with S_vO₂, and BE (p.ß<.ß0.05), while P_v-aCO₂/Ca-vO₂ was significantly associated with Hb, S_vO₂, and BE (p.ß<.ß0.05) but not with cardiac output. Measurements and calculations from mixed and central venous blood were not interchangeable.

P_v-aCO₂ and P_v-aCO₂/C_a-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.