Arterial-venous CO₂ tension gradient (Pa-vCO₂) reflects oxygen debt after cardiopulmonary bypass

INTRODUCTION: Pv-aCO₂ increases with resuscitation from states of shock. We hypothesized that Pv-aCO₂ increases following hypothermic cardiopulmonary bypass (CPB) due to the oxygen debt incurred. The recovery pattern of Pv-aCO₂ was compared to those of O₂ delivery (DO₂), consumption (VO₂), ex-traction (O ₂ER). METHODS: After IRB approval, we enrolled 28 patients undergoing cardiac procedures under hypothermie CPB (28-32×C). Selection criteria included EF>45% and absence of organ dysfunction. Simultaneous arterial & venous blood gases and hemodynamic measures (HR, MAP, PA pressure, CO) were collected under constant mechanical ventilation. Measurement times were pre-CPB, then at 1, 2, 4 and 6 hours post-CPB. CPB factors (duration, cardioplegia vol, and cooling temp) were recorded. Paired t-tests and correlation Z-tests were employed for statistical analyses. Independent variables: patient demographics (age, sex, BSA), CPB factors, hemodynamics, IV fluid intake. Dependent variables: Pv-aCO₂, DO₂, VO₂, O₂ER. RESULTS: Immediately after surgery, Pv-aCO₂ was elevated by 34%, but gradually returned to baseline 2 hrs after CPB (6.5 + 3.2 vs 4.4 + 2.2 mmHg, p=0.05) (Fig 1). The rise in Pv-aCO₂ correlated to an increase in O₂ER (correlat. coeff. = 0.5, p=0.02), which, in turn, was explained by a rise in VO₂ (cc. = 0.54, p<0.01). Cardiac output rose by 61%(p<0.0001). Pv-aCO₂ correlated significantly with intraoperative IV fluid intake and cooling temp. VO₂ was positively correlated to CO and BSA; and was negatively correlated to the patients' age. CONCLUSIONS: High Pv-aCO₂ during early postoperative period may be an indicator of the oxygen debt incurred during hypothermie CPB and a tool to monitor postoperative recovery.