Arterial-venous CO₂ tension difference reflects oxygen debt after cardiopulmonary bypass

Introduction: The gradient between venous and arterial carbon dioxide tension (Pv-aCO₂) increases with resuscitation from states of intravascular depletion such as hemorragic shock. We hypothesized that Pv-aCO₂ might increase following hypothermic cardiopulmonary bypass (CPB) due to the oxygen debt incurred during CPB. The recovery pattern of Pv-aCO₂ was compared to those of O₂ delivery (DO ₂), consumption (VO₂) and extraction (O₂ER). Methods: Following IRB approval, we enrolled 28 patients undergoing elective cardiac procedures under hypothermic CPB (range 28-32°C). Selection criteria included EF>45% and absence of significant organ dysfunction. Simultaneous arterial and venous blood gasses and hypodynamic 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. Two tailed paired t-tests and correlation Z-tests (95% CD were employed for statistical analyses, and P<0.05 was accepted as significant. Independent variables: patient demographics (age, sex, BSA), CPB factors, hemodynamics, and IV fluid intake. Dependent variables: Pv-aCO₂, DO₂, VO₂ and O₂ER. Results: Immeadiately 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) (Figure 1). The rise in Pv-aCO ₂ correlated to an increase in O₂ER (correlat. coeff. = 0.5, p=0.02), which, in turn, was explained by a rise in VO₂ (correlat. coeff. = 0.54, p<0.01). Cardiac output rose by 61% over the first 4 hours post-CPB (p<0.0001). Postoperative Pv-aCO₂ correlated most significantly with intraoperative TV fluid intake (p=0.03) and cooling temp on CPB (p=0.05). In addition, VO₂ was positively correlated to CO and BSA; and was negatively correlated to the patients age (correl coeff = -0.4, p=0.05). Conclusion: High Pv-aCO₂ during early postoperative period may be an indicator of the oxygen debt incurred during hypothermic CPB and a tool to monitor postoperative recovery.