Continuous in-line blood gas monitoring (CILBGM) plays a critical role in managing patients undergoing cardiopulmonary bypass (CPB), where precise oxygenation and ventilation control directly influence outcomes. Traditionally, blood gas values such as arterial oxygen (PaO2) and carbon dioxide (PaCO2) are measured using laboratory analyzers, considered the gold standard. However, newer systems like the Quantum perfusion system estimate these values using algorithms rather than direct measurement. This study investigates whether such calculated values are reliable in clinical practice.
Conducted as a retrospective analysis at the Children’s Hospital of Philadelphia, the study included 81 pediatric patients undergoing cardiac surgery with CPB using the Quantum System and FX05 oxygenator. The investigators compared calculated in-line blood gas values with those obtained from a point-of-care analyzer (i-STAT), focusing on accuracy, timing of discrepancies, and contributing factors such as patient weight and temperature changes.
A key finding was that calculated PaO2 values were significantly overestimated before the first in vivo calibration. At the first blood gas measurement, the average error reached 117 mmHg, with a mean percentage error of 48.3%, far exceeding acceptable clinical thresholds. Notably, 99% of patients showed overestimation, and more than half had errors greater than 100 mmHg. This indicates that prior to calibration, the system’s calculations are unreliable for clinical decision-making.
The study also identified a strong correlation between PaO2 error and patient weight during this early phase, suggesting that the algorithm inadequately accounts for oxygen consumption differences across patient sizes. The authors derived a predictive formula incorporating patient weight to better estimate appropriate oxygen delivery, demonstrating improved alignment with measured values.
Another important observation, illustrated in the graph on page 4, was the near-perfect correlation (R = 0.96) between calculated PaO2 and delivered oxygen fraction (FiO2) before calibration. This suggests that the system relies heavily on FiO2 while underrepresenting physiological variables such as metabolism and perfusion, leading to systematic inaccuracies.
After in vivo calibration, accuracy improved significantly. By the second and third blood gas measurements, errors decreased and fell within acceptable ranges, highlighting the importance of frequent calibration during CPB. However, this improvement was not sustained throughout the procedure.
During the rewarming phase, calculated PaO2 values again became unreliable. The study found a substantial upward drift in PaO2, with average errors exceeding 80 mmHg and more than 80% of patients showing overestimation. As shown in the chart on page 5, this drift moderately correlated with patient weight, suggesting persistent limitations in the algorithm’s adaptability to changing physiological conditions.
PaCO2 performance differed from PaO2. Because its calculation is based largely on measured expired CO2 (FeCO2), it remained within acceptable error ranges during the cooling phase, even without calibration. However, during rewarming, PaCO2 also exhibited an upward drift. The diagram on page 6 demonstrates a moderate correlation between PaCO2 error and temperature gradient, indicating that thermal changes affect gas exchange dynamics and sensor interpretation.
Clinically, these findings raise concerns about relying on calculated blood gas values without validation. The study specifically warns against using the Quantum System’s autoregulation feature for PaO2 before initial calibration and during rewarming, as it may lead to dangerously low actual oxygen levels despite falsely elevated displayed values.
To mitigate these risks, the authors recommend frequent in vivo calibration (“Capture All/Sync”) and monitoring alternative parameters such as oxygen saturation (SaO2), which remained accurate throughout CPB. They also propose predictive formulas to guide FiO2 adjustments based on patient weight, improving initial targeting of PaO2 levels.
Ultimately, the study concludes that while calculated blood gas monitoring offers convenience and continuous data, it cannot yet fully replace direct measurement. Variability in patient size, oxygenator type, and physiological changes during CPB make it difficult for a single algorithm to maintain accuracy across all conditions. Device-specific and context-specific calibration strategies are essential for safe clinical use.
