Non-invasive accurate measurement of arterial PCO2 in a pediatric animal model

The PCO2 in arterial blood (PaCO2) is a good parameter for monitoring ventilation and acid–base changes in ventilated patients, but its measurement is invasive and difficult to obtain in small children. Attempts have been made to use the partial pressure of CO2 in end-tidal gas (PetCO2), as a noninvasive surrogate for PaCO2. Studies have revealed that, unfortunately, the differences between PetCO2 and PaCO2 are too variable to be clinically useful. We hypothesized that end-inspiratory rebreathing, previously shown to equalize PetCO2 and PaCO2 in spontaneously breathing humans, would also be effective with positive pressure ventilation. Eight newborn Yorkshire pigs were mechanically ventilated via a partial rebreathing circuit to implement end-inspiratory rebreathing. Arterial blood was sampled and tested for PaCO2. A variety of alveolar ventilations resulting in different combinations of end-tidal PCO2 (30–50 mmHg) and PO2 (35–500 mmHg) were tested for differences between PetCO2 and PaCO2 (Pet-aCO2). The Pet-aCO2 of all samples was (mean ± 1.96 SD) 0.4 ± 2.7 mmHg. Our study demonstrates that, in ventilated juvenile animals, end-inspiratory rebreathing maintains Pet-aCO2 to what would be a clinically useful range. If verified clinically, this approach could open the way for non-invasive monitoring of arterial PCO2 in critically ill patients.