• Jorn Fierstra, Matthew Machina, Anne Battisti-Charbonney, James Duffin, Joseph Arnold Fisher, and Leonid Minkovich.
• Division of Neurosurgery, University Health Network, Toronto, Canada.
• Intensive Care Med. 2011 Sep 1;37(9):1543-50.

PurposeNoninvasive monitoring of the arterial partial pressures of CO(2) (PaCO(2)) of critically ill patients by measuring their end-tidal partial pressures of CO(2) (PETCO(2)) would be of great clinical value. However, the gradient between PETCO(2) and PaCO(2) (PET-aCO(2)) in such patients typically varies over a wide range. A reduction of the PET-aCO(2) gradient can be achieved in spontaneously breathing healthy humans using an end-inspiratory rebreathing technique. We investigated whether this method would be effective in reducing the PET-aCO(2) gradient in a ventilated animal model.MethodsSix anesthetized pigs were ventilated mechanically. End-tidal gases were systematically adjusted over a wide range of PETCO(2) (30-55 mmHg) and PETO(2) (35-500 mmHg) while employing the end-inspiratory rebreathing technique and measuring the PET-aCO(2) gradient. Duplicate arterial blood samples were taken for blood gas analysis at each set of gas tensions.ResultsPETCO(2) and PaCO(2) remained equal within the error of measurement at all gas tension combinations. The mean ± SD PET-aCO(2) gradient (0.13 ± 0.12 mmHg, 95% CI -0.36, 0.10) was the same (p = 0.66) as that between duplicate PaCO(2) measurements at all PETCO(2) and PETO(2) combinations (0.19 ± 0.06, 95% CI -0.32, -0.06).ConclusionsThe end-inspiratory rebreathing technique is capable of reducing the PET-aCO(2) gradient sufficiently to make the noninvasive measurement of PETCO(2) a useful clinical surrogate for PaCO(2) over a wide range of PETCO(2) and PETO(2) combinations in mechanically ventilated pigs. Further studies in the presence of severe ventilation-perfusion (V/Q) mismatching will be required to identify the limitations of the method.

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