Noninvasive monitoring of the arterial partial pressures of CO2 (PaCO2) of critically ill patients by measuring their end-tidal partial pressures of CO2 (PetCO2) would be of great clinical value. However, the gradient between PetCO2 and PaCO2 (Pet-aCO2) in such patients typically varies over a wide range. A reduction of the Pet-aCO2 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-aCO2 gradient in a ventilated animal model.
Six anesthetized pigs were ventilated mechanically. End-tidal gases were systematically adjusted over a wide range of PetCO2 (30–55 mmHg) and PetO2 (35–500 mmHg) while employing the end-inspiratory rebreathing technique and measuring the Pet-aCO2 gradient. Duplicate arterial blood samples were taken for blood gas analysis at each set of gas tensions.
PetCO2 and PaCO2 remained equal within the error of measurement at all gas tension combinations. The mean ± SD Pet-aCO2 gradient (0.13 ± 0.12 mmHg, 95% CI −0.36, 0.10) was the same (p = 0.66) as that between duplicate PaCO2 measurements at all PetCO2 and PetO2 combinations (0.19 ± 0.06, 95% CI −0.32, −0.06).
The end-inspiratory rebreathing technique is capable of reducing the Pet-aCO2 gradient sufficiently to make the noninvasive measurement of PetCO2 a useful clinical surrogate for PaCO2 over a wide range of PetCO2 and PetO2 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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We thank T. Arenovich for her assistance with the statistical analysis and Dr. S. Iscoe for his valuable contribution to the manuscript. We also thank the veterinary research team, in particular Dr. M. Madden, Dr. A. Goldstein, and S. Lafrance for their assistance and advice during the animal experiments. This work was supported by the Chair fund of the Neurovascular Therapeutics Program and the Department of Anesthesiology at the University Health Network, University of Toronto. The RespirAct™ was made available by Thornhill Research Inc.
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Fierstra, J., Machina, M., Battisti-Charbonney, A. et al. End-inspiratory rebreathing reduces the end-tidal to arterial PCO2 gradient in mechanically ventilated pigs. Intensive Care Med 37, 1543–1550 (2011). https://doi.org/10.1007/s00134-011-2260-y
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