Thomas V Brogan, H Thomas Robertson, Wayne J E Lamm, Jennifer E Souders, Erik R Swenson. J Appl Physiol (1985). 2004 May;96(5):1894-8. doi: 10.1152/japplphysiol.00160.2003. Epub 2003 Dec 5.
 
We have shown previously that inspired CO₂ (3-5%) improves ventilation-perfusion (Va/Q) matching but with the consequence of mild arterial hypercapnia and respiratory acidosis. We hypothesized that adding CO₂ only late in inspiration to limit its effects to the conducting airways would enhance Va/Q matching and improve oxygenation without arterial hypercapnia. CO₂ was added in the latter half of inspiration in a volume aimed to reach a concentration of 5% in the conducting airways throughout the respiratory cycle.

Jorn Fierstra, Matthew Machina, Anne Battisti-Charbonney, James Duffin, Joseph Arnold Fisher, Leonid Minkovich. Intensive Care Med. 2011 Sep;37(9):1543-50. doi: 10.1007/s00134-011-2260-y. Epub 2011 Jun 7.
 
Purpose: 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.

Jorn Fierstra, Jeff D Winter, Matthew Machina, Jelena Lukovic, James Duffin, Andrea Kassner, Joseph A Fisher. J Clin Monit Comput. 2013 Apr;27(2):147-55. doi: 10.1007/s10877-012-9403-8. Epub 2012 Oct 26.
 
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.