The measurement of end-tidal CO2 is a favourite topic of the examiners, eg. in Question 6 from the second paper of 2005, "Critically evaluate the use and limitations of End-Tidal Carbon Dioxide measurement in Intensive Care practice".
Capnography is discussed in greater detail elsewhere:
There is also an excellent site by Prasanna Tilakaratna which explains infra-red absorption spectrophotometry using vividly colourful diagrams.
This specific issue was asked about in Question 9.2 from the second paper of 2008.
The best, most detailed review:
Walsh, Brian K., David N. Crotwell, and Ruben D. Restrepo. "Capnography/Capnometry during mechanical ventilation: 2011." Respiratory care 56.4 (2011): 503-509.
Whitaker, D. K. "Time for capnography–everywhere." Anaesthesia 66.7 (2011): 544-549.
Kodali, Bhavani Shankar. "Capnography outside the operating rooms." Anesthesiology 118.1 (2013): 192-201.
Yamauchi, H., et al. "Dependence of the gradient between arterial and end-tidal PCO2 on the fraction of inspired oxygen." British journal of anaesthesia (2011): aer171.
Razi, Ebrahim, et al. "Correlation of End-Tidal Carbon Dioxide with Arterial Carbon Dioxide in Mechanically Ventilated Patients." Archives of trauma research 1.2 (2012): 58.
Ahrens, Tom, Helen Wijeweera, and Shawn Ray. "Capnography. A key underutilized technology." Critical care nursing clinics of North America 11.1 (1999): 49-62.
Kingston, E. V., and N. H. Loh. "Use of capnography may cause airway complications in intensive care." British journal of anaesthesia 112.2 (2014): 388-389.
Ortega, Rafael, et al. "Monitoring ventilation with capnography." New England Journal of Medicine 367.19 (2012).
Rückoldt, H., et al. "[Pulse oximetry and capnography in intensive care transportation: combined use reduces transportation risks]." Anasthesiologie, Intensivmedizin, Notfallmedizin, Schmerztherapie: AINS 33.1 (1998): 32-36.
Thompson, John E., and Michael B. Jaffe. "Capnographic waveforms in the mechanically ventilated patient." Respiratory care 50.1 (2005): 100-109.
Babik, Barna, et al. "Effects of respiratory mechanics on the capnogram phases: importance of dynamic compliance of the respiratory system." Crit Care 16 (2012): R177.
Additonally, capnography.com has a series of excellent diagrams and is otherwise an indispenasable resource for this topic.