http://www.joms.org/article/0278-2391(87)90078-4/fulltext

In this paper they are using both methods simultaneously and are discussing the difference between methods. Andersson's et. al. conclusion is ..."Consequently, the transcutaneous oxygen monitor does not appear to be optimal as a respiratory monitor in the setting of ultralight general anesthesia where rapid, critical changes in oxygenation must be detected without delay."

Transcutaneous CO2 monitoring may be associated with significant delays in response time,and also is dependent to a degree on skin perfusion,which in excessively nervous individuals in a stressful situation,may be impaired to some degree.

Capnography will be faster,and more reactive,but you also have to consider it's accuracy when using nasal cannulae,in a subject who is not only speaking,but also mouth breathing,and thus nasal capnography will be subject to significant inaccuracies.

Capnography is much more accurate in closed systems such as anaesthetic breathing systems.

Thus,both methods have significant potential for either inaccuracy,and/or slow response times.

My feeling would be that in an open system,using nasal cannulae,will be subject to very wide variations in apparant ETCO2,and you may be best advised to use transcutaneous CO2 as a starting point,with a view to using it as a trend monitor during the time of each study.

i.e. set up the monitor and establish a steady state on the subject prior to running the test period,and then do your test.

Good luck

Thank you Ian for your detailed comment! Best Patrick