In my experience, capnography is usually an accessory output available on a lot of commercial (and relatively expenseive) pulse oximetry machines. In a resuscitation scenario, the lack of a pulse doesn't necessarily mean the heart isn't working. A minimal level of end tidal CO2 concentration of expired air can indicate that exchange is in fact taking place in the lungs/pulmonary vasculature regardless of the presence of a pulse. This CO2 level has been used to predict outcome and help decide when to terminate resuscitation in such events.<p>Like pulse oximetry, capnography typically works by detecting infrared energy absorption at a specific wavelength compared to a reference energy source. One difficulty with the idea of making a cheap device that one might, for example, plug into a smart phone, is that unlike a pulse oximeter, you don't simply slip the energy source/detector over someone's fingertip - the capnographic sensor is usually adapted or otherwise attached to the endotracheal tube with which the person in extremis is intubated.