ETCO₂ measurement must be quantitative (i.e. provide an accurate numeric value). A capnograph is the gold standard, as this provides a reassuring visual display of each breath and can be an additional sign of ventilator disconnection. Lightweight, battery powered, portable monitors are commercially available (e.g. Capnocheck® (Fig. 6.2)), although most pre-hospital defibrillator monitors now include waveform capnography as one of their functions. They use infra-red absorbance spectroscopy to determine the presence and quantity of CO₂ in the exhaled breath. Monitors generally display this information as a figure and as a waveform. The waveforms can be useful for the recognition of trends in ventilation and diagnosing ventilatory problems. The shape of the trace can indicate problems such as a patient starting to breathe against the ventilator, or obstructive breathing (kinked tubing or bronchospasm (Fig. 5.​1)). Some devices may need regular calibration.

A capnometer (e.g. EMMA^TM (Fig. 6.3)) is the next best option. This will give an accurate figure to guide ventilation, but no waveform. The final option is a qualitative ETCO₂ device. These only provide evidence of the presence of CO₂ and usually an idea of whether this is high or low, typically by colour change (e.g. Easy Cap®(Figs. 6.4 and 6.5)). They are disposable and consist of a filter paper impregnated with a pH-sensitive, non-toxic chemical indicator that reversibly changes colour on contact with CO₂. The Easy Cap® provides a breath to breath colour change from purple on inspiration to yellow on expiration, confirming the presence of CO₂ in the exhaled breath. Qualitative devices are only really suitable for confirmation of tracheal placement of a tube in the absence or failure of a quantitative device, and should be replaced as soon as possible.