The Capnographic Waveform

The Capnographic Waveform

The capnographic waveform is displayed on a graph with the partial pressure of carbon dioxide on the vertical axis and time across the horizontal axis.

Because carbon dioxide is present in the atmosphere at about 400ppm or about 0.04%, the baseline of the graph is effectively zero.

(The actual pressure of atmospheric CO2 is about 0.3 mmHg but that is low enough to appear to be zero on this graph.)


There are four phases to a capnograph:

Phase 1 is the inspiratory baseline

Phase 2 is the expiratory upstroke 

Phase 3 is the alveolar plateau

Phase 0 is the inspiratory downstroke

 The alpha and beta angles are also of special significance. In fact, the beat angle is where we measure end-tidal CO2.


We will learn more about the significance of the alpha angle in the next lesson.


The highest value on a normal capnograph coincides with the beta angle and is 35 mmHg – 45 mmHg.


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