Capnography: Riding the Wave!

In the conclusion of my last post – “Capnography: The Intro” – I stated that “today’s technology provides a numerical value and a waveform that is important in the diagnostic process of your patient.” Some people make the analogy of capnography to electrocardiograms (ECG), although, in my opinion, first of all, capnography can be used for many different body system analyses – remember, it provides us with feedback on ventilation, metabolism, and circulation – and second and best of all, the waveform components are easier to learn than the PQRST structure of the ECG.


So with no further ado, here is the “normal” graphic display of capnography, also known as a waveform:

Point A to B represents the inspiratory phase of ventilation and is referred to as the respiratory baseline. In other words, the patient is breathing in. Easy enough!


The almost vertical line from point B to C denotes the sudden increase in CO2 that occurs as alveolar gas enters the capnograph.  This rise in CO2 from B to C occurs when the expired gas from the anatomical deadspace has been washed out and the capnograph senses the arrival of alveolar gas. It is commonly referred to as the expiratory upslope.


The horizontal plateau from point C to D represents the CO2 concentration in the alveolar gas, and is called the expiratory plateau. This plateau should be fairly horizontal.  Any significant upslope of the expiratory plateau signifies an obstruction in the emptying of the alveoli – most often caused by bronchospasm.  This steep upslope is modestly enough called a “shark-fin” waveform, which adequately describes its morphology.  More on that in another post!


However, here is a picture of the "shark-fin" waveform:

The concentration at point D is termed the end-tidal partial pressure of CO2, or simply the EtCO2.  This point represents the peak concentration of CO2 at the end of exhalation.  The EtCO2 is a numerical value provided to you in millimeters of Mercury (mmHg) and is very important in our clinical assessment and treatment of the patient. For now, just remember that under “normal” physiology, the capnography should display an EtCO2 of 35 to 45 mmHg. Higher values indicate hypercarpnia or CO2 retention, and lower levels, well, that’s called low CO2.  Either way, there are a number of reasons for each, stemming from problems in either ventilation, circulation, or (you guessed it) metabolism.


How about that!!!