Continuous capnography (measurement of carbon dioxide) is the gold standard for confirming endotracheal tube placement but is also utilized in the operating room and intensive care unit (ICU) to help guide ventilation in conjunction with blood gases.
The shape of a capnogram can provide valuable insight about pulmonary obstruction (and compliance to a certain degree). The difference between a measured arterial CO2 (PaCO2) and end-tidal CO2 (ETCO2) from the capnogram gives us an idea about alveolar dead space. Changes in ETCO2 can be related to changes in metabolism, pulmonary embolism (increased dead space), or evolution of pulmonary pathologies like pneumonia and acute respiratory distress syndrome (ARDS).
Let’s look at a capnogram. In the following illustration, red and green represent the expiratory and inspiratory phases of respiration, respectively. Initially during expiration (phase 1), gases from the conducting zones (oro/nasopharynx, trachea, proximal bronchi) are exhaled first. This gas has a composition similar to that of the atmosphere, and consequently, does not contain much CO2. Because these regions are not involved in gas exchange, it is termed anatomic dead space.
Phase 2 is a slow upstroke where exhaled gas is now starting to include CO2 from alveoli. Alveoli have varying compliances and degrees of ventilation. For example, alveoli in the lung bases tend to be better ventilated (and perfused, for that matter) compared to alveoli in the apices. Ultimately, the alveolar plateau is reached (phase 3) and represents continued exhalation of CO2-rich alveolar gas. Sometimes phase 3 has an upstroke representing expiratory obstruction as classically seen in chronic obstructive pulmonary disease (COPD). The very end of phase 3 marks the ETCO2. The capnogram then sharply drops off to indicate inspiration since no additional CO2 is being exhaled.
Now what about the angles? Without getting into too much complexity, the α-angle is related to the distribution of fast and slow alveoli. An angle > 90 degrees (upstroke in phase 3) suggests worsening V/Q mismatch. A β-angle > 90 degrees suggests that it takes more time for the CO2 to drop on inspiration (phase 0) which could suggest rebreathing of CO2.
Finally, alveolar dead space represents alveoli which are poorly perfused and therefore do not participate in gas exchange. If a patient has a lot of alveolar dead space, he or she cannot effectively unload CO2 on exhalation (decreased ETCO2) and will therefore retain more CO2 in the arterial blood (PaCO2). This means the PaCO2 – ETCO2 gradient will increase. Since we continuously measure ETCO2 but only check PaCO2 through arterial blood gases occasionally, it’s nice to establish exactly how large this gradient is by drawing a blood gas and recording the ETCO2 at that exact time. The next time you look at the ETCO2, you’ll have a rough idea what the PaCO2 is by adding the same gradient.
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