ETCO2 Notes
Remember:
H+ + HCO3 <--> H2CO3 <--> CO2 + H2O
Kidneys Blood Lungs
What does ETCO2 Show us?:
ETCO2 measures the expiratory volume of CO2 with each breath. This shows us 2 things. First, it shows us the patient's ventilatory status, and Second, it shows us the state of pulmonary perfusion. If a patient has an obstructive airway disease, such as COPD, they have a problem with ventilation. They dont move a lot of air when they breathe. This means that they will have a respiratory rate that is WNL or even tachypneic, and will still have a high ETCO2 volume. In a “normal” patient, we wouldn't see this. Elevated respiratory rates should drop ETCO2 because adequate volumes of air are being exchanged in the lungs. SO ETCO2 tells us how well the patient is ventilating, and whether or not we may want to PPV the patient. In terms of pulmonary perfusion, we can use ETCO2 to evaluate gas exchange in the alveoli, based on the volume of expired CO2, in conjunction with other vital signs. In a PE for example, the patient has a blood clot in the pulmonary artery. Because of this, the distal lung tissue becomes a pathologic dead space. No gas exchange occurs here. The result is a decrease in ETCO2 values, as the CO2 remains in the circulating blood volume. In response we see elevated respiratory rate, tachycardia, and the resulting hypotension. Because we have clear breath sounds, with low SPO2, and low ETCO2, with adequate tidal volume, we know the problem is not ventilation. In this case the problem obviously becomes one not of ventilation, but of perfusion. Our treatment priorities can be adjusted once we understand what the problem is.
The short and easy to remember thing:
Adequate tidal volume with LOW ETCO2 is a perfusion problem.
Poor tidal volume with HIGH ETCO2 is a ventilation problem.
Rapid RR, Shallow volume with LOW ETCO2 suggests acidosis.
Rapid RR Shallow volume with HIGH ETCO2 is a ventilation problem.
Rapid RR, Full volume, LOW ETCO2 AND LOW SPO2 is a perfusion problem.
Rapid RR, Full Volume with LOW ETCO2 and NORMAL SPO2 is totally normal.
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