EtCO2 and PaCO2 are two important measurements in capnography. Capnography involves monitoring the concentration or partial pressure of carbon dioxide (CO2) in respiratory gases. It is a monitoring tool used during anesthesia and intensive care. PaCO2-ETCO2 gradient is the difference between arterial and end-tidal carbon dioxide in capnography, which is normally 0.5 kPa (3.8 mmHg). This value can be changed due to diseases such as pulmonary embolism, infarct or contusion, hypovolemia, hypotension, cardiac failure, pulmonary hypertension, and cardiac arrest.
The key difference between EtCO2 and PaCO2 is their nature. EtCO2 is the partial pressure or maximal concentration of carbon dioxide (CO2) at the end of an exhaled breath, while PaCO2 is the partial pressure of carbon dioxide in arterial blood.
CONTENTS
1. Overview and Key Difference
2. What is EtCO2
3. What is PaCO2
4. Similarities – EtCO2 and PaCO2
5. EtCO2 vs PaCO2 in Tabular Form
6. Summary – EtCO2 vs PaCO2
7. FAQ – EtCO2 and PaCO2
What is EtCO2?
EtCO2 is the CO2 measurement at the end of a tidal volume breath. It represents alveolar gas and does not reflect any dead-phase gas. It is measured at the end of the respiration by using an analyzer. EtCO2 is usually 2-5 mmHg less than PaCO2.
Current guidelines recommend a normal value of 4.0–4.5 kPa (30.0–33.8 mmHg) for ETCO2 as a stand-in for a low-normal PaCO2. Furthermore, the normal expected PaCO2-ETCO2 gradient or difference is 0.5 kPa (3.8 mmHg).
What is PaCO2?
PaCO2 (partial pressure of carbon dioxide) measures the amount of carbon dioxide in the arterial blood and how well carbon dioxide can move out of the lungs. It indirectly reflects the exchange of CO2 gas through the lungs to the outside air. Breathing and lung capacity for freely exchanging CO2 across the alveolar membrane are the two factors that have a significant impact on PaCO2.
The PaCO2-ETCO2 gradient is the difference between arterial and end-tidal carbon dioxide due to the alveolar dead space. This difference is small in healthy children and young people. Under normal physiological conditions, this gradient is around 0.5 kPa (3.8 mmHg). Generally, PaCO2 value is higher than ETCO2. But it is possible to have an ETCO2 higher than the PaCO2 in some situations, like when there is a low functional residual capacity, high tidal volume and low respiratory rate, or lung compliance and if there is rebreathing of expired CO2. A higher ETCO2 value is most commonly seen in pregnancy, obesity, and infants.
Similarities Between EtCO2 and PaCO2
- EtCO2 and PaCO2 are two important measurements in capnography.
- Both values are defined based on the CO2 concentration.
- The difference between PaCO2 and EtCO2 (PaCO2-ETCO2 gradient) represents the pulmonary dead space, which is small and approximately 0.5 kPa (3.8 mmHg) under normal physiological conditions.
- PaCO2-ETCO2 gradient is used to diagnose diseases such as pulmonary embolism, infarct or contusion, hypovolemia, hypotension, and cardiac failure.
Difference Between EtCO2 and PaCO2
Definition
- EtCO2 is the partial pressure or maximal concentration of carbon dioxide (CO2) at the end of an exhaled breath in respiration.
- PaCO2 is the measure of the amount of carbon dioxide in the arterial blood and how well carbon dioxide can move out of the lungs.
Measured by
- EtCO2 is measured by infrared spectroscopy and capnometry devices.
- PaCO2 is measured by capnographic trace through devices like CO2 sensors (INTELLiVENT®‑ASV®).
Lower/Higher
- EtCO2 value is normally lower than PaCO2.
- PaCO2 value is normally higher than PaCo2.
Normal Levels
- The EtCO2 value normally ranges from 30.0–33.8 mmHg.
- The PaCO2 value normally ranges from 33.8- 37.6mmHg.
Importance
- ETCO2 monitors and gives continuous, real-time details about how a person is breathing and helps assess people with acute respiratory distress or other breathing problems, seizures, and sepsis.
- PaCO2 is used to assess people with pulmonary edema, obstructive lung disease, hyperventilation, hypoxia, anxiety, pregnancy, and pulmonary embolism.
The following table summarizes the difference between EtCO2 and PaCO2.
Summary – EtCO2 vs PaCO2
EtCO2 and PaCO2 are two important measurements in capnography. EtCO2 is the level of carbon dioxide released at the end of an exhaled breath, while PaCO2 is the partial pressure or amount of carbon dioxide in arterial blood. This summarizes the difference between EtCO2 and PaCO2.
FAQ: EtCO2 and PaCO2
1. What is capnography used for?
- Capnography is a method for monitoring the concentration or partial pressure of carbon dioxide in gases breathed in and out during respiration. This method is used to diagnose various diseases. It is a continuous, real-time, noninvasive, and rapid monitoring technique.
2. What is a normal EtCO2 level?
- End-tidal CO2 or EtCO2 is a noninvasive technique that represents the partial pressure or maximal concentration of CO2 at the end of exhalation or exhaled breath in respiration. The normal value of EtCO2 is 30.0–33.8 mmHg.
3. What does a high EtCO2 mean?
- Normal EtCO2 levels range from 30.0–33.8 mmHg. However, this may vary based on a patient’s underlying respiratory and metabolic status. Higher EtCO2 levels that rise from a normal baseline to or above 50 mmHg may indicate hypoventilation.
4. What is PCO2 vs PaCO2?
- PCO2 is a term used to describe the partial pressure of carbon dioxide (arterial or venous blood), whereas PaCO2 is a term specifically used to describe the partial pressure of carbon dioxide in arterial blood.
5. What are PaO2 and PaCO2 normal ranges?
- The partial pressure of oxygen in arterial blood (PaO2) ranges from 75 to 100 millimeters of mercury (mm Hg), while the partial pressure of carbon dioxide in arterial blood (PaCO2) ranges from 38 to 42 mm Hg.
Reference:
1. “What Is Etco2?” Capnomask.
2. “ABG (Arterial Blood Gas).” Global Library of Women’s Medicine.
Image Courtesy:
1. “Monitoring” By Behzad39 – Own work (CC BY-SA 4.0) via Commons Wikimedia
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