2 Types of Measurement:
Colorimetric capnography is a qualitative method for measuring expired carbon dioxide (CO2) using a color-changing indicator. It provides a general range of CO2 values, rather than precise measurements, making it primarily suitable for confirming correct endotracheal tube (ETT) placement.
The color change ranges from purple (<4 mmHg CO2) to tan (4-15 mmHg CO2) to yellow (20 mmHg CO2). However, it is important to note that colorimetric capnography cannot rule out bronchial mainstem intubation.
Capnography is a non-invasive method for continuously monitoring the amount of carbon dioxide (CO2) in a patient's exhaled breath. It is measured in millimeters of mercury (mmHg). A normal EtCO2 range is between 35 and 45 mmHg.
EtCO2 is closely correlated with PaCO2, which is the partial pressure of carbon dioxide in arterial blood. However, EtCO2 is typically about 5 mmHg higher than PaCO2 due to the addition of CO2 from the upper airway.
Hypoventilation is suspected if EtCO2 is greater than 50 mmHg or if there is an increase of more than 10 mmHg from baseline.
The Waveform
Phase 1 [A-B] – Dead Space Ventilation
Should contain no CO2
Phase 2 [B-C] – Expiratory Upslope
CO2 raises from alveoli into upper airway
Phase 3 [C-D] – Alveolar Plateau
Value at end of this phase (end-tidal) is that which is reported on monitor
Provides insight into V/Q characteristics of lung
Phase 4 [D-E] – Inspiratory Downslope
Physiologic decline in CO2 partial pressure as patient’s inspire
Sample Abnormal Waveforms
Obstructive Lung Disease – Increased baseline indicates there is some trapping of CO2 within the lungs while the same amount of CO2 is expired each breath
Hypoventilation – Increased amount of CO2 expired each breath, without a change in baseline
Apnea – Serially decreasing amounts of CO2 as decreased amount of CO2 expired
Clinical Applications in the Emergency Department
Spontaneously breathing patients
· Sedated patients
· Metabolic acidosis
· Obstructive lung disease
Ventilated patients/apneic patients
· ET tube placement
· CPR effectiveness/ROSB
References
Tintinalli JE, Stapczynski JS, Ma OJ, Cline D, Meckler GD, Yealy DM. Tintinalli’s emergency medicine: a comprehensive study guide. Eight edition. ed. New York: McGraw-Hill Education; 2016.
Marx JA, Rosen P. Rosen’s emergency medicine : concepts and clinical practice. 8th ed. Philadelphia, PA: Elsevier/Saunders; 2014.
Long B, Koyfman A, Vivirito MA. Capnography in the Emergency Department: A Review of Uses, Waveforms, and Limitations. J Emerg Med. 2017;53(6):829-42. PMID: 28993038