Comprehensive Guide to Capnometry and Capnography

Capnometry Definition: It is defined as the numeric measurement and display of the level of Carbon Dioxide ( $C02$ ).
The Capnometer: This is the specific device used to measure $C02$ .
Measurement Nature: It is a non-invasive measurement of $C02$ found in exhaled respiratory gases.
Scientific Principle: Similar to pulse oximetry, capnometry operates based on the principle that carbon dioxide absorbs infrared light.
Capnometry vs. Capnography:
- Capnometry: Refers to the measurement and display of the $ETC02$ value only, without a waveform.
- Capnography: Refers to the measurement and display of both the $ETC02$ value and the capnogram (the $C02$ waveform).

Infrared Light Absorption: The most common method used in capnometry.
- Exhaled gas is passed through a sampling chamber.
- An infrared light source is positioned on one side of the chamber, with a photodetector on the opposite side.
- Calculation is based on the quantity of infrared light reaching the photodetector:
- High $C02$ levels: Indicated by low infrared light detection.
- Low $C02$ levels: Result in high amounts of infrared light detection.
Alternative Technologies: Other technologies used for measurement include:
- Raman scattering.
- Mass spectroscopy.
- Photoacoustic technology.

Scalar Display: Capnography provides a graphic display of $C02$ levels versus time as they change during breathing.
Continuous Analysis: It comprises the continuous analysis and recording of carbon dioxide concentrations in respiratory gases.
Physiologic Scope: As a non-invasive measurement during inspiration and expiration of the partial pressure of $CO2$ from the airway, it provides critical information on:
- Ventilation.
- Perfusion.
- Metabolism.
Clinical Utility:
- Assessing trends in alveolar ventilation.
- Detecting Ventilation/Perfusion ( $V/Q$ ) imbalances caused by pulmonary or cardiovascular disorders.
- Measuring physiologic deadspace.
- Detecting esophageal intubation.
- Assessing blood flow during cardiac arrest (specifically looking for the return of spontaneous respiration).
- Acting as the "gold standard" for monitoring during conscious sedation.

Initial Exhalation Phase: Initially, the expired $PC02$ is $0\,mm\,Hg$ , which indicates the exhalation of pure dead space gas from the airways.
Mixing Phase: Soon after, alveolar gas begins mixing with dead space gas in the airways, resulting in a rapid increase in expired $PC02$ .
Alveolar Plateau: The $C02$ concentration eventually reaches a plateau, which indicates the exhalation of alveolar gas.
End-Tidal Gas ( $PETC02$ ): Gas sampled at the very end of exhalation is called end-tidal gas. Its partial pressure is abbreviated as $PETC02$ .
The $PETC02$ to $PaCo2$ Gradient:
- In healthy individuals, $PETC02$ averages $3\,to\,5\,mm\,Hg$ less than $PaCo2$ .
- This difference can change dramatically when $V/Q$ ratios vary from normal.
Inhalation Phase: A sharp downstroke and return to baseline occurs immediately after the end-tidal point, indicating the inhalation of fresh gas containing zero $C02$ .

Mainstream Capnography:
- Characterized as invasive and non-diverting.
- Measurement of the $ETCO2$ is performed directly at the airway or sample site.
- Provides a real-time measurement of carbon dioxide pressure.
Sidestream Capnography:
- Characterized as non-invasive and diverting.
- The gas sample is transported from the sample site through a plastic tube to be analyzed in a sample cell.
Microstream Capnography:
- Uses a breath sampling rate of exactly $50\,ml\,per\,minute$ .
- Importance of low sampling rate:
- Permits use for patients of all ages, including neonates (whose small tidal volumes cannot be accurately measured by technologies requiring higher sampling rates).
- Reduces moisture and humidity entering the sampling line, which decreases the potential for sampling line obstruction (a common issue in conventional sidestream technology).

Colorimetric $CO2$ Sensor:
- Features: Qualitative measurement (tells if $C02$ is present, but not the exact amount).
- Usage: Designed specifically to confirm endotracheal intubation; fits between the endotracheal tube and the manual bag-valve- $O2$ device.
- Visual Indicator: Changes color from purple to yellow in the presence of $C02$ to confirm the tube is in the airway.
Technical Care and Troubleshooting:
- For certain sensors, measurements may become unreliable over time; it is recommended to move the target sticker every $3\,to\,4\,hours$ .
- Moisture and humidity can collect in sample lines and alter sampling accuracy.
- Always check equipment and wires to troubleshoot before jumping to clinical conclusions regarding waveform changes.

Hyperventilation:
- Characterized by an increase in waveform frequency or bigger breaths.
- Results in the patient "blowing off" more $C02$ , causing $C02$ levels to fall over time.
Hypoventilation:
- Causes $C02$ levels to rise over time.
- Clinical considerations include whether the patient is febrile or in pain.
Esophageal Intubation:
- Often presents as a flat line on the flow/waveform display.
- May indicate the tube has dislodged (e.g., when rolling a patient) or a complete loss of the airway.
Apnea:
- Characterized by the absence of airflow.
- Capnography can detect both total apnea and severe obstructions where air movement is insufficient.
Obstructed Airway:
- Waveform appears as a "shark fin" shape.
- Severity of the block causes the speed of gas leaving the lungs to slow down.

Causes of Elevated $EtCO2$ :
- Metabolism: Hyperthermia, Pain, Shivering (transferring more out into the blood).
- Respiratory System: Respiratory insufficiency, Respiratory depression, COPD, Analgesia/sedation.
- Circulatory System: Increased cardiac output.
- Medications: Bicarbonate administration.
Causes of Decreased $EtCO2$ :
- Metabolism: Hypothermia, Metabolic acidosis.
- Respiratory System: Alveolar hyperventilation, Bronchospasm, Mucus plugging.
- Circulatory System: Hypotension, Sudden hypovolemia, Cardiac arrest, Pulmonary emboli (common shunt mismatch).