End-tidal carbon dioxide (\(\text{EtCO}_2\)) is a non-invasive measurement of the maximum concentration of carbon dioxide in a person’s breath at the end of exhalation. This measurement provides immediate and continuous insight into three fundamental physiological processes: metabolism, circulation, and ventilation. \(\text{EtCO}_2\) serves as a reliable, real-time indicator of how effectively carbon dioxide is produced in the tissues, transported by the blood, and released by the lungs. Monitoring this value has become a standard practice in a wide range of medical settings for assessing physiological status.
The Physiological Basis of End-Tidal \(\text{CO}_2\)
Carbon dioxide (\(\text{CO}_2\)) is constantly produced by cells as a waste product of cellular metabolism. This gas dissolves into the bloodstream and is transported to the lungs for exhalation. The concentration of \(\text{CO}_2\) in the arterial blood, known as the partial pressure of arterial carbon dioxide (\(\text{PaCO}_2\)), drives the body’s respiratory rate and depth.
The \(\text{EtCO}_2\) measurement serves as a close, non-invasive proxy for \(\text{PaCO}_2\) because the gas originates from the alveoli, the tiny air sacs where gas exchange occurs. In healthy individuals, alveolar \(\text{CO}_2\) rapidly equilibrates with the \(\text{CO}_2\) concentration in the pulmonary capillary blood. The \(\text{EtCO}_2\) value is typically a few millimeters of mercury (mmHg) lower than \(\text{PaCO}_2\), a difference known as the \(\text{PaCO}_2\)–\(\text{EtCO}_2\) gradient.
This slight difference, usually 2 to 5 mmHg, is primarily due to physiological dead space. Dead space refers to lung areas that are ventilated but not perfused with blood, meaning they do not participate in gas exchange. The gas from these areas, which contains little \(\text{CO}_2}\), mixes with the \(\text{CO}_2\)-rich gas from the working alveoli, slightly diluting the final exhaled concentration. Measuring the \(\text{CO}_2\) at the end of the breath ensures that the sample collected is predominantly the \(\text{CO}_2\)-rich gas that originated from the perfused alveoli.
How End-Tidal \(\text{CO}_2\) is Measured
The technology used to measure \(\text{EtCO}_2\) is called capnography, which utilizes infrared spectroscopy to determine the concentration of \(\text{CO}_2\) in the respiratory gas. Carbon dioxide molecules absorb infrared light at a specific wavelength, allowing the device to quantify the amount of gas present in the patient’s breath. Capnography provides both a numerical value (capnometry) and a continuous graphical display (capnogram).
Capnography devices collect the exhaled breath using two primary methods: side-stream or main-stream sampling. Side-stream devices use a small tube to continuously aspirate a gas sample away from the patient’s airway to a sensor located inside the monitor. Main-stream devices place the infrared sensor directly into the breathing circuit at the patient’s airway, providing an instantaneous measurement.
The resulting capnogram waveform is a four-phase representation of the respiratory cycle.
Capnogram Phases
- Phase I is the inspiratory baseline, which is near zero, representing \(\text{CO}_2\)-free inhaled air.
- Phase II is the sharp expiratory upstroke as anatomical dead space gas mixes with \(\text{CO}_2\)-rich alveolar gas.
- Phase III is the alveolar plateau, representing the constant flow of alveolar gas; the peak of this phase is the measured \(\text{EtCO}_2\) value.
- Phase IV is the rapid inspiratory downstroke, where the \(\text{CO}_2}\) level drops back to the baseline as the patient begins to inhale.
Clinical Applications in Patient Monitoring
\(\text{EtCO}_2\) monitoring is a standard of care in numerous clinical situations because it provides immediate feedback on a patient’s status.
Intubation Confirmation
A primary use is confirming proper endotracheal tube (ETT) placement during intubation. The presence of a square-shaped capnogram waveform immediately confirms the tube is correctly positioned in the trachea. If the tube were placed in the esophagus, no \(\text{CO}_2}\) would be reliably detected.
Cardiopulmonary Resuscitation (CPR)
\(\text{EtCO}_2\) is a powerful tool for assessing the effectiveness of CPR during cardiac arrest. Since \(\text{EtCO}_2\) levels depend on blood flow to the lungs, a low reading (below 10 mmHg) during chest compressions indicates inadequate circulation. A sudden, sustained increase in \(\text{EtCO}_2}\) often signals a return of spontaneous circulation (ROSC).
Sedation and Ventilation
In non-intubated patients receiving procedural sedation, continuous \(\text{EtCO}_2}\) monitoring detects respiratory depression before oxygen saturation levels drop. A rising \(\text{EtCO}_2}\) alerts clinicians to hypoventilation caused by sedative medications, allowing for timely intervention. For mechanically ventilated patients, the \(\text{EtCO}_2}\) value is used to precisely adjust ventilator settings, ensuring the patient’s \(\text{CO}_2}\) level remains within a healthy range.
Interpreting Abnormal Readings and Waveforms
The normal range for \(\text{EtCO}_2\) in a healthy adult is between 35 and 45 mmHg. Deviations from this range or changes in the capnogram’s shape offer significant diagnostic information.
Elevated \(\text{EtCO}_2\) (Hypercapnia)
Hypercapnia results from hypoventilation, occurring when a patient is not breathing frequently or deeply enough to eliminate adequate \(\text{CO}_2}\). Causes include narcotic overdose, respiratory muscle fatigue, or certain metabolic conditions that increase \(\text{CO}_2}\) production, such as a high fever.
Low \(\text{EtCO}_2\) (Hypocapnia)
Hypocapnia can be caused by hyperventilation or a problem with circulation. Hyperventilation, often due to anxiety or a compensatory response to metabolic acidosis, causes the patient to rapidly blow off too much \(\text{CO}_2}\). A low reading in a stable-breathing patient may signal a severe decrease in pulmonary blood flow, such as during shock, massive blood loss, or a pulmonary embolism. In these circulatory cases, blood is not reaching the alveoli to drop off \(\text{CO}_2}\), resulting in a low expired gas concentration.
Waveform Changes
Changes in the capnogram’s shape, independent of the numerical value, signal specific airway or mechanical issues. The “shark fin” waveform, where the alveolar plateau slopes upward, is characteristic of obstructive diseases like asthma or chronic obstructive pulmonary disease (COPD). This shape indicates the patient has difficulty exhaling gas due to narrowed airways. An elevated baseline that does not return to zero suggests rebreathing of exhaled \(\text{CO}_2}\), often caused by an issue with the ventilator or breathing circuit.