Capnography is a medical monitoring technique that provides real-time information about the concentration of carbon dioxide (\(\text{CO}_2\)) in a patient’s exhaled breath. This technology assesses the elimination of \(\text{CO}_2\) from the body, providing continuous, non-invasive insight into a person’s ventilation, circulation, and metabolism. Capnography is often described as the “fifth vital sign” alongside temperature, pulse, respiratory rate, and blood pressure. By tracking the patient’s breath-by-breath \(\text{CO}_2\) status, healthcare providers can detect breathing problems much earlier than with traditional monitoring methods, enabling faster, more informed decisions during medical procedures and in emergency situations.
Measuring End-Tidal Carbon Dioxide
The core measurement in capnography is the End-Tidal Carbon Dioxide (\(\text{EtCO}_2\)), which represents the maximum concentration of \(\text{CO}_2\) found at the very end of a patient’s full exhalation. This measurement is particularly significant because the gas expelled at this point comes from the alveoli, the tiny air sacs in the lungs where gas exchange occurs. Therefore, the \(\text{EtCO}_2\) value closely reflects the partial pressure of \(\text{CO}_2\) in the arterial blood, which normally ranges between 35 and 45 millimeters of mercury (\(\text{mmHg}\)).
The device used to perform this measurement is called a capnometer or capnograph, which relies on the principle of infrared absorption spectroscopy. Carbon dioxide molecules absorb infrared light at a specific wavelength, around 4.3 micrometers. The capnograph shines an infrared light beam through the sampled gas and measures how much light is absorbed by the \(\text{CO}_2\) molecules. A higher concentration of \(\text{CO}_2\) means more infrared light is absorbed, and a lower signal reaches the detector. The monitor converts this detected light intensity into the numerical \(\text{EtCO}_2\) value and generates a corresponding waveform.
There are two main methods for sampling the gas: mainstream and sidestream. Mainstream sampling places the sensor directly in the breathing circuit at the patient’s airway, offering a near-instantaneous reading. Sidestream sampling continuously withdraws a small sample of gas through a tube to a sensor located inside the monitor unit, which allows for monitoring of both intubated and non-intubated patients.
Decoding the Capnogram Waveform
The visual output of the capnograph is a graph called the capnogram, which plots the \(\text{CO}_2\) concentration (vertical axis) against time (horizontal axis). A normal capnogram has a characteristic, nearly rectangular shape that is divided into four distinct phases, each corresponding to a different part of the breathing cycle. Analyzing the shape, height, and frequency of this waveform offers significantly more information than the numerical \(\text{EtCO}_2\) value alone.
Phase I, the inspiratory baseline, is a flat line where the \(\text{CO}_2\) concentration is near zero. This occurs during inhalation and the initial part of exhalation, measuring gas from the anatomical dead space where no gas exchange takes place.
Phase II is the expiratory upstroke, where the line rapidly rises as the \(\text{CO}_2\)-free dead space gas mixes with the \(\text{CO}_2\)-rich gas emerging from the alveoli.
Phase III is the alveolar plateau, forming the top, relatively flat part of the waveform. This phase represents the exhalation of gas predominantly from the alveoli, and the highest point at the very end of this plateau is the \(\text{EtCO}_2\) value.
Phase IV is the inspiratory downstroke, a sharp drop in the graph as the patient begins the next inhalation, quickly bringing the \(\text{CO}_2\) concentration back to the zero baseline.
Essential Uses in Patient Monitoring
Capnography serves as a powerful monitoring tool because it reflects a patient’s ventilation, circulation, and metabolism.
Endotracheal Tube Placement
One primary application is the immediate verification of proper placement of a breathing tube, such as an endotracheal tube. Since the esophagus does not contain \(\text{CO}_2\), the presence of a clear, sustained waveform immediately confirms the tube is in the trachea (windpipe). This life-saving check is far more reliable than physical signs.
Monitoring CPR Effectiveness
Capnography is a standard tool for monitoring the effectiveness of cardiopulmonary resuscitation (CPR) during cardiac arrest. The \(\text{EtCO}_2\) level directly correlates with cardiac output and pulmonary blood flow; higher \(\text{EtCO}_2\) readings indicate better quality chest compressions. A sudden, significant increase in the \(\text{EtCO}_2\) value, often above \(35\text{ mmHg}\), can be the earliest sign of Return of Spontaneous Circulation (ROSC).
Sedation Monitoring
Capnography is widely used for patients undergoing conscious sedation. Medications used for sedation can depress a patient’s breathing, leading to hypoventilation. Capnography provides an earlier warning of respiratory depression than pulse oximetry, often detecting the problem 30 to 60 seconds sooner. This early detection allows clinicians to intervene quickly, preventing a dangerous drop in blood oxygen levels.