Capnography is a non-invasive monitoring technique that measures the concentration of carbon dioxide (\(\text{CO}_2\)) in a patient’s exhaled breath. This provides immediate insight into a person’s respiratory function, specifically their ventilation. Nasal capnography uses a specialized cannula placed under the nose to collect this exhaled air, making it a simple, comfortable, and continuous method for monitoring patients who are breathing on their own. By tracking the \(\text{CO}_2\) levels over the entire breathing cycle, this technology helps medical professionals assess gas exchange effectiveness.
How Nasal Capnography Measures Breathing
Measurement begins with a nasal cannula that has small ports positioned near the nostrils. As the patient exhales, a small, continuous sample of the breath is drawn through a thin tube connected to the main capnography unit, a technique known as sidestream sampling. This method is preferred for non-intubated patients because the sensor is located remotely, keeping the monitoring device lightweight and away from the patient’s face. The aspirated gas sample is then delivered to an analyzer inside the monitor for processing.
The core technology relies on infrared (IR) light absorption, a process called infrared spectroscopy. Carbon dioxide molecules absorb specific wavelengths of infrared light, particularly around 4.3 micrometers. Inside the analyzer, a beam of IR light is passed through the sampled gas, and a photodetector measures the amount of light that successfully passes through. The higher the concentration of \(\text{CO}_2\) in the sample, the more IR light is absorbed, resulting in less light reaching the detector.
The analyzer converts this difference in light intensity into a numerical value representing the concentration of \(\text{CO}_2\) in the breath. Because the monitor continuously samples the gas multiple times per second, it tracks \(\text{CO}_2\) concentration throughout the entire respiratory cycle. This breath-by-breath analysis provides a real-time picture of ventilation, allowing for immediate recognition of changes in breathing patterns. The end result is a continuous graph, or capnogram, that visually plots the \(\text{CO}_2\) concentration against time.
Essential Clinical Applications
Nasal capnography provides immediate, continuous feedback on ventilation, making it a valuable tool in various healthcare settings. One of its most common uses is monitoring patients undergoing procedural sedation, such as during endoscopies or minor surgical procedures. Sedation medications can depress the patient’s respiratory drive, and capnography offers an early warning sign by detecting a rise in exhaled \(\text{CO}_2\) before a drop in oxygen levels occurs.
Monitoring with capnography is significantly more responsive than relying solely on pulse oximetry, which measures oxygen saturation. A patient can maintain adequate oxygen saturation for several minutes even after breathing becomes dangerously shallow, especially if receiving supplemental oxygen. Capnography, in contrast, detects a change in ventilation with the very next breath, providing a smaller window for potential complications. This early detection allows medical staff to intervene quickly, often by prompting the patient to take a deeper breath or adjusting the medication dosage.
Beyond procedural sedation, capnography is frequently used in emergency departments and during patient transport. It helps monitor the respiratory status of individuals at risk for respiratory depression due to pain medications, head injuries, or conditions like diabetic ketoacidosis. This application extends to post-operative recovery units, where patients are still under the influence of anesthesia and are at risk for hypoventilation.
Interpreting the Capnogram Waveform
The capnogram is the graph output, plotting \(\text{CO}_2\) concentration on the vertical axis against time on the horizontal axis. A normal capnogram has a characteristic rectangular shape, representing the four phases of a single breath. The numerical reading displayed alongside the graph is the End-Tidal \(\text{CO}_2\) (\(\text{EtCO}_2\)), which is the maximum concentration of \(\text{CO}_2\) measured at the end of exhalation.
The normal \(\text{EtCO}_2\) value falls between 35 and 45 millimeters of mercury (\(\text{mm Hg}\)). The first phase represents inhalation and the beginning of exhalation, where air from the anatomical dead space contains no \(\text{CO}_2\), keeping the line near zero. The line then rises sharply as \(\text{CO}_2\)-rich air from the alveoli begins to be exhaled, leading to the third phase. This third phase is a flattened plateau that ends at the \(\text{EtCO}_2\) value, reflecting the efficient emptying of air from the lungs. The final rapid downstroke marks the beginning of the next inhalation.
Deviations from this normal waveform shape can indicate specific respiratory problems. For example, a gradual, rounded upstroke and a sustained sloped plateau, often described as a “shark fin” pattern, suggests an obstruction to airflow, such as from bronchospasm in asthma or chronic obstructive pulmonary disease. A sudden, sustained drop in the \(\text{EtCO}_2\) value, with the waveform shape remaining normal, signals hyperventilation, where the patient is breathing off too much \(\text{CO}_2\).
Conversely, an increase in the \(\text{EtCO}_2\) value, resulting in a taller waveform, indicates hypoventilation, where the patient is not breathing enough to expel waste \(\text{CO}_2\). If the capnogram baseline does not return to zero, it suggests the patient is rebreathing some of their own exhaled air, which can be caused by a mechanical issue or a specific breathing pattern. By analyzing both the numerical \(\text{EtCO}_2\) value and the shape of the waveform, clinicians can quickly diagnose and respond to a patient’s changing ventilation status.