A capnometer is a specialized medical device used to measure and monitor the concentration of carbon dioxide (\(text{CO}_2\)) in a patient’s exhaled breath. This technology provides healthcare providers with a continuous, breath-by-breath assessment of a person’s ventilatory status. By monitoring \(text{CO}_2\) output, the capnometer offers immediate feedback on gas exchange efficiency in the lungs and \(text{CO}_2\) transport from the body’s tissues. This non-invasive method of continuous respiratory monitoring is standard practice in many clinical environments.
Defining Capnometry and End-Tidal Carbon Dioxide
Capnometry refers to the numerical measurement of carbon dioxide in respiratory gases. Capnography includes this numerical value and a corresponding real-time graphical display called the capnogram. The primary metric tracked is End-Tidal Carbon Dioxide (\(text{EtCO}_2\)), which represents the maximum concentration of \(text{CO}_2\) present at the very end of an exhaled breath.
\(text{EtCO}_2\) is a precise indicator of alveolar ventilation, reflecting the \(text{CO}_2\) concentration in the alveoli where gas exchange occurs. Since \(text{CO}_2\) is a metabolic byproduct transported by the blood, the \(text{EtCO}_2\) value serves as a non-invasive proxy for the partial pressure of \(text{CO}_2\) in the arterial blood (\(text{PaCO}_2\)). Under normal physiological conditions, the \(text{EtCO}_2\) value falls within 35 to 45 millimeters of mercury (mmHg).
Technology Behind the Measurement
Capnometers utilize infrared (IR) spectroscopy to measure \(text{CO}_2\) concentration. The device passes an infrared beam through a gas sample and measures the amount of light absorbed by the \(text{CO}_2\). Higher absorption indicates a higher concentration of \(text{CO}_2\) in the exhaled gas. This technology uses two main sampling methods: mainstream and sidestream.
The mainstream method places the sensor directly into the patient’s airway, often between the breathing tube and the ventilator circuit. This non-diverting design provides an instantaneous, real-time reading without delay. Mainstream sensors are often bulkier and heavier at the airway, which can create drag and may be less suitable for very small patients.
The sidestream method uses a small tube to continuously aspirate a gas sample away from the airway to a remotely housed sensor. This design is less bulky at the patient interface, making it ideal for non-intubated patients using nasal cannulas. A slight time delay occurs as the gas travels through the sampling line, and the system is susceptible to errors from moisture or secretions.
Understanding the Capnography Waveform
The capnography waveform, or capnogram, is a graphical representation of \(text{CO}_2\) concentration over time. A normal capnogram has a distinct shape with four phases corresponding to the respiratory cycle.
Phase I, the inspiratory baseline, is a flat line near zero, representing the inhalation of \(text{CO}_2\)-free air and initial exhalation from the anatomical dead space. Phase II is the sharp expiratory upstroke, where alveolar \(text{CO}_2\)-rich gas mixes with dead space gas, causing a rapid rise in concentration.
Phase III, the alveolar plateau, is the period of exhalation where \(text{CO}_2\) concentration is stable, culminating at the numerical \(text{EtCO}_2\) value. Phase 0, the inspiratory downstroke, shows a rapid drop back to the baseline as the patient inhales fresh air.
Deviations from this normal shape signal specific physiological problems. An obstructed airway, such as during bronchospasm, results in a prolonged, sloped Phase II and III, creating a “shark fin” appearance. A sudden, complete loss of the waveform indicates an immediate issue, such as accidental disconnection of a breathing circuit or dislodgement of an endotracheal tube. Analyzing the shape, rate, and height of the waveform allows clinicians to diagnose and manage respiratory issues.
Essential Clinical Applications
The continuous, real-time feedback provided by capnometry is used across many clinical settings.
Verification of Tube Placement
One primary use is the immediate verification of correct endotracheal tube placement, especially in emergency situations. A persistent \(text{CO}_2\) waveform after intubation confirms the breathing tube is in the trachea, not the esophagus.
Monitoring Sedation and Ventilation
Capnometry is used extensively to monitor patients undergoing procedural sedation, such as during endoscopy. Since medications can depress breathing, the \(text{EtCO}_2\) measurement provides an early warning of hypoventilation, often detecting respiratory compromise 30 to 60 seconds before changes are noted in oxygen saturation levels. In the operating room and intensive care unit, it guides ventilator settings and ensures appropriate ventilation for patients under general anesthesia.
Assessing CPR Effectiveness
During cardiopulmonary resuscitation (CPR) following cardiac arrest, the \(text{EtCO}_2\) value reflects the effectiveness of chest compressions. A higher value suggests better blood flow and perfusion.