Quantitative Waveform Capnography (QWC) is a monitoring tool that measures the level of carbon dioxide (CO2) present in a person’s exhaled breath. This continuous physiological assessment provides real-time information about a patient’s ventilation, circulation, and metabolism. The device delivers two types of output: a numerical value (capnometry) and a graphical display over time (waveform capnography). This non-invasive measurement allows healthcare providers to assess respiratory status with precision, showing how well the lungs move air and how effectively the blood transports CO2.
The Science of Measurement: How Capnography Works
Capnography operates based on the principle that carbon dioxide molecules absorb infrared light at a specific wavelength. The device uses an infrared light source and a photodetector to measure the light passing through the gas sample. As CO2 molecules absorb the infrared energy, less light reaches the detector, allowing the machine to calculate the CO2 concentration.
This method is “quantitative” because it provides a precise, measurable concentration of CO2, typically expressed in millimeters of mercury (mmHg). Measurement is achieved through one of two primary sampling methods. Mainstream capnography places the sensor directly in the patient’s airway circuit, allowing expired gas to flow immediately over the sensor.
Sidestream capnography continuously draws a small gas sample away from the patient’s airway through a narrow tube to an internal analyzer. This infrared technology provides a continuous, real-time analysis of exhaled CO2. This makes it significantly faster at detecting respiratory changes than traditional methods, such as pulse oximetry, which is delayed.
Decoding the Waveform: What the Graph Reveals
The capnograph plots CO2 concentration (vertical axis) against time (horizontal axis) for a single respiratory cycle. A normal capnogram appears rectangular, representing the four phases of a breath. Phase I (A-B) is the inspiratory baseline, which is flat and near zero because the inspired air contains almost no CO2.
Phase II (B-C), the expiratory upstroke, begins as CO2-rich gas from the alveoli mixes with dead space gas, causing a rapid rise in concentration. Phase III (C-D) is the alveolar plateau, representing gas coming exclusively from the alveoli where gas exchange occurs. This plateau ends at the highest point, Point D, which is the End-tidal CO2 (ETCO2) value—the maximum concentration of CO2 at the end of exhalation.
The waveform shape is as important as the numerical ETCO2 value, which normally ranges between 35 and 45 mmHg. Deviations from the normal shape immediately signal specific physiological problems. For example, a “shark fin” appearance, where Phase III slopes steeply upward, is a classic sign of obstructive lung disease like bronchospasm, indicating trapped air. A sudden, complete loss of the waveform, known as a flatline, suggests a severe issue like cardiac arrest, ventilator disconnection, or tube misplacement.
Essential Uses in Patient Care
The continuous, real-time feedback from QWC makes it an indispensable tool in various acute care settings, guiding immediate medical interventions.
Endotracheal Tube Placement
One well-established use is confirming the correct placement of an endotracheal tube (ETT) after intubation. If the ETT is mistakenly placed in the esophagus, the capnograph will show a flatline or minimal CO2. Correct tracheal placement yields a clear, sustained waveform.
Monitoring CPR Effectiveness
ETCO2 levels serve as an immediate measure of chest compression quality during cardiopulmonary resuscitation (CPR). An ETCO2 value below 10 mmHg suggests ineffective compressions, prompting rescuers to adjust their technique. A sudden increase in the ETCO2 value, often rising to the normal range of 35-45 mmHg, is the earliest indicator of the return of spontaneous circulation (ROSC).
Assessing Ventilation Status
QWC provides continuous assessment of a patient’s overall ventilation status, especially for those receiving sedation or mechanical ventilation. An elevated ETCO2 greater than 45 mmHg indicates hypoventilation, meaning the patient is not adequately blowing off CO2. Conversely, a low ETCO2 below 35 mmHg can signal hyperventilation or reduced blood flow due to conditions like a pulmonary embolism.