Capnography is a medical monitoring technique that measures carbon dioxide (CO2) levels during a patient’s breathing. It provides a real-time graphical representation, known as a capnogram, of the CO2 concentration in exhaled breath over time. This continuous measurement offers immediate insights into a patient’s respiratory status, including ventilation, circulation, and metabolism.
The Basics of Capnography
Capnography measures the partial pressure of carbon dioxide at the end of exhalation, known as end-tidal CO2 (EtCO2). This measurement uses infrared light absorption technology, where CO2 molecules absorb specific wavelengths. The amount of light absorbed correlates directly with the CO2 concentration.
A sensor is placed at the patient’s airway, often integrated into a breathing circuit or nasal cannula, to continuously sample exhaled air. This allows for breath-by-breath analysis of CO2 levels. The data is then translated into a waveform displayed on a monitor, providing a continuous visual representation of respiratory gas exchange.
Decoding the Normal Capnography Waveform
A normal capnography waveform presents as a distinct, box-like shape, reflecting the cyclical process of breathing and gas exchange. It is divided into four main phases, each corresponding to a specific part of the respiratory cycle. Understanding these phases is important for interpreting a patient’s respiratory physiology.
Phase I, the inspiratory baseline, represents the initial part of exhalation. During this period, exhaled air is primarily dead space gas, containing virtually no carbon dioxide. The waveform remains at or near zero CO2.
Phase II, the expiratory upswing, marks the rapid increase in CO2 concentration as alveolar gas mixes with dead space gas. This segment reflects the initial contribution of CO2-rich air from the alveoli. The CO2 level quickly rises from the baseline.
Phase III, the alveolar plateau, represents the exhalation of predominantly alveolar gas. The waveform flattens out, indicating a consistent CO2 concentration. The highest point of this plateau is the end-tidal CO2 (EtCO2) value, an important indicator of ventilation adequacy, measured just before the next inspiration.
Phase IV, the inspiratory downstroke, depicts the rapid decrease in CO2 concentration as the patient inhales fresh air. This segment signifies the start of the next breath, with the CO2 level quickly returning to the baseline. The cycle repeats with each subsequent breath, providing a continuous visual display of respiratory function.
Clinical Significance of Capnography
Capnography serves as an important monitoring tool across various healthcare environments, such as operating rooms, intensive care units, and emergency departments. It offers immediate, objective data that guides clinical decisions.
The technology helps confirm proper placement of an endotracheal tube after intubation, as the presence of a CO2 waveform indicates successful airway cannulation. It also assesses the adequacy of ventilation by measuring expired CO2, which reflects gas exchange in the lungs. During cardiopulmonary resuscitation (CPR), capnography helps monitor the effectiveness of chest compressions by reflecting pulmonary blood flow. A sudden increase in EtCO2 during CPR can indicate the return of spontaneous circulation.
Capnography also aids in detecting early changes in a patient’s respiratory status, such as hypoventilation or hyperventilation, often before visible signs. This early detection allows healthcare providers to intervene promptly, improving patient safety and outcomes during procedures like sedation or mechanical ventilation. It helps in adjusting ventilator settings to optimize CO2 levels.
Interpreting Abnormal Waveforms
Deviations from the normal capnography waveform provide important diagnostic clues about a patient’s physiological state. Recognizing these changes allows clinicians to identify and address underlying issues.
An elevated EtCO2, seen as a higher plateau on the waveform, often suggests hypoventilation, meaning the patient is not breathing adequately to expel carbon dioxide. This can result from conditions like opioid overdose or respiratory muscle weakness. Conversely, a decreased EtCO2 typically indicates hyperventilation, where the patient is breathing too rapidly or deeply, or it can point to reduced CO2 production or impaired circulation, such as during cardiac arrest or pulmonary embolism.
A complete loss of the capnography waveform is a serious sign, potentially indicating severe airway obstruction, esophageal intubation (tube in the stomach instead of the airway), or cardiac arrest with no blood flow to the lungs. Sloping or irregular waveforms, rather than the typical box shape, may suggest obstructive airway diseases like asthma or chronic obstructive pulmonary disease (COPD), where air trapping and uneven emptying of the lungs occur.