Capnography is a medical monitoring tool that non-invasively measures the concentration of carbon dioxide (\(\text{CO}_2\)) in a patient’s exhaled breath over time. This technique provides a continuous, real-time assessment of ventilation, offering immediate insight into a patient’s respiratory function, blood circulation, and cellular metabolism. The primary measurement recorded is End-Tidal \(\text{CO}_2\) (\(\text{EtCO}_2\)), which represents the maximum \(\text{CO}_2\) concentration reached at the very end of the expiratory phase. This measurement is displayed numerically and graphically as a waveform, known as a capnogram, allowing for breath-by-breath evaluation.
The Science Behind Capnography
The fundamental principle of capnography relies on the physical property of carbon dioxide molecules absorbing infrared light. A capnograph device works by shining an infrared light beam through a sample of the patient’s exhaled gas. As the \(\text{CO}_2\) molecules pass through the light path, they absorb a specific wavelength of the infrared radiation.
A detector measures the amount of light that passes through the gas sample without being absorbed. The amount of light absorbed is directly proportional to the concentration of \(\text{CO}_2\) present in the breath. This measurement is then converted into a partial pressure value, typically expressed in millimeters of mercury (\(\text{mmHg}\)).
The resulting \(\text{EtCO}_2\) value, normally ranging between 35 and 45 \(\text{mmHg}\) in a healthy individual, serves as a reliable proxy for the partial pressure of \(\text{CO}_2\) in the arterial blood (\(\text{PaCO}_2\)). This close relationship occurs because the gas exhaled at the end of a breath comes from the alveoli, the primary sites of gas exchange.
Understanding the Capnogram Waveform
The capnogram is a time-based graph of \(\text{CO}_2\) concentration, displaying the four distinct phases of a normal breathing cycle as a characteristic, nearly rectangular shape. Phase I, the inspiratory baseline, is a flat line where the \(\text{CO}_2\) level is near zero, representing the inhalation of \(\text{CO}_2\)-free gas.
Phase II, the expiratory upstroke, begins as exhalation starts and the \(\text{CO}_2\) concentration rapidly rises. This phase reflects the transition from exhaling gas from the conducting airways, which do not participate in gas exchange, to the \(\text{CO}_2\)-rich air from the alveoli.
Phase III is the alveolar plateau, where the waveform flattens out as mostly alveolar gas is being exhaled. The \(\text{EtCO}_2\) value is recorded at the very end of this plateau, representing the maximum \(\text{CO}_2\) concentration for that breath.
Phase IV, the inspiratory downstroke, is the sudden, sharp drop back to the zero baseline. This drop signifies the start of the next inhalation, as the patient begins to draw in fresh air.
Primary Applications of Capnography
Capnography is used across various healthcare settings, including emergency medicine, operating rooms, and critical care units. One primary application is confirming the correct placement of an endotracheal tube (ETT) immediately following intubation. A sustained, square-shaped capnogram confirms the ETT is properly situated in the trachea, as the lungs are the only source of significant \(\text{CO}_2\).
The technology is also used for monitoring patients undergoing conscious sedation for minor procedures. Sedation can depress a patient’s respiratory drive, and capnography can provide an earlier warning of hypoventilation than pulse oximetry, which monitors oxygen levels.
During cardiopulmonary resuscitation (\(\text{CPR}\)), \(\text{EtCO}_2\) is a direct indicator of the effectiveness of chest compressions. Effective compressions circulate blood to the lungs, allowing \(\text{CO}_2\) to be exhaled, and an \(\text{EtCO}_2\) reading consistently above 10 \(\text{mmHg}\) suggests adequate blood flow. A sudden, significant rise in \(\text{EtCO}_2\), often to above 45 \(\text{mmHg}\), can be the first indication that the patient has achieved a return of spontaneous circulation (\(\text{ROSC}\)).
What Abnormal Readings Indicate
Deviations from the normal capnogram shape and numerical value signal specific physiological problems requiring immediate attention. A sudden, complete loss of the waveform indicates a catastrophic event, such as the endotracheal tube being displaced into the esophagus or a ventilator circuit disconnection.
An elevated \(\text{EtCO}_2\) reading, typically above 45 \(\text{mmHg}\), suggests hypoventilation. This means the patient is not breathing frequently or deeply enough to eliminate \(\text{CO}_2\) effectively. This pattern can be seen in patients who have received narcotic pain medication or have an underlying head injury. Conversely, a low \(\text{EtCO}_2\), below 35 \(\text{mmHg}\), points to hyperventilation, where the patient is blowing off \(\text{CO}_2\) too quickly, often due to pain, anxiety, or a metabolic issue.
A distinct “shark fin” appearance, where the Phase III plateau slopes steeply upward, is characteristic of a small airway obstruction, such as bronchospasm in asthma or chronic obstructive pulmonary disease (\(\text{COPD}\)). This shape occurs because air-trapping causes the \(\text{CO}_2\) to be exhaled slowly. A sudden, sharp decrease in \(\text{EtCO}_2\) with an otherwise normal waveform shape can be an early sign of a sudden loss of pulmonary blood flow, potentially caused by a massive pulmonary embolism or a rapid drop in cardiac output.