Continuous End-Tidal Carbon Dioxide Monitoring
End-tidal carbon dioxide provides a noninvasive continuous measurement of ventilation12 or inhaled and exhaled carbon dioxide concentration commonly referred to as capnography. A capnograph depicts this measurement as a graphic picture or waveform tracing of each respiratory cycle. The partial pressure of end-tidal CO2 is assumed to represent alveolar gas, which under normal ventilation/perfusion matching in the lungs closely parallels arterial levels of CO2.
PREREQUISITE NURSING KNOWLEDGE
• Capnography provides the clinician with a calculation of airway respiratory rate (RR), and the combination of both end-tidal carbon dioxide (PetCO2) and RR can provide clinicians with one of the earliest indications that ventilation is hindered. The carbon dioxide waveform changes immediately on any degradation in quality of breathing and some of the most pertinent real-time information provided to give the caregiver an immediate alert to respiratory compromise, such as hypoventilation, airway obstruction, or cessation of breathing.6 PetCO2 monitoring is the earliest indicator of airway compromise, and in the aforementioned study, abnormal PetCO2 findings were observed with many acute respiratory events. In another study, acute respiratory events were found to cause PetCO2 abnormalities seen before oxygenation desaturation or observed hypoventilation.3
• Capnography can be thought of as the “ventilation vital sign” because it provides breath-to-breath feedback and generates a respiratory rate that is measured at the airway. The clinician now has the ability to measure respiratory frequency and detection of respiratory depression in patients who are not intubated sooner than with traditional monitoring techniques, which allows for safer titration of medications.6
• Ventilation is the bulk movement of gases into and out of the lung and is composed of two distinct processes: inspiration and expiration.
During inspiration, gas is delivered to the alveoli, at which time it participates in gas exchange. Oxygenation occurs when the oxygen diffuses across the alveolar membrane into the blood. CO2 exchange occurs during this time as it diffuses across the alveolar membrane into the alveoli. Oxygenated blood is then distributed to and metabolized by the cells of muscles and organs. Oxygen saturation can be evaluated with a blood gas machine (oxygen saturation [SaO2]) or pulse oximetry (SpO2).
During expiration, alveolar gas is exhaled, which results in the elimination of CO2. Cells produce carbon dioxide (CO2) as a by-product of metabolism; this CO2 is transported by the vascular system to the lungs where it is eliminated through exhalation. For exhaled CO2 to be detected, adequate circulation must carry CO2-laden blood from the peripheral tissues to the lungs and adequate ventilation must carry CO2 from the lungs to the mouth.6 CO2 elimination can also be evaluated with a blood gas machine (SaO2) PetCO2 monitor/capnography.
• Just as capnography cannot measure oxygenation, pulse oximetry cannot directly measure ventilation or alveolar ventilation (but SpO2 can provide some directional reflection of ventilation changes if the patient is breathing room air). To assist in the complete monitoring of the patient’s respiratory status, the two parameters must be used together: pulse oximetry to assess how well oxygen has moved across the alveolar capillary membrane into the blood to be transported to the tissues, and capnography to determine how well the patient is ventilating through the process of moving air in and out of the lungs and exhaling carbon dioxide.6
• For a patient who is not intubated who needs capnography, a specialized nasal cannula delivers supplemental oxygen and measures PetCO2, apneic events, and respiratory rate. Placing the capnography cannula is the same as initiating a nasal cannula for supplemental oxygen. Breath samples are obtained through both nostrils, and oxygen is delivered through the nasal prongs (design depends on manufacturer). An extension in the front of the mouth can be used for patients who breathe by mouth.6 PetCO2 can be monitored in patients who are intubated; the sensor is directly connected to the ventilator circuit.
• The principles of arterial blood gas sampling (see Procedure 64) and interpretation should be understood.
• Indications for continuous end-tidal CO2 monitoring include the following:
Determine a baseline CO2 waveform and PetCO2.1,15
Continuously monitor the patency of the airway and the presence of breathing.
Provide mechanism for early detection of changes in waveform pattern or PetCO2 value that may accompany a sudden or gradual change in CO2 production or elimination (permissive hypercapnia, hyperthermia, hypoventilation [extubation], hyperventilation therapy), or reduction in circulation (pulmonary blood flow).1,4,11,13,15
• Basic principles of PetCO2 monitoring should be understood. The end-tidal CO2 monitor may be a stand-alone system, a module incorporated into the patient’s bedside physiologic monitor or incorporated into a mechanical ventilator. An infrared capnograph passes light through an expiratory gas sample and, with a photodetector, measures absorption of that light by the gas. The capnograph determines the amount of CO2 in the gas sample based on the absorption properties of CO2
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