• 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.

• 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:

• 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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