Monitoring of Patients with Airway/Respiratory Disease
Capnography: A Valuable Tool for Airway Management

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Capnography provides continuous, dynamic assessment of the ventilatory status of patients. Carbon dioxide physiology and the technology utilized in end-tidal carbon dioxide monitor devices are reviewed. Clinical applications with regard to ventilation and airway management are discussed, including: verification of endotracheal tube placement, continuous monitoring of tube position, monitoring during procedural sedation and in the obtunded patient, and assessment of patients with respiratory illnesses. Current guidelines for use of capnography within emergency medicine are included. Potential future applications are also presented.

Section snippets

Terminology

The maximum partial pressure of CO2 obtained at the end of an exhaled breath is referred to as end–tidal CO2 (EtCO2). A capnometer reports the result as a numeric value. A capnograph adds a graphic display of a waveform representing expired CO2 as a function of either volume or time, the latter being used most commonly in emergency medicine and emergency medical services (EMS). Such a waveform is referred to as the capnogram (Fig. 1). Capnography provides two distinct advantages over using

Technology

The initial technology that brought capnography into widespread clinical use was mass spectrometry. Because of cost and complexity, however, this has not been used outside of the operating room. The more recent introduction of infrared spectroscopy has allowed for the development of capnographs, which are used in the ED and EMS settings.12 Capnography based on infrared spectroscopy works on a similar principle as pulse oximetry. A beam of filtered infrared radiation is sent from a light source

Carbon dioxide physiology

Reviewing the production, transport, and elimination of carbon dioxide provides a framework for interpreting capnographic results, as alterations in any of these physiologic processes can effect EtCO2.

CO2 is produced in tissues as a by-product of aerobic metabolism. Therefore, any stimulus prompting increased metabolism will result in additional CO2 production. This may include pathophysiologic conditions (eg, fever, sepsis, hyperthyroidism, trauma, and burns) and nonpathologic conditions (eg,

Physiology of the waveform

Providing a graphic representation of the concentration of CO2 over either time or volume creates a waveform. Here, the authors discuss only time-based capnograms.

A normal waveform has a trapezoidal appearance, comprised of four different phases of the respiratory cycle (see Fig. 1).20 At the onset of exhalation, CO2-poor atmospheric air at the sensor gives a baseline reading of zero (I). Shortly after exhalation begins and the air from anatomic dead space has cleared, synchronous release of

Confirming Endotracheal Intubation

There are numerous clinical indications for using capnography with regard to airway management. The most common of these is for confirmation of endotracheal intubation. The ability to distinguish between endotracheal and esophageal tube placement is crucial; failure to recognize and promptly correct a misplaced tube can be catastrophic. Studies have shown that 5% to 10% of intubation attempts in emergent settings result in esophageal tube placement, with even higher rates reported when

Ventilatory Monitoring During Procedural Sedation and Analgesia

Management of pain and anxiety surrounding urgent and emergent procedures is a priority for EMS and in the ED. Consequently, procedural sedation and analgesia (PSA) have become integral to the practice of emergency medicine. Many PSA agents have the potential to cause respiratory depression and airway compromise. Early recognition and response to such potentially serious events is paramount. Detection traditionally has relied on continuous monitoring of heart rate, respiratory rate, and oxygen

Future advanced airway uses

Novel uses of capnography for airway management continue to emerge. Blind nasal intubation for EMS and in the ED has become rare since the advent of newer technologies; however, this remains a rescue option for the unanticipated difficult airway.34 Capnography can be a useful adjunct for this technique, as passage of the tube toward the esophagus leads to declining EtCO2 values and loss of the waveform, which suggests the need for tube redirection.73 Similarly, small case series have described

Metabolism and perfusion

This article has focused on the applications of capnography related to ventilation and airway management. It is worth noting, however, that as a reflection of CO2 physiology, capnography can assess changes in perfusion and metabolism. Numerous studies in the emergency medicine literature have demonstrated the utility of capnography as a means of assessing perfusion during cardiac arrest. In particular, EtCO2 monitoring can be used to evaluate the adequacy of cardiopulmonary resuscitation (CPR),

Summary

Capnography provides a noninvasive means for measuring a patient's ventilatory status. It has been shown to be an effective clinical tool for:

  • Confirming and monitoring endotracheal intubations

  • Monitoring patients who have depressed mental status and during procedural sedation

  • Evaluating those who have obstructive airway diseases such as asthma

Many of these indications are endorsed by national position statements and guidelines. Additional novel uses for capnography as an adjunctive tool during

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