End-Tidal Carbon Dioxide Monitoring: A VITAL Sign Worth Watching

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Pathophysiology

CO2 is produced as an end product of aerobic metabolism and oxygen use by the cells. During inspiration, air that contains minimal CO2 is drawn into the alveoli and dilutes the CO2 present in the functional residual capacities of the lung. At the same time, as the CO2 increases within the cells, CO2 diffuses into the tissue capillaries and is carried by the blood to the lungs. Mixed venous blood returns to the pulmonary capillaries with high levels of CO2, and then diffuses from the pulmonary

Arterial carbon dioxide—end-tidal carbon dioxide gradient

In normal lungs, exhaled CO2 levels closely approximate Paco2 levels [12]. “Carbon dioxide is first exhaled from the anatomic dead spaces, which have nearly infinite ventilation-perfusion (V˙/Q˙) ratios. Gas then comes from low-resistance lung areas with relatively high V˙/Q˙ ratios and low carbon dioxide levels. Finally, high-resistance lung units with lower V˙/Q˙ ratios and the highest carbon dioxide levels are emptied” [5]. At the end of expiration, the maximum partial pressure of CO2 (Petco2

Capnography use in critical care

Petco2 monitoring has many clinical applications in the critical care environment, including verifying and maintaining correct ETT placement, evaluating the effectiveness of cardiopulmonary resuscitation (CPR) compressions and identifying the return of spontaneous circulation, monitoring continuously during procedural sedation and anesthesia, assessing V˙/Q˙ status, evaluating ventilation status during weaning and following extubation, validating end expiration during hemodynamic waveform

Capnography use in other settings

Capnography is also a useful monitoring tool in settings other than critical care units. In prehospital use with emergency medical systems and transport, capnography has been used to verify and revalidate correct ETT placement in this highly mobile environment, evaluate CPR efforts for effectiveness, quickly recognize the return of a spontaneous pulse, and predict outcomes of survival or death in prehospital CPR [4], [14], [17], [22]. In the emergency department, capnography has also been used

Capnography equipment

There are three types of capnography equipment: mainstream, sidestream, and microstream. Mainstream capnography is used in intubated patients. A heated sensor is placed in the airway circuit between the ETT and the ventilator tubing. The exhaled gas flows directly over the sensor, providing an almost instantaneous measurement of Petco2. Disadvantages of microstream capnography are the weight of the sensor (increasing the risk of accidental extubation), the additional dead space within the

The normal capnography waveform

The capnography waveform, or capnogram, provides information about the patient's ventilatory status beyond the CO2 numeric display (Fig. 1). The waveform is read from left to right and represents the CO2 concentration during the respiratory cycle, with each wave representing a single respiratory cycle. The waveform elevates on expiration, representing the increased CO2 concentration in the exhaled gas. The capnogram consists of four phases: baseline, expiratory upstroke, expiratory or alveolar

Why capnography is important: abnormal waveforms

When the ETT is correctly placed in the trachea, a normal capnogram can be seen; however, incorrect placement reveals a flat line and no waveform. A sudden loss of Petco2 to a waveform at baseline or near baseline indicates immediate danger because no respirations are detected (Fig. 2), which could be due to extubation, acute ventilator malfunction, tubing disconnection from the ventilator, a kinked or obstructed ETT, or cardiac arrest [5], [9], [10], [12], [24]. When no waveform is observed,

Summary

Not only should clinicians be aware of the benefits of capnography but they also should be using it in their practices. Learning to analyze a capnography waveform is as easy as monitoring an EKG waveform or a hemodynamic waveform, and the capnography waveform gives much more information than the numeric value alone.

The advantages of capnography in the critical care setting are immense. For patients who are intubated or nonintubated, the nurse can use the shape of the capnogram to determine

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