The capnographic waveform is rich in physiological information.
End-tidal PCO2 lies roughly 2-5 mm Hg below the arterial PCO2 in health, but the relationship is quite unreliable during critical illness.
In steady-state, end-tidal PCO2 is unrelated to cardiac output but, when the circulation is changing, a relationship develops.
Capnography is useful in confirming endotracheal tube position, predicting fluid-responsiveness, guiding resuscitation from cardiac arrest, and monitoring conscious sedation.
Capnography relies on the unique property of CO2 to absorb infrared radiation at a specific wavelength.1 This allows the continuous and quantitative measurement of CO2 in the airway gas, illustrated as a capnographic waveform. For mainstream capnography, the CO2 detector is located within the breathing circuit, between the endotracheal tube and Y connector, and is only suitable for patients with an artificial airway. Often, an additional flow sensor is incorporated: integration of flow yields volume, facilitating the display of PCO2 versus volume (volume-capnography). In contrast, sidestream devices continuously draw a sample from the airway gas, which is then transported to an adjacent sensor. Sidestream capnography can be used for ventilated patients and also for those without artificial airways. Since exhaled volume cannot be measured with this approach, sidestream devices display only PCO2 versus time (time-capnography).
Time-capnography, which is less complex and less expensive than volume-capnography, is now commonplace in ICUs. Because it can be used in nonintubated patients, it is finding increasing use also for monitoring conscious sedation and patients judged at high risk of occult ventilatory insufficiency. Volume-capnography, on the other hand, yields much more information, including total exhaled CO2 volume and various dead space calculations.
The time capnogram has four phases: (1) the period of inspiration and very early expiration when airway PCO2 is zero; (2) the ascending phase as anatomic dead space gas, increasingly mixed with CO2-containing alveolar gas, passes the sensor; (3) the alveolar plateau consisting of alveolar gas; and (4) the inspiratory drop as fresh gas again passes the sensor (Fig. 47-1). The maximal PCO2 value is usually at end-expiration (end-tidal CO2 or ET-CO2). Volume capnography shows only phases I-III, ending as expiratory flow ceases (once the lung reaches FRC or the subsequent breath begins). The value of ET-CO2, the shape of the capnogram (width and amplitude), and the various slopes and angles measured have clinical correlations as discussed below.
Schematic plot of a time capnogram, showing all 4 phases, α angle and ET-CO2.
The clinical value of capnography finds its basis in the determinants of alveolar PCO2 (PACO2). In ...