Capnography: A Feasible Tool in Clinical and Experimental Settings.

Capnography is the monitoring of the partial pressure of alveolar carbon dioxide (CO2) in the respiratory gases. It is a useful noninvasive clinical tool for assessing efficiency and optimizing mechanical ventilation.1 The use of capnography for monitoring surgical patients during anesthesia and in the emergency department to confirm artificial airway placement is well-established and recommended.2 Capnography’s importance as a standard of monitoring and patient safety in the ICU has been confirmed in recent years.1,3 Clinical uses of capnography in the ICU also include indirect assessment of cardiac output during weaning from cardiopulmonary bypass in patients without significant lung disease, monitoring of patients during changes in bed positioning, prognostic indicator of outcome in cardiac arrest, adjustment of the trigger sensitivity, and assessment of pulmonary circulation, recognizing the presence of pulmonary embolism as well as the effectiveness of chemical thrombolysis.4-8 Expiratory partial pressure of CO2 can be plotted against time (time-based capnography) or expired volume (volumetric capnography) on each breath. The capnogram is a graphical representation of the concentration or partial pressure of inhaled and exhaled CO2. Measurement of PaCO2 in arterial blood gas, although a true reflection of ventilatory efficiency, is far from ideal because of its invasive and intermittent nature. The main advantage of capnography over arterial blood gas is the ability to provide continuous CO2 monitoring, thus making trend assessments feasible. Although capnography does not replace arterial blood gas analysis (PaCO2), it may decrease the required frequency. Physiologically based indices derived from the capnographic waveform give direct information about ventilatory misdistributions and functional disturbances. Changes in the morphology of the capnogram indicate ventilation disturbances, and several indices based upon the geometry of the curve were developed in order to quantify ventilation homogeneity distribution.9-11 Moreover, capnography provides an assessment of ventilation/perfusion ratio (V̇/Q̇) mismatch through measurements of end-tidal CO2 (PETCO2). The difference between PaCO2 and PETCO2 in healthy subjects is very small, so PETCO2 may reflect PaCO2. 12 Many clinical situations could change PETCO2 values, influencing the arterial-alveolar CO2 difference, which in healthy individuals is 5 mm Hg.12 This difference can be significantly increased in the presence of lung diseases and some cyanotic heart diseases. Then, in an absence of V̇/Q̇ mismatch, the PETCO2 can be correlated to PaCO2.