Correct analysis of a capnogram requires systematic evaluation of the following steps. First, it is important to determine whether or not there is exhaled CO 2. This is very important for early recognition of esophageal intubation. Second, the four phases of the waveform should be analyzed. The capnogram represents the total CO 2 eliminated by the lungs given that no gas exchange occurs in the airways. Expired gas contains CO 2 from three sequential compartments: phase I contains gas from the apparatus and anatomical dead-space, phase II represents increasing CO 2 concentration resulting from progressive emptying of alveoli, and phase III represents essentially alveolar gas. Phase III is often referred to as the plateau, as it is flat or has a small positive slope ( Fig, 1). The highest point is the end-tidal PCO2

(PetCO2). Therefore the expiratory capnogram is a technique that provides qualitative information on the waveform patterns associated with mechanical ventilation and a quantitative estimation of arterial PCO2 (PaCO2) from PetCO2. Additionally, the minimum PCO2 should be determined and the PaCO2 - PetCO2 gradient should be calculated. When all this information is analyzed, we can search for the causes of hypo- or hypercapnia, if present, such as inadequate alveolar ventilation, CO 2 rebreathing, or excessive CO2 output.

Fig. 1 Expired capnograms from patients receiving mechanical ventilation: (a) a normal capnogram; (b) a capnogram produced during an episode of airway obstruction in a patient with chronic obstructive pulmonary disease.

The shape of the capnogram can be characteristic in many clinical situations. The absence of an alveolar plateau is the result of the presence of alveoli with long time constants that continuously contribute to increasing the concentration of CO 2 in the airways (Fig, 1). This usually occurs in patients with chronic obstructive pulmonary disease or asthma. A cleft just before end-expiration may be seen in patients fighting the ventilator or recovering from neuromuscular blockade. Oscillations during phase III are caused by beating of the heart against the lungs and are generally observed in patients ventilated at lower respiratory rates. Moreover, continuous capnographic monitoring at low-speed recording provides information about disturbances in the pulmonary circulation, and exhaled CO 2 tracings are also useful for detecting episodes of hypoventilation during mechanical ventilation ( Hess.19.9.0).

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