Fig. 6

Illustration of thoracic distension mechanism based on airway pressure, flow and CO₂ analysis. This figure illustrates from top to bottom, airway pressure (Paw), flow at airway opening (Flow) and expired CO₂ (CO₂) tracings obtained in cadavers (panel A), bench (panel B) and animals (panel C). The left column illustrates thoracic distension, while the right column represents regular pattern. For each situation, the two gray vertical tilted lines define the time for the lung volume to return to FRC (time with thorax above FRC), while the two black vertical tilted lines define the expiration time (time between two insufflations). Positive flow indicates decompression or insufflation. Negative flow indicates compression or exhalation. Please note the exact time correspondence between flow and CO₂ oscillations whatever the situation. During expiration, in case of thoracic distension (left column), the flow does not return to zero line during a couple of CC indicating that the thorax is still above FRC even during the decompression phase. CO₂ oscillations resume only once the flow crosses the zero line, thus indicating the return of lung volume to FRC. On the contrary, the right column obtained with a smaller Vt illustrates that the flow induced by CC crosses the zero line immediately after insufflation generating CO₂ full oscillations. This specific full oscillating CO₂ pattern indicates that chest compressions operate close to FRC