Figure 14 The work of breathing in asthma (A) and interstitial fibrosis (B), as depicted by the

Figure 14 The work of breathing in asthma (A) and interstitial fibrosis (B), as depicted by the area of a pressure-volume (PV) diagram (in this case, transpulmonary pressure is recorded as a negative number representing pleural pressure minus alveolar pressure). In asthma, there is a normal amount of elastic work of breathing (area bounded by the volume axis and the solid PV line); however, severe airway narrowing results in an increase in the flow-resistive work of both inspiration (area bounded by the dashed line to the right of the solid line) and expiration (area bounded by the dashed line to the left of the solid line). Notice that the expiratory flow work loop falls outside the limits of elastic work (dashed line portion to the left of the volume axis), indicating that active expiratory muscle force must be used to generate expiratory flow. In fibrosis, elastic work is greatly increased as shown by a shift of the solid PV line down and to the right, with an increase in the elastic work area (area bounded by the volume axis and the solid line); however, flow-resistive work (area within the dashed lines) is normal. (From Chemiack RM. Pulmonary Function Testing. Philadelphia: Saunders, 1977, Fig. 2-19.)

Figure 15 Increased energy expenditures in normal and diseased respiratory states. The plot shows the level of oxygen consumption in relation to the minute ventilation. Notice the markedly increased energy requirement of increased minute ventilation in emphysema, obesity, and congestive heart failure compared with the normal condition. (From Cherniack RM. Pulmonary Function Test­ ing. Philadelphia: Saunders, 1977, Fig. 2-21.)