Fig. 3. A. The work of breathing in relation to vital capacity (VC). B. Point a represents a VT of 500 mL with a VC of 5 L; point b represents a VT of 500 mL with a VC of 1 L. (Shapiro BA, Harrison RA, Cane RD et al: Clinical Application of Respiratory Care, 4th ed. Chicago, Mosby-Year Book Medical Publishers, 1991.)