Clinical measurement of end-expiratory lung volume on the intensive care unit

➥ normal inspiration and expiration. In adults it is about 700 ml (10 ml/kg). Expiratory reserve volume (ERV) is the gas volume additionally exhaled during forced expiration and inspiratory reserve volume (IRV) is that after a forced inspiration. The sum of VT and IRV is the inspiratory capacity (IC). Even after a maximum expiratory effort, some air is left in the lung; no lung region normally collapses. This persisting gas volume is called residual volume (RV) and amounts to 2–2.5 litres in adults. Expiration stops at residual volume for two reasons: distal airways of diameter ≤2 mm will close before the alveoli collapse;14 the chest wall, rib cage and diaphragm cannot be distorted to the point where all gas in the lung has been exhaled. The EELV – the gas volume remaining in the lungs after an ordinary expiration – amounts to about 3–4 litres, depending on sex, age, height, and weight.15 The existence of this persisting gas volume prevents collapse of the alveoli and ensures that the variation of gas concentration inside the alveoli is minimised. The gas volume in the lung after a maximum inspiration is called the total lung capacity (TLC). In adults, typically, it is 6–8 litres. The volume expired after a maximal inspiration is called the vital capacity (VC) and is therefore equal to the difference between TLC and RV.