Lung volumes

Lung volumes are classically described as composed of four components: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume

(Fig 1). Combinations of these volumes form four capacities. All the lung volumes taken together constitute the total lung capacity. The inspiratory reserve volume, tidal volume, and expiratory reserve volume form the vital capacity. The expiratory reserve volume and the residual volume form the functional residual capacity. The functional residual capacity is the volume of gas in the lungs at the end of normal exhalation. It represents a balance between the elastic nature of the lungs to collapse inwards and of the chest wall to spring outwards. The closing capacity approximates the residual volume in normal individuals, representing the volume at which small airways close, inhibiting further flow of gas. The relationship between the closing capacity and the functional residual capacity forms the basis for the pathophysiological ventilatory changes found in the postoperative setting. Normally the functional residual capacity is always greater than the closing capacity. During a tidal breath the small airways remain open throughout inspiration and expiration. If the closing capacity becomes greater than the functional residual capacity, a tidal breath may open small airways only partially or not at all. This results in areas of atelectasis and mismatched ventilation and perfusion (low ventilation-to-perfusion ratio) (Fig 1).

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Fig. 1 Normal lung volumes are composed of the tidal volume (Vt), the inspiratory reserve volume (IRV), the expiratory reserve volume (ERV), and the residual volume (RV). The capacities are the vital capacity (VC), the inspiratory capacity (IRC), and the total lung capacity (TLC). The closing capacity (CC) is the volume of gas in the lungs at which the small airways begin to close. Normally it is very close to the residual volume. Surgery produces a restrictive ventilatory defect, reducing all the lung volumes, with the functional residual capacity and the total capacity sometimes becoming less than the closing capacity. As a result, portions of the lung are not ventilated because the airways remain closed throughout inspiration and expiration. The gas in unventilated alveoli is absorbed and atelectasis occurs.

Thoracic and abdominal surgery produces a restrictive ventilatory defect. All the lung volumes are reduced. The vital capacity and functional residual capacity can be reduced as much as 50 to 70 per cent during the first 24 h after surgery. These changes occur shortly after anesthetic induction and can last as long as 7 to 14 days postoperatively. Pain and splinting can contribute to the reduced lung volumes; although effective pain control with epidural analgesia and nerve blocks may not restore lung volumes to presurgery levels, these can help. General anesthesia induces a number of changes that reduce lung volumes. These include relaxation of the diaphragm, chest wall relaxation, loss of respiratory compliance, and shift of extracellular fluid volume from the chest into the abdomen.

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