Physiologic basis for improved pulmonary function after lung volume reduction

Abstract

It is not readily apparent how pulmonary function could be improved by resecting portions of the lung in patients with emphysema. In emphysema, elevation in residual volume relative to total lung capacity reduces forced expiratory volumes, increases inspiratory effort, and impairs inspiratory muscle mechanics. Lung volume reduction surgery (LVRS) better matches the size of the lungs to the size of the thorax containing them. This restores forced expiratory volumes and the mechanical advantage of the inspiratory muscles. In patients with heterogeneous emphysema, LVRS may also allow space occupied by cysts to be reclaimed by more normal lung. Newer, bronchoscopic methods for lung volume reduction seek to achieve similar ends by causing localized atelectasis, but may be hindered by the low collateral resistance of emphysematous lung. Understanding of the mechanisms of improved function after LVRS can help select patients more likely to benefit from this approach.

Figure 1.

(A) Effects of lung volume reduction surgery (LVRS) that removes only cysts and bullae. The dashed line represents the static relationship between pleural pressure and lung volume, as might be recorded as a subject with emphysema makes a very slow (quasi-static) inspiration from residual volume (RV) to total lung capacity (TLC). VC is represented by the difference on the ordinate between TLC and RV. Maximal elastic recoil pressure is shown by the double-headed arrows at TLC. The slope of the relationship is lung compliance. The line labeled “inspiratory muscle capacity” represents the chest wall pressure–volume relationship during maximal inspiratory muscle contraction. It could be recorded by measuring the maximal negative pleural pressure as a subject makes a series of inspiratory efforts against an occluded airway at various lung volumes. TLC is reached when the increasing recoil of the lung equals the diminishing maximal recoil of the chest wall. Effects of LVRS are shown by the thin vertical line. Because this LVRS removed only destroyed lung, which does not contribute to lung elastic properties, compliance is unchanged. RV is reduced, and TLC is reduced by a lesser amount because the muscles can stretch the remaining lung further. The difference between them, the VC, increases. Recoil pressure also increases, but this does not cause the increase in VC. (B) Effects of LVRS in a patient with diffuse emphysema. In this example, the resected lung includes parenchyma, which has some elastic recoil. Its removal decreases the compliance of the lung left behind. Note that now the recoil pressure rises by more than in (A), but the VC improves by less. If LVRS impairs intrinsic muscle function, the curve labeled “inspiratory muscle capacity” would shift downward. This would also limit the improvement in VC.

Figure 1.

(A) Effects of lung volume reduction surgery (LVRS) that removes only cysts and bullae. The dashed line represents the static relationship between pleural pressure and lung volume, as might be recorded as a subject with emphysema makes a very slow (quasi-static) inspiration from residual volume (RV) to total lung capacity (TLC). VC is represented by the difference on the ordinate between TLC and RV. Maximal elastic recoil pressure is shown by the double-headed arrows at TLC. The slope of the relationship is lung compliance. The line labeled “inspiratory muscle capacity” represents the chest wall pressure–volume relationship during maximal inspiratory muscle contraction. It could be recorded by measuring the maximal negative pleural pressure as a subject makes a series of inspiratory efforts against an occluded airway at various lung volumes. TLC is reached when the increasing recoil of the lung equals the diminishing maximal recoil of the chest wall. Effects of LVRS are shown by the thin vertical line. Because this LVRS removed only destroyed lung, which does not contribute to lung elastic properties, compliance is unchanged. RV is reduced, and TLC is reduced by a lesser amount because the muscles can stretch the remaining lung further. The difference between them, the VC, increases. Recoil pressure also increases, but this does not cause the increase in VC. (B) Effects of LVRS in a patient with diffuse emphysema. In this example, the resected lung includes parenchyma, which has some elastic recoil. Its removal decreases the compliance of the lung left behind. Note that now the recoil pressure rises by more than in (A), but the VC improves by less. If LVRS impairs intrinsic muscle function, the curve labeled “inspiratory muscle capacity” would shift downward. This would also limit the improvement in VC.