Initially, it seemed a paradox that the surgical removal of
lung tissue in patients with severe emphysema resulted in improvement
in pulmonary mechanics, symptoms, and exercise capacity. The data now
emerging have explained this largely on the basis of an increase in
lung elastic recoil and improvement in respiratory muscle function.
Changes in lung structure due to emphysema leads to a “mismatch” in
the size of the patient’s lung and chest wall. The lung is too large
for the chest wall, placing the diaphragm in a flat and short position,
which is suboptimal for force generation. Conversely, the chest wall is
too small to allow the lung to expand to a volume at which elastic
recoil pressures are high enough to maintain adequate flow rates and
tether airways to avoid air trapping. The removal of lung tissue allows
the remaining lung to expand enough to achieve a more normal recoil
pressure, maintaining the patency of surrounding airways, and
increasing the driving pressures for expiratory flow.
FEV1, FVC, and measures of hyperinflation, most
particularly the ratio of residual volume (RV) to total lung capacity
(TLC), all improve. The smaller lung allows recovery to normal
diaphragmatic curvature and force generation.