In contrast to the disagreement concerning the resistance to flow offered by small airways 2 mm in diameter in normal lungs, all three groups that have made direct measurements of small airways resistance in COPD (including the Belgian group) reported that the small conducting airways 2 mm in diameter become the major site of increased resistance in COPD.
COPD
On theoretical grounds, the substantial increase in peripheral airways resistance measured in COPD is easier to explain by generalized narrowing of most of small airways lumens rather than by a reduction in their number because lumen narrowing increases resistance in proportion to the reduction in the radius raised to the fourth power. However, in a system of tubes arranged in parallel, total resistance at each generation is obtained by addition according to the equation 1/RT = 1/R1+1/R2+1/R3+… + 1/Rn meaning that removal of one-half of the tubes arranged in parallel is required to double their resistance.
Therefore, if we accept the theory on the behavior of tubes arranged in parallel summarized by the above equation and begin with the value 0.70 ± 0.26 cm H2O/L/s measured by Yanai et al in living humans with normal lung function, removal of one-half of the small airways will only increase their resistance to 1.4 cm H2O/L/s. A value well below the 2.78 and 4.59 cm H2O/L/s measured in living people with COPD in the same study. Furthermore, removal of one-half of the remaining airways (ie, a reduction to 25% of their starting numbers) would only increase small airways resistance to 2.8 cm H2O/L/s which is barely within the lower range of the resistance they measured in COPD. A reduction to 12.5% of their starting number is required to increase small airways resistance to 5.6 cm H2O/L/s which falls in the upper range of the values measured in COPD by Yanai et al.
Bignon et al provided the first histologic evidence in support of the theory that the airways narrowed in COPD by showing that the proportion of bronchioles 400 mm in diameter increased in lungs from patients that died of respiratory failure caused by severe emphysema. Although Matsuba and Thurlbeck reported a similar trend in people with less severe emphysema that did not reach statistical significance, they added the caveat that complete destruction of the smaller bronchioles might have buffered the downward shift in small airways diameters measured in their study.
