| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
LETTER TO THE EDITOR The following is the abstract of the article discussed in the subsequent letter:
We hypothesized that structural airway remodeling contributes to airways hyperresponsiveness (AHR) in asthma. Small, medium, and large airways were analyzed by computed tomography in 21 asthmatic volunteers under baseline conditions (FEV1 = 64% predicted) and after maximum response to albuterol (FEV1 = 76% predicted). The difference in pulmonary function between baseline and albuterol was an estimate of AHR to the baseline smooth muscle tone (BSMT). BSMT caused an increase in residual volume (RV) that was threefold greater than the decrease in forced vital capacity (FVC) because of a simultaneous increase in total lung capacity (TLC). The decrease in FVC with BSMT was the major determinant of the baseline FEV1 (P < 0.0001). The increase in RV correlated inversely with the relaxed luminal diameter of the medium airways (P = 0.009) and directly with the wall thickness of the large airways (P = 0.001). The effect of BSMT on functional residual capacity (FRC) controlled the change in TLC relative to the change in RV. When the FRC increased with RV, TLC increased and FVC was preserved. When the relaxed large airways were critically narrowed, FRC and TLC did not increase and FVC fell. With critical large airways narrowing, the FRC was already elevated from dynamic hyperinflation before BSMT and did not increase further with BSMT. FEV1/FVC in the absence of BSMT correlated directly with large airway luminal diameter and inversely with the fall in FVC with BSMT. These findings suggest that dynamic hyperinflation caused by narrowing of large airways is a major determinant of AHR in asthma. Overestimation of lung volume
Overestimation of lung volume
To the Editor: The assumptions, speculations, and conclusions developed in the paper of Brown et al. (1) are of interest. However, there are two major points of contention.
The authors have reported in their obstructive asthmatic subjects following bronchodilation usual findings, i.e., increase in forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), and FEV1/FVC, but also an unusual one, a rather large decrease (0.64 liter) in total lung capacity (TLC). TLC was measured with the plethysmographic method. In obstructive patients, thoracic gas volume is overestimated by the plethysmographic method (4, 8, 9). The overestimation is related to the degree of airway obstruction (4, 8, 9), frequency of panting (6, 7), and compliance of upper airways (3, 9). According to these data and recent international recommendations (10), there is no overestimation of lung volume when panting is done at 1 Hz or <1 Hz. Surprisingly, the authors have measured TLC at panting frequencies between 1 and 3 Hz. In chronic obstructive pulmonary disease patients, TLC between 1.5 and 2 Hz was overestimated on the average by 0.58 liter and between 2.5 and 3 Hz by 1.49 liters (7). Similarly, in asthmatic subjects during induced bronchospasm, at a panting frequency of 2.7 Hz, thoracic gas volume was overestimated on the average by 1.2 liters (6). Therefore, one may conclude that changes in TLC in this paper are in error, an explanation also advanced by the accompanying editorial (2). For a given frequency, TLC overestimation increases with the degree of airway obstruction (6). Since airway obstruction was more severe at baseline than following bronchodilation, the actual TLC would be lower than that reported. Therefore, there would be less or no change in TLC following albuterol. Functional residual capacity and residual volume are also, necessarily, biased: they are derived from TLC measurements. The higher TLC before albuterol was attributed by the authors (1) to a change in the balance between the outward force of the thorax and the elastic recoil of the lung. In acute induced asthma, spurious loss of lung recoil pressure and shift to the left of the static pressure-volume curve were shown to be the consequences of a spurious increase in plethysmographic measured TLC (5).
The second point of contention is the measurement of lung function and of airways size, by computed tomography, in two different body positions. Well aware of changes in functional residual capacity and airway resistance from upright to supine position, the authors "... do not think it affected the validity of our comparisons between patients ... since it occurred in all volunteers" (italics mine). Comparisons were, however, not done between patients but in all patients in two different positions. Position was linked with procedure: in a given patient airways were visualized in supine position, but lung volumes were measured in upright position, therefore biasing the comparison.
REFERENCES
nescu DC. Demonstration of failure of body plethysmography in airway obstruction. J Appl Physiol 52: 949–954, 1982.nescu DC. Reassessment of lung volume measurement by helium dilution and by body plethysmography in chronic air-flow obstruction. Am Rev Respir Dis 126: 1040–1044, 1982.[ISI][Medline]nescu DC. Elastic properties of the lung in acute induced asthma. J Appl Physiol 54: 152–158, 1983.nescu DC. Frequency dependence of plethysmographic volume in healthy and asthmatic subjects. J Appl Physiol 54: 159–165, 1983.nescu DC, Rodenstein DO, Cauberghs M, and Van de Woestijne KP. Failure of body plethysmography in bronchial asthma. J Appl Physiol 52: 939–948, 1982.nescuThis article has been cited by other articles:
|
|
 |
|
  P. Macklem Commentary on "The role of the large airways on smooth muscle contraction in asthma" J Appl Physiol, October 1, 2007; 103(4): 1459 - 1459. [Full Text] [PDF]
|
|
|
|
|
|
R. H. Brown, D. B. Pearse, G. Pyrgos, M. C. Liu, A. Togias, and S. Permutt REPLY FROM DRS. BROWN, PEARSE, PYRGOS, LIU, TOGIAS, AND PERMUTT J Appl Physiol, December 1, 2006; 101(6): 1813 - 1813. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
