Heterogeneous airway tone in asthmatic
subjects
G. JULIA-SERDA, N. A. MOLFINO, K. R. CHAPMAN, P. A. McCLEAN, N. ZAMEL, A. S. SLUTSKY, AND V. HOFFSTEIN Respiratory Division, Department of Medicine, University of Toronto, Toronto, Ontario M5B 1 W8, Canada JULIA-SERDA, G., N. A. MOLFINO, K. R. CHAPMAN, P. A. MCCLEAN,N.ZAMEL, A.S. SLUTSKY,AND V. HOFFSTEIN.H~~erogeneous airway tone in asthmatic subjects. J. Appl. Physiol. 73(6): 2328-2332, 1992.---We examined the effect of volume history on the dynamic relationship between airways and lung parenchyma (relative hysteresis)in 20 asthmatic subjects.The acoustic reflection technique was employed to evaluate changesin airway cross-sectionalareasduring a slow continuousexpiration from total lung capacity to residual volume and inspiration back to total lung capacity, Lung volume was measuredcontinuously during this quasi-static maneuver.We studied three anatomic airway segments:extra- and intrathoracic tracheal and main bronchial segments.Plots of airway area vs. lung volume were obtained for each segmentto assessthe relative magnitude and direction of the airway and parenchymal hysteresis. We also performed maximal expiratory flow-volume and partial expiratory flow-volume curves and calculated the ratio of maximal to partial flow rates (M/P) at 30% of the vital capacity. We found that 10 subjects (group I) showeda significant predominanceof airway over parenchymal hysteresis (P < 0.005) at the extra- and intrathoracic tracheal and main bronchial segments;these subjectshad high M/P ratios [I.53 t 0.27 (SD)]. The other 10 subjects (group II) showed similar airway and parenchymal hysteresis for all three segments and significantly lower M/P ratios (1.16 t 0.20, P < 0.01). We conclude that the effect of volume history on the relative hysteresis of airway and lung parenchyma and M/P ratio at 30% of vital capacity in nonprovoked asthmatic subjects is variable. We suggestthat our findings may result from heterogeneousairway tone in asthmatic subjects. hysteresis;airways; lungs; asthma; acoustic reflections THE EFFECT OF A DEEP inspiration
on airway tone has been studied extensively (10, 12, 18, 29, 33, 34). In healthy subjects, maximum inspiration produces no significant change or a slight reduction in airway resistance (Raw) (10,17,29). However, when bronchoconstriction is induced in normal volunteers, deep inspiration causes transient reduction in Raw (16, 29). In stable nonprovoked asthmatic subjects, the airway response to a maximal inspiration is variable: in some patients Raw increases (15, 31), whereas in others it decreases (2) or remains unchanged (24). In the presence of induced bronchoconstriction, some asthmatic subjects show bronchodilation after maximal inhalation (2, 16, 31), whereas others display no significant change (11, 31). Moreover, when bronchospasm is spontaneous, deep inspiration provokes further obstruction (23,35). Although different explanations have been proposed (2,8,X&25), this heterogeneous response remains poorly understood. Because no direct measurements of bronchomotor tone 2328
0161-7567/92
$2.00 Copyright
or airway hysteresis have been performed in asthmatic subjects, it is possible that this apparent heterogeneity reflects, in part, the differences in technique employed to assess airway responses to volume history (e.g., Raw, flow-volume curves, response to bronchoactive agents). Airway hysteresis has been measured in animal models with use of pressure-volume curves (26). Human studies aimed at assessing the effect of volume history on airway tone have been performed employing indirect methods such as Raw (15), specific airway conductance (sGaw) (ll), volume of anatomic dead space (VD) (14), or maximal and partial expiratory flow-volume (MEFV and PEFV) curves (6, 33). A more direct approach for measuring hysteresis of the airway relative to that of the lung parenchyma was proposed by Froeb an .d Mead (1 4). They measured VD at different lung volumes in normal individuals and found that VD was slightly greater after an inspiration to total lung capacity (TLC) than after expiration to residual volume (RV), suggesting that airway hysteresis exceeds parenchymal hysteresis. The purpose of the present study was to examine airway tone in asthmatic subjects. We assessed airway tone by use of measurements of airway hysteresis. To accomplish this, we employed the acoustic reflection technique to investigate the effect of a maximal inhalation on the relationship between airway area and lung volume in asthmatic subjects. This technique allows us to measure airway areas at different lung volumes during quasi-static inspiratory and expiratory maneuvers and, thus, determine relative contributions of airway and parenchyma1 hysteresis. METHODS
Subjects. We studied 20 asthmatic
subjects (6 men and
14 women), aged 18-43 yr [30 t 6.9 (SD)], who satisfied
the American Thoracic Society criteria for asthma (1). Measurement of airway areas. Tracheal and bronchial cross-sectional areas were measured using the acoustic reflection technique. This method has been described in detail (4, 13, 19, 20). Briefly, it employs acoustic pulses that are launched from a loudspeaker into the subject’s respiratory tract. As the waves propagate through the airways, they are reflected by changes in cross-sectional areas. From the knowledge of the intensities of the incident and reflected waves, the area at a given distance from the microphone is computed, and a plot of crosssectional area as a function of distance is obtained (Fig. 1). A spirometer (Med-Science Electronics, St. Louis, MO) located at the en .d of the wave tube monitors changes in lung volume.
0 1992 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.215.017.188) on January 14, 2019.
HETEROGENEOUS
AIRWAY
TONE
12.00 r
I
2 4.80
0.00 L 0.0
I
1
24.0 36.0 48.0 60.0 Distance (cm) FIG. 1. Representative area-distance plot illustrating regional airway segments: ET, extrathoracic; IT, intrathoracic; MB, main bronchial. 12.0
Initially, subjects breathed from a reservoir bag containing 20% O,-80% He. End-tidal N, was sampled continuously (Nitralyzer 505, Med-Science Electronics) to ensure adequate washout with O,-He. When the concentration of N, was
subjects
G. JULIA-SERDA, N. A. MOLFINO, K. R. CHAPMAN, P. A. McCLEAN, N. ZAMEL, A. S. SLUTSKY, AND V. HOFFSTEIN Respiratory Division, Department of Medicine, University of Toronto, Toronto, Ontario M5B 1 W8, Canada JULIA-SERDA, G., N. A. MOLFINO, K. R. CHAPMAN, P. A. MCCLEAN,N.ZAMEL, A.S. SLUTSKY,AND V. HOFFSTEIN.H~~erogeneous airway tone in asthmatic subjects. J. Appl. Physiol. 73(6): 2328-2332, 1992.---We examined the effect of volume history on the dynamic relationship between airways and lung parenchyma (relative hysteresis)in 20 asthmatic subjects.The acoustic reflection technique was employed to evaluate changesin airway cross-sectionalareasduring a slow continuousexpiration from total lung capacity to residual volume and inspiration back to total lung capacity, Lung volume was measuredcontinuously during this quasi-static maneuver.We studied three anatomic airway segments:extra- and intrathoracic tracheal and main bronchial segments.Plots of airway area vs. lung volume were obtained for each segmentto assessthe relative magnitude and direction of the airway and parenchymal hysteresis. We also performed maximal expiratory flow-volume and partial expiratory flow-volume curves and calculated the ratio of maximal to partial flow rates (M/P) at 30% of the vital capacity. We found that 10 subjects (group I) showeda significant predominanceof airway over parenchymal hysteresis (P < 0.005) at the extra- and intrathoracic tracheal and main bronchial segments;these subjectshad high M/P ratios [I.53 t 0.27 (SD)]. The other 10 subjects (group II) showed similar airway and parenchymal hysteresis for all three segments and significantly lower M/P ratios (1.16 t 0.20, P < 0.01). We conclude that the effect of volume history on the relative hysteresis of airway and lung parenchyma and M/P ratio at 30% of vital capacity in nonprovoked asthmatic subjects is variable. We suggestthat our findings may result from heterogeneousairway tone in asthmatic subjects. hysteresis;airways; lungs; asthma; acoustic reflections THE EFFECT OF A DEEP inspiration
on airway tone has been studied extensively (10, 12, 18, 29, 33, 34). In healthy subjects, maximum inspiration produces no significant change or a slight reduction in airway resistance (Raw) (10,17,29). However, when bronchoconstriction is induced in normal volunteers, deep inspiration causes transient reduction in Raw (16, 29). In stable nonprovoked asthmatic subjects, the airway response to a maximal inspiration is variable: in some patients Raw increases (15, 31), whereas in others it decreases (2) or remains unchanged (24). In the presence of induced bronchoconstriction, some asthmatic subjects show bronchodilation after maximal inhalation (2, 16, 31), whereas others display no significant change (11, 31). Moreover, when bronchospasm is spontaneous, deep inspiration provokes further obstruction (23,35). Although different explanations have been proposed (2,8,X&25), this heterogeneous response remains poorly understood. Because no direct measurements of bronchomotor tone 2328
0161-7567/92
$2.00 Copyright
or airway hysteresis have been performed in asthmatic subjects, it is possible that this apparent heterogeneity reflects, in part, the differences in technique employed to assess airway responses to volume history (e.g., Raw, flow-volume curves, response to bronchoactive agents). Airway hysteresis has been measured in animal models with use of pressure-volume curves (26). Human studies aimed at assessing the effect of volume history on airway tone have been performed employing indirect methods such as Raw (15), specific airway conductance (sGaw) (ll), volume of anatomic dead space (VD) (14), or maximal and partial expiratory flow-volume (MEFV and PEFV) curves (6, 33). A more direct approach for measuring hysteresis of the airway relative to that of the lung parenchyma was proposed by Froeb an .d Mead (1 4). They measured VD at different lung volumes in normal individuals and found that VD was slightly greater after an inspiration to total lung capacity (TLC) than after expiration to residual volume (RV), suggesting that airway hysteresis exceeds parenchymal hysteresis. The purpose of the present study was to examine airway tone in asthmatic subjects. We assessed airway tone by use of measurements of airway hysteresis. To accomplish this, we employed the acoustic reflection technique to investigate the effect of a maximal inhalation on the relationship between airway area and lung volume in asthmatic subjects. This technique allows us to measure airway areas at different lung volumes during quasi-static inspiratory and expiratory maneuvers and, thus, determine relative contributions of airway and parenchyma1 hysteresis. METHODS
Subjects. We studied 20 asthmatic
subjects (6 men and
14 women), aged 18-43 yr [30 t 6.9 (SD)], who satisfied
the American Thoracic Society criteria for asthma (1). Measurement of airway areas. Tracheal and bronchial cross-sectional areas were measured using the acoustic reflection technique. This method has been described in detail (4, 13, 19, 20). Briefly, it employs acoustic pulses that are launched from a loudspeaker into the subject’s respiratory tract. As the waves propagate through the airways, they are reflected by changes in cross-sectional areas. From the knowledge of the intensities of the incident and reflected waves, the area at a given distance from the microphone is computed, and a plot of crosssectional area as a function of distance is obtained (Fig. 1). A spirometer (Med-Science Electronics, St. Louis, MO) located at the en .d of the wave tube monitors changes in lung volume.
0 1992 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.215.017.188) on January 14, 2019.
HETEROGENEOUS
AIRWAY
TONE
12.00 r
I
2 4.80
0.00 L 0.0
I
1
24.0 36.0 48.0 60.0 Distance (cm) FIG. 1. Representative area-distance plot illustrating regional airway segments: ET, extrathoracic; IT, intrathoracic; MB, main bronchial. 12.0
Initially, subjects breathed from a reservoir bag containing 20% O,-80% He. End-tidal N, was sampled continuously (Nitralyzer 505, Med-Science Electronics) to ensure adequate washout with O,-He. When the concentration of N, was
