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On the relationship between response to inhaled methacholine and airway dimensions
Journal of Aerosol Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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On the relationship between response to inhaled methacholine and airway dimensions Owen R. Moss a,n, Michael J. Oldham b,1 a b
POK Research, Apex, NC, USA Virginia Commonwealth University, VA, USA
a r t i c l e i n f o
Keywords: Methacholine Inhalation Lung Morphometry Dosimetry Clinical-trials
abstract Site-specific dosimetry was used to address the hypothesis that, given the airway dimensions of one subject, the airway dimensions of a second subject can be obtained by comparing responses to a modified bronchoconstrictor inhalation-protocol. Fourteen healthy, nonsmoking subjects received a high-resolution computed tomography (HRCT) scan prior to performing a methacholine challenge and measurement of change in forcedexpiratory-volume-in-one-second, (ΔFEV1). Scale factors of average length (L) and diameter (d) of the first six tracheobronchial airway generations were used to construct typical path lungs. The Multiple Path Particle Dosimetry (MPPD) model calculated surface density of methacholine. Airway circumference change, ΔC/C0, was used to calculate ΔFEV1 in terms of airway smooth muscle sensitivity, K (cm/mg/cm2). Virtual protocols were modeled such that fractional changes in ΔFEV1, “ΔY”, were independent of K. Eight subjects responded to methacholine with ΔFEV1 4 3% and Ks from 0.011 to 35.8. For one virtual protocol, the linear relations between ΔY and length and diameter scale factors had R2s respectively of 0.49 and 0.57. Within the limits of these measurements, the hypothesis is demonstrated; — any continuously increasing function of ΔY vs. scale factor being sufficient to allow (without HRCT scans) classification of subjects according to airway size. & 2016 Elsevier Ltd. All rights reserved.
1. Introduction Deposition of inhaled particulate matter in the human respiratory tract primarily depends on properties of the aerosol and physiology (i.e., breathing pattern and airway anatomy). If an aerosol, that provokes a smooth muscle response, is inhaled by separate individuals, each having different but unique breathing patterns and lung-airway anatomy, then any difference in biological response should be explained by a combination of two factors; site-specific dose and airway smooth muscle sensitivity. By normalizing biological response to airway smooth muscle sensitivity, some insight should be possible into airway anatomy. In a previously reported human pilot study (Oldham, Clinkenbeard & Moss, 2013), subjects inhaled methacholine and their response (change in forced expiratory volume in one second, ΔFEV1) was compared to lung airway anatomy. Based on the data available from this previous report, the research described here extends their findings by using site-specific
n
Corresponding author. 1
Current address: Altria Client Services
http://dx.doi.org/10.1016/j.jaerosci.2016.01.010 0021-8502/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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dosimetry to quantitate airway smooth muscle sensitivity to methacholine. This quantitation of airway smooth muscle sensitivity is then used to address the hypothesis that given the airway dimensions of one subject, the airway dimensions of a second subject can be obtained by comparing responses to an inhaled bronchoconstrictor.
2. Materials and methods 2.1. Overview of the human study in the current analysis For each subject, a methacholine challenge protocol and two lung scans were completed within 1 day: Upon arrival, the first set of high-resolution computed tomography (HRCT) images of the lungs were collected; this was followed by administration of the methacholine challenge, where change in pulmonary function was assessed by measuring forced expiratory volume in one second (ΔFEV1). Following administration of a bronchodilator, a second HRCT image was collected. For each set of HRCT scans, every airway in the first six airway generations (trachea ¼ 0) was measured for airway gravity angle, branch angle, length, and diameter. These measurements combined with the data of Phalen, Oldham, Beaucage, Crocker, and Mortensen (1985) and Yeh and Schum (1980) were used here to construct a 25-generation, symmetrically branching typical path lung. This symmetrical lung was then used to calculate methacholine, site-specific dosimetry which allowed estimation of airway circumference change, airway resistance change, and airway smooth muscle sensitivity (K). And finally, the value of K for each subject was incorporated into models of exposure/response scenarios; where between-subject differences in response were independent of K and primarily due to each subject's airway anatomy. 2.2. Materials The methacholine solutions used in this study were as follows: Methacholine (provocholine powder [C8H18ClNO2; m.w. 185.7 g/mole]; Methapharm, Coral Springs, FL, USA) in saline solution at concentrations of 0.0, 0.025, 0.25, 2.5, 10, and 25 mg/mL. The bronchiodilator administered to the subjects was Albuterol (Salbutamol, inhaler): two inhalations from the inhaler. 2.3. Methods 2.3.1. Human subjects The human study protocol (protocol no. RJRCSM001) was approved by an Institutional Review Board (IRB). The IRB and all human subject procedures and measurements were conducted by Piedmont Medical (Winston-Salem, NC). A minimum of 10 and preferably 20 subjects were to be enrolled in the study based on the expected probability of seeing a population distribution due to tracheobronchial airway size in the first six airway generations. All subjects were required to have an FEV1 4 80% predicted in a screening pulmonary function test. 2.3.2. Methacholine challenge Each methacholine challenge was delivered as an aerosol generated from a disposable nebulizer (PARI LC Plus, PARI, Midlothian, VA, USA; operated continuously at 12.5 psi for an output of 3.2 L/min; and calibrated with Spraytec spray droplet size measurement system, Malvern Insterments, LtD., Worchestershire, UK; mass median aerodynamic diameters, MMAD¼6.2, and geometric standard deviation, GSD ¼2.2, were estimated by assuming that the density of each particle was near 2.2 g/ml.). For each concentration introduced into the nebulizer, the subject followed a five-breath dosimeter protocol (American Thoracic Society, 2000; American Association for Respiratory Care, 2001). Subjects were asked to breathe normally (resting ventilation) and not to pause between inhalation and exhalation. Prior to the methacholine challenge, subjects performed a baseline FEV1 maneuver (monitored with a Spirolab II spirometer, MIR, Waukesha WI). Following the fifth inhalation of methacholine aerosol, the subject performed FEV1 maneuvers. This five-breath methacholine challenge was repeated up to five times, where in each subsequent challenge, the concentration of methacholine in the nebulized solution was increased by a factor of 2 or more (i.e., for concentrations of 0.0, 0.025, 0.25, 2.5, 10, and 25 mg/mL). In order to keep the cumulative effect of methacholine relatively constant, the time interval between the start of two subsequent concentrations was kept at 5 min. The challenges were repeated until the subject inhaled the aerosol generated from the solution with the highest methacholine concentration, until the subject's FEV1 decreased by more than 10%, or the subject refused to continue. If at the highest exposure (25 mg/mL introduced into the nebulizer), there was less than 3% decrease in FEV1, the subject was identified as non-responsive (NR); otherwise as responsive (R). Following the final methacholine challenge, the subject had two inhalations from an albuterol inhaler and waited 10 min prior to performing one or more FEV1 maneuvers. When FEV1 returned to at least baseline, the subject was immediately positioned for imaging. 2.3.3. Imaging The lung of each subject was imaged in the supine position using HRCT (GE Lightspeed þ; GE Healthcare, Chalfont St. Giles, UK). The acquisition parameters were 120 Kv, 1.03 ms exposure time, 0.0° of detector tilt, and 1.0 mm slice thickness. Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Two CT imaging sessions were scheduled: one before methacholine challenge and one after albuterol inhalation. A reproducible image immediately after the methacholine challenge was not possible due to the rapidity of recovery from the effects of methacholine. For each imaging session, a physical mark on the body was used to allow alignment of the images. Although a small difference exists between male and female in the average distance needed to collect data on the first six airway generations, the required data were generally obtained from CT images collected 35 mm above and 35 mm below the carina (first bifurcation of the trachea). The scans from each HRCT imaging session were segmented and reconstructed (Clinkenbeard et al., 2002) in order to extract the characteristic branching, tree-like image of the first six tracheobronchial airway generations. The resulting three dimensional image could be rotated and tilted. 2.3.4. Airway dimensions The analysis reported here is based on data from the first HRCT scan. From this scan, every airway in the first six tracheobronchial airway generations (G0–G5) was measured (trachea¼0). The reconstructed three-dimensional image of lung airways was used to obtain, from each individual airway, the following four values: airway length, diameter, branch angle, and gravity angle (Parallax was minimized by rotating and tilting the image.). These measurements were used to construct a typical path lung model (TPLM) using the scheme of Weibel (1963). For G0 to G5, the TPLM was constructed by averaging the dimensions of the airways of each airway generation. This six-generation TPLM-6 was used to construct a 25-generation, symmetrically branching typical path lung by applying scale factors (FP) to Phalen's (1985) algorithm relating subject height to airway dimension (G6 to G15), and scale factors (FY) to the Yeh and Schum (1980) report of airway dimensions and branch angles of the lung cast from a 60 year old male (G16 to G24). The approach was as follows: The scale factor for length (FPL) was calculated from the average ratio of TPLM-6 lengths to Phalen's predicted lengths—[and likewise for the scale factor for diameter (FPd)]. The scale factors were used to scale Phalen's predicted dimensions for generations G6 to G15 resulting in a new 16-generation TPLM (TPLM-16). And finally, to construct airway generations G16 to G24, a scale factor for length (FYL) was calculated from the average ratio of TPLM-16 lengths to Yeh and Schum's measured lengths—[and likewise for the scale factor for diameter (FYd)]. These scale factors were used to scale the Yeh and Schum dimensions for generations G16 to G24 resulting in the final 25 generation TPLM (TPLM-25) 2.3.5. Site-specific dosimetry For each airway (i), site specific deposition, Xdep(i) (mg/cm2), was not measured. Instead, Xdep(i) was estimated using MPPD, the multiple path particle deposition model (Anjilvel & Asgharian, 1995; Subramaniam, Asgharian, Freijer, Miller, & Anjilvel, 2003; available through Dr. Asgharian at http://www.ara.com/products/mppd.htm access verified 19 November 2014). Prior to running the program, a subject's TPLM-25 was inserted into files hum_a.dat (for the trachea) and hum_s.dat (for the remaining airway dimensions; MPPD default branch angles and inclination to gravity angles were kept.). Within the program, the Yeh and Schum symmetric lung model was selected, the aerosol properties and subject breathing parameters inserted, and the value for functional reserve capacity adjusted so that the MPPD-airway-dimension scale-factor was 1.0. After running MPPD, the airway specific deposition fractions were multiplied by the inhaled mass of methacholine in order to calculate Xdep(i). 2.3.6. Modeling response to inhaled methacholine In the conducting airways (G0 to G15), methacholine causes smooth muscle surrounding an airway to constrict; resulting in a decrease in airway circumference. In order to model a subject's response to methacholine, the fractional change in airway circumference (ΔC(i)/C0(i)) was written as a function of [1] airway smooth muscle sensitivity to methacholine, K (cm of smooth muscle constriction per concentration of methacholine on the airway surface), and [2] physical properties such as aerosol size, the physiology of breathing, and airway dimensions as follows: ! X depðiÞ ΔC ðiÞ ¼ sin ð∅0ðiÞ ÞK C 0ðiÞ π 2 L0ðiÞ d2 0ðiÞ
where, for airway generation (i), L0(i), d0(i), and C0(i) are respectively the initial length, diameter, and circumference; ΔC(i) is the change in circumference; and ϕ0(i) is the angle the smooth muscle band makes with the axis of the cylindrical airway. For the simplification of ϕ0(i) ¼ π/2, the change in FEV1, (as a function of ΔC(i)/C0(i)), becomes the following: 8 9 > > 15 > > P > > L 1 0ðiÞ > > > > < = 2ðiÞ d40ðiÞ i¼0 ΔFEV 1 ¼ 1 15 > > P 1 L0ðiÞ > > > > 1 > > > :i ¼ 0 2ðiÞ d40ðiÞ 1 ΔC ðiÞ 4 > ; C 0ðiÞ
where ΔFEV1 is the measured value. From this relation for ΔFEV1, each subject's sensitivity to methacholine was estimated by incrementally changing K until the calculated value of ΔFEV1 equaled the measured value. Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Table 1 Subject demographics (“R” indicates response and “NR” non-response). ID Sex Age (yr) Height (cm) Weight (kg) Breathing frequency (breaths/ min)
Minute ventilation (L/ min)
Response to methacholine R or NR
1 2 3 4 5 6 7 8 9 10 11 12 13 14
43.9 31.9 40.9 49.3 22.8 49.0 47.0 25.0 65.5 29.8 19.0 26.5 26.8 17.0
R R R R R R R R NR NR NR NR NR NR
F M M F F F M F M F M M F F
40 39 27 31 37 29 38 27 28 35 40 41 38 36
163.0 183.5 175.8 166.8 165.0 160.0 175.9 154.0 186.4 165.8 167.6 176.6 154.0 160.8
120.9 86.4 91.3 85.6 99.2 65.7 87.3 59.3 114.8 81.1 100.9 86.4 63.9 81.0
15.2 13.8 14.9 18.9 19.1 16.5 18.7 16.9 12.5 16.4 22.1 15.9 19.2 15.5
For the inhalation protocol that was performed, this value of K was then used (along with the size distribution of the methacholine aerosol, the subject's breathing parameters, and estimated lung morphology) to create a response curve of ΔFEV1 vs. the inhaled mass of methacholine. In addition, the value of K for each subject was used to simulate response curves for variations of the original inhalation protocol. The simulated protocols were based on the observation that when a subject repeats the methacholine challenge protocol by inhaling the same mass of methacholine but at a different volumetric flow rate, the fractional difference in ΔFEV1 is independent of airway smooth muscle sensitivity. The response curve, based on the protocol that was performed, was assigned as Case-1a. In virtual Case 1b, the calculated response curve was based on the simulation of a protocol in which the subject inhaled the same amount of methacholine (as in Case-1a) but at twice the subject's original volumetric flow rate. In virtual Case 2, calculated response curves were based on two additional simulated protocols. In simulated protocol 2a, the subject inhaled at a volumetric flow rate, Va (based on the average minute ventilation of responding subjects, Table 1). And at this volumetric flow rate, the subject inhaled a mass of methacholine sufficient to produce a 5% decrease in FEV1. In simulated protocol 2b, the subject inhaled this same mass of methacholine but at twice Va. Fractional differences in ΔFEV1 (ΔY1 ¼ [Case 1b ΔFEV1 - Case 1a ΔFEV1]/[Case 1a ΔFEV1] and ΔY2 ¼ [Case 2b ΔFEV1 - Case 2a ΔFEV1]/[Case 2a ΔFEV1]) were compared against the Yeh and Schum scale factors for length, FYL, and diameter, FYd; — any continuously increasing function of ΔY vs. scale factor being sufficient to allow (without HRCT scans) classification of subjects according to airway size.
3. Results Subject demographics are listed in Table 1. Eight of the 14 subjects were responsive to methacholine (5 women and 3 men). Of these eight, their breathing frequencies differed by a factor of 1.4 and their minute ventilations by a factor of 2.1. For all 14 subjects, Table 2 lists the typical path length and diameter dimensions for airway generations G0 to G5. These dimensions were used to produce the scale factors for the Phalen et al. (1985) algorithm and Yeh and Schum (1980) lung cast (Table 3). For the eight responsive subjects with ΔFEV1s greater than 3% the mass of inhaled methacholine differed by a factor of over 1400x while the maximum ΔFEV1s differed by only a factor of 4x. Although individual methacholine deposition patterns had a slight effect, the major difference in these two factors was reflected in the inverse relation between the mass of inhaled methacholine and the airway smooth muscle sensitivity constant (K), which differed between subjects by a factor of 3200x (Table 4). For each responsive subject, these values of K produced the exposure response curves shown in Fig. 1. And when input to the original protocol was modified to match the simulated protocols in part “b” of Case 1 and parts “a” and “b” of Case 2, the calculated fractional change (ΔY) in ΔFEV1 varied between subjects in Case 1 up to 33x and in Case 2 up to 2.8x (Table 5). The difference, in these variations, was reflected in the linear relations between ΔY and (Yeh & Schum, 1980) length- and diameter- scale factors (Fig. 2). For the comparison between the protocol that was performed (Case 1a) and virtual protocol Case 1b, the linear relation between ΔY and scale factors for length had an R2 ¼ 0.22, and for diameter an R2 ¼ 0.42. For the virtual protocols in Case 2, the linear relation for length had an R2 ¼ 0.49 and the linear relation for diameter an R2 ¼ 0.57 (Fig. 2). For comparing between-subject airway size, Case 1 and Case 2 indicate that a methacholine inhalation protocol can be modified such that ΔY, the fractional change in ΔFEV1 is independent of airway smooth muscle sensitivity. Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Table 2 Typical path lung model from first HRCT scan (generations G0 to G5; L & d in cm). ID
Sex
L(0)
L(1)
L(2)
L(3)
L(4)
L(5)
d(0)
d(1)
d(2)
d(3)
d(4)
d(5)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
F M M F F F M F M F M M F F
9.17 10.45 10.43 10.18 8.40 9.99 10.12 9.89 10.06 10.45 9.88 11.20 7.48 8.44
4.07 3.22 4.37 4.02 4.46 3.16 4.03 3.29 4.53 4.05 4.51 4.20 3.12 4.28
1.54 1.76 1.55 1.51 1.39 1.68 1.62 1.39 1.82 1.89 1.82 1.84 1.65 1.51
0.96 1.07 1.38 1.00 1.07 1.05 1.33 0.97 1.16 1.03 1.37 1.40 1.06 1.05
0.69 1.02 0.83 0.72 0.78 0.85 0.84 0.57 0.98 0.97 0.84 0.89 0.79 0.86
0.60 0.63 0.56 0.49 0.65 0.64 0.63 0.59 0.69 0.66 0.67 0.67 0.55 0.59
1.67 1.77 1.83 1.39 1.37 1.28 1.78 1.21 1.39 1.57 1.50 2.03 1.47 1.51
1.03 1.15 1.36 1.16 1.09 0.87 1.29 0.86 1.04 0.98 1.03 1.28 1.07 1.04
0.65 0.84 0.78 0.68 0.74 0.72 0.82 0.59 0.77 0.73 0.77 0.85 0.71 0.64
0.54 0.64 0.62 0.53 0.58 0.56 0.64 0.50 0.58 0.58 0.63 0.62 0.55 0.56
0.33 0.42 0.36 0.33 0.39 0.33 0.34 0.28 0.40 0.38 0.37 0.45 0.39 0.35
0.22 0.25 0.27 0.22 0.25 0.25 0.24 0.20 0.24 0.26 0.23 0.30 0.26 0.24
Table 3 Length and diameter scale factors. ID
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Sex
Height (cm)
F M M F F F M F M F M M F F
Phalen et al. (1985)
163.0 183.5 175.8 166.8 165.0 160.0 175.9 154.0 186.4 165.8 167.6 176.6 154.0 160.8
Yeh and Schum (1980)
FPL ¼ Ave (L/LPhalen)
FPd ¼Ave (d/dPhalen)
FYL ¼ Ave (L/LYeh)
FYd ¼ Ave (d/dYeh)
1.024 1.025 1.080 1.001 1.047 1.090 1.079 1.004 1.085 1.161 1.183 1.158 1.032 1.077
0.846 0.896 0.930 0.818 0.885 0.830 0.903 0.759 0.797 0.883 0.880 1.003 0.935 0.866
0.667 0.719 0.744 0.664 0.688 0.702 0.741 0.633 0.770 0.763 0.788 0.797 0.651 0.697
0.495 0.561 0.569 0.484 0.518 0.477 0.554 0.428 0.502 0.519 0.521 0.613 0.526 0.500
Table 4 Response (R) or non-response (NR) to inhaled methacholine ("—" indicates that the value was not calculated; K is the airway smooth muscle sensitivity to methacholine). ID
Sex
Measured ΔFEV1 (%)
R or NR
Calculated Minhaled (lg)
Calculated K (cm/lg/cm2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
F M M F F F M F M F M M F F
6.7% 18.8% 12.5% 9.5% 8.0% 6.6% 6.7% 4.7% 2.7% 1.0% 0.1% 0.1% 0.1% 2.5%
R R R R R R R R NR NR NR NR NR NR
0.21 2.49 21.19 265.84 262.95 304.60 268.69 296.09 400.19 304.84 226.51 315.07 261.67 323.89
35.880 10.151 0.776 0.041 0.037 0.059 0.032 0.011 — — — — — —
4. Conclusions Models of virtual (untried) methacholine inhalation-protocols were used to demonstrate the hypothesis that given the airway dimensions of one subject, the airway dimensions of a second subject could be estimated based on each subjects
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Fig. 1. Methacholine Protocol Response Curves. Original protocol response curves for subjects S1 to S8. (S1 ¼ left side solid line; S2 ¼ long dashes þ 2 dots; S3 ¼ long dashes þ single dots; S4 ¼ medium dashes þ single dots; S5 ¼ medium dashes; S6 ¼ short dashes; S7 ¼ dots; S8 ¼ right side solid line.) Circles indicate maximum Y for each subject. Horizontal dots indicate 10% and 5% vales of Y. Table 5 Fractional change (ΔY) in ΔFEV1 between parts “a” and “b” in virtual Cases 1 and 2: In “Case 1a”, the subject was asked to repeat the original protocol; and in “Case 1b”, the subject was asked to inhale the same mass of methacholine as in “Case 1a” but at twice the subject’s original volumetric flow rate. In “Case 2a”, the subject was asked to inhale (at a volumetric flow rate, Va, based on the average volumetric flow rate of responding subjects in the original protocol) a mass of methacholine sufficient to produce a 5% decrease in FEV1; and in “Case 2b”, the subject was to asked to inhale this same mass of methacholine but at 2Va). ID
Sex
Virtual ΔY(Case
1 2 3 4 5 6 7 8
F M M F F F M F
61% 21% 40% 63% 2% 67% 31% 61%
1)
Virtual ΔY(Case
2 )
58% 46% 40% 57% 65% 73% 26% 64%
Fig. 2. Relation of Airway Scale Factors to ΔY. Yeh and Schum scale factors for length, FYL {triangles}, and diameters, FYd {Squares}, vs. ΔY, the fractional change in ΔFEV1 for Virtual Cases 1 and 2 (See Tables 3 and 5). Virtual Case 1 (Grey) and Virtual Case 2 (Black). Solid grey line, FYd ¼ 0.134 ΔY þ 0.569 (R2 ¼ 0.42); Solid black line, FYd ¼ 0.239 ΔY þ 0.639 (R2 ¼ 0.57); Dashed grey line, FYL ¼ 0.078 ΔY þ 0.728 (R2 ¼ 0.22); Dashed black line, FYL ¼ 0.178 ΔY þ 0.790 (R2 ¼ 0.49).
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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fractional change in forced expiratory volume in one second, ΔFEV1. The models depended on knowing the sensitivity of airway smooth muscle to methacholine (K). K was estimated by first using HRCT scans to construct, for each subject, lung dimensions of the first five airway generations. Simple ratios of these dimensions with previously published dimensions (Phalen et al., 1985; Yeh & Schum, 1980) allowed construction of a 25-generation typical path lung. This completed typical path lung model (plus exposure conditions and breathing parameters) was entered into MPPD (Anjilvel & Asgharian, 1995) in order to obtain the mass distribution of inhaled methacholine on the surface of the conducting airways. Given this mass distribution and measured ΔFEV1, iterative calculations were used to estimate K and thus create subject-specific models of the actual as well as virtual methacholine inhalation protocols. The protocol-models provided the observation that when a subject is asked to repeat the inhalation protocol by inhaling the same mass of methacholine but at a different volumetric flow rate, the fractional difference in ΔFEV1, ΔY, was independent of airway smooth muscle sensitivity. Based on this observation, virtual protocols demonstrated a linear relation between ΔY and airway dimension scale factors applied to measurements of a human lung cast (Yeh & Schum, 1980). The virtual methacholine inhalation protocols simulated in Case 2 can be used to demonstrate an application that meets the hypothesis: A subjects airway dimensions can be predicted from ΔY. For example consider the diameter scale factor predicted for subject 4. In Case 2, the simulation of subject 4 resulted in a ΔY of 57% (Table 5). For this value of ΔY the linear curve (Fig. 2) predicts a Yeh and Schum scale factor for airway diameter of 0.502 [compared to 0.484 based on diameter measurements (Table 3)]. If the predicted factor of 0.502 is used to calculate airway diameters using the Yeh and Schum lung morphology data, the difference from measured diameters (by generation) is the following: G0 ( 28%), G1 ( 32%), G2 ( 18%), G3 ( 21%), G4 (þ/ 0%), G5 (þ32%). If the Case-2-protocol is now followed by a new subject who has not received an HRCT scan, and thus who's airway diameter scale factor is unknown, then the airway dimensions of the new subject can be compared with the airway dimensions of subject 4. If the new subject responds with a ΔY ¼ 38%. Then, according to Case 2 in Fig. 2, the new subject will have a diameter scale factor of 0.548. Compared to subject 4's scale factor of 0.502, the new subject's typical path airways, on average, would be 9% larger (and for equal inspirational airflow rates, the new subject's airways would have 30% less resistance to airflow). Moreover, when two subjects (both without HRCT scans) follow such a modified protocol, the subject with the greater value of ΔY will also be the subject with the larger airway dimensions: allowing a cohort to be separated into groups based on airway size. This potential reduction in response variability, due to more consistent airway anatomy, might reduce the number of subjects required in clinical efficacy trials. Limitations to demonstrating this linear relation between ΔY and scale factor are the size of this pilot study and the detection limit in measuring FEV1. More reproducible data should be possible with a less effort-dependent measurement such as FEF25-50; although for the Case 2 protocol, a low-effort, reproducible method needs to be developed for measuring change in conducting airway resistance.). Other limitations are the dependence on accurately measuring aerosol properties (including particle density) and on measuring subject breathing parameters (during inhalation of methacholine). In addition to addressing these limitations, the relation between ΔY and scale factor can be improved by accounting for the following: – the amount and direction of smooth muscle spiraling around the airway, – clearance/deactivation times for methacholine at the smooth muscle receptors, – liquid/tissue between smooth muscle and airway lumen In spite of these limitations in the approach of this analysis, the estimation of K and the application of the subsequent subject specific models to predict responses to inhaled methacholine provide a direction to consider in simplifying and/or improving the methacholine challenge, modeling related disease states, validating site-specific aerosol dosimetry algorithms, or reducing clinical trial variability of medicinal aerosols. Acknowledgments The previously reported Human Protocol (Oldham et al., 2013) was funded by a grant to the first author from RJR Tobacco; this additional analysis was supported individually by the authors.
References American Association for Respiratory Care. (2001). AARC clinical practice guidline – methacholine challenge testing: 2001 revision & update. Respiratory Care, 46(5), 523–530. American Thoracic Society. (2000). Guidelines for methacholine and exercise challenge testing––1999. American Journal of Respiratory and Critical Care Medicine, 161(1), 309–329. Anjilvel, S., & Asgharian, B. (1995). A multiple-path model of particle deposition in the rat lung. Fundamental and Applied Toxicology, 28, 41–50. Clinkenbeard, R. E., Johnson, D. L., Parthasarathy, R., Altan, C., Tan, K. H., Crawford, R. H., et al. (2002). Replication of human tracheobronchial hollow airway models using a selective laser sintering rapid prototyping technique. American Industrial Hygiene Association Journal, 63, 141–150. Oldham, Michael J., Clinkenbeard, Rodney E., & Moss, Owen R. (2013). Influence of morphometry and airway patency on response to inhaled methacholine. Aerosol Science and Technology, 47(5), 574–579. Phalen, R. F., Oldham, M. J., Beaucage, C. B., Crocker, T. T., & Mortensen, J. D. (1985). Postnatal enlargement of human tracheobronchial airways and implications for particle deposition. Anatomical Record, 212, 368–380.
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Subramaniam, R. P., Asgharian, B., Freijer, J. I., Miller, F. J., & Anjilvel, S. (2003). Analysis of lobar differences in particle deposition in the human lung. Inhalation Toxicology, 15, 1–21. Weibel, E. R. (1963). Morphometry of the Human Lung (pp. 136–140)Berlin: Springer-Verlag136–140. Yeh, H. C., & Schum, G. M. (1980). Models of human lung airways and their application to inhaled particle deposition. Bulletin of Mathematical Biology, 42, 461–480.
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Contents lists available at ScienceDirect
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On the relationship between response to inhaled methacholine and airway dimensions Owen R. Moss a,n, Michael J. Oldham b,1 a b
POK Research, Apex, NC, USA Virginia Commonwealth University, VA, USA
a r t i c l e i n f o
Keywords: Methacholine Inhalation Lung Morphometry Dosimetry Clinical-trials
abstract Site-specific dosimetry was used to address the hypothesis that, given the airway dimensions of one subject, the airway dimensions of a second subject can be obtained by comparing responses to a modified bronchoconstrictor inhalation-protocol. Fourteen healthy, nonsmoking subjects received a high-resolution computed tomography (HRCT) scan prior to performing a methacholine challenge and measurement of change in forcedexpiratory-volume-in-one-second, (ΔFEV1). Scale factors of average length (L) and diameter (d) of the first six tracheobronchial airway generations were used to construct typical path lungs. The Multiple Path Particle Dosimetry (MPPD) model calculated surface density of methacholine. Airway circumference change, ΔC/C0, was used to calculate ΔFEV1 in terms of airway smooth muscle sensitivity, K (cm/mg/cm2). Virtual protocols were modeled such that fractional changes in ΔFEV1, “ΔY”, were independent of K. Eight subjects responded to methacholine with ΔFEV1 4 3% and Ks from 0.011 to 35.8. For one virtual protocol, the linear relations between ΔY and length and diameter scale factors had R2s respectively of 0.49 and 0.57. Within the limits of these measurements, the hypothesis is demonstrated; — any continuously increasing function of ΔY vs. scale factor being sufficient to allow (without HRCT scans) classification of subjects according to airway size. & 2016 Elsevier Ltd. All rights reserved.
1. Introduction Deposition of inhaled particulate matter in the human respiratory tract primarily depends on properties of the aerosol and physiology (i.e., breathing pattern and airway anatomy). If an aerosol, that provokes a smooth muscle response, is inhaled by separate individuals, each having different but unique breathing patterns and lung-airway anatomy, then any difference in biological response should be explained by a combination of two factors; site-specific dose and airway smooth muscle sensitivity. By normalizing biological response to airway smooth muscle sensitivity, some insight should be possible into airway anatomy. In a previously reported human pilot study (Oldham, Clinkenbeard & Moss, 2013), subjects inhaled methacholine and their response (change in forced expiratory volume in one second, ΔFEV1) was compared to lung airway anatomy. Based on the data available from this previous report, the research described here extends their findings by using site-specific
n
Corresponding author. 1
Current address: Altria Client Services
http://dx.doi.org/10.1016/j.jaerosci.2016.01.010 0021-8502/& 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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dosimetry to quantitate airway smooth muscle sensitivity to methacholine. This quantitation of airway smooth muscle sensitivity is then used to address the hypothesis that given the airway dimensions of one subject, the airway dimensions of a second subject can be obtained by comparing responses to an inhaled bronchoconstrictor.
2. Materials and methods 2.1. Overview of the human study in the current analysis For each subject, a methacholine challenge protocol and two lung scans were completed within 1 day: Upon arrival, the first set of high-resolution computed tomography (HRCT) images of the lungs were collected; this was followed by administration of the methacholine challenge, where change in pulmonary function was assessed by measuring forced expiratory volume in one second (ΔFEV1). Following administration of a bronchodilator, a second HRCT image was collected. For each set of HRCT scans, every airway in the first six airway generations (trachea ¼ 0) was measured for airway gravity angle, branch angle, length, and diameter. These measurements combined with the data of Phalen, Oldham, Beaucage, Crocker, and Mortensen (1985) and Yeh and Schum (1980) were used here to construct a 25-generation, symmetrically branching typical path lung. This symmetrical lung was then used to calculate methacholine, site-specific dosimetry which allowed estimation of airway circumference change, airway resistance change, and airway smooth muscle sensitivity (K). And finally, the value of K for each subject was incorporated into models of exposure/response scenarios; where between-subject differences in response were independent of K and primarily due to each subject's airway anatomy. 2.2. Materials The methacholine solutions used in this study were as follows: Methacholine (provocholine powder [C8H18ClNO2; m.w. 185.7 g/mole]; Methapharm, Coral Springs, FL, USA) in saline solution at concentrations of 0.0, 0.025, 0.25, 2.5, 10, and 25 mg/mL. The bronchiodilator administered to the subjects was Albuterol (Salbutamol, inhaler): two inhalations from the inhaler. 2.3. Methods 2.3.1. Human subjects The human study protocol (protocol no. RJRCSM001) was approved by an Institutional Review Board (IRB). The IRB and all human subject procedures and measurements were conducted by Piedmont Medical (Winston-Salem, NC). A minimum of 10 and preferably 20 subjects were to be enrolled in the study based on the expected probability of seeing a population distribution due to tracheobronchial airway size in the first six airway generations. All subjects were required to have an FEV1 4 80% predicted in a screening pulmonary function test. 2.3.2. Methacholine challenge Each methacholine challenge was delivered as an aerosol generated from a disposable nebulizer (PARI LC Plus, PARI, Midlothian, VA, USA; operated continuously at 12.5 psi for an output of 3.2 L/min; and calibrated with Spraytec spray droplet size measurement system, Malvern Insterments, LtD., Worchestershire, UK; mass median aerodynamic diameters, MMAD¼6.2, and geometric standard deviation, GSD ¼2.2, were estimated by assuming that the density of each particle was near 2.2 g/ml.). For each concentration introduced into the nebulizer, the subject followed a five-breath dosimeter protocol (American Thoracic Society, 2000; American Association for Respiratory Care, 2001). Subjects were asked to breathe normally (resting ventilation) and not to pause between inhalation and exhalation. Prior to the methacholine challenge, subjects performed a baseline FEV1 maneuver (monitored with a Spirolab II spirometer, MIR, Waukesha WI). Following the fifth inhalation of methacholine aerosol, the subject performed FEV1 maneuvers. This five-breath methacholine challenge was repeated up to five times, where in each subsequent challenge, the concentration of methacholine in the nebulized solution was increased by a factor of 2 or more (i.e., for concentrations of 0.0, 0.025, 0.25, 2.5, 10, and 25 mg/mL). In order to keep the cumulative effect of methacholine relatively constant, the time interval between the start of two subsequent concentrations was kept at 5 min. The challenges were repeated until the subject inhaled the aerosol generated from the solution with the highest methacholine concentration, until the subject's FEV1 decreased by more than 10%, or the subject refused to continue. If at the highest exposure (25 mg/mL introduced into the nebulizer), there was less than 3% decrease in FEV1, the subject was identified as non-responsive (NR); otherwise as responsive (R). Following the final methacholine challenge, the subject had two inhalations from an albuterol inhaler and waited 10 min prior to performing one or more FEV1 maneuvers. When FEV1 returned to at least baseline, the subject was immediately positioned for imaging. 2.3.3. Imaging The lung of each subject was imaged in the supine position using HRCT (GE Lightspeed þ; GE Healthcare, Chalfont St. Giles, UK). The acquisition parameters were 120 Kv, 1.03 ms exposure time, 0.0° of detector tilt, and 1.0 mm slice thickness. Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Two CT imaging sessions were scheduled: one before methacholine challenge and one after albuterol inhalation. A reproducible image immediately after the methacholine challenge was not possible due to the rapidity of recovery from the effects of methacholine. For each imaging session, a physical mark on the body was used to allow alignment of the images. Although a small difference exists between male and female in the average distance needed to collect data on the first six airway generations, the required data were generally obtained from CT images collected 35 mm above and 35 mm below the carina (first bifurcation of the trachea). The scans from each HRCT imaging session were segmented and reconstructed (Clinkenbeard et al., 2002) in order to extract the characteristic branching, tree-like image of the first six tracheobronchial airway generations. The resulting three dimensional image could be rotated and tilted. 2.3.4. Airway dimensions The analysis reported here is based on data from the first HRCT scan. From this scan, every airway in the first six tracheobronchial airway generations (G0–G5) was measured (trachea¼0). The reconstructed three-dimensional image of lung airways was used to obtain, from each individual airway, the following four values: airway length, diameter, branch angle, and gravity angle (Parallax was minimized by rotating and tilting the image.). These measurements were used to construct a typical path lung model (TPLM) using the scheme of Weibel (1963). For G0 to G5, the TPLM was constructed by averaging the dimensions of the airways of each airway generation. This six-generation TPLM-6 was used to construct a 25-generation, symmetrically branching typical path lung by applying scale factors (FP) to Phalen's (1985) algorithm relating subject height to airway dimension (G6 to G15), and scale factors (FY) to the Yeh and Schum (1980) report of airway dimensions and branch angles of the lung cast from a 60 year old male (G16 to G24). The approach was as follows: The scale factor for length (FPL) was calculated from the average ratio of TPLM-6 lengths to Phalen's predicted lengths—[and likewise for the scale factor for diameter (FPd)]. The scale factors were used to scale Phalen's predicted dimensions for generations G6 to G15 resulting in a new 16-generation TPLM (TPLM-16). And finally, to construct airway generations G16 to G24, a scale factor for length (FYL) was calculated from the average ratio of TPLM-16 lengths to Yeh and Schum's measured lengths—[and likewise for the scale factor for diameter (FYd)]. These scale factors were used to scale the Yeh and Schum dimensions for generations G16 to G24 resulting in the final 25 generation TPLM (TPLM-25) 2.3.5. Site-specific dosimetry For each airway (i), site specific deposition, Xdep(i) (mg/cm2), was not measured. Instead, Xdep(i) was estimated using MPPD, the multiple path particle deposition model (Anjilvel & Asgharian, 1995; Subramaniam, Asgharian, Freijer, Miller, & Anjilvel, 2003; available through Dr. Asgharian at http://www.ara.com/products/mppd.htm access verified 19 November 2014). Prior to running the program, a subject's TPLM-25 was inserted into files hum_a.dat (for the trachea) and hum_s.dat (for the remaining airway dimensions; MPPD default branch angles and inclination to gravity angles were kept.). Within the program, the Yeh and Schum symmetric lung model was selected, the aerosol properties and subject breathing parameters inserted, and the value for functional reserve capacity adjusted so that the MPPD-airway-dimension scale-factor was 1.0. After running MPPD, the airway specific deposition fractions were multiplied by the inhaled mass of methacholine in order to calculate Xdep(i). 2.3.6. Modeling response to inhaled methacholine In the conducting airways (G0 to G15), methacholine causes smooth muscle surrounding an airway to constrict; resulting in a decrease in airway circumference. In order to model a subject's response to methacholine, the fractional change in airway circumference (ΔC(i)/C0(i)) was written as a function of [1] airway smooth muscle sensitivity to methacholine, K (cm of smooth muscle constriction per concentration of methacholine on the airway surface), and [2] physical properties such as aerosol size, the physiology of breathing, and airway dimensions as follows: ! X depðiÞ ΔC ðiÞ ¼ sin ð∅0ðiÞ ÞK C 0ðiÞ π 2 L0ðiÞ d2 0ðiÞ
where, for airway generation (i), L0(i), d0(i), and C0(i) are respectively the initial length, diameter, and circumference; ΔC(i) is the change in circumference; and ϕ0(i) is the angle the smooth muscle band makes with the axis of the cylindrical airway. For the simplification of ϕ0(i) ¼ π/2, the change in FEV1, (as a function of ΔC(i)/C0(i)), becomes the following: 8 9 > > 15 > > P > > L 1 0ðiÞ > > > > < = 2ðiÞ d40ðiÞ i¼0 ΔFEV 1 ¼ 1 15 > > P 1 L0ðiÞ > > > > 1 > > > :i ¼ 0 2ðiÞ d40ðiÞ 1 ΔC ðiÞ 4 > ; C 0ðiÞ
where ΔFEV1 is the measured value. From this relation for ΔFEV1, each subject's sensitivity to methacholine was estimated by incrementally changing K until the calculated value of ΔFEV1 equaled the measured value. Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Table 1 Subject demographics (“R” indicates response and “NR” non-response). ID Sex Age (yr) Height (cm) Weight (kg) Breathing frequency (breaths/ min)
Minute ventilation (L/ min)
Response to methacholine R or NR
1 2 3 4 5 6 7 8 9 10 11 12 13 14
43.9 31.9 40.9 49.3 22.8 49.0 47.0 25.0 65.5 29.8 19.0 26.5 26.8 17.0
R R R R R R R R NR NR NR NR NR NR
F M M F F F M F M F M M F F
40 39 27 31 37 29 38 27 28 35 40 41 38 36
163.0 183.5 175.8 166.8 165.0 160.0 175.9 154.0 186.4 165.8 167.6 176.6 154.0 160.8
120.9 86.4 91.3 85.6 99.2 65.7 87.3 59.3 114.8 81.1 100.9 86.4 63.9 81.0
15.2 13.8 14.9 18.9 19.1 16.5 18.7 16.9 12.5 16.4 22.1 15.9 19.2 15.5
For the inhalation protocol that was performed, this value of K was then used (along with the size distribution of the methacholine aerosol, the subject's breathing parameters, and estimated lung morphology) to create a response curve of ΔFEV1 vs. the inhaled mass of methacholine. In addition, the value of K for each subject was used to simulate response curves for variations of the original inhalation protocol. The simulated protocols were based on the observation that when a subject repeats the methacholine challenge protocol by inhaling the same mass of methacholine but at a different volumetric flow rate, the fractional difference in ΔFEV1 is independent of airway smooth muscle sensitivity. The response curve, based on the protocol that was performed, was assigned as Case-1a. In virtual Case 1b, the calculated response curve was based on the simulation of a protocol in which the subject inhaled the same amount of methacholine (as in Case-1a) but at twice the subject's original volumetric flow rate. In virtual Case 2, calculated response curves were based on two additional simulated protocols. In simulated protocol 2a, the subject inhaled at a volumetric flow rate, Va (based on the average minute ventilation of responding subjects, Table 1). And at this volumetric flow rate, the subject inhaled a mass of methacholine sufficient to produce a 5% decrease in FEV1. In simulated protocol 2b, the subject inhaled this same mass of methacholine but at twice Va. Fractional differences in ΔFEV1 (ΔY1 ¼ [Case 1b ΔFEV1 - Case 1a ΔFEV1]/[Case 1a ΔFEV1] and ΔY2 ¼ [Case 2b ΔFEV1 - Case 2a ΔFEV1]/[Case 2a ΔFEV1]) were compared against the Yeh and Schum scale factors for length, FYL, and diameter, FYd; — any continuously increasing function of ΔY vs. scale factor being sufficient to allow (without HRCT scans) classification of subjects according to airway size.
3. Results Subject demographics are listed in Table 1. Eight of the 14 subjects were responsive to methacholine (5 women and 3 men). Of these eight, their breathing frequencies differed by a factor of 1.4 and their minute ventilations by a factor of 2.1. For all 14 subjects, Table 2 lists the typical path length and diameter dimensions for airway generations G0 to G5. These dimensions were used to produce the scale factors for the Phalen et al. (1985) algorithm and Yeh and Schum (1980) lung cast (Table 3). For the eight responsive subjects with ΔFEV1s greater than 3% the mass of inhaled methacholine differed by a factor of over 1400x while the maximum ΔFEV1s differed by only a factor of 4x. Although individual methacholine deposition patterns had a slight effect, the major difference in these two factors was reflected in the inverse relation between the mass of inhaled methacholine and the airway smooth muscle sensitivity constant (K), which differed between subjects by a factor of 3200x (Table 4). For each responsive subject, these values of K produced the exposure response curves shown in Fig. 1. And when input to the original protocol was modified to match the simulated protocols in part “b” of Case 1 and parts “a” and “b” of Case 2, the calculated fractional change (ΔY) in ΔFEV1 varied between subjects in Case 1 up to 33x and in Case 2 up to 2.8x (Table 5). The difference, in these variations, was reflected in the linear relations between ΔY and (Yeh & Schum, 1980) length- and diameter- scale factors (Fig. 2). For the comparison between the protocol that was performed (Case 1a) and virtual protocol Case 1b, the linear relation between ΔY and scale factors for length had an R2 ¼ 0.22, and for diameter an R2 ¼ 0.42. For the virtual protocols in Case 2, the linear relation for length had an R2 ¼ 0.49 and the linear relation for diameter an R2 ¼ 0.57 (Fig. 2). For comparing between-subject airway size, Case 1 and Case 2 indicate that a methacholine inhalation protocol can be modified such that ΔY, the fractional change in ΔFEV1 is independent of airway smooth muscle sensitivity. Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Table 2 Typical path lung model from first HRCT scan (generations G0 to G5; L & d in cm). ID
Sex
L(0)
L(1)
L(2)
L(3)
L(4)
L(5)
d(0)
d(1)
d(2)
d(3)
d(4)
d(5)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
F M M F F F M F M F M M F F
9.17 10.45 10.43 10.18 8.40 9.99 10.12 9.89 10.06 10.45 9.88 11.20 7.48 8.44
4.07 3.22 4.37 4.02 4.46 3.16 4.03 3.29 4.53 4.05 4.51 4.20 3.12 4.28
1.54 1.76 1.55 1.51 1.39 1.68 1.62 1.39 1.82 1.89 1.82 1.84 1.65 1.51
0.96 1.07 1.38 1.00 1.07 1.05 1.33 0.97 1.16 1.03 1.37 1.40 1.06 1.05
0.69 1.02 0.83 0.72 0.78 0.85 0.84 0.57 0.98 0.97 0.84 0.89 0.79 0.86
0.60 0.63 0.56 0.49 0.65 0.64 0.63 0.59 0.69 0.66 0.67 0.67 0.55 0.59
1.67 1.77 1.83 1.39 1.37 1.28 1.78 1.21 1.39 1.57 1.50 2.03 1.47 1.51
1.03 1.15 1.36 1.16 1.09 0.87 1.29 0.86 1.04 0.98 1.03 1.28 1.07 1.04
0.65 0.84 0.78 0.68 0.74 0.72 0.82 0.59 0.77 0.73 0.77 0.85 0.71 0.64
0.54 0.64 0.62 0.53 0.58 0.56 0.64 0.50 0.58 0.58 0.63 0.62 0.55 0.56
0.33 0.42 0.36 0.33 0.39 0.33 0.34 0.28 0.40 0.38 0.37 0.45 0.39 0.35
0.22 0.25 0.27 0.22 0.25 0.25 0.24 0.20 0.24 0.26 0.23 0.30 0.26 0.24
Table 3 Length and diameter scale factors. ID
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Sex
Height (cm)
F M M F F F M F M F M M F F
Phalen et al. (1985)
163.0 183.5 175.8 166.8 165.0 160.0 175.9 154.0 186.4 165.8 167.6 176.6 154.0 160.8
Yeh and Schum (1980)
FPL ¼ Ave (L/LPhalen)
FPd ¼Ave (d/dPhalen)
FYL ¼ Ave (L/LYeh)
FYd ¼ Ave (d/dYeh)
1.024 1.025 1.080 1.001 1.047 1.090 1.079 1.004 1.085 1.161 1.183 1.158 1.032 1.077
0.846 0.896 0.930 0.818 0.885 0.830 0.903 0.759 0.797 0.883 0.880 1.003 0.935 0.866
0.667 0.719 0.744 0.664 0.688 0.702 0.741 0.633 0.770 0.763 0.788 0.797 0.651 0.697
0.495 0.561 0.569 0.484 0.518 0.477 0.554 0.428 0.502 0.519 0.521 0.613 0.526 0.500
Table 4 Response (R) or non-response (NR) to inhaled methacholine ("—" indicates that the value was not calculated; K is the airway smooth muscle sensitivity to methacholine). ID
Sex
Measured ΔFEV1 (%)
R or NR
Calculated Minhaled (lg)
Calculated K (cm/lg/cm2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
F M M F F F M F M F M M F F
6.7% 18.8% 12.5% 9.5% 8.0% 6.6% 6.7% 4.7% 2.7% 1.0% 0.1% 0.1% 0.1% 2.5%
R R R R R R R R NR NR NR NR NR NR
0.21 2.49 21.19 265.84 262.95 304.60 268.69 296.09 400.19 304.84 226.51 315.07 261.67 323.89
35.880 10.151 0.776 0.041 0.037 0.059 0.032 0.011 — — — — — —
4. Conclusions Models of virtual (untried) methacholine inhalation-protocols were used to demonstrate the hypothesis that given the airway dimensions of one subject, the airway dimensions of a second subject could be estimated based on each subjects
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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Fig. 1. Methacholine Protocol Response Curves. Original protocol response curves for subjects S1 to S8. (S1 ¼ left side solid line; S2 ¼ long dashes þ 2 dots; S3 ¼ long dashes þ single dots; S4 ¼ medium dashes þ single dots; S5 ¼ medium dashes; S6 ¼ short dashes; S7 ¼ dots; S8 ¼ right side solid line.) Circles indicate maximum Y for each subject. Horizontal dots indicate 10% and 5% vales of Y. Table 5 Fractional change (ΔY) in ΔFEV1 between parts “a” and “b” in virtual Cases 1 and 2: In “Case 1a”, the subject was asked to repeat the original protocol; and in “Case 1b”, the subject was asked to inhale the same mass of methacholine as in “Case 1a” but at twice the subject’s original volumetric flow rate. In “Case 2a”, the subject was asked to inhale (at a volumetric flow rate, Va, based on the average volumetric flow rate of responding subjects in the original protocol) a mass of methacholine sufficient to produce a 5% decrease in FEV1; and in “Case 2b”, the subject was to asked to inhale this same mass of methacholine but at 2Va). ID
Sex
Virtual ΔY(Case
1 2 3 4 5 6 7 8
F M M F F F M F
61% 21% 40% 63% 2% 67% 31% 61%
1)
Virtual ΔY(Case
2 )
58% 46% 40% 57% 65% 73% 26% 64%
Fig. 2. Relation of Airway Scale Factors to ΔY. Yeh and Schum scale factors for length, FYL {triangles}, and diameters, FYd {Squares}, vs. ΔY, the fractional change in ΔFEV1 for Virtual Cases 1 and 2 (See Tables 3 and 5). Virtual Case 1 (Grey) and Virtual Case 2 (Black). Solid grey line, FYd ¼ 0.134 ΔY þ 0.569 (R2 ¼ 0.42); Solid black line, FYd ¼ 0.239 ΔY þ 0.639 (R2 ¼ 0.57); Dashed grey line, FYL ¼ 0.078 ΔY þ 0.728 (R2 ¼ 0.22); Dashed black line, FYL ¼ 0.178 ΔY þ 0.790 (R2 ¼ 0.49).
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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fractional change in forced expiratory volume in one second, ΔFEV1. The models depended on knowing the sensitivity of airway smooth muscle to methacholine (K). K was estimated by first using HRCT scans to construct, for each subject, lung dimensions of the first five airway generations. Simple ratios of these dimensions with previously published dimensions (Phalen et al., 1985; Yeh & Schum, 1980) allowed construction of a 25-generation typical path lung. This completed typical path lung model (plus exposure conditions and breathing parameters) was entered into MPPD (Anjilvel & Asgharian, 1995) in order to obtain the mass distribution of inhaled methacholine on the surface of the conducting airways. Given this mass distribution and measured ΔFEV1, iterative calculations were used to estimate K and thus create subject-specific models of the actual as well as virtual methacholine inhalation protocols. The protocol-models provided the observation that when a subject is asked to repeat the inhalation protocol by inhaling the same mass of methacholine but at a different volumetric flow rate, the fractional difference in ΔFEV1, ΔY, was independent of airway smooth muscle sensitivity. Based on this observation, virtual protocols demonstrated a linear relation between ΔY and airway dimension scale factors applied to measurements of a human lung cast (Yeh & Schum, 1980). The virtual methacholine inhalation protocols simulated in Case 2 can be used to demonstrate an application that meets the hypothesis: A subjects airway dimensions can be predicted from ΔY. For example consider the diameter scale factor predicted for subject 4. In Case 2, the simulation of subject 4 resulted in a ΔY of 57% (Table 5). For this value of ΔY the linear curve (Fig. 2) predicts a Yeh and Schum scale factor for airway diameter of 0.502 [compared to 0.484 based on diameter measurements (Table 3)]. If the predicted factor of 0.502 is used to calculate airway diameters using the Yeh and Schum lung morphology data, the difference from measured diameters (by generation) is the following: G0 ( 28%), G1 ( 32%), G2 ( 18%), G3 ( 21%), G4 (þ/ 0%), G5 (þ32%). If the Case-2-protocol is now followed by a new subject who has not received an HRCT scan, and thus who's airway diameter scale factor is unknown, then the airway dimensions of the new subject can be compared with the airway dimensions of subject 4. If the new subject responds with a ΔY ¼ 38%. Then, according to Case 2 in Fig. 2, the new subject will have a diameter scale factor of 0.548. Compared to subject 4's scale factor of 0.502, the new subject's typical path airways, on average, would be 9% larger (and for equal inspirational airflow rates, the new subject's airways would have 30% less resistance to airflow). Moreover, when two subjects (both without HRCT scans) follow such a modified protocol, the subject with the greater value of ΔY will also be the subject with the larger airway dimensions: allowing a cohort to be separated into groups based on airway size. This potential reduction in response variability, due to more consistent airway anatomy, might reduce the number of subjects required in clinical efficacy trials. Limitations to demonstrating this linear relation between ΔY and scale factor are the size of this pilot study and the detection limit in measuring FEV1. More reproducible data should be possible with a less effort-dependent measurement such as FEF25-50; although for the Case 2 protocol, a low-effort, reproducible method needs to be developed for measuring change in conducting airway resistance.). Other limitations are the dependence on accurately measuring aerosol properties (including particle density) and on measuring subject breathing parameters (during inhalation of methacholine). In addition to addressing these limitations, the relation between ΔY and scale factor can be improved by accounting for the following: – the amount and direction of smooth muscle spiraling around the airway, – clearance/deactivation times for methacholine at the smooth muscle receptors, – liquid/tissue between smooth muscle and airway lumen In spite of these limitations in the approach of this analysis, the estimation of K and the application of the subsequent subject specific models to predict responses to inhaled methacholine provide a direction to consider in simplifying and/or improving the methacholine challenge, modeling related disease states, validating site-specific aerosol dosimetry algorithms, or reducing clinical trial variability of medicinal aerosols. Acknowledgments The previously reported Human Protocol (Oldham et al., 2013) was funded by a grant to the first author from RJR Tobacco; this additional analysis was supported individually by the authors.
References American Association for Respiratory Care. (2001). AARC clinical practice guidline – methacholine challenge testing: 2001 revision & update. Respiratory Care, 46(5), 523–530. American Thoracic Society. (2000). Guidelines for methacholine and exercise challenge testing––1999. American Journal of Respiratory and Critical Care Medicine, 161(1), 309–329. Anjilvel, S., & Asgharian, B. (1995). A multiple-path model of particle deposition in the rat lung. Fundamental and Applied Toxicology, 28, 41–50. Clinkenbeard, R. E., Johnson, D. L., Parthasarathy, R., Altan, C., Tan, K. H., Crawford, R. H., et al. (2002). Replication of human tracheobronchial hollow airway models using a selective laser sintering rapid prototyping technique. American Industrial Hygiene Association Journal, 63, 141–150. Oldham, Michael J., Clinkenbeard, Rodney E., & Moss, Owen R. (2013). Influence of morphometry and airway patency on response to inhaled methacholine. Aerosol Science and Technology, 47(5), 574–579. Phalen, R. F., Oldham, M. J., Beaucage, C. B., Crocker, T. T., & Mortensen, J. D. (1985). Postnatal enlargement of human tracheobronchial airways and implications for particle deposition. Anatomical Record, 212, 368–380.
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i
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O.R. Moss, M.J. Oldham / Journal of Aerosol Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Subramaniam, R. P., Asgharian, B., Freijer, J. I., Miller, F. J., & Anjilvel, S. (2003). Analysis of lobar differences in particle deposition in the human lung. Inhalation Toxicology, 15, 1–21. Weibel, E. R. (1963). Morphometry of the Human Lung (pp. 136–140)Berlin: Springer-Verlag136–140. Yeh, H. C., & Schum, G. M. (1980). Models of human lung airways and their application to inhaled particle deposition. Bulletin of Mathematical Biology, 42, 461–480.
Please cite this article as: Moss, O.R., & Oldham, M.J. On the relationship between response to inhaled methacholine and airway dimensions. Journal of Aerosol Science (2016), http://dx.doi.org/10.1016/j.jaerosci.2016.01.010i