American Journal of Industrial Medicine 22155-162 (1992)
Small Airways Function of Silica-Exposed Workers K.S. Chia, MBBS, MSc(OM), T.P. Ng, MBBS, MSc(OM), MFPHM, FACOM, and J. Jeyaratnam, MBBS, MSc(OM), PhD, MFCM, FFOM
Small airways obstruction may be present for many years before chronic airway obstruction becomes evident. Several spirometric indices, especially flow rates at low lung volumes, may reflect the status of small airways. Time domain indices, by using moments analysis of the volume time spirogram, have also been shown to be sensitive indicators of small airways obstruction. In this study, we have applied the various spirometric indices as well as time domain indices to a group of granite quarry workers without radiographic evidence of silicosis or physiological evidence of obstruction to the larger airways. The aim was to evaluate small airways function in relation to dust exposure in subjects with normal ratio of the forced expiratory volume in one second to the forced vital capacity (FEVJFVC) and normal FVC. The volume-time spirograms of 140 quarry workers were digitized using an electronic digitizer connected to a microcomputer where flow and time domain indices were computed. The workers were divided into three exposure groups based on their occupational history. With adjustment for age, height, and smoking status, all the time domain indices showed significant small airways obstruction with increasing dust exposure. Smokers had greater degree of airways obstruction than the non-smokers, with a similar trend of increase in small airways obstruction in relation to higher exposure. Our present study suggests that small airways obstruction is present among silica exposed workers in the absence of radiological evidence of silicosis and large airways obstruction. There was also evidence of increasing small airways obstruction in higher dust exposure group. Our study also suggests that time domain indices are more sensitive to small airways obstruction. o 1992 Wiley-Liss, hc. Key words: small airways impairment, time-domain indices, PFTs, healthy worker selection, silica exposure
INTRODUCTION
Resistance in airways smaller than 2 mm in internal diameter is a small component of total pulmonary resistance, and significant small airways obstruction may be present with little effect on total pulmonary resistance [Macklem and Mead, 1967; Brown et al., 19691. It is postulated that small airways obstruction may be present for many years before it becomes evident [Hogg et al., 1968; Woolcock et al., 19691. The method most commonly used for measuring small airways obstruction is freDepartment of Community, Occupational and Family Medicine, National University of Singapore, Singapore, Republic of Singapore. Address reprint requests to K.S. Chia, Department of Community, Occupational and Family Medicine, National University of Singapore, Lower Kent Ridge Road, Singapore 051 1, Republic of Singapore. Accepted for publication January 2, 1992.
0 1992 Wiley-Liss, Inc.
156
Chia et al.
quency dependence of compliance. This is, however, unsuitable for routine measurements. Several spirometric indices, especially flow rates at low lung volumes, may reflect the status of small airways. These would include forced expiratory flow at 50% and 75% of forced vital capacity (FEFmSand FEF,,,), forced expiratory flow between 25% and 75%, 75% and 85% of forced vital capacity (FEF2,-,,,, FEF,,-,,,) [Morris et al., 1975; McFadden and Linden, 1972; Miller, 19861. Time domain indices, by using moments analysis of the volume time spirogram, have also been shown to be sensitive indicators of small airways obstruction [Pride, 1979; Tockman et al., 1976; Permutt and Menkes, 1979; Webster et al., 1977; Neuburger et al., 1976; Liang et al., 1979; Chia et al., 19871. There have been few studies utilizing time domain indices to assess the small airways function of dust exposed workers. In this study, we have applied the various spirometric indices, as well as time domain indices, in a group of granite quarry workers without radiographic evidence of silicosis or physiological evidence of obstruction to the larger airways. The aim was to evaluate their small airways function in relation to their dust exposure. MATERIALS AND METHODS Subjects The volume-time spirograms of 153 currently employed granite quarry workers were selected. Each spirogram has at least three satisfactory tracings. In an attempt to exclude those with large airways obstruction, only those with a ratio of the forced expiratory volume in one second (FEV,) to the forced vital capacity (FVC) of greater than 0.75, and a FVC greater than 75% of the predicted FVC were selected for the study. The tracing with the highest FVC was digitized using an electronic digitizer. The digitized data were stored in a microcomputer, and volume-time, as well as flow-volume curves, were plotted on the computer screen for a visual check. Flow was calculated by least square regression using pairs of volume-time data in an interval of 0.05 seconds on either side of a given time point. This method of digitizing volume-time tracings had been shown to be an accurate and useful way of deriving volume-time and flow-volume indices from volume-time tracings [O’Donnel et al., 1987; Chia et al., 1988, 19901. Early termination increases the error in indices at low lung volumes [Moms et al., 19751. Early truncation increases the variability of time domain indices [Miller and Pincock, 1982; Miller et al., 19851. Hence, only spirograms that had a flow rate of less than 0.05 l/s in the last 0.5 seconds were selected for further analysis [Fenis, 19781. Of the 153 spirograms, only 140 satisfied the above criteria. The FVC is the volume of the last pair of volume-time data. The FEV, is obtained by intrapolation between the two data points before and after one second. Similar intrapolation techniques are used for the FEF2,-,,,, FEF,,-,,,. These indices are then corrected to body temperature, at atmospheric pressure fully saturated with water (BTPS). FEF,,, and FEF,,, are derived using least square regression, using pairs of volume-time data in an interval of 0.05 seconds on either side of 50% and 75% of FVC. For the analysis of time domain indices, the spirogram is considered to be made up of many small volume increments (dV), each having a transit time (tt) which is the average time taken for that volume increment to be expressed at the mouth (Fig. 1).
Small Airways Function of Silica-Exposed Workers
157
Volume (liters)
-
_---
,
-
I -
~~
dV
I
7
-1 1
I
-'
I
1 I
~
n
I
I
A" =
I
E(dV x tt)
FVC
I
Time (seconds)
tt
Fig. 1.
Derivation of moments from the volume-time spirogram.
The spirogram is hence a cumulative distribution of transit times [Permutt and Menkes, 19791. Using statistical moments analysis, the first, second, and third moments (A1, A2, A3) of the volume-time spirogram, standardized for volume, could be derived. The mean transit time (MTT), coefficient of skewness of transit times (CoVlT), the index of skewness of transit times (IoS'IT), and the moments ratio (MR) are derived from these three moments: MTT
=A1
Small Airways Function of Silica-Exposed Workers K.S. Chia, MBBS, MSc(OM), T.P. Ng, MBBS, MSc(OM), MFPHM, FACOM, and J. Jeyaratnam, MBBS, MSc(OM), PhD, MFCM, FFOM
Small airways obstruction may be present for many years before chronic airway obstruction becomes evident. Several spirometric indices, especially flow rates at low lung volumes, may reflect the status of small airways. Time domain indices, by using moments analysis of the volume time spirogram, have also been shown to be sensitive indicators of small airways obstruction. In this study, we have applied the various spirometric indices as well as time domain indices to a group of granite quarry workers without radiographic evidence of silicosis or physiological evidence of obstruction to the larger airways. The aim was to evaluate small airways function in relation to dust exposure in subjects with normal ratio of the forced expiratory volume in one second to the forced vital capacity (FEVJFVC) and normal FVC. The volume-time spirograms of 140 quarry workers were digitized using an electronic digitizer connected to a microcomputer where flow and time domain indices were computed. The workers were divided into three exposure groups based on their occupational history. With adjustment for age, height, and smoking status, all the time domain indices showed significant small airways obstruction with increasing dust exposure. Smokers had greater degree of airways obstruction than the non-smokers, with a similar trend of increase in small airways obstruction in relation to higher exposure. Our present study suggests that small airways obstruction is present among silica exposed workers in the absence of radiological evidence of silicosis and large airways obstruction. There was also evidence of increasing small airways obstruction in higher dust exposure group. Our study also suggests that time domain indices are more sensitive to small airways obstruction. o 1992 Wiley-Liss, hc. Key words: small airways impairment, time-domain indices, PFTs, healthy worker selection, silica exposure
INTRODUCTION
Resistance in airways smaller than 2 mm in internal diameter is a small component of total pulmonary resistance, and significant small airways obstruction may be present with little effect on total pulmonary resistance [Macklem and Mead, 1967; Brown et al., 19691. It is postulated that small airways obstruction may be present for many years before it becomes evident [Hogg et al., 1968; Woolcock et al., 19691. The method most commonly used for measuring small airways obstruction is freDepartment of Community, Occupational and Family Medicine, National University of Singapore, Singapore, Republic of Singapore. Address reprint requests to K.S. Chia, Department of Community, Occupational and Family Medicine, National University of Singapore, Lower Kent Ridge Road, Singapore 051 1, Republic of Singapore. Accepted for publication January 2, 1992.
0 1992 Wiley-Liss, Inc.
156
Chia et al.
quency dependence of compliance. This is, however, unsuitable for routine measurements. Several spirometric indices, especially flow rates at low lung volumes, may reflect the status of small airways. These would include forced expiratory flow at 50% and 75% of forced vital capacity (FEFmSand FEF,,,), forced expiratory flow between 25% and 75%, 75% and 85% of forced vital capacity (FEF2,-,,,, FEF,,-,,,) [Morris et al., 1975; McFadden and Linden, 1972; Miller, 19861. Time domain indices, by using moments analysis of the volume time spirogram, have also been shown to be sensitive indicators of small airways obstruction [Pride, 1979; Tockman et al., 1976; Permutt and Menkes, 1979; Webster et al., 1977; Neuburger et al., 1976; Liang et al., 1979; Chia et al., 19871. There have been few studies utilizing time domain indices to assess the small airways function of dust exposed workers. In this study, we have applied the various spirometric indices, as well as time domain indices, in a group of granite quarry workers without radiographic evidence of silicosis or physiological evidence of obstruction to the larger airways. The aim was to evaluate their small airways function in relation to their dust exposure. MATERIALS AND METHODS Subjects The volume-time spirograms of 153 currently employed granite quarry workers were selected. Each spirogram has at least three satisfactory tracings. In an attempt to exclude those with large airways obstruction, only those with a ratio of the forced expiratory volume in one second (FEV,) to the forced vital capacity (FVC) of greater than 0.75, and a FVC greater than 75% of the predicted FVC were selected for the study. The tracing with the highest FVC was digitized using an electronic digitizer. The digitized data were stored in a microcomputer, and volume-time, as well as flow-volume curves, were plotted on the computer screen for a visual check. Flow was calculated by least square regression using pairs of volume-time data in an interval of 0.05 seconds on either side of a given time point. This method of digitizing volume-time tracings had been shown to be an accurate and useful way of deriving volume-time and flow-volume indices from volume-time tracings [O’Donnel et al., 1987; Chia et al., 1988, 19901. Early termination increases the error in indices at low lung volumes [Moms et al., 19751. Early truncation increases the variability of time domain indices [Miller and Pincock, 1982; Miller et al., 19851. Hence, only spirograms that had a flow rate of less than 0.05 l/s in the last 0.5 seconds were selected for further analysis [Fenis, 19781. Of the 153 spirograms, only 140 satisfied the above criteria. The FVC is the volume of the last pair of volume-time data. The FEV, is obtained by intrapolation between the two data points before and after one second. Similar intrapolation techniques are used for the FEF2,-,,,, FEF,,-,,,. These indices are then corrected to body temperature, at atmospheric pressure fully saturated with water (BTPS). FEF,,, and FEF,,, are derived using least square regression, using pairs of volume-time data in an interval of 0.05 seconds on either side of 50% and 75% of FVC. For the analysis of time domain indices, the spirogram is considered to be made up of many small volume increments (dV), each having a transit time (tt) which is the average time taken for that volume increment to be expressed at the mouth (Fig. 1).
Small Airways Function of Silica-Exposed Workers
157
Volume (liters)
-
_---
,
-
I -
~~
dV
I
7
-1 1
I
-'
I
1 I
~
n
I
I
A" =
I
E(dV x tt)
FVC
I
Time (seconds)
tt
Fig. 1.
Derivation of moments from the volume-time spirogram.
The spirogram is hence a cumulative distribution of transit times [Permutt and Menkes, 19791. Using statistical moments analysis, the first, second, and third moments (A1, A2, A3) of the volume-time spirogram, standardized for volume, could be derived. The mean transit time (MTT), coefficient of skewness of transit times (CoVlT), the index of skewness of transit times (IoS'IT), and the moments ratio (MR) are derived from these three moments: MTT
=A1
