The impact of asbestos-related (AR) pleural thickening (PT) on pulmonary function has become increasingly apparent in the last decade. It is by now well recognized that diffuse PT may result in restrictive ventilatory impairment[1,2] and death[3] from respiratory failure even when AR interstitial pulmonary fibrosis (asbestosis) is minimal or absent. In addition, statistically significant decrements in vital capacity have been ascribed to circumscribed PT (pleural plaque) when it is present alone or together with asbestosis.[4-8]

With the widespread efforts to identify individuals with AR disease, more and more patients are diagnosed as having PT alone, with little or no impairment in results of standard tests of lung function despite frequent complaints of dyspnea. Since dyspnea in the presence of normal resting pulmonary function is a classic indication for incremental exercise testing, we have been interested in applying this approach to evaluate the types and mechanisms of functional impairment in these patients. This article reports our findings in 23 patients, 10 with diffuse and 13 with circumscribed PT. All had normal lung fields on radiographic examination and normal (n=12) or minimally reduced lung function, five restrictive and six obstructive.

METHODS

All patients were referred to the Mount Sinai Medical Center in New York for evaluation of AR disease. All had PT (of varying extent) and normal pulmonary parenchyma on radiographs read by an expert ("B") reader (A.M.) according to the latest International Labour Office (ILO) Classification of Pneumoconioses.[9] Overall extent of PT was quantitated using an integrative index.[10] Dyspnea was defined as shortness of breath or difficulty breathing on climbing [is less than or equal to] 2 flights of stairs.

Spirometry and single breath diffusing capacity of the lung for carbon monoxide ([Dco.sub.SB]) were performed according to American Thoracic Society guidelines.[11,12] Predicted values were those published by the authors using similar methods.[13,14] The forced vital capacity (FVC) and [Dco.sub.SB] were considered normal when they were [is greater than or equal to] 80 percent and minimally decreased when they were between 64 and 79 percent of predicted. The forced expiratory volume in 1 s ([FEV.sub.1])/FVC was considered normal when it was [is greater than or equal to0.70 and minimally decreased when it was between 0.60 and 0.69. Disease was classified as obstructive when the [FEV.sub.1]/FVC was decreased. Those patients with greater decrements in lung function were not included.

Exercise testing was performed on a breath-by-breath-system (model 2000 or CPX Medical Graphics, St. Paul, Minn) under continuous clinical, electrocardiographic, and oximetric (Ohmeda Biox 3700 pulse oximeter, Boulder, Colo) monitoring. The work load was increased each minute at a rate (generally 15 to 20 W) sufficient to reach maximum [Vo.sub.2] in approximately 8 to 10 min. Testing was terminated when [Vo.sub.2] or heart rate was [is greater than or equal to] 75 percent of predicted maximum values or the patient indicated intolerable dyspnea, fatigue, or leg pain. Arterial blood gases were sampled at rest and at each 1-min interval, allowing calculation of [O.sub.2] saturation, alveolar-arterial difference in [PO.sub.2] (P[A-a][O.sub.2]) and physiologic dead space (VD). Physiologic dead space was also estimated from the end-tidal [PCO.sub.2].[15] [TABULAR DATA OMITTED]

Table 1 summarizes the many physiologic parameters analyzed, along with the normal values for our laboratory. The latter generally follow Hansen et al[16] and Wasserman et al,[17] including the resting value for VD/VT ([is less than or equal to] 0.42, which is 1.65 SD above the mean value of their reference subjects.[16] Following the suggestion of Cotes et al[18] to evaluate exercise parameters at a "standard submaximal work load" equivalent to 1.0 L [Vo.sub.2], we have adopted the normal value at this workload of [is less than or equal to] 0.25 of Jones et al.[15] To confirmthese values for VD/VT, we measured estimated VD/VT in 21 normal adults (age range, 23 to 53 years), all of whom achieved a peak [Vo.sub.2] [is greater than or equal to] 75 percent of predicted. Of the 21, 19 were nonsmokers while 2 had discontinued smoking 1.5 and 4 years earlier; 15 were male. The mean value for VD/VT at rest was 0.337 [+ or -] 0.071 and at 1.0 L [Vo.sub.2] was 0.200 [+ or -] 0.039. Only one subject exceeded 0.25 at 1 L [Vo.sub.2] (estimated VD/VT=0.26). The mean value for ventilatory response ([Delta]VE/[Delta]Vo.sub.2]), using the method of Reebuck et al[19] (linear regression up to the anaerobic threshold [AT] was 0.212 [+ or -] 0.051 in our normal subjects.

Statistical significance was assessed by the two-tailed Student's t test (for mean values) and by Fisher's exact test (for frequencies).

RESULTS

Of the patients studied, 21 were male subjects who had been occupationally exposed to asbestos as insulators or in shipyards beginning 18 or more years earlier, when workplaces were less controlled. Durations of exposure were variable (from less than 1 year to more than 30 years). Two patients were wives of such workers ("household exposure"). Mean age of all patients was 61 years (range, 37 to 74 years). Five were lifetime nonsmokers and the remaining 18 were former smokers, a distribution of smoking histories similar to large cohorts of these trades.[1] Durations of smoking ranged from 10 to 38 years. Of the 18 former smokers, 17 had discontinued 5 or more years (range, 5 to 30 years).

The profusion score for small parenchymal opacities was 0/1 or 0/0 for all patients. Ten had diffuse PT defined by obliteration of an ipsilateral costophrenic angle, and 13 had circumscribed PT. The extent of PT ranged from minimal (unilateral B,2 or face on 1 or bilateral A,1, face on 1) to severe (bilateral [is greater than or equal to]B,2 plus face on), with integrative indices[10] ranging from 2 to 26.

Table 2 shows mean values for pulmonary function and exercise parameters in groups 1 and 2. The 12 patients in group 1 had normal values for spirometry (mean FVC, 92 percent of predicted; [FEV.sub.1]/FVC, 0.79) and [Dco.sub.SB] (mean, 116 percent of predicted) while the 5 patients in group 2A had slight restriction (mean FVC for these 5, 75 percent of predicted) and the 6 in group 2B had slight obstruction (mean [FEV.sub.1]/FVC for these 6, 0.63). Of the 11 patients in group 2, Dco was slightly decreased in 6 (mean value for these 6, 72 percent of predicted).

There was no difference in extent or type of PT between those with normal pulmonary function (group 1) and those with minimal impairment (group 2), 5 of the 12 patients in group 1 and 5 of the 11 in group 2 had diffuse PT. The FVC was lower in the patients with diffuse (mean, 77.2 percent of predicted) as compared with circumscribed PT (mean, 88.2 percent of predicted) whether they were in group 1 or group 2. Values for [FEV.sub.1]/FVC were similar in both types of PT.

Mean values for ventilatory responses (VE/[Vo.sub.2] at AT and [Delta]VE/[Delta][Vo.sub.2]) were somewhat more impaired in group 2B, although the differences were not statistically significant. Ventilatory reserve was significantly less in groups 2A and 2B, reflecting in part their lower ventilatory capacity. There was no difference in VD/VT at exercise, in peak [Vo.sub.2], or in AT.