4.6. Discussion
Although both FEV i and FVC for PF subjects in group A were within the "normal" range they wK’e, nevertheless, significantly reduced compared to the matched control group. This observation together with the reduced flow rates for the tests of small airways function (MMFR25.7 5, Vmax50 and Vmax25) may well reflect the early stages of onset of chronic
bronchitis which has been reported to be present in a proportion of pigeon fanciers (Bourke et al., 1989). Since both total lung capacity and total diffusing capacity were within the normal range for PF subjects in group A then it is unlikely that restrictive defect, a well known phenomenon in pigeon fanciers, was present in this study population. PF subjects in group B, with absence of circulating precipitins and minimal chest symptoms, demonstrated normal lung function tests with the exception of Vmax25 which was on average only 54 % of predicted.
The clearance of radioaerosoi from the lungs depends on its initial site of deposition within the airways (Agnew et al., 1981). In general radioaerosoi deposited proximally will be cleared more rapidly from the lungs than radioaerosoi deposited peripherally within the lungs. In this study, it was possible to ensure that the radioaerosoi deposition within the lungs (i.e. alveolar deposition) of the PF subjects was similar to that of the healthy control subjects in both groups A and B.
The use of historical controls is not ideal. However, it is not feasible to study a control group prospectively since it is impossible to ensure similar radioaerosoi deposition within the lungs for the healthy subjects as for the PF subjects. Historical controls were therefore selected from a bank of data for a group of healthy subjects who inhaled radioaerosoi at different flow rates to achieve a range of alveolar depositions.
Tracheobronchial clearance of the PF subjects in both groups were found to be statistically significantly reduced compared to their respective healthy control groups (figures 4.1-4.3). This finding indicates that the presence or absence of circulating precipitins is not related to defective mucociliary clearance. The presence of circulating precipitins was correlated with exposure and there was a progressive tendency towards sensitisation (Fink et al., 1972; Banham et a l, 1986). However, the findings in this study population would suggest that level of exposure is immaterial to the degree of impairment in tracheobronchial clearance. Taking all PF subjects as a whole group (fig. 4.4), similar retardation was seen in tracheobronchial clearance at a higher level of statistical significance because of the increased number of subjects.
It is unlikely that cigarette smoking may have confounded the interpretation of the observed data since tracheobronchial clearance in the non-smoking PF subjects was slower (significantly) than the corresponding control group. It is of interest to note that the on average retardation in tracheobronchial clearance in the current smoking PF subjects compared to their controls was greater in magnitude than in the non-smoking group (154 vs. 93 %.h). Although the number of subjects in each group is low it is possible that smoking and exposure to pigeons may have a cumulative adverse effect on tracheobronchial clearance. The apparent lack of a difference in tracheobronchial clearance between smoking groups within the PF subjects can be attributed to differences in physical characteristics and alveolar deposition (e.g. AD for the non-smokers 42±6 % vs. 33+5 % for the smokers).
Cough appears to have played a small part in the overall tracheobronchial clearance with only five PF subjects producing sputum during the 6 hour observation period. This finding together with any help in tracheobronchial clearance due to unproductive coughing (chapter six) would result in an underestimate of the degree of reported impairment in lung mucociliary clearance. The lack of any significant enhancement in tracheobronchial clearance at 6 hours following instructed coughing would suggest that the undue retention of the radioaerosoi was beyond the reach of productive coughing (Leith, 1968) i.e. probably beyond the seventh generation on Weibel's model of the human lung (Weibel, 1963) and/or due to lack of appropriate hypersecretion which is necessary for productive coughing (Camner et al., 1979; Puchelle et al., 1980).
It is of interest to note that the degree of lung mucociliary clearance impairment in PF subjects within group B was comparable to that seen for those subjects in group A with circulating precipitins, reduced lung function and increased symptomatology. This is consistent with the reported apparent lack of relationship between degree of impairment of lung mucociliary clearance and pulmonary function indices in patients with chronic bronchitis (Pavia, 1984a).
This study has demonstrated that lung mucociliary clearance is compromised in pigeon fanciers. This is further evidence of involvement of the large airways in these groups of subjects. Any reduction of lung mucociliary clearance will tend to increase the residence time of any inhaled antigen. Inhaled antigen has been shown to result in reduced lung mucociliary clearance (Ahmed et al., 1981) and undue retention of lung secretions has been reported to result in atelectasis thereby giving rise to an increased incidence of chest infections (Gamsu et al., 1976).
Avian precipitins have been shown to have a role, as a non-invasive test, in the diagnosis of bird breeder's disease (Reynaud et a l, 1990).
This study using an objective, non-invasive technique has demonstrated that lung mucociliary clearance is compromised in pigeon fanciers irrespective of the presence or absence of avian precipitins. The data, albeit on a small number of subjects, appears to demonstrate that mucociliary clearance may have at best a limited relationship to immune activation in PF subjects.
The detection of an abnormal clearance in PF subjects with absence of avian precipitins may be akin to the observation by Lourenco and associates (1971) of abnormal mucociliary transport in asymptomatic smokers. Smokers who develop chronic bronchitis, where chest symptoms are present, are known to have impaired mucociliary clearance (Agnew et al., 1982). As such, abnormal mucus transport may be an early indicator of the disease process. Delayed clearance may represent one aspect of a vicious cycle (Cole, 1984) with prolonged retention at airway sites of inhaled antigen itself promoting a further decline in the efficacy of the mucociliary mechanism. This in turn could cause undue retention of lung secretions and thereby an increased incidence of chest infections (Pavia, 1987b).
The clearance deficit demonstrated in this study provides further evidence that pigeon fanciers suffer lung abnormalities involving the conducting airways. It is worthy of note that this degree of clearance dysfunction was detected in pigeon fanciers who had agreed to volunteer while at a meeting to do with their interest - and not when seeking medical attention.