In this study, we observed that restrictive and moderate–severe obstructive impairments were associated with several measures of poor HRQOL in unadjusted models. However, when we adjusted for chronic respiratory symptoms and other covariates in the analyses, most of the significant associations were attenuated or no longer significant, with the exception of fair/poor perceived health. On the other hand, all the HRQOL measures were significantly associated with any self-reported respiratory symptoms. These results suggest that although both pulmonary function and respiratory symptoms may contribute to poor HRQOL, respiratory symptoms may disproportionately impact more aspects of HRQOL than does pulmonary function. Aside from the mere presence of any respiratory symptoms, we also observed different associations between the specific symptoms and poor HRQOL measures. When adjusting for the presence of the other respiratory symptoms, frequent cough was no longer associated with increased likelihood of any poor HRQOL measures. Frequent phlegm was associated with increased likelihood of fair/poor health and frequent physical distress. Past year wheeze was associated with increased likelihood of all poor HRQOL measures except frequent activity limitation.
Some research has previously shown an association between pulmonary function and HRQOL among patients with COPD, asthma, and other respiratory diseases [3, 9]. The relationship is generally stronger with the physical component of HRQOL than the mental or psychological component [3, 7, 9, 11, 12]. These findings are similar to our unadjusted results, where we observed a higher unadjusted prevalence of frequent physically unhealthy days among adults with restrictive or moderate–severe obstructive impairment, but no difference in prevalence of frequent mental distress based on pulmonary function category. However, once we adjusted for sociodemographic variables, chronic illnesses, smoking status, and BMI, we no longer observed a difference in frequent physically unhealthy days by pulmonary function category. These earlier studies also did not evaluate the association between respiratory symptoms and HRQOL septely from pulmonary function, and were therefore not able to ascertain how much poor HRQOL was attributable to impaired pulmonary function versus respiratory symptoms.
Our findings regarding respiratory symptoms are supported by several other studies. In an Australian adult population cohort, chronic cough was more common among those with severe psychological disturbance. This result persisted when the analyses were limited to individuals without identifiable respiratory disease and non-smokers. Compared to those without chronic cough at any time, adults reporting cough at follow-up were more likely to report more severe psychological disturbance, as well as poorer HRQOL (both physical and mental components) . Another population study in Norway (N = 2306) showed that after adjustment for respiratory symptoms, the physical component scale score was significantly lower (worse) in individuals with severe COPD, and the mental component scale score was significantly higher (better) in individuals with COPD . Similar to our study, both physical and mental components of HRQOL were negatively impacted by respiratory symptoms. The primary focus of that study was COPD and restrictive impairment and other obstructive impairment were not evaluated . Among Swedish patients with asthma, symptom severity was associated with worse physical and mental well-being among women but not among men . Other studies have found that respiratory symptoms were related to poorer HRQOL among COPD, asthma, and sarcoidosis patients, while pulmonary function was not [8, 21]. Finally, in the European Community Respiratory Health Survey, individuals with respiratory symptoms had poorer physical and mental HRQOL even in the absence of asthma and COPD and after adjustment for bronchial hyperresponsiveness and FEV1.
Mild obstruction was not associated with any HRQOL measures in our study. Likewise, asymptomatic mild obstruction was not associated with poorer HRQOL in a Swiss cohort when compared to asymptomatic individuals with normal lung function. However, those with symptomatic mild obstruction not only had poorer HRQOL, they also had a faster decline in FEV1 and greater utilization of respiratory care . This further emphasizes the importance of respiratory symptoms in overall health. In another study, individuals with mild obstruction were more likely to have depressive symptoms than adults without obstruction . Unfortunately, data regarding respiratory symptoms was not included in that analysis. Since these individuals were selected for that study because they had been treated for COPD in the previous year, it is possible that they had more respiratory symptoms than if they had been selected from the general population.
The HRQOL questions used in this study were developed by CDC to provide generic measures of HRQOL rather than condition-specific measures in order to allow comparison across all population age groups and chronic conditions. They have been validated in general populations as well as numerous subpopulations and various conditions [18, 24–26]. These measures are compble to subscales of other generic HRQOL instruments such as the Medical Outcomes Study Short Form-36 . Although there is some literature on the association of pulmonary function and respiratory symptoms with HRQOL using condition-specific questionnaires such as the St. George’s Respiratory Questionnaire and the Airways Questionnaire 20, items used to assess the presence or severity of respiratory symptoms are overrepresented and therefore do not assess global measures of physical and mental health.
There are some limitations to this study. First, this was a cross-sectional study. Therefore, we cannot determine causality in the association between pulmonary function, respiratory symptoms, and HRQOL. Although adverse respiratory health likely leads to poor HRQOL, it is also possible that poor HRQOL could lead to worse management of symptoms or to behaviors that might be detrimental to respiratory health. Second, symptoms included in the questionnaire were limited. One important symptom that was not addressed was shortness of breath or dyspnea. The respiratory symptoms questions were also asked of a limited age group (aged ≥40 years). Therefore, our results may not be generalizable to younger populations.
Another limitation of this study lay in the large proportion of adults without spirometric results who were omitted from our analyses. There were 1224 adults with complete data on symptoms, HRQOL measures, and confounders, but missing spirometry data. Of these, 592 were safety exclusions, 315 underwent spirometric testing but of insufficient quality, and 317 did not undergo testing for other reasons such as participant refusal or time limitations. Compared to the study population, the safety exclusions were older, had more chronic illnesses, and were more likely to have a history of smoking. Similarly to the moderate-severe obstruction group, this group had a higher likelihood of reporting frequent cough, frequent phlegm, and past year wheeze. The persons who underwent spirometry but with insufficient quality results were also older, but were more likely to be of minority ethnic/racial groups, and to be never smokers. This group had a higher likelihood of reporting past year wheeze, but not frequent cough or frequent phlegm, than those with normal lung function. Participants excluded for other reasons were more predominantly female and had a higher proportion of minority ethnic/racial groups. This group also had a higher likelihood of reporting past year wheeze, but not frequent cough or frequent phlegm, than those with normal lung function. All three groups excluded from our analyses due to inadequate or no spirometry data had a higher likelihood of reporting fair/poor health, frequent physical distress, and frequent activity limitation (in fully adjusted models) than adults with normal lung function. The safety exclusion group also had a higher prevalence of frequent mental distress. Many of the participants in the safety exclusion group likely had severe lung function impairment, such as might require supplemental oxygen (see Table 1), but other problems may not have been associated with poor lung function at all.