Quality of life in smokers: focus on functional limitations rather than on lung function?

INTRODUCTION

There are several ways to assess the severity of chronic obstructive pulmonary disease (COPD). In almost all guidelines the classification of the severity of COPD is based on airflow limitation alone, that is, the level of forced expiratory volume in one second (FEV1) without reference, for instance, to the severity of respiratory symptoms.1^,2 Importantly, the correlation between airflow limitation and health-related quality of life, that is, an individual's satisfaction or happiness with domains of life insofar as these affect or are affected by health, is modest.3^–8

Dyspnoea is the main symptom limiting functional status in patients with chronic obstructive pulmonary disease (COPD). Functional status is the individuals' ability to perform normal daily activities in the different domains of life.3 Limitations of the functional status measured on a dyspnoea-scale seem to be more strongly correlated with quality of life than pulmonary function.9 In addition, patients usually attend the physician because of worsening of respiratory symptoms and not because of lung function decline. Therefore, some authors have criticised the one-dimensional grading of COPD severity based on pulmonary function alone, as proposed by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) Guidelines.7 In the daily management of smokers, including those with early phases of COPD, there is a need for a simple and standardised instrument to assess health-related quality of life to guide interventions. The Medical Research Council (MRC) dyspnoea scale could represent a useful instrument, because of its ability to categorise patients with severe COPD in terms of their disability.6 However, the relationship of the MRC dyspnoea scale with quality of life in smokers at risk of developing COPD or with early stages of COPD, is unknown, as is the correlation of airflow limitation with quality of life in this large population. The aim of this study was to study these relations.

METHOD

In 1998 702 male smokers (aged 40–65 years) enlisted with a general practice in IJsselstein, a town in the centre of the Netherlands, participated in a screening study to identify undetected airflow limitation. Only men were included because of the higher prevalence of COPD in males than in females and due to limited study resources.10 In 2003, a follow-up survey was performed in this sample of relatively healthy middle-aged smokers to examine the quality of life and to assess the incidence of moderate COPD. A total of 601 subjects (86% of 702) were still eligible, the lower number being mainly due to non-participation of one of the GPs (n = 65) and to a minor extent due to removal of the practice list (n = 30) or severe illness or death (n = 6). Eventually, 436 of the 601 eligible individuals (73%) participated in the second survey.

Spirometry was performed by means of a hand-held Jaeger spirometer. Details of the procedure are described elsewhere.10 Briefly, each subject had to perform at least three acceptable forced vital capacity (FVC) manoeuvres while seated. The results were shown on a computer screen and the procedure was supported by computer software. If the FEV1 was less than 85% of the predicted value, the bronchodilator response was tested 15 minutes after inhalation of four ‘puffs’ of salbutamol [100 mcg] through an inhalation chamber. In subjects aged 60 years or over, the bronchodilator response was tested 30 minutes after inhalation of two puffs of ipratropium bromide [20 mcg]. Experienced and specially trained lung function assistants employed by a primary care diagnostic centre performed all measurements. The spirometer was calibrated daily with a 1-litre syringe at the start of a series of measurements. Two investigators independently assessed the quality of the flow-volume curves and time-volume curves according to the criteria of the American Thoracic Society.11

Predicted values of FVC and FEV1 were computed using the regression equations of the European Coal and Steel Community (ECSC).12

According to the GOLD guidelines COPD is defined by a postbronchodilator FEV1/FVC ratio <0.7.1^,2 The severity of COPD can be distinguished in four stages according to postbronchodilator FEV1 values (Table 1).

Before each lung function test, height and weight were measured and the body mass index (kg/m²) was calculated. The number of pack years was computed as the number of cigarettes smoked per day divided by 20 and multiplied by the number of years of smoking.

Limitations of physical functioning were assessed by a Dutch language version of the MRC dyspnoea scale.13 The MRC dyspnoea scale measures limitations caused by dyspnoea graded at five levels from grade 1; ‘Not troubled with breathlessness except with strenuous exercise’ to grade 5, ‘Too breathless to leave the house’ (Supplementary Table 1). The scale has been used for many years and is simple to administer.14

Generic quality of life was assessed by means of the Short Form Health Survey questionnaire (SF–36) and disease-specific quality of life by means of the Quality Of Life in Respiratory Illness Questionnaire (QOLRIQ).15^,16 Both self-administered questionnaires were completed at home in the 4 weeks before the survey. The SF–36 is composed of 36 questions, organised into eight multi-item scales: physical functioning, role limitations due to physical functioning, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems, and mental health. The scores were linearly transformed to a 0–100 scale, with higher scores indicating a better quality of life. The SF–36 is broadly used and validated.15^,17^,18 The disease-specific quality of life questionnaire was developed and validated for patients with mild to moderate asthma or COPD treated primarily in general practice. It comprises 55 items classified into a total score and seven subscales: breathing problems, physical problems, emotions, general activities, situations triggering or enhancing breathing problems, daily and domestic activities and social activities, relationships and sexuality. For every item, patients were asked to answer, on a 7-point Likert-type scale, to what degree they were troubled due to pulmonary complaints. The response categories of all items ranged from 1 (not troubled at all) to 7 (very much troubled). In case of missing data, less than 50% of missing items were allowed per subscale, and one missing subscale was allowed for the calculation of the total score. The total score of the QOLRIQ and subscales scores of both questionnaires were computed by adding the item scores and dividing the sum by the number of valid items. The scores of the QOLRIQ were transformed in such a way that a lower score indicates a reduced quality of life in order to facilitate the comparison of the scores of the QOLRIQ with the scores of the SF–36.

Comorbidity, defined as a diagnosis of cardiovascular, musculoskeletal or renal disease, cancer or diabetes mellitus, was extracted from the GP medical records. A disease was classified as present when a diagnosis by a primary or secondary care physician was found in the records or in the medical correspondence. In total 17 of 409 (4.2%) medical records were missing. Chronic cough was considered present if the patient responded ‘yes’ when asked whether they were coughing almost every day for the previous 3 months.

RESULTS

A total of 395 male smokers (91% of the 436 participants) performed adequate spirometry and completed the questionnaires. Twenty-seven subjects (6%) did not perform adequate spirometry and 14 participants (3%) did not sufficiently complete the questionnaires. The mean age of the participants was 55.4 years (standard deviation [SD 6.3]) and the mean smoking history was 27.1 pack years (SD 19.3) (Table 2). Airflow limitation was found in 40.2% (159/395) and limitations of physical functioning due to dyspnoea — MRC dyspnoea scale I to V — in 25.1% (99/395) of the participants. (Table 2) The MRC dyspnoea scale correlated moderately (r_s = 0.52) with the overall score of the disease-specific questionnaire while the severity of airflow limitation correlated weakly (r_s = 0.22) with this score (data not shown).

The MRC dyspnoea scale was weakly to moderately correlated to all separate component scores of both questionnaires (r_s = 0.19–0.58) while airflow limitation was only weakly associated with all component scores of the disease-specific questionnaire (r_s = 0.12–0.28) and some component scores of the generic questionnaire (r_s = 0.13–0.20). The MRC dyspnoea scale correlated best with quality of life components measuring impairment in daily activities, that is ‘physical functioning’ from the generic questionnaire (r_s = 0.53) and ‘daily and domestic activities’ from the disease specific questionnaire (r_s = 58). In addition, in bivariate logistic regression analysis the correlations of the MRC dyspnoea scale with all quality-of-life components were stronger than the correlations of airflow limitation with all quality-of-life subscales (Table 3). For example, the odds ratio (OR) of having a worse score of the ‘physical functioning’ component of the SF–36 was approximately eightfold higher in subjects with moderate or severe limitations on the dyspnoea scale than in those with mild limitations on this scale (OR = 8.4; 95% confidence interval [CI] = 4.6–15.5), while the corresponding OR was only slightly higher (1.3; 1.0–1.5) in patients with moderate compared to those with mild COPD according to the GOLD criteria (Table 3).

DISCUSSION