This study is part of the Groningen Longitudinal Aging Study (GLAS), a population-based prospective follow-up study of the determinants of health-related QoL of older people . Available data for the present study have been collected and organized since 1993 (T0). A total of 5,279 persons, aged 57 years and older, were recruited to participate in the baseline assessment. These participants were monitored for selected diseases by their general practitioners between 1993 and 1998. After CHD emerged, the patients received a letter from the research team asking them to participate in the follow-up study, covering three follow-up assessments at 2, 6, and 12 months (T1, T2, and T3). This study was approved by the Medical Ethical Committee of the University Medical Center of Groningen.
Data for two types of CHD were collected: acute myocardial infarction and congestive heart failure, according to the criteria of the International Classification of Primary Care .
Four hundred ninety-four patients with a new episode of acute myocardial infarction or congestive heart failure after the baseline were recruited. Two hundred two patients who completed all follow-up assessments were included in the present study. Figure 1 shows the flowchart of the cases after the baseline assessment. Participants were compared with nonparticipants (patients who dropped out, died, or could not participate to all the assessments for other reasons) at the premorbid stage for sociodemographic characteristics and QoL. The two groups did not significantly differ in most of the measurements (gender, depressive feelings, anxiety, and role and physical functioning), except for the fact that participants were significantly younger (on average, 71.9 versus 74.7 years; F = 15.1, p < 0.001) and reported higher levels of social functioning (on average, 74.8 versus 68.5; F = 5.5, p < 0.05).
Data at all assessment points were collected through semistructured interviews and by means of self-report questionnaires.
A weighted sum index combining three major indicators of SES, namely, educational level, income, and occupational prestige, was created at the baseline for the entire GLAS sample.
Educational level is the highest level of education attained by the patients with scores ranging from 1 (elementary school not completed) to 6 (higher education, second phase). The level of education for the respondents is based on the International Standard Classification of Education .
The income of the respondents was measured by asking them their net monthly household income. For respondents who were married or living with a partner, the monthly after-tax income for both the respondent and the partner was recorded. This household income was converted to an individual income (six equivalent categories) on a scale ranging from category 1, up to 522 euros per month, to category 6, 795 euros per month or higher.
Occupational prestige was derived by coding the last profession of the respondent according to the classification of the Dutch Central Bureau for Statistics . These nominal codes correspond with the International Standard Classification of Occupations . The occupational codes were converted into prestige scores with an interval level of measurement from 0 to 100 . This scale was developed in The Netherlands on the basis of 116 occupations classified by a random sampling of 500 persons. According to Dahl , using the occupation of the male partner for married, divorced, or widowed women provides the best results when researching health among older women, even compared to their own occupational score. Therefore, we used the information of the male partners for female participants who were living with their partners, widowed, or divorced.
To compute an overall index, the scores on all three indicators of SES were transformed into standardized Z scores. We performed an unrotated factor analysis to determine whether the three indicators all loaded on one factor, which turned out to be the case. The factor loadings of the three variables were high and comparable: educational level (0.82), income (0.76), and occupational prestige (0.77). We then multiplied the score for each variable with its factor loading and summed them up to a weighted index for SES. Missing data for income (N = 26) and occupational prestige (N = 14) were replaced by using the mean standardized income score or the mean standardized occupational prestige score calculated for participants with the corresponding value in educational level. The new variable, ranging from −4.36 to 4.64, was finally recoded into low and high SES using the median score of the index as the cut-off point (−0.047). Classification in two categories was preferred over classification in three categories in order to increase power. A closer inspection of the two SES groups on the basis of their composite scores revealed the following composition of the sample. Of the patients in the low SES group, 90% had a lower education (from elementary school to vocational education, lower level), 65% had a small income (less than 658 euros per month), and 91% had low professional prestige (below 50 on the Sixma and Ultee scale). Conversely, 69% of those in the high SES group had a high educational level (from advanced education, higher level to higher education, second phase), 91% had a high income (more than 658 euros per month), and 61% had high job prestige (scoring more than 50 on the Sixma and Ultee scale).
Gender, age, and severity of the disease were related to cardiac disease outcome in the present data and in previous research [10, 20]. Disease severity was assessed according to the New York Heart Association (NYHA) classification at the first follow-up assessment . The NYHA classification indicates the severity of cardiac symptoms by the level of complaints of breathlessness in relation to physical activities. It ranges from I (mild symptoms) to IV (severe symptoms).
Five indicators representing the three domains of QoL (psychological, physical, and social) were used at both baseline and follow-up assessments.
Depressive feelings and anxiety were assessed with the two subscales of the Hospital Anxiety and Depression Scale (HADS) . Both the anxiety (Cronbach’s α = 0.83 at baseline) and depressive feelings (Cronbach’s α = 0.71 at baseline) subscales were composed of seven items varying from 0 to 21 (higher scores indicating more symptoms). HADS has been validated for an older Dutch population .
The participants’ social functioning, role functioning, and physical functioning were quantified using three subscales of the Medical Outcomes Study Short Form 20 (MOS SF-20) . The social functioning subscale measures the extent to which health interferes with normal social activities such as having contacts and visiting friends (one item). The role functioning subscale measures the extent to which health interferes with usual daily activities such as housework or the professional job (two items, Cronbach’s α = 0.87 at baseline). The physical functioning subscale provides a global indication for physical limitations such as walking uphill or eating and dressing (six items, Cronbach’s α = 0.79 at baseline). All three subscales range from 0 to 100 and higher scores indicate better functioning. The psychometric properties of the Dutch version of the MOS were approved in a previous study .
In order to examine the relationship between SES and psychological distress, as well as role, social, and physical functioning at different assessment points, mean values of the considered variables were compared between the two socioeconomic groups using a one-way analysis of variance (one-way ANOVA) with SES as an independent factor. A multivariate analysis of variance (MANOVA) investigated the similarities or differences between the paths of adaptation of the two levels of SES. All the results were controlled for covariates. All analyses were performed using SPSS version 14.0.