Longitudinal evaluation of the effects of illness perceptions and beliefs about cardiac rehabilitation on quality of life of patients with coronary artery disease and their caregivers

Patients’ illness perceptions were assessed using the Brief Illness Perception Questionnaire (B-IPQ) [25], which consists of eight items: consequences (i.e. how much does your illness affect your life?); timeline (i.e. how long do you think your illness will continue?); personal control (i.e. how much control do you feel you have over your illness?); treatment control (i.e. how much do you think your treatment can help your illness?); identity (i.e. how much do you experience symptoms from your illness?); illness concern (i.e. how concerned are you about your illness?); coherence (i.e. how well do you understand your illness?)) and emotional representation (i.e. how much does your illness affect you emotionally e.g. does it make you angry, scared, upset or depressed?). Caregivers’ perceptions of the patient’s illness were assessed by them answering each question according to their view about their partner’s (i.e. the patient’s) illness (CAD.) Each item on the B-IPQ is scored from 0 to 10. Increases in item scores represent linear increases in each of the dimension measured [25], i.e. effect of illness on life, duration of illness, perceived control over illness, perceived efficacy of treatment, symptom experience, level of illness concern, understanding of illness and emotional impact of illness. The cumulative score for items 1–8 gives a score range of 0 to 80. In order to compute the overall score, items 3, 4, and 7 were reverse coded. A higher total score reflects a more threatening (negative) view of the illness [25]. The B-IPQ also has a causal representation item (item 9), which requires an open-ended response (not reported in this paper). The B-IPQ has shown good validity [11, 25], in research with cardiac patients [26, 27], and spouses [28]. A slight modification was made to the B-IPQ to fit the context relevant to caregivers [28]. This involved rewording the introduction to indicate to the caregivers how to answer the questions appropriately with respect to the patient’s illness and not themselves. Cronbach’s alpha for the B-IPQ (total score) was 0.75 for patients and 0.65 for caregivers.

Patients’ and caregivers’ beliefs about the patient’s CR were assessed using the 13 item Beliefs about Cardiac Rehabilitation Questionnaire (BCR-Q) [29], a self-administered tool containing four sub-scales: perceived necessity (5 items); concerns about exercise (3 items); practical barriers (3 items); and perceived suitability (2 items). All items were rated on a 5-point Likert scale, from 1 (= strongly disagree) to 5 (= strongly agree), with the exception of one item on the necessity scale, that is ‘some aspects of the CR programme are unnecessary for me’, which was reversed scored. For each sub-scale, the scores were summed and means obtained for: necessity (range 9–25); concerns about exercise (range 3–15); practical barriers (range 3–15); and perceived suitability (range 2–10). Higher scores by subscale indicate the individual is more likely to agree that CR is necessary; has more concerns about exercise; perceives more practical barriers; and that CR is probably more suitable for a younger, more active person [29]. The BCR-Q has been shown to be a valid and reliable measure of beliefs about CR [29, 30]. In this study, a modification was made to the BCR-Q to fit the context relevant to caregivers, which involved rewording the introduction to indicate to them how to answer the questions appropriately with respect to the patient’s CR. Cronbach’s alpha for the 4 sub-domains of the BCR-Q were 0.68–0.76 for patients and 0.62–0.76 for caregivers.

Patients’ and caregivers’ quality of life was assessed using the Medical Outcomes Short-Form 12 (version 2) Health Survey (SF-12v2) [31], which is composed of 12 items; which are aggregated into two summary components: the physical component score (PCS) and mental component score (MCS). Rated items reflect what the individual could do functionally, how they felt and how they evaluated their own health status. Quality of life was regarded as a multi-dimensional construct to include subjective evaluation of the individual’s physical and mental health and social functioning. The SF-12v2 scores were calculated following the norm-based scoring algorithm, using weights derived from confirmatory factor analysis [32]. The measure has demonstrated good validity and reliability [31, 33, 34], and it has been used in studies of cardiac patients [2, 35‐39], and patient-caregiver dyads [40‐42]. Cronbach’s alpha for the PCS was 0.77 for patients and 0.89 for caregivers; and the MCS was 0.81 for patients and 0.91 for caregivers.

We collected socio-demographic and clinical data to describe the sample characteristics. Occupation was identified by the Office of National Statistics (ONS 1998). The Carstairs index [39], provided social deprivation categories based on the postcode region in Scotland. Scores range from 1 to 7, with higher categories indicating greater deprivation (i.e. lower socio-economic status). Diagnosis, revascularisation, left ventricular ejection fraction, cardiac history, co-morbidity (i.e. hypertension, diabetes), other cardiovascular disease risk factors and current medications were identified from the patient’s clinical records.

Paired sample t-tests were used to examine differences in patients’ illness perceptions (and caregivers’ perceptions of the patient’s illness), beliefs about CR and quality of life, compared within two dyad members at baseline and 6 months. For the second specific aim, longitudinal multi-level dyadic regression modelling, the Actor-Partner Inter-dependence Model (APIM) for distinguishable dyads was used [43, 44]. Actor effects refers to the impact of an individual’s characteristics i.e., the patient’s illness perceptions (and caregiver’s perceptions of the patient’s illness) and beliefs about CR at baseline on their own quality of life at 6 months, while we controlled for individuals’ quality of life at baseline. Partner effects refers to the impact of an individual’s characteristics i.e., then patient’s illness perceptions (and caregiver’s perceptions of the patient’s illness) and beliefs about CR at baseline on his or her partner’s quality of life at 6 months while controlling the quality of life at baseline.

For the dyadic analysis, all data were restructured to a pairwise dyadic data set. Grand-mean centred scores were created that were standardized using z scores to obtain unstandardized and standardized regression coefficients for the actor and partner effects [43]. Thirteen separate APIM models were computed for the physical component score (PCS) and 13 APIM models were computed for the mental component score (MCS). Physical health (PCS) and mental health (MCS) at 6 months were regressed on baseline illness perceptions (B-IPQ, total score and 8 individual items) and beliefs about CR (4 sub-domains), controlling for baseline physical health (PCS) and mental health (MCS). All analyses were performed using SPSS version 21.0 for Windows, with p < 0.05 indicating statistical significance.