Using data from the regional myocardial infarction registry RHESA, we demonstrated that participating in Disease Management Programs (DMPs) does not result in lower rates of cardiac events whereas participation in cardiac rehabilitation (CR) after discharge from hospital was associated with a distinctly lower hazard rate of MACE compared to non-participants.
One main goal of DMPs is to educate patients in everyday behavior in order to reduce the occurrence of complications and contain the effect of risk factors in patients with chronic disease. Therefore, modules like nutritional advices, assistance in smoking cessation or concepts to increase daily physical activity complement the general practitioner’s (GP) guideline care [15]. DMPs were established and repeatedly adapted since 2003, so there could be a spillover effect on the outpatient treatment by GPs on all of their patients, regardless being DMP-enrollees or not [17, 20]. GPs who are working with methods of DMPs over years possibly have improved the process quality and structure of secondary prevention in patients.
Surprisingly, patients with risk factors like a previous AMI, diabetes mellitus, smoking or obesity, tended to be less common DMP participants. Smokers and obese people in our cohort depicted an unexpected lower chance to be enrolled in DMPs. On this account, several key components of the DMPs probably could not unfold their full effect, because patients who would probably benefit the most were participating less often in DMPs. The slightly lower hazard ratio for MACE in the crude model is probably due to the DMP participants being already healthier before the DMP than the control group.
It is also remarkable, that the proportion of enrollment in DMP of about one quarter is substantially lower than the 77%, which was found eight years ago in a comparable study in the region of Augsburg by Laxy et al. [16]. Similar results (enrollment rate of 72%) were found throughout Germany in 2013 by Röttger et al. [23] in patients with coronary heart disease. Possible explanations could be regional socio-economic differences and health characteristics of the respective cohorts as well as the time span between the studies [21, 23]. In conclusion, it is apparent that the DMPs cannot reach their required target group in Saxony-Anhalt. While only about one third of all in RHESA registered patients took part in the baseline survey with at max. 70% answering in the respective follow-up, often participants are more health conscious.
Thus, our results indicate that the process of DMP participant acquisition may be one of the reasons for the lack of effects on MACE in patients after myocardial infarction. This is especially important, considering the higher rates of cardiac mortality, risk factor distribution and demographic structure in our regional study population [37]. These observations are in line with a study by Schäfer et al. about selection effects in current DMP research [35]. While our results match the conclusions of similar studies [16, 17], health insurance evaluations repeatedly described protective effects [36]. The explanation could be a different comparison group.
In contrast, participation in CR after discharge from hospital displayed a distinctly lower hazard rate of MACE compared to non-participants. In an observational study, the question is if this might be related to selection of participants. On the one hand, we found that many of those who smoked at the time of AMI, stopped smoking before starting rehabilitation. The effect of rehabilitation on MACE in this group was stronger than amongst participants that stopped smoking at the time of rehabilitation or never quit smoking during follow up time. This indicates that effects of stopping smoking on subsequent outcomes should not be falsely attributed to the CR. On the other hand, adjusting for the relevant risk factors did not pertinently change the estimated effect of CR in the direction of the null.
According to our results, smoking at the time of the initial AMI shows a weak association with prolonged survival time. This well-known ‘smoker’s paradox’ was reported in several studies [38, 39]. The main suspected reason is that smoker experience AMI at younger age, thus the relation with mortality is diluted.
Strengths and limitations
The strength of the study lies in the prospective, population-based design with a cohort of patients severely affected by the elevated risk of multiple complications after their myocardial infarction. Also, the time dependent covariate design in survival time analysis as well as the possibility to adjust the regression to multiple important co-morbidities and patient’s characteristics with relevant influence on the total effect adds to the novelty and importance in current research.
Immortal time bias is common in prospective cohort studies, but our method of implementing DMP status as time dependent covariate in the analysis can strongly reduce biased treatment effect estimates [32, 42]. This enables our analysis to account for DMP time whether or not the patient enrolled before or after the time of the initial AMI. Hence, our study is not limited by a time fixed control group status which would ignore late onset DMP enrollment.
The findings of our study are limited by the follow-up time of only 2 years, thus later outcomes are not considered. A longer period of time with greater, Germany-wide data could add to our results. Also, the main source of our data are questionnaires or telephone interviews which can cause recall bias or erroneous answers [40]. Selection bias can be caused by more motivated patients responding to the survey as well as the described effect of not covering equal distribution of risk factors in the intervention and control groups [21, 41].