Ischemic heart disease is a common cause of death in industrialized countries and accounts for a large proportion of hospital admissions in Norway [1]. Approximately 13,000 men and women are diagnosed annually with acute myocardial infarction (AMI) [2]. Secondary preventive drug therapy, e.g. platelet inhibitors, statins, beta-blockers and angiotensin-converting enzyme inhibitors (ACEI)/angiotensin receptor II blockers (ARB), is recommended following AMI to reduce the risk of new cardiovascular events and death [3‐7]. However, an underuse of secondary preventive drugs has previously been observed following AMI, especially in patients not undergoing percutaneous coronary intervention (PCI) [8]. Despite their elevated cardiovascular risk [9], still many AMI patients are not treated according to guidelines [10]. This may be related to under-prescription, reduced adherence, and/or under-dosing of secondary preventive drug therapy [11, 12]. A potential source for the underuse of recommended secondary preventive drugs could be the shift of treatment responsibility from the hospitals to the general practitioners in the primary care setting. The extent to which the hospital-initiated treatment is continued as initially prescribed, the doses adjusted or drugs switched to another type of drug within the same drug class, is not known. Comprehensive analyses of initiation and adherence in different patient populations are essential to improve long-term use of secondary preventive drugs and cardiovascular outcomes [13].

The aim of this nationwide cohort study was to examine the initiation and long-term use of secondary preventive drug in patients hospitalized with AMI in Norway during the years 2009 to 2013.

A total of 57,106 individuals were admitted to Norwegian hospitals for AMI during the study period, of whom 45,838 (80.3 %) were younger than 85 years. Of these, 42,707 (93.2 %) were alive 30 days after hospital discharge and could be included in the study (Fig. 1). Overall, 70 % of the patients were men and mean age was 65.8 years (standard deviation 11.8) (Table 1). A total of 58 % of the patients underwent PCI, with an increasing proportion during the study period (from 53 % to 63 %) (Additional file 1, Table 1). Patients undergoing PCI were younger and more often male compared with the medically treated patients (Table 1).

This nationwide observational cohort study, including all patients younger than 85 years surviving an AMI in Norway during the years 2009 to 2013, showed a generally high long-term adherence to the prescribed treatment with antiplatelet drugs, statins, beta-blockers and ACEI/ARB. The shift of responsibility for prescribing secondary preventive drugs from hospital to primary health care showed a sustained high use of the originally prescribed drugs. Only few patients switched to another type of statin or changed the dose of statin, beta-blocker and ACEI/ARB during the first two years after the AMI. For patients not undergoing PCI, a smaller proportion was discharged with secondary preventive drugs following their AMI compared with patients undergoing PCI, and less than half were prescribed DAPT at discharge.

Contemporary national level data describing long-term adherence to secondary preventive medications, i.e. antiplatelet drugs, statins, beta-blockers, and ACEI/ARB in AMI populations comparing younger vs. older and PCI vs. non-PCI patients are scarce. Furthermore, little is known on the impact of change in drug treatment responsibilities as AMI patients are transferred from hospital care to primary health care.

The initiation of secondary preventive drugs in our study was considerably higher than was found in a previous Danish nationwide study [12]. In patients admitted with a first AMI between 1995 and 2002 in Denmark, only 58 % received beta-blockers, 29 % ACE-inhibitors, and 34 % statins at discharge [12]. Although long-term compliance was reasonably good, patients who did not start treatment shortly after discharge had a low probability of starting treatment later [12]. In a more recent Swedish study, both initiation and long-term adherence to secondary preventive drug therapy was higher and similar to our findings: 82 % of AMI patients were still on aspirin 12 months post AMI, 73 % on statins and 80 % on beta-blockers [18]. A high degree of initiation of secondary preventive drug therapy was also observed in a recent study from the United States [13]. However, when patients were divided into risk groups based on the Global Registry of Acute Coronary Event (GRACE) risk score at hospital discharge, high-risk patients had a lower likelihood of receiving all appropriate therapies at discharge compared with low-risk patients.

Our data may indicate that if secondary preventive drug therapy after AMI is not prescribed at hospital discharge, the likelihood of receiving such treatment is limited. Thus, initiation of guideline-recommended secondary preventive drug therapy after AMI seems to depend on the hospital physicians, in accordance with the previous observations from Denmark [12]. The present study further demonstrates that only minor dose adjustments for drugs prescribed at discharge were performed by the primary care physicians during follow-up. Thus, Norwegian primary care physicians seemed reluctant to changing already prescribed secondary preventive drug therapies after AMI.

This finding further underscores the importance of drug prescription prior to discharge of AMI patients from hospitals. Not only should AMI patients be prescribed guideline-recommended secondary preventive drugs, but the hospital physicians could also advice on further up-titration and the target doses of the prescribed drugs in the discharge summary sent to the patient’s general practitioner. It is also important that the general practitioners are updated on current guidelines and take the responsibility for optimizing secondary preventive treatment after AMI. Interestingly, patients not undergoing PCI were less likely to receive guideline-recommended secondary preventive drugs compared with patients undergoing PCI. For example, a 10–20 % higher use of statins and beta-blockers was seen in PCI patients vs. patients not undergoing PCI. Furthermore, a larger portion of patients not undergoing PCI was discharged without a P2Y₁₂ inhibitor (48 % and 55 %, respectively, in patients ≤75 or 76–84 years) compared with PCI patients (4 % and 7 %). The reasons for this undertreatment of non-PCI patients are not known. We might speculate that a higher degree of comorbidities might be present in these patients, making the physicians more selective in their prescription of secondary preventive drugs.

The mean doses of both ACEI, ARB and beta-blockers in our study were lower than the target doses in randomized trials studying the efficacy and safety of these drugs. The reasons for the choice of these lower doses, or the lack of up-titration of doses after hospital discharge, are unknown. These drugs are used for a variety of indications, and since we have limited data on weight, blood pressure, heart rate, ejection fraction, comorbidities and on the specific indications for the various drugs (e.g. for ACEI/ARB), we find it difficult to draw any firm conclusions regarding target doses and whether the doses used in our study were too low or not. With respect to statin treatment, it should be noted that during most of our study period, no specific statin dose was recommended and statins were prescribed mainly according to LDL-cholesterol levels.

We observed a generally short duration of P₂Y₁₂ inhibitor treatment for patients not undergoing PCI (Fig. 4), with a significant proportion of patients treated for only three months as also observed in a recent study from Sweden [19]. One possible reason for the longer treatment duration after PCI may be that P2Y₁₂ inhibition is regarded particularly important after stent implantation to avoid stent thrombosis. However, ESC guidelines recommend P₂Y₁₂ inhibition for 12 months in all ACS patients [3‐6].

A large proportion of patients discontinued P₂Y₁₂ inhibitor after nine months; mainly patients on clopidogrel treatment. One likely explanation to this finding was the nine months’ limited reimbursement of clopidogrel in Norway until September 2011 [20]. Prasugrel and ticagrelor have not had such reimbursement limitations. This finding demonstrates the influence the health authorities have on medical treatment by determining the prescription rules for reimbursements of various drugs.

This nationwide observational study, including all patients in Norway below 85 years of age being alive 30 days after AMI during the years 2009 to 2013, showed a generally high long-term adherence to antiplatelet therapy, as well as treatment with statins, beta-blockers and ACEI/ARB. To a large extent, PCI patients received guideline-recommended treatment with secondary preventive drugs. Patients not undergoing PCI were less likely to be discharged with the recommended drugs. The shift of responsibility for prescribing drug treatment from hospital to primary health care did not to any major extent alter the already prescribed treatments. Thus, the majority of AMI patients remained on the secondary preventive treatment originally prescribed, further underlining the importance of prescribing guideline-recommended drug treatment at hospital discharge, and preferably including specialist guidance on future target doses in the discharge summary. The present study also indicates a need for more careful attention to secondary preventive drug therapy in AMI patients not undergoing PCI.

ACEI, angiotensin-converting enzyme inhibitor; AMI, acute myocardial infarction; ARB, angiotensin II receptor blocker; ASA, acetylsalicylic acid; DAPT, dual antiplatelet therapy; ICD-10-CM, International Classification of Diseases, 10th revision, Clinical Modification; OAC, oral anticoagulant; PCI, percutaneous coronary intervention.