Early assessment and treatment of acute coronary syndrome (ACS) are major contributors to decreasing the morbidity burden of cardiovascular disease and to avoiding the development of potentially preventable complications [1]. In Sweden every ambulance has to be manned with at least one ambulance nurse (AN) who is a registered nurse with or without specialist training in prehospital emergency care [2]. Ambulance nurses are thus required to assess patients’ symptoms correctly [3, 4], making advanced clinical decisions in order to convey the patients to emergency departments (EDs) or other levels of care [5, 6].

Contradictory suggestions are reported about symptoms that could predict the severity of ACS. European guidelines declare that severe or ongoing chest pain lasting 20 min or more is a symptom indicating high risk of ACS [7]. However it has been established that the severity of chest pain is a poor predictor of imminent complications such as cardiac arrest [8]. In one study, more than 50% of patients with chest pain at the ED were found to have cardiovascular conditions such as myocardial infarction, angina pectoris, pulmonary embolism and heart failure [9]. Others, however, reported that only 9 % of patients admitted to the ED with chest pain had an underlying myocardial infarction [10]. Chest pain symptoms resulting in calling for an ambulance are variable and perceived very differently by different individuals [11].

Earlier studies on patients with acute myocardial infarction have reported a variety of emotional reactions [12] and large variations in patients’ conceptions of the event [13]. It is the patients who provide reliable information about their subjective experiences of symptoms and signs [4]. The localisation of chest pain as reported by patients can vary as well as its intensity, which provides evidence of ANs’ way of taking patients’ subjective experiences into account [14].

These conditions underline the importance of additional and deepened knowledge about early assessment and clinical decisions concerning patients suffering suspected ACS. Reducing time from onset of ACS symptoms to arrival in the coronary care unit at the hospital continues to be a challenge to the healthcare system [15]. For example in the prehospital setting, a more rapid revascularisation has been reported in patients with ST-elevation acute myocardial infarction after prehospital recognition and treatment followed by fast-tracking to a coronary care unit [16].

Experiences from Sweden suggest that more than half of all patients with a final diagnosis of ACS dial 112 for transport by ambulance to hospital. Among these patients a very large proportion has an electrocardiogram (ECG) recorded on scene that often is sent directly to the nearest cardiology department for further evaluation. Among patients with ECG signs of myocardial ischaemia a direct transport to the catheterisation laboratory or coronary care unit is today a routine procedure [16].

The pathophysiology behind the chest pain and the variability in patients’ experiences of chest pain when suffering from ischaemic heart diseases remains obscure. However, in a retrospective study on chest pain in patients admitted to hospital, ambulance patients received the final diagnosis of acute myocardial infarction more often than patients not transported by ambulance [10]. In a retrospective study measuring the intensity of chest pain in the prehospital setting, a shorter prehospital delay was more likely for patients who reported a higher pain intensity than others [17]. Since a shorter prehospital delay has been associated with a higher likelihood of an underlying myocardial infarction [18], these findings suggest that at least some patients intuitively recognise their pain as being of a more severe nature.

However, not much is known on the subject of symptoms such as chest pain, discomfort, anxiety and various other clinical findings from the prehospital setting regarding the predictable severity of ischaemic heart diseases including the risk of complications. Here, there is a gap in knowledge and therefore the aim of this study has been to explore the possible connection between one aspect of chest pain, i.e. the patients’ estimated intensity of chest pain when first seen by the ambulance crew in suspected ACS, and clinical findings and complications before and after arrival in hospital.

All in all, 1836 patients were included in the main study [19]. Of these, 1767 (96%) patients were eligible for the present study. In 1640 individuals, information on VAS when the ambulance team arrived was received, and of these 1603 had a value of ≥4 at randomisation and constituted the population analysed.

This is to the best of our knowledge the largest study reporting on the connection between the intensity of chest pain according to subjective assessment on ambulance arrival and clinical findings before and after arrival in hospital among patients with acute chest pain raising suspicion of ACS.

The principal research question being addressed is the degree of association between the patient-reported intensity of an acute symptom at an early assessment and the subsequent outcome. We found significant but mostly weak associations between the patients’ estimated severity of chest pain on admission of the ambulance clinicians and a number of indicators reflecting the severity of the disease. These findings are new and suggest that the severity of chest pain as assessed by patients using the VAS may give some indirect indications of the severity of the disease underlying the symptom presentation in prehospital assessments. Some of the study’s outcomes are strictly time dependent conditions (e.g. myocardial infarction) and some are not (e.g. angina pectoris). Our findings could be related to a Norwegian study showing that a majority of ambulance patients with chest pain were admitted for further assessment in hospital, but only a quarter were assessed in the prehospital setting as having a severe illness [25]. These facts together draw attention to the importance of ANs’ ability to make prehospital assessments using the patients’ experience of pain as important information. However, pain may be understood as a complex phenomenon not fully encompassed by measuring pain intensity using the VAS.

The present results show a connection between estimated intensity of chest pain and AV block/bradyarrhythmias in the prehospital setting. In general, the proportion of various complications prior to hospital admission was very low, contributing to the failure of finding any significant association between the reported intensity of chest pain and the majority of complications prior to arrival to hospital. The ambulance care organisation is affected by an ambition to reduce time delay, which may lead to stress with a detrimental effect on patient care [26]. However, rapid transport to hospital care allows less time for patients’ clinical complications to develop. Today guidelines are considered important to help accomplish fast prehospital assessments of patients with ST-elevation ACS in order to reduce time delay to in-hospital treatment [3]. However, the present results may support the development of prehospital guidelines to assist assessment of ACS without ST-elevation as well [27].

After admission to hospital, the proportion of various haemodynamic complications as well as supraventricular arrhythmias increased, whereas ventricular arrhythmias and bradyarrhythmias remained infrequent. No significant association between the early assessment of the intensity of chest pain in the prehospital setting and these complications was found with exception for heart failure. Hence this may indicate a weak association between the early intensity of chest pain prior to arrival to hospital and the severity of the disease. Galinski et al. found no significant difference regarding the severity of pain between ambulance transported patients with acute myocardial infarction and no myocardial infarction [28]. They argued for chest pain severity as a useless factor for distinguishing acute myocardial infarctions from other conditions. In addition, Thuresson et al. found that only half of the patients with ST-elevation ACS reported that chest pain appeared suddenly and rapidly reached a high intensity [29]. However, this contrasted with our findings of a significant association between estimated intensity of chest pain and the proportion of a final diagnosis of acute myocardial infarction. This is in agreement with a previous study showing that when the dispatchers scored more severe chest pain based on patient interviews, then the likelihood of acute myocardial infarction was much higher [30].Their results indicated that the severity of the pain was one of the most important questions to address in order to optimise the priority at the dispatch centre. However, this finding does not exclude the risk of an underlying myocardial infarction or complications even among patients with a lower intensity of pain.

The present finding of a significant association between estimated intensity of chest pain and the duration of hospitalisation may be explained by a higher proportion of acute myocardial infarction with increasing severity of pain. This is in line with an earlier study by Goodacre et al., showing pain as a predictor of acute myocardial infarction and ACS in patients with normal or non-diagnostic ECG [31]. Patients who did not have relief of chest pain after 15 min and on admission to hospital constituted a minority in the present study.

Our results showed an association between the reported initial intensity of chest pain and anxiety as well as chest pain after admission to hospital. This may correspond to myocardial infarction patients’ vulnerability due to lacking control of the situation, experiencing anxiety [13] and fearing death as a possible outcome [32]. One factor causing anxiety after arrival in hospital may be the reduced presence of medical staff in the ED compared with the ambulance environment [33], resulting in a feeling of isolation and insecurity [34]. The present findings corroborate this interpretation and may indicate the importance of taking the patients’ whole life situation into account when assessing their physical disorder [4]. Forslund et al. found in a study on ambulance patients that acute chest pain is a subjective experience that accompanies fear and anxiety [35].

In patients with chest pain raising suspicion of ACS on admission by the ambulance clinicians and then transported by ambulance, the estimated intensity of chest pain was associated with complications prior to hospital admission, heart failure, anxiety and chest pain after arrival in hospital, final diagnosis and the number of days in hospital. One implication of this study might be that when ANs suspect ACS the likelihood of an underlying acute myocardial infarction is somewhat higher if the patient in addition scores high on VAS.

However, the association between early assessment and the subsequent outcome was relatively weak as patients with lower intensity (VAS < 4) of chest pain were not included in the study and therefore cannot be commented on. Hence, this study provides knowledge about patients reporting prehospital chest pain measuring VAS 4 or higher. In order to generate a wider picture of the association between reported intensity of prehospital chest pain and the subsequent outcome, studies on patients reporting lower intensity of pain are important as a complement to the present study. In addition, risk scores for patients with acute chest pain might be developed specifically for the prehospital setting in which the intensity of the initial chest pain could be one of a number of different aspects of the clinical picture.