WHAT IS ALREADY KNOWN ON THIS TOPIC
HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY
Introduction
International guidelines and quality indicators for emergency cardiovascular care recommend the performance of a 12-lead ECG by Emergency Medical Services (EMS) personnel during assessment of patients with suspected acute coronary syndrome (ACS).1 2 Information gained from a prehospital 12-lead ECG (PHECG) informs decision-making in three components of immediate care: targeted prehospital treatment, transportation to an appropriate receiving hospital, and activation of the receiving cardiac catheter laboratory when ST-segment elevation myocardial infarction (STEMI) is identified.3
Research has focused on the association of PHECG with process-of-care quality descriptors, such as ‘call-to-reperfusion’ time.4 A systematic review and meta-analysis confirmed the association of PHECG with improved clinical outcomes.5 However, PHECG was underused, particularly in older patients, women and people with comorbidities. Our previous work assessing use and impact of PHECG was undertaken in England and Wales when fibrinolytic therapy was the principal reperfusion strategy for STEMI.6 Primary percutaneous coronary intervention (pPCI) became widely available in England and Wales in 2009, with 95% of eligible patients receiving pPCI by 2012/2013.7 This change in practice necessitated changes in EMS care of STEMI patients, with direct transportation to pPCI capable hospitals replacing prior strategies of either prehospital or in-hospital fibrinolytic treatment.
The purpose of this study was to assess PHECG use, and related outcomes, since the introduction of pPCI. Specifically, our three ‘research questions’ were: whether the proportion of eligible patients receiving PHECG changed since the national rollout of pPCI networks; whether patients who received PHECG differed from those who did not; and how patients who did and did not receive PHECG differed with respect to prehospital times, reperfusion strategy, and 30-day and 1-year mortality.
Methods
The study protocol has been published previously.8 We performed a population-based, linked cohort study using 2010–2017 UK national heart attack data from the Myocardial Ischaemia National Audit Project (MINAP), a comprehensive registry of ACS hospitalisations mandated by the Department of Health and Social Care, to update evidence on care and outcomes for patients eligible for PHECG. At the time of the study, MINAP provided patient demographic and clinical details across 122 data items.9
Patient and public involvement
Three patient and public representative has been involved since study inception. They played a role in the development of relevant research questions, the study proposal and funding application, oversight activities, review and interpretation of results and dissemination of findings.
Inclusion/exclusion criteria
Patients aged 18 years or older when attended by EMS between 1 January 2010 and 31 December 2017 and admitted with ACS to a hospital in England and Wales participating in MINAP were included. For patients with multiple admissions, only the first ACS event in this period was included. Analysis was by final diagnosis recorded.
Data collection
MINAP data are collected prospectively using a secure electronic system, encrypted and transferred online to a central database which protects patient identity. Data were anonymised and imported into the Secure eResearch Platform UK trusted research environment and deterministically linked with Office for National Statistics (ONS, civil registry) mortality data from NHS England. The analysis team did not have access to data that would allow records to be linked back to named individuals.
Statistical analysis
To explore any change in the proportion of patients receiving PHECG over time, we tabulated the proportion of patients receiving PHECG and conducted a linear regression analysis. Data are presented overall and by EMS.
For our second and third questions, we compared demographics, comorbidities and medical histories prior to the index ACS event, incorporating these core factors: age (dichotomised as 18–74 years/75+ years); sex; previous validated episode of acute myocardial infarction (MI); previous coronary artery bypass graft (CABG); diabetes and previous chronic heart failure (CHF). Factors were selected for consistency with previous work and are listed with each table.6 ORs, with 95% CI, are presented throughout, being more informative than p values, which were generally highly significant even for small absolute differences due to cohort size.
We used logistic regression to investigate any association between receipt of PHECG and prespecified demographic and clinical factors. The dependent variable was receipt of PHECG. In addition to the core factors, we also adjusted for ethnicity (Caucasian/Asian/other); hypertension; peripheral vascular disease (PVD); whether the patient currently smokes; dyslipidaemia; prior PCI; prior stroke; chronic kidney disease (CKD); prior angina; and asthma/chronic obstructive pulmonary disease (COPD).
Prehospital haemodynamic data are not available in MINAP. Instead, we conducted unadjusted comparisons of heart rate at, and first systolic blood pressure after, hospital admission using Mann-Whitney U tests. As post-PHECG data, these were not incorporated into the logistic regression investigating use of PHECG.
We used logistic regression to investigate any association between 30-day and 1-year mortality and PHECG use. In addition to the core factors, we adjusted for ethnicity (as above), receipt of aspirin, raised cardiac markers, CKD and prior stroke/cerebrovascular disease. We used Cox regression models to investigate differences in the time from call for help to arrival at hospital, and from ambulance arrival to arrival at hospital, between patients who did, and did not, receive PHECG. In addition to the core factors above, we adjusted for ethnicity (as above), CKD and prior stroke. The proportional hazards assumption was verified using Schoenfeld residual plots and log(-log(survival)) plots. Time intervals were right-censored at 1 day.
For STEMI patients, we used logistic regression to investigate any association between provision of reperfusion therapy and PHECG. The dependent variable was receipt of reperfusion. We adjusted for core factors and incident year only.
For STEMI patients receiving reperfusion, we used Cox regression to investigate whether there was a difference in time to reperfusion treatment associated with PHECG. The dependent variable was time to reperfusion (minutes) from call for help, from ambulance arrival and from hospital door. We adjusted for core factors, ethnicity (as above), CKD and prior stroke. The proportional hazards assumption was verified using Schoenfeld residual plots and log(-log(survival)) plots. Time intervals were right-censored at 1 day. We also used logistic regression, adjusted as per the Cox regression, to investigate whether PHECG use was associated with receipt of reperfusion within 90 min of the call for help. The dependent variable was receipt of reperfusion within 90 min.
Treatment of missing data
We used multiple imputation to mitigate against bias due to missing data, generating five sets of imputations using Fully Conditional Specification using the full MINAP dataset. The imputation model included the following variables, chosen for consistency with the previous work6: core factors as above; STEMI; conveyance by EMS; ethnicity (as above); prior angina; hypertension; dyslipidaemia; PVD; prior stroke/cerebrovascular disease; asthma/COPD; CKD; whether seen by a cardiologist; prior PCI; current smoker; mini-Global Registry of Acute Coronary Events10 risk score tercile; Index of Multiple Deprivation quartile; systolic blood pressure quartile; heart rate at admission quartile; whether a delay in treatment was recorded in MINAP; raised cardiac markers. Imputed data were used for all analyses, except use of PHECG over time, for which data were complete.
Statistical software
Data were analysed using SPSS, V.26.
Results
The MINAP registry (2010–2017) included 730 886 ACS records (figure 1). We excluded 138 472 records (18.9%) because the patient or event was ineligible, we could not link MINAP and ONS data, or data were otherwise not analysable. Most of these exclusions (125 996 records) were the second or subsequent ACS event for that patient within the study period. The remaining 592 414 records were linked to mortality data with a further 201 650 excluded where not conveyed by EMS, or where mode of conveyance was unknown.
Figure 1Flow diagram. EMS, Emergency Medical Service; MINAP, Myocardial Ischaemia National Audit Project; nSTEMI, non-STEMI; PCI, percutaneous coronary intervention; PHECG, prehospital ECG; STEMI, ST-elevation myocardial infarction.
Of the remaining 390 764 patients conveyed by EMS, 60 051 were excluded because whether they received PHECG (28 569 patients) or final diagnosis (31 482) was unknown. This resulted in an analysis cohort of 330 713 patients.
Overall, four-fifths (79.7%) of patients received PHECG. Table 1 shows overall PHECG use increased from 74.2% in 2010 to 85.0% in 2017, an annual increase of 1.59%-points. Although more patients conveyed by each EMS received PHECG in 2017 vs 2010, the proportion varied by EMS, and in some cases fell year-on-year.
Table 1Proportion of patients receiving PHECG per year
Patients receiving PHECG were typically younger (median age 70 years (IQR 59–80) vs 75 (64–84)), less frequently female and less likely to have diabetes, hypertension or PVD (table 2). They were more likely to be current smokers, have dyslipidaemia or previous PCI, but less likely to have other medical history recorded.
Table 2Comparison of patients conveyed by EMS who do and who do not receive a PHECG
A multivariate model to account for mutual confounding found that being older than 75 years, female and recording of PVD, diabetes and history of prior MI, heart failure, stroke, CKD, angina and asthma or COPD were all associated with lower odds of receiving PHECG, while current smokers, dyslipidaemia and prior PCI were associated with higher odds. PHECG was more common in patients with a final diagnosis of STEMI (90.3%) than those with nSTEMI (71.4%).
Patients receiving PHECG had a lower median heart rate (77/min (IQR 65–90) vs 80 (68–96), Mann-Whitney U test statistic −46.21, p<0.01) and systolic blood pressure (135 mm Hg (117–154) vs 137 (119–156), Mann-Whitney U-test statistic −15.45, p<0.01) at admission than those without PHECG.
Patients with PHECG recorded had lower mortality than those without PHECG at 30 days (7.1% vs 10.9%; adjusted OR (aOR) 0.77; 95% CI 0.75 to 0.80) and 1 year (14.2% vs 23.2%; aOR 0.69; 95% CI 0.68 to 0.71) (table 3). A mortality benefit was observed for STEMI patients alone, nSTEMI alone and for STEMI patients who received reperfusion.
Table 330-day and 1-year mortality by receipt of PHECG
The median time from call for help to arrival at hospital was 3 min longer for patients with PHECG than those without (65 min (IQR 51–84) vs 62 min (IQR 47–85)). However, adjusted HRs marginally favour PHECG (1.04; 95% CI 1.03 to 1.05), suggesting some of the difference may be explained by other factors (online supplemental figure 1).
The median time from ambulance arrival to hospital admission was 5 min longer for patients with PHECG than those without (50 min (IQR 38–66) vs 45 min (IQR 33–65)). Adjusted HRs suggest patients without PHECG spend less time under EMS care (adjusted HR 0.92, 95% CI 0.91 to 0.93).
STEMI patients who received PHECG were more likely to receive reperfusion than those who did not after adjusting for confounding factors (aOR 4.37, 95% CI 4.20 to 4.54) (figure 2; online supplemental table 1). Reperfusion was less likely in patients aged 75+, females, with diabetes or prior MI, CABG or CHF. There was no association between incident year and likelihood of reperfusion. While pPCI was more common than thrombolysis for patients with, and without, PHECG, a higher proportion of patients with PHECG received pPCI (103 741/109 998 (94.3%) vs 6830/7,659 (89.2%), aOR 1.30, 95% CI 1.20 to 1.40).
Figure 2ORs for receipt of reperfusion in patients with STEMI. CABG, coronary artery bypass graft; CHF, chronic heart failure; MI, myocardial infarction; PHECG, prehospital ECG; STEMI, ST-elevation MI.
For STEMI patients receiving reperfusion, the median time from ambulance arrival to reperfusion was 6 min shorter for patients with PHECG than without (97 min (IQR 78–120) vs 103 min (IQR 78–144) (table 4, online supplemental figure 2). Patients with PHECG were also more likely to receive reperfusion within 90 min of the call for help (24.5% vs 21.8%, aOR 1.18, 95% CI 1.11 to 1.25).
Table 4Time to reperfusion in STEMI patients undergoing reperfusion
Discussion
International clinical guidelines for the management of ACS extant towards the end of our study supported performance of PHECG by EMS personnel.1 Current guidelines recommend that EMS ambulances are equipped with ECG recorders and that PHECG should be performed as soon as possible when ACS is suspected to determine the initial treatment pathway and inform conveyance decisions.11 Those with STEMI and those with continuing ischaemic symptoms without ST-segment elevation would normally be transported to hospitals providing immediate coronary interventions. Further, PHECG performance and interpretation consistently appears among descriptors of good quality care of both STEMI and nSTEMI.2 12
The proportion of patients receiving PHECG increased from 74.2% in 2010 to 85.0% in 2017. This continues the trend we first reported in the ‘thrombolysis-era’, from 51% in 2005 to 64% in 2009, prior to implementation of the existing pPCI approach to STEMI management.6 However, 20.3% did not receive PHECG during this study. Patients who did not receive PHECG were older, female and likely to have comorbidities; those receiving PHECG were more likely to have prior PCI, dyslipidaemia and be smokers.
Although consistent with our previous work, it is not clear from MINAP data why some patients are less likely to receive PHECG.6 It may be that symptoms experienced and/or reported more frequently by females and by people with these comorbidities do not ‘trigger’ the performance of an ECG. In a German registry covering the same period as our study, patients with diabetes experiencing their first MI, particularly those older than 55 years, were less likely than those without diabetes to experience the type of chest pain historically considered ‘typical’ of MI presentation, and more likely to experience breathlessness; though much of this difference was attenuated when accounting for co-existence of poor ventricular function and renal impairment.13 Interviews with diabetes patients hospitalised following acute MI revealed that while the majority experienced chest pain during the event, their overall experience did not live up to their expectation of a ‘heart attack’ and they interpreted it as a hypoglycaemic event—which, if accepted by the attending EMS, may reduce the likelihood of PHECG.14
Our findings are consistent with others in relation to gender disparities in prehospital ACS care.15 16 A systematic review of the signs and symptoms of MI reported that both sexes experienced chest pain, but females more often presented with other symptoms such as nausea, vomiting and breathlessness, and prodromes of vague sleep disturbance and fatigue—which may distract the attending paramedic from the need to perform an ECG.17 Semistructured interviews of paramedics revealed concerns about gender-concordance (or lack thereof) between practitioner and patient, with male paramedics expressing hesitancy in exposing a woman’s chest.18 Safety investigations into the training and competence of British paramedics in PHECG extend these concerns to include religious and cultural considerations and difficulties removing electrodes from older patients’ skin.19
We found systolic blood pressure at hospital admission was marginally lower in patients with PHECG (135 mm Hg) than those without (137 mm Hg), as was heart rate on admission (77 beats/min vs 80). If these differences indicate what was present before any EMS intervention, they suggest PHECG is offered to lower risk patients with respect to heart rate, but higher risk patients with respect to systolic blood pressure. This suggests it is unlikely EMS personnel simply ‘scoop and run’ with more ill-looking patients.
Current quality indicators for ACS management include unadjusted all-cause in-hospital mortality as a single expression of outcome.12 We chose to present the previously recommended quality indicator—adjusted 30-day mortality and found that a PHECG recorded in MINAP is associated with reduced 30-day and 1-year mortality for both STEMI and nSTEMI patients, regardless of reperfusion strategy for STEMI patients. If PHECGs were recorded at the 2017 rate for the entire study period, an estimated 17 686 additional patients would have received one. With a 30-day mortality of 7.1% in patients receiving PHECG compared with 10.9% in patients without PHECG, we would expect approximately 672 fewer deaths within 30 days over the study period. If, instead, patients received PHECG at the highest level recorded in a single EMS in a single year, an estimated 53 679 additional patients would have received one, corresponding to approximately 2040 fewer deaths within 30 days over the study period. This demonstrates the importance of addressing the variations in care reported in the literature. Simms et al found missed opportunities were significantly associated with mortality, and prehospital missed opportunities predicted later failures associated with adverse outcomes.20
A lower proportion of STEMI patients with PHECG received thrombolysis compared with those without. However, it is not clear whether PHECG influenced the type of reperfusion, or if there is a mutual confounding factor (eg, incident year).
The median time from ambulance arrival to hospital was longer for patients with PHECG recorded, consistent with meta-analysis findings.21 However, STEMI patients with PHECG were more likely to receive reperfusion, and more timely reperfusion; the median time from call to reperfusion was 10 min faster in patients with PHECG, widening to 29 min at the upper quartile.
Less than a quarter of STEMI patients received reperfusion within 90 min of their call for help, only marginally higher in patients with PHECG than without (24% vs 21%). This may suggest that the 90 min European Society of Cardiology quality indicator target is too ambitious, and the recent guideline of 120 min is more realistic.11 12
The focus of PHECG is identification of ACS patients with STEMI, whose outcomes can be improved by rapid reperfusion via pPCI. It is less clear by what mechanism PHECG is associated with better 30-day survival in those with a final diagnosis of nSTEMI for whom routine immediate reperfusion is not mandated. However, we note the 2023 ECS guidelines recommend almost all nSTEMI patients receive angiography, and some within 24 hours, or even 2 hours, based on risk.11
Strengths and limitations
The main strengths of this study are the very large population and its multifaceted, multicentre approach to understanding barriers and facilitators to PHECG.
The main limitation of the study is its observational, cross-sectional nature, which precludes inferring causal relationships. Although findings are consistent with our previous work, this analysis used final diagnosis to determine patient eligibility, whereas that work used initial diagnosis.5 This does not impact generalisability, but does mean the two works are not fully comparable. This may partly explain the difference in proportion of PHECGs reported between 2009 in the previous work and 2010 herein. Age was dichotomised to maximise comparability with the previous study, where treating it as a continuous variable would otherwise be preferable. We did not adjust for reperfusion in STEMI patients on the same basis; although subgroup analyses for STEMI patients with/without reperfusion are presented and suggest this would be an important factor for future research. While we investigated time to reperfusion for STEMI patients, we did not investigate time to angiography for nSTEMI patients, despite 2023 ACS guidelines recommending angiography during hospitalisation.11 MINAP does not record ACS symptoms or prehospital haemodynamic measurements; therefore, we were unable to investigate any association with PHECG provision. Finally, we are unable to ascertain the extent to which patients under EMS care received PHECG that was not recorded in MINAP.
Conclusions
Patients receiving PHECG were younger, with fewer comorbidities, than those without PHECG. Although the proportion of ACS patients receiving PHECG was fairly high, and increased during the study period, systematic inequalities in administration associated with important differences in processes and outcomes of care persist. EMS providers must address these variations in care to avoid increasing health inequalities.
Data availability statement
Data may be obtained from a third party and are not publicly available. Data may be obtained from the UK Myocardial Ischaemia National Audit Project, MINAP and are not publicly available.
Ethics statements
Patient consent for publication
Not applicable.
Ethics approval
We received favourable opinion from the London-Hampstead Research Ethics Committee on 29 October 2018 (ref: 18LO1679). We obtained Section 251 support for the use of patient data without patient consent from the Confidentiality Advisory Group (ref: 18CAG0164) and ethical approval issued on 15 February 2019 by the Health Research Authority in accordance with English and Welsh Law.
Acknowledgments
The authors acknowledge with thanks the contribution of Janette Turner, School of Health and Related Research (ScHARR), University of Sheffield, UK, to the project in her role as independent chair of the study steering committee, and to Janet Holah and Robert Harris-Meyes for their expert patient and public input to study oversight. This work uses data provided by patients and collected by the National Health Service as part of their care and support. Data have been provided by the Healthcare Quality Improvement Partnership from the National Cardiac Audit Programme.
