Prehospital Vital Signs, Including SpO₂, Are Significantly Associated With Mortality and Mechanical Circulatory Support Requirement in Acute Myocardial Infarction Patients

Abstract

Background: Early risk stratification for acute myocardial infarction (AMI) using prehospital vital signs, including percutaneous oxygen before oxygen supply (pre-SpO₂), remains underexplored.

Methods and Results: We analyzed 3,006 AMI patients from the Mie ACS Registry. Patients were stratified by the prehospital Simple Risk Index (pre-SRI = [heart rate × (age / 10)²] / systolic blood pressure) and pre-SpO₂ levels. The primary outcome was 30-day death. Combined assessment of pre-SRI and pre-SpO₂ was superior to individual parameters for predicting 30-day death. In the multivariate analysis, combined assessment (SRI ≥34 and SpO₂ ≤94%) was the strongest independent predictors of 30-day death (hazard ratio 3.98, P<0.001).

Conclusions: Prehospital vital signs including pre-SpO₂ enable early high-risk identification, improving clinical decision-making for hospital selection.

Central Figure

The prognosis of acute myocardial infarction (AMI) has improved with the advent of early percutaneous coronary intervention (PCI). However, the incidence of cardiogenic shock is increasing with mortality rates approaching 50%. Mechanical circulatory support (MCS) devices, such as Impella, reduce deaths in AMI-related cardiogenic shock cases, but complexity and costs necessitate treatment in specialized centers, emphasizing the need for accurate prehospital risk stratification and appropriate hospital selection.

Existing risk stratification tools (GRACE scores) require hospital-based information, limiting prehospital utility. The Simple Risk Index (SRI = [heart rate × (age / 10)²] / systolic blood pressure) offers a simple, accurate tool using basic vital signs.¹ We previously demonstrated that the prehospital SRI (pre-SRI) effectively predicts patient outcomes.²

Oxygen saturation (SpO₂) is a crucial vital sign. Previous study reported that baseline SpO₂ <94% correlates with AMI prognosis.³ However, prehospital SpO₂ before oxygen supply (pre-SpO₂) for risk stratification remains unexplored. Combining pre-SRI with pre-SpO₂ may enable more accurate risk stratification and optimal hospital selection, potentially improving outcomes.

Methods

From January 2013 to June 2022, 7,058 consecutive acute coronary syndrome patients from the Mie ACS Registry were evaluated.² Cases of AMI with direct ambulance transport were included, excluding out-of-hospital cardiac arrest. Patients were stratified by pre-SRI and pre-SpO₂ levels. The pre-SRI ≥34 cutoff was determined from receiver operating characteristic curves for 30-day death prediction. Pre-SpO₂ categories (<90%, 90–94%, >94%) were defined from the previous study.³ Prehospital hemodynamic parameters were recorded by the emergency medical service at first medical contact. The primary outcome was 30-day death; secondary outcomes included the need for MCS or ventilation support.

Results

Among 7,058 ACS patients, 3,006 AMI patients met the inclusion criteria. Patients were further divided into subgroups combining the pre-SRI and pre-SpO₂ categories. Combined pre-SRI+pre-SpO₂ assessment (area under the curve 0.82, P<0.001) demonstrated superior prognostic performance compared to individual parameters (P<0.01), and showed similar performance to the GRACE score and Killip classification.

For 30-day death prediction, patients were categorized into 4 groups (Figure A). The highest-risk group exhibited a 30-day mortality rate of 24.7%, representing a 25-fold increase compared to the lowest-risk group (P<0.001). In the multivariate analysis, combined assessment (SRI ≥34 and SpO₂ ≤94%) was the strongest predictor with a hazard ratio of 3.98 (P<0.001).

Figure.

Clinical outcomes stratified by pre-SRI and pre-SpO₂. (A) 30-day mortality rates across 4 groups stratified by pre-SRI (cutoff 34) and pre-SpO₂ (cutoff 94%). (B) Need for MCS (gray bars) and MCS or ventilation support (black bars) across 6 groups stratified by pre-SRI (cutoff 31) and pre-SpO₂ categories. MCS, mechanical circulatory support; SRI, Simple Risk Index.

For MCS/ventilation support prediction, a pre-SRI cutoff of 31 was optimal (AUC 0.66). Patients with SRI ≥31 and SpO₂ <90% required MCS/ventilation support in 63.8% of cases and MCS alone in 44.1% (Figure B).

Discussion

This study provides the first demonstration that prehospital vital signs, including SpO₂, have prognostic significance in AMI patients. Combined pre-SRI+pre-SpO₂ assessment offers superior risk stratification compared to individual parameters, achieving 30-day death prediction comparable to established in-hospital scores.

Although primary PCI has reduced AMI mortality rates, the prognosis for severe AMI remains poor. Specialized centers with board-certified cardiologists and advanced MCS capabilities improve outcomes, emphasizing the need for appropriate patient triage. In suburban regions such as Mie Prefecture, where few hospitals provide stable Impella and ECMO management, accurate prehospital risk stratification becomes crucial.

Adding pre-SpO₂ to pre-SRI significantly improved the accuracy of risk stratification compared to pre-SRI alone because pre-SRI alone is inadequate for identifying AMI patients with heart failure. A combined pre-SRI and pre-SpO₂ assessment effectively identifies these high-risk patients requiring specialized care.

Predicting MCS/ventilation support needs represents another clinical advance. However, age inclusion in pre-SRI may limit MCS prediction accuracy because elderly patients often avoid such interventions. Future development of age-corrected indices may improve prediction.

Study Limitations

Single-prefecture study limits generalizability, requiring multi-regional validation.

Conclusions

Prehospital vital signs, including SpO₂, provide an effective and accurate risk stratification tool for AMI patients. This approach enhances early identification of high-risk patients and improves clinical decision-making for MCS and hospital selection, potentially leading to better patient outcomes.

Source of Funding

Non-profit organization Mie Cardiovascular and Renal Disease Network (http://www.medic.mie-u.ac.jp/miecrnet/).

Disclosure

K.D. received research grants from Otsuka Pharmaceutical Co., Ltd., Nippon Boehringer Ingelheim Co., Ltd. Sumitomo Pharma Co., Ltd. and Abbott Medical Co., Ltd. in 2024; lecture fees from Novartis Pharma K.K., AstraZeneca K.K., and Medtronic Japan Co., Ltd. in 2024. K.D. received research grants from Otsuka Pharmaceutical Co., Ltd., and DAIICHI SANKYO Co., Ltd., in 2023; lecture fees from Bayer Co., Ltd., AstraZeneca K.K., DAIICHI SANKYO Co., Ltd., and Nippon Boehringer Ingelheim Co., in 2023.

IRB Information

Mie University Hospital IRB (Reference no.2881).

Data Availability

Individual deidentified participant data, figure, and the study protocol will be shared and available for 1 year after acceptance. The data will be shared as Microsoft Excel via email.

References

• 1.   Morrow DA, Antman EM, Giugliano RP, Cairns R, Charlesworth A, Murphy SA, et al. A simple risk index for rapid initial triage of patients with ST-elevation myocardial infarction: An InTIME II substudy. Lancet 2001; 358: 1571–1575.
• 2.   Moriwaki K, Kurita T, Hirota Y, Ito H, Ishise T, Fujimoto N, et al. Prognostic impact of prehospital simple risk index in patients with ST-elevation myocardial infarction. Circ J 2023; 87: 629–639.
• 3.   James SK, Erlinge D, Herlitz J, Alfredsson J, Koul S, Fröbert O, et al. Effect of oxygen therapy on cardiovascular outcomes in relation to baseline oxygen saturation. JACC Cardiovasc Interv 2020; 13: 502–513.