Which patients with acute coronary syndrome need the cardiac intensive care unit: tuning the tools for risk stratification

In contemporary health systems, cardiac critical care is resource intensive.1,2 The increasing complexity of the patient population and cardiac treatments is placing new demands on the cardiac intensive care unit (CICU).2 Moreover, there is a lack of specialized healthcare workers to meet this rising burden in the CICU.3 For these reasons, it is necessary to allocate CICU beds more wisely. Some conditions that have traditionally been routinely triaged to the CICU may instead be safely cared for in other hospital wards. In particular, among patients with acute coronary syndromes (ACSs) who are revascularized and are at low risk of mechanical complications or shock, it may be time to break the habit of CICU monitoring for all.4 However, to achieve such practice, we need adequate tools to identify patients with ACSs who will develop shock or severe heart failure later after their initial presentation. In this issue of the European Heart Journal: Acute Cardiovascular Care, three reports help illustrate a path towards refined risk stratification in this population.5,–7 A focused clinical history and basic physical and laboratory findings remain remarkably useful for risk stratification of patients with suspected ACSs. Previously, the thrombolysis in myocardial infarction (TIMI) simple risk index (heart rate (HR) × (age/(10))2/systolic blood pressure (SBP)) was developed to stratify early mortality risk in ST-segment elevation myocardial infarction (STEMI) patients.8 In addition, the Global Registry of Acute Coronary Events (GRACE) risk score uses clinical, ECG, and biomarker parameters (age, HR, SBP, Killip class, cardiac arrest at presentation, ST-segment deviation, serum creatinine, troponin) to estimate in-hospital mortality in ACSs.9 In contrast, to predict late-onset cardiogenic shock in STEMI patients after primary percutaneous coronary intervention (PCI), the Observatoire Régional Breton sur l’Infarctus (ORBI) risk score uses age greater than 70 years, previous stroke/transient ischemic attack, presentation with cardiac arrest, anterior myocardial infarction, first medical contact to PCI delay greater than 90 minutes, Killip class, heart rate greater than 90 beats/minute on admission, SBP less than 125 mmHg and pulse pressure less than 45 mmHg, glycemia greater than 10 mmol/l, culprit lesion of the left main, and post-PCI TIMI flow less than 3.10 In patients with non-ST-segment elevation myocardial infarction (NSTEMI), the Acute Coronary Treatment and Intervention Outcomes Network (ACTION) intensive care unit (ICU) risk score uses nine variables on admission (age, serum creatinine, HR, SBP, initial troponin, signs/symptoms of heart failure, ST depression, prior revascularization, chronic lung disease) to estimate the likelihood that an initially stable NSTEMI patient will develop a complication requiring critical care.11 An ACTION ICU risk score of 2 or less identifies 15% of the NSTEMI population with a risk of less than 5% or a score of 5 or less captures nearly 50% of the population with a less than 10% probability of complications during the index admission (cardiac arrest, shock, high-grade atrioventricular block, respiratory failure, stroke, or death). Each of these tools uses readily available clinical data at the time of admission to stratify risk effectively. In this issue, El-Menyar et al. report a negative predictive value greater than 97% for new onset of shock in ACS patients using the shock index, an easy and intuitive bedside tool derived from the TIMI risk index.5 The shock index, calculated as admission HR divided by the SBP (normal range between 0.5 and 0.7), was evaluated in 24,636 ACS patients. In this population with 47% STEMI, a shock index of 0.8 or greater was present in 16.5% of cases and was associated with a 17% risk of in-hospital cardiogenic shock and an 11% risk of ventricular arrhythmia. Although not as accurate for in-hospital mortality prediction as GRACE or the TIMI risk index, the shock index is appealing in its simplicity. In another important report from this issue, Frydland et al. demonstrate the application of novel biomarkers to enhance the performance of the ORBI score.6 In particular, Frydland et al. found that in 2247 initially stable patients with STEMI, copeptin, mid-regional pro-adrenomedullin (MRproADM) and pro-atrial natriuretic peptide (proANP) added to the ORBI score for predicting late-onset cardiogenic shock.6 Each of these biomarkers had a negative predictive value of 98% or greater and, when adjusted for the ORBI risk score, were associated with late-onset shock (e.g. adjusted odds ratio 3.13 and 1.82 for MRproADM and proANP, respectively). The study illustrates the potential value of an integrated approach to risk assessment using the clinical presentation and biomarkers to stratify risk more effectively, and builds on the emergency use of high-sensitivity troponin to identify very low-risk patients with suspected ACSs.7,11 Imagine a patient less than 70 years old, with no previous stroke, SBP of 125 mmHg, heart rate 97 beats/minute, presenting with an anterior STEMI without left main coronary involvement, and successfully treated (TIMI flow 3) with primary PCI in less than 90 minutes, without heart failure signs (Killip I), hyperglycemia (<180 mg/dL) or arrhythmia at admission. In this case, the estimated risk of shock is less than approximately 5% with the simple shock index and is 1.4% using the ORBI risk score (https://www.orbiriskscore.com/). Therefore, the patient appears to be a reasonable candidate for a telemetry unit. Moving forward, decision-making at the bedside should integrate clinical history and signs, ECG, biomarkers, and may use available risk scores to risk-stratify patients for short-term, in-hospital serious complications. A low simple shock index, ORBI risk score, or hemodynamic biomarker measurements can help identify STEMI patients who are at low risk of major complications and may not need monitoring in a CICU (Figure 1).6 In NSTEMI, a low ACTION ICU risk score or GRACE risk score might help to select a low-risk group.11 Future advances may include the integration of novel biomarkers, multimarker profiling and advanced imaging to help direct the right patient to the right bed. Nevertheless, diverse structures and staffing of non-ICU units will continue to require clinicians to be aware of their own institutional capacity (e.g. monitoring capabilities and nurse-to-patient ratios) to care for ACS patients outside an ICU. In conclusion, not all stable ACS patients, including those with STEMI, require triage to a CICU for monitoring. A joint analysis of clinical–ECG–biomarker risk scores can help select patients who are at low risk of in-hospital complications, and who are reasonable candidates for a less costly telemetry unit. Future research should continue to refine these tools towards an ideal goal of patient-centered, practical and accurate strategies for triage to cardiac critical care. Decision tree for CICU triage of ACS patients. ACS: acute coronary syndrome; CICU: cardiac intensive care unit; STEMI: ST-segment elevation myocardial infarction; NSTEMI: non-ST-segment elevation myocardial infarction; ORBI: Observatoire Régional Breton sur l’Infarctus; ACTION: Acute Coronary Treatment and Intervention Outcomes Network; ICU: intensive care unit. Decision tree for CICU triage of ACS patients. ACS: acute coronary syndrome; CICU: cardiac intensive care unit; STEMI: ST-segment elevation myocardial infarction; NSTEMI: non-ST-segment elevation myocardial infarction; ORBI: Observatoire Régional Breton sur l’Infarctus; ACTION: Acute Coronary Treatment and Intervention Outcomes Network; ICU: intensive care unit.