In-Ambulance Troponin Measurements
By Michael H. Crawford, MD, Editor
SYNOPSIS: A study of triaging suspected non-ST-elevation acute coronary syndrome patients by employing in-ambulance troponin measurements augmented the predictive value for 45-day major adverse cardiac events. This could help identify very high-risk patients who would benefit from urgent coronary angiography.
SOURCE: van Dongen DN, Fokkert MJ, Tolsman RT, et al. Value of prehospital troponin assessment in suspected non-ST-elevation acute coronary syndrome. Am J Cardiol 2018;122:1610-1616.
It is now feasible to measure troponin levels in the ambulance. Troponin is a key component of the HEART score (history, ECG, age, risk factors, troponin), which has shown prognostic value in patients with suspected non-ST-elevation acute coronary syndrome (NSTE-ACS).
Van Dongen et al sought to determine whether prehospital troponin measurements added value to the prehospital HEAR score (a risk score without the troponin component of the HEART score) for predicting major adverse coronary events (MACE) within 45 days in patients triaged for suspected NSTE-ACS in two hospitals and 33 ambulances. MACE was defined as cardiac ischemia, all-cause mortality, or coronary revascularization. Patients with coma, cognitive impairment, shock, sustained ventricular tachycardia, and end-stage renal disease were excluded. Cardiac troponin T (TnT) was performed in the ambulance using the Roche device, which provides a value in about 10 minutes. TnT < 40ng/L (limit of detection) was scored 0 on the HEART scale and > 40 ng/L was scored 2.
Of 823 eligible patients, 700 had complete data and were included in the analysis. The median duration of symptom onset until TnT measurement was 150 minutes (range, 65-435 minutes). Using the HEAR score, 26% of patients were classified as low risk (< 3 points). Using the HEART score, 25% of patients were classified as low risk. No patient classified as low risk under either scoring system died within 45 days. MACE within 45 days occurred in 17% of patients (six patients died). Using the HEAR scale, 7% of low-risk patients experienced a MACE. Using the HEART scale, 3% of low-risk patients experienced a MACE (P < 0.001). The area under the curve (AUC) for predicting MACE using the HEAR score was 0.65 (confidence interval [CI], 0.6-0.71). The AUC using the HEART score was 0.74 (CI, 0.69-0.79; P < 0.001). The AUC for prehospital TnT alone was 0.67 (CI, 0.60-0.73). The authors concluded that the prehospital TnT component of the HEART score added predictive value for 45-day MACE in patients with suspected NSTE-ACS.
The HEART score is a useful and well-validated tool for stratifying patients with suspected NSTE-ACS into high, low, and intermediate risk for MACE. Low-risk patients often are discharged from the ED, high-risk patients are admitted to the hospital, and intermediate patients typically are held in a chest pain unit for further noninvasive testing. Traditionally, the HEART score was determined in the ED, but pressure to triage the higher-risk patients more quickly moved the “HAR” part of the score to the ambulance, with higher-risk patients undergoing an ECG within 10 minutes of ED arrival. As ECG technology moved to the field, a “HEAR” score could be calculated in the ambulance or even pretransport. Point of care troponin has been available for some time but has not achieved widespread use in the United States outside of EDs. While moving it to prehospital could increase the efficiency of triaging chest pain patients rather than waiting for someone to run troponin in the ED or laboratory, would it really make a difference?
Van Dongen sought to compare the triage accuracy of the HEAR score vs. the complete HEART score in the field or ambulance. The hazard ratio for MACE with a HEAR score > 3 was 3.57 compared to 8.89 for a HEART score > 3. Not surprisingly, adding the TnT in the field added considerable predictive value. In studies of the HEART score in the hospital, the AUC for MACE was higher (0.83) than that reported in this study for prehospital use (0.74). One explanation for this finding may be that the added value of troponin is related to the time from symptom onset until when the test is performed. Obviously, this time would be less in the field than at the ED. In this study, 73 patients who experienced MACE tested negative on TnT in the field, and prehospital TnT alone had an AUC less than the full HEART score (0.67 vs. 0.74, respectively). Thus, prehospital HEART is better for identifying high-risk patients rather than low-risk patients.
The strength of this study is that the authors used one urban and one rural hospital. Also, there was a 99.6% follow-up rate. But there also were weaknesses. TnT could not be obtained in 157 eligible patients because of various technical problems. While the authors stated that the assay needs improvement, they used the fourth-generation assay, not the new high-sensitivity troponin assay. Finally, the ambulance system only employed critical care-trained bachelor’s degree nurses who have used the HEART score since 2012. This level of expertise and experience may be hard to duplicate in other countries.
Full HEART score determination in the field probably is most useful if the patient has symptoms for > 120 minutes. Also, in-the-field troponin measurements are likely to increase the sensitivity for detecting ACS but decrease the positive predictive value. This increase in false-positives may create extra work for the hospital. On the other hand, in-the-field troponin measurements may augment the identification of the very high-risk non-NSTE-ACS patient who may need urgent coronary angiography.
A study of triaging suspected non-ST-elevation acute coronary syndrome patients by employing in-ambulance troponin measurements augmented the predictive value for 45-day major adverse cardiac events. This could help identify very high-risk patients who would benefit from urgent coronary angiography.
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