Utility of shock index in 24,636 patients presenting with acute coronary syndrome

Background: Shock index is a bedside reflection of integrated response of the cardiovascular and nervous systems. We aimed to evaluate the utility of shock index (heart rate/systolic blood pressure) in patients presenting with acute coronary syndrome (ACS).

Methods: We analyzed pooled data from seven Arabian Gulf registries; these ACS registries were carried out in seven countries (Qatar, Bahrain, Kuwait, UAE, Saudi Arabia, Oman and Yemen) between 2005 and 2017. A standard uniform coding strategy was used to recode each database using each registry protocol and clinical research form. Patients were categorized into two groups based on their initial shock index (low vs. high shock index). Optimal shock index cutoff was determined according to the receiver operating characteristic curve (ROC). Primary outcome was hospital mortality.

Results: A total of 24,636 ACS patients met the inclusion criteria with a mean age 57±13 years. Based on ROC analysis, the optimal shock index was 0.80 (83.5% had shock index <0.80 and 16.5% had shock index ≥0.80). In patients with high shock index, 55% had ST-elevation myocardial infarction and 45% had non-ST-elevation myocardial infarction. Patients with high shock index were more likely to have diabetes mellitus, late presentation, door to electrocardiogram >10 min, symptom to Emergency Department > 3 h, anterior myocardial infarction, impaired left ventricular function, no reperfusion post-therapy, recurrent ischemia/myocardial infarction, tachyarrhythmia and stroke. However, high shock index was associated significantly with less chest pain, less thrombolytic therapy and less primary percutaneous coronary intervention. Shock index correlated significantly with pulse pressure (r= -0.52), mean arterial pressure (r= -0.48), Global Registry of Acute Coronary Events score (r =0.41) and Thrombolysis In Myocardial Infarction simple risk index (r= -0.59). Shock index ≥0.80 predicted mortality in ACS with 49% sensitivity, 85% specificity, 97.6% negative predictive value and 0.6 negative likelihood ratio. Multivariate regression analysis showed that shock index was an independent predictor for in-hospital mortality (adjusted odds ratio (aOR) 3.40, p<0.001), heart failure (aOR 1.67, p<0.001) and cardiogenic shock (aOR 3.70, p<0.001).

Conclusions: Although shock index is the least accurate of the ones tested, its simplicity may argue in favor of its use for early risk stratification in patients with ACS. The utility of shock index is equally good for ST-elevation myocardial infarction and non-ST-elevation acute coronary syndrome. High shock index identifies patients at increased risk of in-hospital mortality and urges physicians in the Emergency Department to use aggressive management.