Shock index and modified shock index for prediction of myocardial damage and clinical outcome of STEMI
Shock index is the ratio of heart rate to systolic blood pressure which allows rapid bedside risk stratification . Modified shock index is the ratio of heart rate to mean arterial pressure . Reinstadler SJ et al  used these indices to predict the myocardial damage and clinical outcome of ST segment elevation myocardial infarction (STEMI). They found that STEMI patients with elevated shock index had more severe myocardial and microvascular damage and it was associated with major adverse cardiac events (MACE) at 1 year.
This multicenter study analyzed around eight hundred patients dichotomized to an admission shock index of less than 0.62 or more. Forty percent of their patients had a shock index above 0.62 and they had significantly larger area of myocardium at risk, larger infarct size and larger extent of microvascular obstruction. Cardiac magnetic resonance imaging was used to assess the infarct size and microvascular obstruction. They noted a worse thrombolysis in myocardial infarction (TIMI) flow before and after primary percutaneous coronary intervention (PPCI) in those with shock index of 0.62 or more.
It may be noted that others have used a shock index cut-off value of 0.7  while this study used 0.62. The current study authors noted similar results on a secondary analysis using cut-off of 0.7 also. Similarly, they noted no difference in the predictive value between shock index and modified shock index. But other authors have found modified shock index to be more accurate than shock index . In that study, shock index cut-off of 0.7 and modified shock index cut-off of 1.4 were used. They noted that odds ratios of increased modified shock index for all cause mortality, cardiogenic shock, life-threatening arrhythmias and MACE were higher than those of increased shock index. The study had used shock index at presentation to emergency department and studied the seven day outcomes for STEMI in 160 patients.
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- Shangguan Q, Xu JS, Su H, Li JX, Wang WY, Hong K, Cheng XS. Modified shock index is a predictor for 7-day outcomes in patients with STEMI. Am J Emerg Med. 2015 Aug;33(8):1072-5.