From the Guidelines
The shock index is a critical clinical tool calculated by dividing heart rate by systolic blood pressure, with values exceeding 1.0 indicating significant shock requiring immediate intervention, as defined by the British Society of Gastroenterology guidelines 1. The shock index is particularly valuable in emergency settings for identifying patients with occult shock, as tachycardia often precedes hypotension in hemorrhagic or hypovolemic conditions.
- A normal shock index value is typically considered to be between 0.5 to 0.7.
- Values above 0.9 suggest early shock and hemodynamic instability.
- The shock index is calculated by dividing the heart rate by the systolic blood pressure, making it a simple yet effective tool for quickly identifying patients who may be compensating for blood or fluid loss before obvious hypotension develops. The physiological basis for the shock index relates to the body's compensatory mechanisms - as blood volume decreases, heart rate increases to maintain cardiac output while blood pressure may initially remain normal due to vasoconstriction, making the ratio between these parameters more sensitive than either vital sign alone, as supported by the World Journal of Emergency Surgery guidelines 1. For example, a patient with a heart rate of 120 beats per minute and a systolic blood pressure of 100 mmHg would have a shock index of 1.2, indicating significant concern and the need for immediate intervention, as emphasized by the Critical Care guidelines 1.
From the Research
Definition of Shock Index
- The shock index (SI) is defined as the heart rate (HR) divided by systolic blood pressure (SBP) 2, 3, 4, 5.
- It has been studied in patients either at risk of or experiencing shock from a variety of causes, including trauma, hemorrhage, myocardial infarction, pulmonary embolism, sepsis, and ruptured ectopic pregnancy 2.
Calculation and Thresholds
- The SI is calculated as HR/SBP 2, 3, 4, 5.
- A SI threshold value ≥1 has been fixed to define two groups: SI<1 and SI≥1 3.
- Different studies have suggested various threshold values for SI, including ≥1.0 2, ≥1.3 4, ≥0.7, and ≥0.9 4.
- A high diastolic shock index (heart rate/diastolic blood pressure) ≥3 has also been suggested as a marker for significant injury in trauma patients 6.
Clinical Significance
- SI >1.0 has been widely found to predict increased risk of mortality and other markers of morbidity, such as need for massive transfusion protocol activation and admission to intensive care units 2.
- A SI ≥1 represents a predictive factor of hospital mortality at 7 days and one month in severe trauma patients 3.
- SI>1.3 was associated with a clinically significant increase in both the likelihood of hospital admission and inpatient mortality in a general emergency department population 4.
- The shock index has been suggested as a useful marker for significant injury in trauma patients, with an optimal SI threshold of 0.83 for predicting any of the severity measures 5.