Shock Index (SI)

Cautions

• Heart rate divided by systolic blood pressure. Heart rate and systolic blood pressure can look normal during the compensatory phase while the index has already risen, so it is used as an aid that keeps the assessment from resting on blood pressure alone.
• Cut-off values are not fixed. The studies in the review alone use different values by condition and purpose. The normal range is given as 0.5 to 0.7 (based on pediatric studies, and some studies accept up to 0.9). For massive transfusion in trauma, values above 0.9 are used; for trauma triage, values above 1.0; for ST-elevation myocardial infarction (STEMI), 0.8 or more; for pulmonary embolism, 1 or more; and for sepsis, values above 0.7. For this reason the tool gives no bands and shows only the value.
• For obstetric hemorrhage, the Japanese "Guide for responding to obstetric critical hemorrhage 2022" (Japan Society of Obstetrics and Gynecology and other societies) uses SI as a management index. If SI reaches 1 in bleeding at delivery, a primary facility should consider transfer to a higher-level facility and prepare for transfusion. If, despite the measures taken, any one of the following applies, "obstetric critical hemorrhage" is declared: bleeding continues with abnormal vital signs, SI is 1.5 or more, the obstetric DIC score is 8 or more, or fibrinogen is below 150 mg/dL. In pregnant women an SI of 1 is estimated to correspond to a blood loss of about 1.5 L, and an SI of 1.5 to about 2.5 L. These cut-offs are for obstetric bleeding.
• Age, antihypertensive drugs, drugs that slow the heart rate such as beta-blockers and calcium channel blockers, beta-agonists and other factors change the heart rate and blood pressure and so the meaning of the index. Older adults are less likely to develop tachycardia. Children have different normal heart rates and blood pressures at each age, so age-adjusted indices have been proposed.
• Do not use the index alone to diagnose or to rule out a serious condition. The review also warns against using it alone.
• It cannot be calculated with a systolic blood pressure of 0 (division by zero). A result of 0.00 with a heart rate of 0 appears as a number but has no meaning.