What is the Shock Index defined as?

• A. Heart Rate Divided by Diastolic Blood Pressure
• B. Heart Rate Divided by Systolic Blood Pressure
• C. Systolic Blood Pressure Divided by Heart Rate
• D. Heart Rate Minus Systolic Blood Pressure

Answer: (B)

A quick way to gauge circulatory compromise is to combine heart rate and blood pressure into a single ratio. The Shock Index is defined as heart rate divided by systolic blood pressure. The logic is simple: in shock, the heart often beats faster to compensate, while systolic pressure tends to drop as blood volume or vascular tone declines. So the index climbs as worsening perfusion occurs, providing a handy bedside signal of instability.

This uses systolic pressure, not diastolic, because SBP is the primary driver of peak arterial pressure the heart can generate and it changes predictably with volume loss and vasodilation. The alternative of subtracting SBP from heart rate would not reflect the balance between heart workload and arterial pressure. Similarly, reversing the ratio (SBP divided by heart rate) would yield a value that tends to decrease with worsening shock, which is the opposite of what the Shock Index is meant to indicate.