What is the formula for calculating shock index?

Shock Index Formula

The shock index is calculated by dividing the heart rate by the systolic blood pressure (Heart Rate ÷ Systolic Blood Pressure). 1, 2

Clinical Calculation

• Formula: SI = Heart Rate / Systolic Blood Pressure 1, 2
• The calculation produces a dimensionless ratio that serves as a hemodynamic marker 1
• For example, a patient with heart rate of 110 bpm and systolic blood pressure of 100 mmHg would have a shock index of 1.1 2

Critical Threshold Values

• SI >1.0 indicates hemodynamic instability and is used as a threshold for identifying patients requiring aggressive intervention 1, 2
• In lower gastrointestinal bleeding, SI >1 defines an "unstable" patient requiring immediate hospital-based management 1
• In cardiogenic shock assessment, SI >1.0 serves as one of several supplementary hemodynamic markers alongside cardiac power output and other parameters 1, 2
• Normal SI values are typically <0.9, though this threshold has limitations in sensitivity 2, 3

Clinical Applications by Setting

Trauma and hemorrhagic shock: The European trauma guidelines recommend using shock index to assess the degree of hypovolemic shock and guide transfusion requirements (Grade 1C recommendation) 1, 2

Gastrointestinal bleeding: British Society of Gastroenterology guidelines use SI >1 as the defining criterion for unstable lower GI bleeding, triggering immediate resuscitation and intervention pathways 1

Cardiogenic shock: The 2022 ACC/AHA/HFSA heart failure guidelines include SI >1.0 as one of multiple hemodynamic considerations in shock assessment, though it is not a primary diagnostic criterion 1, 2

Important Limitations and Pitfalls

• SI is too insensitive to rule out major injury or disease when values appear normal - a normal SI should never lower clinical suspicion for serious pathology 1, 2
• The shock index requires field calculation, and its value for prehospital triage decisions remains unclear compared to using heart rate and blood pressure separately 1, 2
• Age-related physiologic differences alter normal SI ranges, particularly in older adults and pediatric patients, though specific adjusted thresholds are not well-defined in standard practice 1, 2
• SI performs best when combined with other clinical parameters including mechanism of injury, anatomical findings, lactate levels, and response to resuscitation rather than used in isolation 1, 2

Practical Implementation

• Calculate SI immediately upon patient presentation using the first available vital signs 2, 3
• Use SI >1.0 as a threshold triggering heightened concern and aggressive management 1, 2
• Recognize that SI <0.9 does not exclude significant pathology and should not reduce clinical vigilance 1, 2
• Integrate SI trends with other hemodynamic markers including lactate, base deficit, urine output, and mental status 1, 2
• Consider more sophisticated hemodynamic monitoring (cardiac index, pulmonary capillary wedge pressure) in refractory shock states where initial SI-guided interventions prove insufficient 1, 2