What is the significance of the Shock Index (SI) and Modified Shock Index (MSI) in assessing hypotension and circulatory status?

From the Guidelines

The Shock Index (SI) is a significant predictor of hypotension and circulatory status, with a value of ≥ 0.9 indicating an increased risk of shock and poor outcomes, as evidenced by a recent study published in Critical Care in 2023 1. The shock index is calculated by dividing heart rate by systolic blood pressure, with a normal range of 0.5 to 0.7.

Key Points to Consider:

• A SI ≥ 0.9-1.0 has been retrospectively associated with increased massive transfusion (MT), interventional radiology, and operative intervention in bleeding trauma patients 1.
• The SI has been shown to be an independent predictor of mortality and blood transfusion, with an odds ratio of 3.57 1.
• A narrowed pulse pressure (< 40/< 30 mmHg) is also an indicator of hemodynamic instability and has been associated with transfusion, resuscitative thoracotomy, and emergent surgery 1.

Using the Shock Index in Practice:

1. Calculate shock index = Heart rate / Systolic BP
2. Interpret results:
   • Shock index > 0.9: Consider fluid resuscitation, further monitoring
   • Normal values: Continue standard care The shock index is a valuable tool for quickly assessing a patient's hemodynamic status and risk of critical illness, and can detect subtle changes in hemodynamics before obvious clinical signs of shock appear 1. However, it should be used in conjunction with other clinical findings and not as a standalone diagnostic tool.

Additional Considerations:

• The modified shock index, which incorporates diastolic blood pressure, may also be useful in assessing hemodynamic instability, but its use is not as well established as the traditional shock index 1.
• The shock index has been used in various clinical settings, including trauma and gastrointestinal bleeding, and has been shown to be a useful predictor of outcomes in these populations 1.

From the Research

Significance of Shock Index (SI) and Modified Shock Index (MSI)

• The Shock Index (SI) is defined as the heart rate (HR) divided by systolic blood pressure (SBP) and has been studied in patients at risk of or experiencing shock from various causes 2.
• 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.
• The Modified Shock Index (MSI) has also been explored, which is calculated as heart rate/mean blood pressure, and has been found to be associated with adverse outcomes in ward patients experiencing clinical deterioration 3.

Clinical Utility of SI and MSI

• SI has been found to be a useful tool in predicting critical bleeding post-trauma, with an optimal cut-off of ≥0.9 4.
• SI has also been found to be associated with hospital admission and inpatient mortality in a US national database of emergency departments, with a cut-off of >1.3 being clinically significant 5.
• MSI has been found to be associated with ICU admission, vasopressor therapy, and in-hospital mortality in ward patients experiencing clinical deterioration 3.
• SI and MSI have been found to be correlated with other parameters such as pulse pressure, mean arterial pressure, and Global Registry of Acute Coronary Events score 6.

Limitations and Future Directions

• The clinical utility of SI and MSI is limited by their poor discrimination between survivors and non-survivors, with area under the curve values of <0.60 3.
• Further research is needed to evaluate the utility of SI and MSI in different patient populations and to determine their optimal cut-offs for predicting adverse outcomes 2, 6, 3, 4, 5.