Shock Index and Its Role in Emergency Trauma

Shock Index

Shock Index is a tool that’s used by paramedics and EMTs to predict outcomes after physical trauma. The Shock Index measurement has been shown to be especially reliable at predicting hospital outcomes in rural areas where routes to the hospital are longer, and has been shown to provide a good indication of mortality when compared to other methods such as taking vital signs.

Though the prehospital Shock Index has received some criticism, studies have shown Shock Index to be a much better indicator of the first signs of shock in trauma patients with catastrophic blood loss than vital signs in isolation. It is a method that can be helpful for determining if a patient needs intensive care, and also for identifying patients with higher chances of survival in cases of multiple trauma.

What Is the Shock Index?

The Shock Index is a number that indicates how likely it is that a patient who is in shock will survive. According to the National Association of Emergency Medical Technicians’ AMLS course handbook, “shock” refers to “a progressive state of cellular hypoperfusion that occurs when insufficient oxygen is available to meet tissue demands” [1]. In the case of hypovolemic shock, this state of cellular hypoperfusion occurs due to life-threatening bleeding, which—if it’s not stemmed and treated in time—often leads to death.

How Is Shock Index Calculated?

Shock Index (SI) is calculated by dividing the patient's heart rate by their systolic blood pressure. The normal Shock Index figure is considered to be in the range of 0.5-0.7; higher rates than this indicate a much higher risk of occult shock (early hypoperfusion), a greater need for blood transfusion, and a greater likelihood of post-intubation hypotension.

Modified Shock Index Calculation

Modified Shock Index (MSI) is calculated by dividing heart rate by mean arterial pressure. In one study that was published in 2019, there were “non-statistical trends of greater accuracy of MSI over SI.”

Shock Index, Pediatric-Adjusted Calculation

Shock Index, Pediatric Age-Adjusted (SIPA) is calculated by dividing the maximum normal heart rate by the minimum normal systolic blood pressure according to age. In a study published in 2021, higher SIPA values after 24 hours in the ICU accurately predicted poor outcomes and high mortality rates in non-traumatic children.

How Is Shock Index Used in Emergency Trauma?

Emergency medical responders (EMRs) use the Shock Index measurement to identify patients who are at risk of shock, which can be especially difficult in patients who are suffering from heavy internal bleeding (as opposed to visible external bleeding). They can then use the results to decide which patients to treat first and to inform the emergency medicine techniques they use for each patient.

As a bystander to an accident or trauma, your efforts should be focused on stopping traumatic bleeding rather than calculating the Shock Index. Stopping severe bleeding using the equipment in a bleeding control kit (such as a tourniquet and hemostatic gauze) reduces the patient’s chances of going into shock and buys them more time until they can get definitive care in an emergency department or trauma center.

Strengths of Using the Shock Index Tool as a Predictor of Mortality in Emergency Trauma

Studies have been conducted to examine the efficacy of the Shock Index with regard to trauma patients, and the results support its use as an effective predictor of shock. 

  • There is a positive correlation between Shock Index values and a need for further care. The timely provision of such care can help to prevent mortality from sepsis, organ failure, and other secondary effects of shock.
  • The Shock Index has also shown positive results in classifying hypovolemic shock in patients with severe traumatic brain injury (which is often not visible externally).

How Does Shock Index Compare to Other Methods of Predicting Mortality?

Shock Index is only one tool that emergency responders can use when predicting mortality. Other tools include the Rapid Emergency Medicine Score (REMS), the Revised Trauma Score (RTS), conventional vital signs, and the Injury Severity Score (ISS). Of these, comparative studies are available for SI, RTS, and ISS vs. REMS, and SI vs. conventional vital signs.

Shock Index vs. REMS

Whereas SI takes two parameters into account, REMS takes six factors into account: respiratory rate, heart rate, mean arterial pressure, Glasgow Coma Scale, age, and pressured oxygen saturation.

Imhoff et al. (2014)

A study by Imhoff et al. (2014) found that REMS is a more effective tool than RTS, ISS, and SI to predict in-hospital mortality. While this is interesting, it's worth noting that the scoring system used in this study doesn't differentiate between injury types, which is a known factor in predicting mortality.

Shock Index vs. Conventional Vital Signs

Traditionally, heart rate and systolic blood pressure (along with pulse rate and respiration rate) have been used in emergency departments to assess critically-ill patients' hemodynamic status after major trauma. 

However, this approach has downsides because these parameters can appear to be normal, even in seriously ill patients. If only vital signs are taken into account, critical interventions may be delayed, leading to an increased need for intensive care as well as an elevated rate of morbidity and mortality.

Examples of Patients for Whom Vital Signs May Not Be Accurate Indicators of Shock

Geriatric trauma patients, particularly, may display different signs to adult trauma patients. Furthermore, patients with chronic hypertension may not display typical signs of hemodynamic distress like tachycardia and hypotension. Traumatically injured patients may also display heart rate and systolic blood pressure within normal limits even after losing nearly a pint of blood.

Rady et al. (1994)

Rady et al (1994) observed that using Shock Index in tandem with measuring vital signs gave a much more reliable indication of patients' conditions than using vital signs alone.

Koch et al. (2019)

In a study published by Koch et al. (2019), Shock Index was concluded to be more valuable in predicting hypovolemic shock or the need for massive transfusion than more traditional measures like tachycardia or hypotension. 

This study observed that patients’ heart rates and systolic blood pressures were still within normal limits after 450 ml of blood loss, whereas SI was constantly elevated at both one and five minutes.

Gupta et al. (2021)

Gupta et al. (2021) went further into this analysis, explaining that a critically ill patient displays compensatory mechanisms in his or her vital signs; the body keeps blood pressure from falling despite decreased circulating blood volume, stroke volume, and cardiac output. This skews the results, making the patient appear to be more stable than they actually are. Another limitation is a concurrent rise in HR and SBP which occurs in response to pain and anxiety.

In these scenarios, Shock Index works better as an early indicator of extreme distress than conventional vital signs alone. Shock Index is also the easiest and quickest risk-mitigation calculation that can be taken. In extreme cases where time is of the essence, this is crucial.

What Are The Limitations of Shock Index in Emergency Trauma?

Koch et al. concluded that, although SI can be a valuable tool in the ER as a predictor of mortality in some cases, the utility of SI is somewhat lower in the elderly, febrile patients, and patients whose chronic conditions such as hypertension may affect baseline hemodynamics. 

Additionally, medications such as beta-blockers, beta-agonists, or other antihypertensives affect vital signs, which then skews the connection between SI and mortality.

The impact of Shock Index on hospital mortality also merits further study, as there are many situations and populations that haven't been studied, for example, patients who are suffering from burns or cardiogenic shock.

Recognizing and Treating Shock in Trauma Patients

The most important factor in preventing hypovolemic shock is to prevent traumatic blood loss. However, after stemming severe blood loss in yourself or someone else, there are additional things that you can do to minimize the effects of shock.

What Are the Signs of Shock?

The first important step is to know how to recognize the signs that a patient may be going into shock. When checking for shock, medical personnel will typically look for:

Skin Signs

  • Cool, clammy skin
  • A pale or ashen skin color
  • A gray or bluish tint to the fingernails

Mental Status of the Patient

  • Anxiety or restlessness

Other Possible Telltale Signs of Shock

  • A weak, fast pulse rate (tachycardia)
  • Fast breathing (tachypnea), or labored or irregular breathing
  • Weak or absent peripheral pulses
  • Dilated pupils
  • Increased thirst
  • Nausea and vomiting
  • Hives, mottling, or chest pains

Although there are several clear signs of shock, some patients will present with normal vital signs. For this reason, constant vigilance, and knowing the signs of shock, are essential for increasing the patient’s chances of surviving after severe trauma.

How to Treat Shock in Cases of Severe Blood Loss

Hypovolemic shock can happen when the body loses a large amount of blood—usually more than 20% of the person’s blood volume—and there’s not enough blood left for the heart to pump it to the vital organs effectively. This can lead to organ failure and ultimately death if it’s not treated in time.

It is estimated that 20 percent of people who have died from traumatic injuries could have survived with fast action to control bleeding. If you or another person suffers an accidental amputation, gunshot wound, or other traumatic injury, taking swift and decisive action can help to prevent hemorrhagic shock.

What to Do in the Case of Severe Bleeding with a Risk of Hypovolemic Shock

  1. Call the emergency services immediately.
  2. Treat the cause of severe bleeding. Use a bleeding control kit that includes a tourniquet for bleeding from extremities, or hemostatic gauze to stem bleeding from extremities (after applying a tourniquet) or from the abdomen or junctures such as the groin or neck. Maintain pressure for at least three minutes when compressing the wound with hemostatic gauze for best results.
  3. Help the casualty lie down, and raise their legs to improve the blood supply to their vital organs.
  4. Help to keep them warm by covering them with blankets or spare clothes.
  5. Try to keep the patient calm because fear and anxiety can make the symptoms of shock worse.
  6. Monitor their responses. If the patient becomes unresponsive, prepare to perform CPR.

Shock Index: A Helpful Tool for Providing Strategic Care to Trauma Patients

Shock Index is an effective and efficient tool that medical personnel use to anticipate the care needed by traumatically injured patients, and helps them to take the swiftest action possible. The strength of the SI measurement comes from its ability to produce fast results, which is a huge benefit in emergency care.

Before medical professionals arrive on the scene, however, the actions of bystanders can be critical in deciding the outcome of the trauma. For this reason, knowing how to help someone who is suffering from severe bleeding, as well as having the right tools to stop the bleeding, can go a long way to preventing shock and increasing the patient’s chances of survival.

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