What is considered hemodynamically unstable?

Hemodynamic Instability: Definition and Clinical Criteria

Hemodynamic instability is defined as a systolic blood pressure <90 mmHg and heart rate >120 bpm, with evidence of skin vasoconstriction (cool, clammy, decreased capillary refill), altered level of consciousness and/or shortness of breath, according to the Advanced Trauma Life Support (ATLS) definition. 1

Core Criteria for Hemodynamic Instability

Hemodynamic instability can be identified by the presence of any of the following:

• Systolic blood pressure <90 mmHg 1
• Heart rate >120 bpm 1
• Evidence of skin vasoconstriction (cool, clammy skin with decreased capillary refill) 1
• Altered level of consciousness 1
• Shortness of breath 1

Additional Criteria for Hemodynamic Instability

Even with systolic blood pressure >90 mmHg, a patient may still be considered hemodynamically unstable if they demonstrate:

• Requirement for bolus fluid infusions/transfusions to maintain blood pressure 1
• Need for vasopressor drugs to maintain adequate blood pressure 1
• Base deficit >5-6 mmol/L 1
• Shock index >1 (heart rate divided by systolic blood pressure) 1
• Transfusion requirement of at least 4-6 units of packed red blood cells within the first 24 hours 1

Transient Responders

Patients who show an initial response to adequate fluid resuscitation but then develop signs of ongoing blood loss and perfusion deficits are classified as "transient responders" and should be considered hemodynamically unstable. 1

Clinical Assessment of Hemodynamic Instability

When evaluating a potentially unstable patient, consider:

• Vital signs trending over time rather than single measurements 2
• Response to initial fluid resuscitation (transient vs. sustained improvement) 1
• Signs of end-organ hypoperfusion (altered mental status, decreased urine output) 1
• Laboratory markers of tissue hypoperfusion (lactate, base deficit) 1

Diagnostic Approach to Hemodynamically Unstable Patients

• Bedside echocardiography is the preferred initial diagnostic tool for rapid assessment of cardiac function and volume status in unstable patients 3
• Passive leg raising combined with measurement of cardiac output parameters is the most reliable test for predicting fluid responsiveness in hemodynamically unstable adults 4
• Respiratory variation in vena cava diameter measured by ultrasound (distensibility index >15%) can predict fluid responsiveness in patients without spontaneous respiratory efforts 4

Clinical Pitfalls to Avoid

• Relying solely on blood pressure as an indicator of hemodynamic stability - patients may maintain "normal" blood pressure despite significant ongoing blood loss through compensatory mechanisms 1
• Failing to recognize transient responders who initially stabilize with fluid resuscitation but later decompensate 1
• Delaying definitive intervention in unstable patients - hemodynamic instability despite resuscitation suggests ongoing bleeding requiring immediate intervention 1
• Overlooking the need for serial assessments - hemodynamic status can change rapidly, requiring frequent reassessment 2

Special Considerations

• In pediatric patients, hemodynamic stability is considered a systolic blood pressure of 90 mmHg plus twice the child's age in years 1
• An acceptable hemodynamic response in children is considered a positive response to fluid resuscitation after 3 boluses of 20 mL/kg of crystalloid 1
• Trauma patients with hemodynamic instability require immediate intervention (surgery or angioembolization) if they show no or only transient response to resuscitation 1