What is the classification of hypovolemic shock?

Classification of Hypovolemic Shock

Hypovolemic shock is classified into four distinct classes based on the percentage of blood volume lost, with each class characterized by specific physiological parameters and clinical manifestations.

Class I (Mild) Hypovolemic Shock

• Blood loss: Up to 15% of total blood volume (approximately 750 mL in a 70 kg adult) 1
• Clinical presentation:
  • Normal vital signs or minimal changes
  • Heart rate: Normal or slight increase (<100 beats/min)
  • Blood pressure: Normal
  • Pulse pressure: Normal
  • Respiratory rate: Normal (14-20 breaths/min)
  • Urine output: Normal (>30 mL/hour)
  • Mental status: Normal, possibly mild anxiety
  • Capillary refill: Normal (<2 seconds) 1

Class II (Moderate) Hypovolemic Shock

• Blood loss: 15-30% of total blood volume (approximately 750-1500 mL in a 70 kg adult) 1
• Clinical presentation:
  • Tachycardia (heart rate 100-120 beats/min)
  • Normal systolic blood pressure with narrowed pulse pressure
  • Respiratory rate: Increased (20-30 breaths/min)
  • Urine output: Mildly decreased (20-30 mL/hour)
  • Mental status: Mild anxiety, restlessness
  • Capillary refill: Delayed (>2 seconds)
  • Skin: Cool, pale extremities 1

Class III (Severe) Hypovolemic Shock

• Blood loss: 30-40% of total blood volume (approximately 1500-2000 mL in a 70 kg adult) 1
• Clinical presentation:
  • Marked tachycardia (heart rate >120 beats/min)
  • Hypotension (systolic BP <90 mmHg)
  • Respiratory rate: Tachypnea (>30 breaths/min)
  • Urine output: Oliguria (5-20 mL/hour)
  • Mental status: Confusion, agitation
  • Capillary refill: Significantly delayed
  • Skin: Cold, pale, clammy 1

Class IV (Critical) Hypovolemic Shock

• Blood loss: >40% of total blood volume (>2000 mL in a 70 kg adult) 1
• Clinical presentation:
  • Extreme tachycardia (>140 beats/min) or relative bradycardia in terminal stages
  • Severe hypotension (systolic BP <70 mmHg)
  • Respiratory rate: Marked tachypnea (>35 breaths/min)
  • Urine output: Anuria (<5 mL/hour)
  • Mental status: Lethargy to unconsciousness
  • Capillary refill: Absent
  • Skin: Cold, mottled, cyanotic 1

Assessment Tools for Hypovolemic Shock

• Shock Index (SI): Heart rate divided by systolic blood pressure

  • SI ≥0.9-1.0 is associated with increased need for massive transfusion and interventions 1
  • More sensitive than individual vital signs in detecting early shock 1

• Ultrasound Assessment:

  • Small, collapsing inferior vena cava (IVC)
  • Small cardiac chamber sizes with intraventricular obliteration during systole
  • These findings strongly indicate severe hypovolemia 1

Special Considerations

• Pediatric Patients: Initial resuscitation of hypovolemic shock should begin with infusion of isotonic crystalloids or albumin, with boluses of up to 20 mL/kg over 5-10 minutes 1

• Compensatory Mechanisms: Early stages of shock may show minimal changes in traditional vital signs due to compensatory mechanisms that maintain perfusion to vital organs 2

• Base Excess: Included in modern shock classification systems as it provides additional information about tissue perfusion and metabolic derangements 1

Clinical Pitfalls

• Relying solely on blood pressure can be misleading, as it may remain normal until late stages of shock due to compensatory mechanisms 2

• Children can maintain normal blood pressure despite significant volume loss through vasoconstriction and increased heart rate; once hypotension occurs, cardiovascular collapse may follow rapidly 1

• The traditional ATLS classification system provides only a rough estimate of blood loss and should be used in conjunction with other clinical parameters 1

• Certain medications (beta-blockers, calcium channel blockers) may mask tachycardia, leading to underestimation of shock severity 3