How is systemic vascular resistance managed following hemorrhage?

Management of Systemic Vascular Resistance Following Hemorrhage

In hemorrhagic shock, fluid resuscitation should be the first-line therapy to restore mean arterial pressure and systemic blood flow, with vasopressors like norepinephrine added only when fluid therapy is insufficient to maintain adequate tissue perfusion. 1

Initial Hemodynamic Assessment and Management

Fluid Resuscitation Strategy

• Begin with crystalloids as the initial fluid of choice for volume replacement 1
• Target a systolic blood pressure of 80-90 mmHg to maintain tissue perfusion without exacerbating bleeding 1
• Consider permissive hypotension (mean arterial pressure around 65 mmHg) in patients without traumatic brain injury or spinal injuries 1
• Avoid aggressive fluid resuscitation which may:
  • Increase hydrostatic pressure on wounds
  • Dislodge blood clots
  • Dilute coagulation factors
  • Cause undesirable cooling 1

Vasopressor Therapy

When fluid resuscitation alone is inadequate:

1. Norepinephrine is the vasopressor of choice for hemorrhagic shock 1

   • Mechanism: Predominantly vasoconstrictive effects through α-adrenergic stimulation
   • Benefits:
     • Increases arterial resistance
     • Induces venoconstriction in splanchnic circulation
     • Shifts blood volume from capacitance vessels to systemic circulation
     • Recruits blood from venous unstressed volume 1
   • Dosing: Start at 2-4 mcg/min (0.5-1 mL/min of standard dilution) and titrate to response 2

2. Inotropic Support

   • Add dobutamine or epinephrine if myocardial dysfunction is present 1
   • Assess cardiac function during initial ultrasound examination to identify dysfunction 1

Physiological Considerations

Vascular Resistance Changes During Hemorrhage

• Following hemorrhage, resistance vessels (small arteries) constrict significantly to approximately 52% of baseline diameter 3
• Capacitance vessels (small veins) constrict to about 70% of baseline 3
• This selective vasoconstriction helps maintain mean arterial pressure and redistributes blood from skin to vital organs 3

Vasopressor Effects on Pulmonary vs. Systemic Circulation

• Vasopressin decreases pulmonary-to-systemic vascular resistance ratio by 45% compared to phenylephrine, potentially improving oxygenation 4
• Norepinephrine may induce pulmonary hypertension and increase pulmonary vascular resistance, potentially worsening hypoxemia 4

Special Considerations

Contraindications to Permissive Hypotension

• Traumatic brain injury
• Spinal cord injury
• Elderly patients
• Patients with chronic arterial hypertension 1

Monitoring Parameters

• Assess cardiac function with ultrasound to identify myocardial dysfunction
• Monitor for signs of inadequate circulation:
  • Relative tachycardia
  • Relative hypotension
  • Oxygen extraction >50% 1

Potential Pitfalls

1. Early vasopressor use without adequate volume resuscitation

   • May be deleterious compared to aggressive volume resuscitation 1
   • Should not substitute for appropriate fluid resuscitation 1

2. Excessive vasopressor administration

   • May increase cardiac afterload, especially if left ventricular function is impaired 1
   • Can impair tissue perfusion despite normalized blood pressure

3. Aggressive fluid resuscitation

   • May increase risk of secondary abdominal compartment syndrome 1
   • Can worsen coagulopathy (observed in >40% of patients receiving >2000 mL, >50% with >3000 mL, and >70% with >4000 mL) 1

Remember that the goal of managing systemic vascular resistance following hemorrhage is to restore adequate tissue perfusion while avoiding the adverse effects of both excessive vasoconstriction and excessive fluid administration.