Distributive Shock

Distributive shock is circulatory failure from profound vasodilation and abnormal distribution of blood flow, so tissues are underperfused despite normal or even high cardiac output. Vascular resistance is low, and it is the most common form of shock. The most frequent cause is sepsis, and others are anaphylaxis, neurogenic injury, adrenal crisis, toxic shock syndrome, post-cardiac surgery vasoplegia, drug and toxin effects, liver failure and pancreatitis.

Mechanisms: inflammatory mediators (nitric oxide, prostaglandins) and relative vasopressin and cortisol deficiency, glycocalyx damage and capillary leak, and loss of sympathetic tone (spinal injury, anesthesia, autonomic neuropathy). Adrenal insufficiency from chronic steroid use is an important and treatable cause.

Septic shock is defined by sepsis with persistent hypotension needing vasopressors to hold a mean arterial pressure of 65 mmHg or higher and lactate over 2 mmol/L despite fluids. Hospital mortality is about 35 to 45%. Complications: acute respiratory distress syndrome, acute kidney injury, disseminated intravascular coagulation, myocardial depression, ischemic limbs and multiorgan failure.

• Warm, flushed skin (early), bounding pulses, wide pulse pressure with low diastolic pressure, and tachycardia
• Fever or hypothermia, tachypnea, altered mental state and oliguria
• Later cold shock with mottling when myocardial depression and hypovolemia add
• Source clues: cough and consolidation, dysuria, abdominal peritonism, cellulitis, line sites, meningism
• Anaphylaxis: urticaria, angioedema and wheeze. Neurogenic: bradycardia and warm dry skin. Adrenal crisis: hyponatremia, hyperkalemia, abdominal pain and hyperpigmentation
• Red flags: systolic pressure under 90 mmHg, lactate over 4 mmol/L, purpura, and children with weak pulses and prolonged capillary refill

1. Clinical assessment with the full set of vital signs, and a rapid screen for a source.
2. Blood cultures (two sets) before antibiotics without delaying them, and cultures of urine, sputum, wounds and other sites.
3. Laboratory tests: lactate, full blood count, procalcitonin, C-reactive protein, creatinine, liver tests, coagulation and fibrinogen, glucose, blood gases, random cortisol, tryptase (anaphylaxis).
4. Echocardiography: hyperdynamic ventricle in early septic shock, or septic cardiomyopathy.
5. Imaging of the likely source: chest radiograph, abdominal CT, ultrasound of the biliary tract and kidneys.
6. Central venous oxygen saturation (often high or normal in distributive states).

Differentials: cardiogenic, hypovolemic and obstructive shock, and endocrine causes.

1. Fluids: balanced crystalloid 30 mL/kg IV in the first 3 hours (children 10 to 20 mL/kg boluses up to 40 to 60 mL/kg with reassessment for overload), then guided by dynamic measures.
2. Antibiotics within 1 hour of recognition:
   • Piperacillin-tazobactam 4.5 g IV every 6 hours (children 100 mg/kg of piperacillin every 8 hours), or meropenem 1 g IV every 8 hours (children 20 mg/kg every 8 hours) with risk of resistant organisms.
   • Add vancomycin 25 to 30 mg/kg IV loading dose, then 15 to 20 mg/kg every 8 to 12 hours, for suspected methicillin-resistant Staphylococcus aureus, with troughs checked.
   • Toxic shock: add clindamycin 600 to 900 mg IV every 8 hours (children 10 to 13 mg/kg every 8 hours) to suppress toxin synthesis.
3. Source control within 6 to 12 hours: drain abscesses, remove infected lines, debride necrotic tissue, decompress obstructed systems.
4. Vasopressors: norepinephrine 0.05 to 0.5 micrograms/kg/min IV (through a peripheral vein at first if needed) for a mean arterial pressure over 65 mmHg. Add vasopressin 0.03 units/min IV when norepinephrine exceeds 0.25 micrograms/kg/min. Epinephrine 0.05 to 0.5 micrograms/kg/min for persistent low output or in children with cold shock.
5. Hydrocortisone 50 mg IV every 6 hours (children 2 mg/kg then 1 mg/kg every 6 hours) for ongoing vasopressor need (norepinephrine 0.25 micrograms/kg/min or more for 4 hours), and 100 mg IV then 50 mg every 6 hours for proven or suspected adrenal crisis.
6. Vasoplegia after cardiac surgery: methylene blue 1 to 2 mg/kg IV over 20 to 60 minutes, or angiotensin II 20 nanograms/kg/min IV (titrated to 5 to 40) in refractory vasodilatory shock. Avoid methylene blue in G6PD deficiency and with serotonergic drugs.
7. Anaphylaxis: epinephrine 0.5 mg IM (children 0.01 mg/kg, maximum 0.5 mg) into the anterolateral thigh, repeated every 5 minutes, then infusion 0.05 to 0.1 micrograms/kg/min.
8. Neurogenic shock: maintain mean arterial pressure at 85 to 90 mmHg, and use atropine 0.5 to 1 mg IV for bradycardia.
9. Supportive care: lung-protective ventilation (6 mL/kg ideal body weight), renal replacement therapy for refractory acidosis or hyperkalemia, glucose control to 8 to 10 mmol/L, enoxaparin 40 mg SC daily, and red cell transfusion when hemoglobin falls below 70 g/L.

Targets: lactate falling, urine output over 0.5 mL/kg/h, and capillary refill normalizing. Reassess antibiotics at 48 to 72 hours with culture results, and counsel on recovery and post-intensive care syndrome.