13. Shock

1. Definition

• Shock → acute circulatory failure with inadequate tissue perfusion and oxygen delivery, leading to cellular hypoxia
• Not defined by blood pressure alone → hypotension is a late sign

2. Classification

A. Hypovolaemic

• Haemorrhagic → trauma, gastrointestinal bleeding, ruptured aneurysm
• Non-haemorrhagic → burns, vomiting, diarrhoea, bowel obstruction, pancreatitis
• Most common type in surgery

B. Cardiogenic

• Pump failure → myocardial infarction, arrhythmias, cardiomyopathy, valve rupture, myocardial contusion

C. Obstructive

• Obstruction to flow outside the heart:
  • Tension pneumothorax
  • Cardiac tamponade
  • Massive pulmonary embolism (PE)

D. Distributive

• Vasodilatation, maldistribution of flow:
  • Septic shock
  • Anaphylactic shock
  • Neurogenic shock → spinal cord injury above T6; hypotension with bradycardia

E. Endocrine

• Adrenal insufficiency (Addisonian crisis), hypothyroidism

3. Pathogenesis

Compensated shock

• Fall in cardiac output or vascular tone
• Baroreceptor reflex → sympathetic activation, catecholamines
• Tachycardia, vasoconstriction → skin, gut, kidney
• Centralisation of the circulation → blood redirected to brain and heart
• Renin-angiotensin-aldosterone, antidiuretic hormone → retain water and sodium
• Fluid shift from interstitium into vessels

Decompensated shock

• Cellular hypoxia → anaerobic metabolism → lactic acidosis
• Failure of the sodium-potassium pump → cell swelling
• Capillary leak, microcirculatory stasis, sludging
• Disseminated intravascular coagulation (DIC)
• Systemic inflammatory response syndrome (SIRS)

Irreversible shock

• Cell death, multi-organ dysfunction syndrome (MODS)
• Shock organs:
  • Lung → acute respiratory distress syndrome (ARDS)
  • Kidney → acute tubular necrosis
  • Gut → bacterial translocation
  • Liver → ischaemic hepatitis
  • Heart → depression of contractility
  • Brain → confusion, coma

4. Clinical picture

• Tachycardia → earliest sign
• Tachypnoea
• Cold, pale, clammy skin; delayed capillary refill > 2 s
• Narrow pulse pressure
• Oliguria → < 0.5 mL/kg/h
• Anxiety → confusion → coma
• Hypotension → late, after loss of > 30% blood volume
• Septic shock → warm, flushed skin early (warm shock)
• Neurogenic shock → bradycardia, warm dry skin
• Obstructive and cardiogenic → raised jugular venous pressure
• Shock index → heart rate ÷ systolic blood pressure; > 1 suggests significant shock

5. Diagnosis

Laboratory

• Lactate → > 2 mmol/L abnormal; > 4 mmol/L severe
• Arterial blood gas → metabolic acidosis, base deficit
• Full blood count, coagulation, crossmatch
• Urea, creatinine, electrolytes, liver function
• Troponin, blood cultures

Monitoring and imaging

• Electrocardiogram (ECG), continuous monitoring
• Urinary catheter → hourly urine output
• Central venous pressure, arterial line
• Focused assessment with sonography for trauma (FAST), echocardiography
• Chest X-ray, CT when stable

6. Treatment

General principles

• ABCDE approach; high-flow oxygen
• Treat the cause → stop bleeding, drain the pericardium, decompress the pneumothorax, control the septic source
• 2 large-bore intravenous cannulas
• Targets → mean arterial pressure ≥ 65 mmHg, urine output ≥ 0.5 mL/kg/h, falling lactate

Hypovolaemic

• Haemorrhage control → pressure, tourniquet, surgery, angioembolisation
• Permissive hypotension in bleeding trauma → systolic about 80–90 mmHg until bleeding controlled (not in head injury)
• Early blood products (1:1:1) rather than large crystalloid volumes
• Tranexamic acid within 3 h
• Non-haemorrhagic → crystalloids

Cardiogenic

• Cautious fluids
• Inotropes → dobutamine
• Coronary revascularisation
• Mechanical support → intra-aortic balloon pump

Obstructive

• Tension pneumothorax → needle decompression, then chest drain
• Tamponade → pericardiocentesis or thoracotomy
• Massive PE → thrombolysis

Septic

• Sepsis bundle within 1 h:
  1. Measure lactate
  2. Blood cultures before antibiotics
  3. Broad-spectrum antibiotics
  4. Crystalloid 30 mL/kg for hypotension or lactate ≥ 4 mmol/L
  5. Vasopressors (noradrenaline) if MAP < 65 mmHg after fluids
• Source control → drain, debride, resect

Anaphylactic

• Adrenaline 0.5 mg intramuscularly (0.5 mL of 1:1000), repeat after 5 min
• Fluids, oxygen; antihistamines and steroids second line

Neurogenic

• Fluids, vasopressors, atropine for bradycardia

7. Complications

• Acute kidney injury
• ARDS
• DIC
• Multi-organ failure
• Death