Study Notes: On Circulatory Shock

Definition:

• Circulatory shock = decreased tissue perfusion due to an absolute or relative decrease in cardiac output (CO).

• Even with normal or high CO, if demand > supply → relative shock.

Determinants of Blood Pressure

• Systolic BP → determined by cardiac output.

• Diastolic BP → determined by peripheral resistance.

• ↓CO or ↓resistance → ↓BP → ↓tissue perfusion.

Determinants of Cardiac Output

• Heart pumping ability.

• Venous return (depends on blood volume).

Types & Causes of Circulatory Shock

1. Cardiogenic Shock (pumping failure of heart)

   • Causes: Myocardial infarction, arrhythmias, severe heart failure.

2. Hypovolemic Shock (↓blood volume → ↓venous return)

   • Causes: Hemorrhage, trauma, severe burns (plasma loss).

3. Obstructive Shock (blood flow blocked out of heart)

   • Causes: Pulmonary embolism, cardiac tamponade, tension pneumothorax.

4. Distributive Shock (↓peripheral resistance, vasodilation)

   • Anaphylactic shock (allergic vasodilation).

   • Septic shock (endotoxins → vasodilation).

   • Neurogenic shock (loss of vasomotor tone, e.g., deep anesthesia, spinal cord injury, spinal anesthesia).

Stages of Shock

1. Non-Progressive (Compensated) Shock

• Body compensates to maintain BP.

• Mechanisms:

  • Neural reflexes:

    • Baroreflex (fast, seconds).

    • Chemoreflex (low BP, O₂ detection).

  • Hormonal responses:

    • RAAS → ↑Angiotensin II → vasoconstriction + aldosterone + ADH.

    • ↑Na⁺/water retention → restore volume & venous return.

  • Intermediate mechanisms:

    • Fluid shift (↓capillary hydrostatic pressure → fluid moves into capillaries).

    • Reverse stress relaxation of vessels.

• Effective if blood loss ≤10%.

2. Progressive Shock

• Compensation fails; requires medical intervention.

• Positive feedback loops worsen shock:

  • ↓Coronary blood flow → ↓heart pumping.

  • Kidney retains too much fluid → heart overload → pulmonary congestion → hypoxia.

  • Hypoxia → ↑capillary permeability → fluid loss → worsening hypovolemia.

  • Tissue ischemia → cell death → release toxins → further depress heart.

• Interventions:

  • Avoid fluid overload (diuretics).

  • Inotropes (improve contractility).

  • Oxygen therapy.

  • Glucocorticoids (stabilize lysosomal membranes).

  • Vasoconstrictors only helpful in distributive shock (e.g., anaphylaxis).

  • Correct underlying cause (blood/plasma for hypovolemia, reperfusion for MI, etc.).

3. Irreversible Shock

• Shock persists despite treatment.

• Cause:

  • Severe ATP depletion (from hypoxia).

  • ADP → adenosine → vasodilation worsens BP.

  • Creatine phosphate & ATP stores exhausted → cannot be replenished.

• Result: Organ failure, death.

Summary

• Circulatory shock = ↓tissue perfusion from ↓effective CO.

• Types: cardiogenic, hypovolemic, obstructive, distributive.

• Stages:

  • Non-progressive → compensated by body.

  • Progressive → positive feedback, needs intervention.

  • Irreversible → ATP depletion, organ failure, death.