Shock classification

• True hypovolemia
  • Bleeding i.e hemorrhagic shock
  • Fluid loss (e.g. diarrhoea, vomiting, bowel obstruction, ‘third’ spacing)
• Effective hypovolemia (distributive) where the vascular space increases but insufficient fluid is there to fill it.
  • Anaphylaxis
  • Neurogenic shock
  • Septic shock
• Pump problems
  • Cardiogenic shock (e.g. post-MI), cardiomyopathy
  • Mechanical (e.g. cardiac tamponade, tension pneumothorax), aortic stenosis
  • Electrical eg VT or AF or most fundamentally VF

In 1972 Hinshaw and Cox suggested the following classification which is still used today. It uses four types of shock: hypovolaemic, cardiogenic, distributive and obstructive shock:^[1]

• Hypovolemic shock – This is the most common type of shock and based on insufficient circulating volume. Its primary cause is loss of fluid from the circulation from either an internal or external source. An internal source may be hemorrhage. External causes may include extensive bleeding, high output fistulae or severe burns.
• Cardiogenic shock – This type of shock is caused by the failure of the heart to pump effectively. This can be due to damage to the heart muscle, most often from a large myocardial infarction. Other causes of cardiogenic shock include arrhythmias, cardiomyopathy, congestive heart failure (CHF), contusio cordis or cardiac valve problems.
• Distributive shock – As in hypovolemic shock there is an insufficient intravascular volume of blood. This form of “relative” hypovolemia is the result of dilation of blood vessels which diminishes systemic vascular resistance. Examples of this form of shock are:
  • Septic shock – This is caused by an overwhelming infection leading to vasodilation, such as by Gram negative bacteria i.e. Escherichia coli, Proteus species, Klebsiella pneumoniae which release an endotoxin which produces adverse biochemical, immunological and occasionally neurological effects which are harmful to the body. Gram-positive cocci, such as pneumococci and streptococci, and certain fungi as well as Gram-positive bacterial toxins produce a similar syndrome.
  • Anaphylactic shock – Caused by a severe anaphylactic reaction to an allergen, antigen, drug or foreign protein causing the release of histamine which causes widespread vasodilation, leading to hypotension and increased capillary permeability.
  • Neurogenic shock – Neurogenic shock is the rarest form of shock. It is caused by trauma to the spinal cord resulting in the sudden loss of autonomic and motor reflexes below the injury level. Without stimulation by sympathetic nervous system the vessel walls relax uncontrolled, resulting in a sudden decrease in peripheral vascular resistance, leading to vasodilation and hypotension.
• Obstructive shock – In this situation the flow of blood is obstructed which impedes circulation and can result in circulatory arrest. Several conditions result in this form of shock.
  • Cardiac tamponade in which blood in the pericardium prevents inflow of blood into the heart (venous return). Constrictive pericarditis, in which the pericardium shrinks and hardens, is similar in presentation.
  • Tension pneumothorax. Through increased intrathoracic pressure, blood flow to the heart is prevented (venous return).
  • Massive pulmonary embolism is the result of a thromboembolic incident in the blood vessels of the lungs and hinders the return of blood to the heart.
  • Aortic stenosis hinders circulation by obstructing the ventricular outflow tract

Recently a fifth form of shock has been introduced:

• Endocrine shock based on endocrine disturbances.
  • Hypothyroidism, in critically ill patients, reduces cardiac output and can lead to hypotension and respiratory insufficiency.
  • Thyrotoxicosis may induce a reversible cardiomyopathy.
  • Acute adrenal insufficiency is frequently the result of discontinuing corticosteroid treatment without tapering the dosage. However, surgery and intercurrent disease in patients on corticosteroid therapy without adjusting the dosage to accommodate for increased requirements may also result in this condition.
  • Relative adrenal insufficiency in critically ill patients where present hormone levels are insufficient to meet the higher demands