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PH-FD07

Shock

Shock is circulatory failure that leaves cells short of oxygen. This lesson explains its four mechanisms, how the body compensates, and how prolonged hypoperfusion injures cells and organs.

By the end, you can

  • Relate blood pressure and oxygen delivery to cardiac output, vascular resistance and blood volume.
  • Distinguish hypovolaemic, cardiogenic, obstructive and distributive shock by what fails.
  • Explain the sympathetic, hormonal and renal responses that compensate in early shock.
  • Explain how prolonged hypoperfusion leads to anaerobic metabolism, lactic acidosis and a vicious circle.
  • Relate the signs of shock and organ failure to the underlying mechanism.
  • Recognise the organ changes, classifications and tests examiners ask about, and explain how fluids, blood, vasopressors, inotropes and cause-specific treatments act on the failing circulation.

What it is

Shock is acute circulatory failure in which oxygen delivery to the tissues is inadequate for their needs, causing cellular dysfunction and, if it persists, cell death. It is defined by poor tissue perfusion, not by a particular blood pressure: low blood pressure is common but can be a late sign.

4 types to tell apart
Hypovolaemic
Too little circulating volume: bleeding, or loss of fluid from vomiting, diarrhoea, burns or high urine output.
Cardiogenic
The pump fails: a large myocardial infarction, an arrhythmia, acute valve failure or myocarditis.
Obstructive
Filling or emptying of the heart is blocked: cardiac tamponade, tension pneumothorax or a massive pulmonary embolism.
Distributive
Vessels dilate and leak, so the circulation is too big for the blood in it: sepsis, anaphylaxis and loss of sympathetic tone after spinal cord injury.

US spelling: litres → liters.

Normal function

Pressure, flow and oxygen delivery

The pumps blood into the ; the amount per minute is the cardiac output (heart rate × stroke volume), about 5 litres at rest. Mean arterial pressure is roughly cardiac output × systemic vascular resistance, the resistance set mainly by the tone of the arterioles.

Stroke volume depends on how well the ventricles fill (preload), how forcefully they contract, and the pressure they pump against. Filling depends on venous return through the into the .

Oxygen delivery to the tissues is cardiac output × the oxygen content of arterial blood. Baroreceptors in the carotid sinus and aortic arch sense pressure every beat and adjust heart rate and vessel tone to keep it steady.

Point to it on the model