Shock is a state in which the circulation cannot deliver enough oxygen to meet the needs of the tissues. Without treatment, cells switch to anaerobic metabolism, lactate builds up and organs begin to fail.
Hypovolemic shock
The tank is low: bleeding, vomiting, diarrhea or burns reduce blood volume. Preload falls, so stroke volume and cardiac output fall.
Cardiogenic shock
The pump fails, most often after a large heart attack. The heart cannot eject enough blood, and pressure backs up into the lungs.
Distributive shock
The vessels dilate too much and may leak, as in sepsis, anaphylaxis or spinal cord injury. Blood pools in the periphery, and the skin may be warm early on.
Obstructive shock
Something blocks filling or outflow, such as a tension pneumothorax, cardiac tamponade or a large pulmonary embolism. Treatment is to relieve the blockage.
Compensation
Early on, the body raises heart rate and tightens vessels to protect blood pressure. Fast heart rate, cool pale skin, anxiety and low urine output appear first; low blood pressure is a late sign.
At the bedside
Look for shock in every sick patient by checking mental status, skin, capillary refill, heart rate and urine output, not just blood pressure. A shock index (heart rate divided by systolic pressure) above 1 is a warning sign.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).