Shock is most commonly due to hypotension, which is a systolic blood pressure less than 90 mm Hg, or a mean arterial pressure less than 65 mm Hg, that leads to inadequate tissue perfusion and injury to various organ systems, like the brain, heart, kidneys, and liver.
If left untreated, shock can cause irreversible multi-organ failure and death.
Shock can be thought of with the following equation. The mean arterial pressure is equal to the cardiac output times the systemic vascular resistance.
The cardiac output is equal to the heart rate times the stroke volume, and the stroke volume is dependent on the preload and the contractility.
So, the causes of shock can be either due to a decline in the cardiac output or the systemic vascular resistance. When one parameter declines, normally the other increases to try to compensate.
A decrease in the cardiac output can be due to a decreased preload, decreased contractility or a heart rate that’s too fast or too slow.
Decreased preload can be due to extracellular fluid volume depletion, such as from diarrhea, excessive diuresis, dehydration, or hemorrhage. These are all causes of hypovolemic shock.
Preload can also decrease if there is something obstructing the entry of blood into or out of a cardiac chamber, and this is called obstructive shock.