Cardiovascular System
Distributive shock and low total peripheral resistance
Distributive shock is inadequate blood flow caused by widespread vasodilation (widening of the blood vessels), which lowers total peripheral resistance, the overall resistance to flow.
Remember
In distributive shock, widespread vasodilation lowers total peripheral resistance, so blood pressure falls even though the blood volume is essentially normal.
Shock is a state of inadequate blood flow to the tissues. Blood pressure depends on cardiac output (the volume the heart pumps each minute) multiplied by total peripheral resistance, the overall resistance the blood vessels offer to flow. So shock can arise in two ways: cardiac output falls, or total peripheral resistance falls.
In distributive shock the fault is a fall in total peripheral resistance. The vessels lose their tone (the ability of their walls to stay narrowed) and the widespread vasodilation opens them wide. The blood volume is essentially normal and the heart is pumping, but the blood is spread through vessels that are now too wide, so it seems too little. Picture the same water in a much wider hosepipe, and the pressure is lower. Volume loss may contribute as well, since fluid can leak out of the circulation in anaphylaxis, but it is not the main mechanism.
Cardiac output is often normal or even raised early on, so the pump is not the main problem. The skin vessels are open, so the skin is often warm and flushed. Later, cardiac output may fall and the skin may turn cool.
The two main types are septic shock, where infection triggers a massive inflammatory response that widens the vessels, and anaphylactic shock, where a severe allergic reaction makes mast cells (immune cells) release histamine, a chemical that widens the vessels.
The other types start differently: cardiogenic shock (the pump fails), mechanical (obstructive) shock (the heart's filling or outflow is blocked) and hypovolaemic shock (blood volume is lost). When perfusion (blood flow to the tissues) is inadequate, cells are short of oxygen and several organs may fail.
Key facts
- Definition
- a fall in total peripheral resistance from widespread vasodilation, with an essentially normal blood volume.
- Blood pressure
- mean arterial blood pressure (the average pressure in the arteries) equals cardiac output multiplied by total peripheral resistance.
- Blood volume
- it is essentially normal, but it is spread through vessels that are now too wide, so it seems too little.
- Cardiac output
- it may be normal or raised early, and it may fall later as shock progresses.
- Skin
- the skin is often warm and flushed early, in contrast to the cold, clammy skin of hypovolaemic shock.
- Types
- septic shock and anaphylactic shock are the main types.
Exam traps
- A failing pump is cardiogenic shock. In distributive shock the cardiac output is often normal or even raised in the early stages.
- Losing blood volume is hypovolaemic shock. In distributive shock the circulating volume is essentially normal, but the vessels are too dilated for it.
- A slow heart rate can lower cardiac output, which makes it tempting. Distributive shock usually causes a fast heart rate, and the low pressure comes from falling vascular resistance.
- Vasoconstriction (narrowing of the vessels) is the body's response when cardiac output falls, and it makes the skin pale and cool. Raised resistance would tend to increase blood pressure rather than lower it.
- Thickening of the arterial wall in chronic hypertension is a long-term change. It does not explain acute hypotension and poor tissue perfusion.
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