15. Distributive shock: Causes and hemodynamics
In distributive shock the distribution of the cardiac output is affected rather than the blood volume or cardiac function. CO will be high or normal in these cases.
Some parts of the body will have accumulation of blood or fluid while other parts will suffer inefficient perfusion because of this.
To mechanisms can lie in the background of distributive shock:
- Capillary leakage – fluid is lost to the interstitium
- Systemic vasodilation – the blood pools in the periphery
There are three types of distributive shock:
- Septic shock
- Neurogenic shock
- Anaphylactic shock
We distinguish between an early (warm) phase and a late “cold” phase, and both will be discussed in detail later. Let’s look at some causes for a distributive shock to happen.
Causes
- Neurogenic shock
- Spinal cord injury
- Spinal anaesthesia
- Traumatic brain injury
- Septic shock
- Sepsis
- Anaphylactic shock
- Anaphylaxis
- Polytrauma
- Accidents often lead to the early phase of distributive shock
- Heat stroke
- Acute pancreatitis
"Warm" phase
If the distributive shock is caused by systemic vasodilation the first phase will be the “warm” phase. This causes formation of arteriovenous shunts, making the cardiac output skip the capillaries and cause ischaemia of tissues. Since there is peripheral vasodilation will the skin feel warm, hence “warm” shock.
Ischaemia will eventually cause endothelial damage and fluid to be lost though the capillary wall to the interstitium. This decreases the plasma volume.
"Cold" phase
Decreased plasma volume and low blood pressure stimulates vasoconstrictors such as catecholamines, angiotensin and vasopressin. Vasoconstriction causes the skin to become cool, hence “cold” shock.
Haemodynamics
| Early phase | Late phase | |
|---|---|---|
| Blood volume | Normal | Decreased |
| Vascular resistance (TPR) | Low | High (because of vasoconstriction) |
| Blood pressure | Low | Very low |
| Stroke volume | Nearly normal | Low |
| Heart rate | High | Very high |
| Cardiac output | High | Low |
| Central venous pressure | Nearly normal | Low |
| EDV and EDp | Normal | Low because of low blood volume |
| Flow velocity | Increased | Slow |