Inadequate perfusion for metabolic requirements of body tissue
Therefore can involve decreased circulatory function, excessive tissue metabolic activity or both.
It is one of the key issues to address in resuscitation
Both a conceptual and clinical way of approaching the cause of circulatory dysfunction can be divided into
- Hypovolaemic – decrease in circulating blood volume e.g. major bleeding
- Distributive – vasodilation resulting relative decrease in blood volume e.g. spinal anaesthesia
- Cardiogenic – reduced heart pump function e.g. myocardial infarction
- Obstructive – major obstruction to circulatory flow e.g. saddle pulmonary embolus
In response to the insult, circulatory system can compensate in several ways via activation of the baroceptor reflex and the renin-aldosterone-angiotensin system.
- Increased myocardial contractility and heart rate
- Sodium/Water retention and increase of circulating volume
The response seen will vary according to which part of the compensatory system has been impaired e.g. loss of vasoconstriction in sepsis, inability to increase myocardial function after cardiac insult
Depending on the magnitude and duration of the insult it is helpful to conceptualise three stages of shock
- Compensated shock – homeostatic responses active and blood pressure/perfusion maintained
- Decompensated shock – homeostatic responses maximally activated and blood pressure/perfusion falls
- Irreversible shock – prolonged hypo-perfusion results in permanent damage to circulatory system such that even correction inevitably results in death.
The important point is that falling blood pressure is a late sign when shock is well established and should not be the only trigger to take action
- Vasoconstriction – pallor, decreased capillary refill, peripheral cyanosis
- Increased heart rate
- Evidence of decreased perfusion to vital organs e.g. altered mental state, oliguria
- Biochemical evidence of stressed circulation e.g. low bicarbonate, metabolic acidosis, increased lactate
There is no one parameter that can be used to diagnose shock since each one in isolation may have alternative causes. They need to be considered together and in context of the situation.
See also Practical Approach To Shock