1. Hypovolemic shock

This is the most common type of shock and based on insufficient circulating volume. Its primary cause is loss of fluid from the circula­tion (most often “hemorrhagic shock”). Causes may include internal bleeding, traumatic bleeding, high output fistulae or severe burns.

2. Cardiogenic shock

This type of shock is caused by the failure of the heart to pump effectively. This can be due to damage to the heart muscle, most often from a large myocardial infarction. Other causes of cardiogenic shock include arrhythmias, cardiomyopathy, congestive heart failure (CHF), or cardiac valve problems.

3. Distributive shock

As in hypovolaemic shock there is an insufficient intravascular volume of blood. This form of “relative” hypovolaemia is the result of dilation of blood vessels which diminishes systemic vascular resistance. Examples of this form of shock are:

Septic shock

It is caused by an overwhelming systemic infection resulting in vasodilation leading to hypotension. Septic shock can be caused by Gram-negative bacteria which release an endotoxin which produces adverse biochemical, immunological and occasionally neurological ef­fects which are harmful to the body, and other Gram-positive cocci, such as pneumococci and streptococci, and certain fungi as well as Gram-positive bacterial toxins. Septic shock also includes some ele­ments of cardiogenic shock.

Anaphylactic shock

It is caused by a severe anaphylactic reaction to an allergen, anti­gen, drug or foreign protein causing the release of histamine which causes widespread vasodilation, leading to hypotension and increased capillary permeability.

Neurogenic shock

Neurogenic shock is the rarest form of shock. It is caused by trauma to the spinal cord resulting in the sudden loss of autonomic and motor reflexes below the injury level. Without stimulation by the sympathetic nervous system the vessel walls relax uncontrollably, re­sulting in a sudden decrease in peripheral vascular resistance, leading to vasodilation and hypotension.

4. Obstructive shock

In this situation the flow of blood is obstructed which impedes circulation and can result in circulatory arrest.

The severity of shock can be graded 1—4, based on the physical signs. This approximates to the effective loss of blood volume.

Grade 1. Up to about 15 % loss of effective blood volume (~ 750 ml in an average adult who is assumed to have a blood volume of 5 liters). This leads to a mild resting tachycardia and can be well tolerated in otherwise healthy individuals. In the elderly or those with underlying conditions such as ischaemic heart disease the additional myocardial oxygen demands may not be tolerated so well.

Grade 2. Between 15—30 % loss of blood volume (750—1500 ml) will provoke a moderate tachycardia and begin to narrow the pulse pressure. The time taken for the capillaries to refill after 5 seconds of pressure (capillary refill time) will be extended.

Grade 3. At 30—40 % loss of effective blood volume (1500—2000 ml) the compensatory mechanisms begin to fail and hypotension, tachy­cardia and low urine output (< 0.5 ml/kg/hr in adults) are seen.

Grade 4. At 40—50 % loss of blood volume (2000—2500 ml) pro­found hypotension will develop and if prolonged will cause end-organ damage and death.

Shock requires immediate interventions to preserve life. Therefore, the early recognition and treatment is essential even before a specific diagnosis is made. Most forms of shock seen in trauma or sepsis re­spond initially to aggressive intravenous fluids.

The prognosis of shock depends on the underlying cause and the nature and extent of concurrent problems. Hypovolemic, anaphylactic and neurogenic shocks are readily treatable and respond well to med­ical therapy. Septic shock however, is a grave condition and with a mortality rate between 30 % and 50 %. The prognosis of cardiogenic shock is even worse.

Unit 17 INJURIES