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Shock is a condition where the circulatory system is not able to perfuse the vital organs adequately, due to a discrepancy between the circulating blood volume and the volume of the vascular bed.
Shock can be caused by failure of different mechanisms.
Cardiogenic shock. When the heart cannot contract effectively, forward propulsion of blood in the circulation decreases, and organs such as the brain and lungs do not receive any blood, and therefore oxygen and other nutrients. This causes stagnation hypoxia. A massive myocardial infarct or heart attack is the most common cause of cardiogenic shock. Insufficient or decreased cardiac function further negatively affects the heart muscle, as perfusion of the heart muscle itself also decreases, and the heart cannot contract adequately.
Hypovolemic shock. Severe blood loss or loss of other body fluids, either externally (dehydration) or internally (for instance in the abdominal cavity in cases of peritonitis or inflammation of the abdominal cavity), can decrease the circulating blood volume. Trauma with blood loss, for instance due to multiple injuries sustained in a motor-vehicle accident, or due to stab wounds, is a common cause of hypovolemic shock. This results in anaemic hypoxia of the tissue.
Neurogenic shock. Loss of the normal tone of blood vessels can result in dilatation of the vascular bed, which then increases dramatically in volume. The circulating blood volume is not sufficient to fill the dilated vascular bed. This type of shock is seen in spinal cord injuries, and sometimes during anaesthetic procedures.
Septic shock. Micro-organisms, especially bacteria, can produce sub-stances which cause shock through certain mechanisms. The substances are usually part of the structure of the micro-organism, and are then called endotoxins. If the micro-organism produces and releases the substance into the blood, they are called exotoxins. They affect different mechanisms, for example blood-clotting mechanisms, resulting in DIC (diffuse intravascular coagulopathy).
Anaphylactic shock. Individuals who are highly allergic to substances like penicillin or bee venom, will also experience a dramatic dilatation of the vascular bed. The blood volume is not sufficient to fill this dilated vascular bed. There is often additional loss of fluid due to increased permeability of vessels.
Shock is therefore the result of an imbalance between the circulating blood volume and the vascular bed which has to be perfused.
A person in shock is characteristically cold and sweaty, with a grey colour, low blood pressure and weak pulse. The pulse rate is rapid, but as shock progresses and becomes irreversible, the pulse rate will gradually drop. Shock is initially reversible if managed optimally and timeously.
Shock affects many organs, but the degree of damage to vital organs will determine whether or not the person in shock will survive. The normal response of the body is to redistribute blood from the less important organ systems, such as the skin and abdominal organs, to the more important vital organs such as the brain and heart.
Some of the brain cells in specific regions of the brain (eg the neurons in the hippocampus) are more susceptible to a lack of oxygen (hypoxia) than elsewhere in the brain. In addition the watershed regions are also more susceptible to hypoxia due to decreased blood flow, and are therefore often involved in cases of severe hypotension (low blood pressure). The watershed areas are present at the boundaries between the distribution areas of two major cerebral arteries.
Because it is essential to fulfil the oxygen needs of the brain, blood flow to the brain will be maintained almost until death occurs, unfortunately with the sacrifice of other organ systems, for instance the lungs, kidneys and gastro-intestinal system.
Lung changes are also known as diffuse alveolar damage (DAD), shock lung or adult respiratory distress syndrome (ARDS). Due to the changes oxygen cannot be sufficiently absorbed by the small blood vessels surrounding the lung sacs (alveolar capillaries). These patients often have to be ventilated, and are susceptible to lung infections.
Decreased blood flow to the kidneys causes cell death in the small tubuli in the kidneys. This is also known as acute tubular necrosis (ATN), and it causes renal failure.
There are other causes of acute tubular necrosis which are not necessarily associated with shock in the initial stages, such as poisons or the products released by damaged red blood cells and muscle cells (haemoglobin and myoglobin). The latter condition is often seen during marathons, and is the combined effect of dehydration and damage to red blood cells and muscle elements in the athletes' feet. ATN also commonly occurs after assault causing multiple blunt injuries. These victims often have severe bruises or contusions, causing two problems: blood loss into the soft tissue, with subsequent hypovolemia, and at the same time destruction of the red blood cells trapped in the tissue. Two or more days after the assault the victim presents in a shocked state, with no urinary output, and dies.
Stomach ulcers can cause severe bleeding, which can be prevented by administering antacids to the person in shock.
There can be necrosis of the intestinal mucosa, and sometimes even of the entire bowel wall due to rechannelling of blood away from the gastro-intestinal system towards the vital organs.
In a patient in shock the liver presents with a patchy red pattern against a whitish background. The patchy pattern is caused by congestion of the central blood vessels due to poor drainage against the background of early fatty changes due to decreased oxygen supply. In some of the textbooks this appearance is described as a nutmeg pattern.
There can be haemorrhages beneath the endocardium (the innermost membrane covering the heart surface), especially in the left ventricle. These sub-endocardial haemorrhages are also described in arsenic poisoning and head injuries (see photo 49).
The adrenals can show haemorrhages and fat depletion. The latter is the result of the release of the fat-containing stress hormones (adrenalin and cortisone).
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