shock

shock has many meanings in common usage. Most often it refers to a sudden mental or emotional experience ranging from a trivial unpleasant surprise to the deep disturbance of personal disaster or bereavement; ‘shell-shock’ refers to distress and disturbed behaviour in the aftermath of battle. When the media report someone as ‘suffering from shock’ this may vaguely imply ‘only’ shock, without physical injury, whereas a clinician will use the term for a potentially dangerous condition with quite specific physical features. This last will be the main topic under this heading.

In medical terms, shock occurs when the blood supply to the tissues is inadequate to meet the requirements of the body. It is a sudden, or acute, failure of the circulation.

Causes of shock

The simplest and most frequent example of acute failure of the circulation is fainting. The cause may be a sudden emotional or painful experience — a physical reaction, linked to a mental ‘shock’ via the autonomic nervous system, which causes slowing of the heart rate. When the heart slows excessively, it does not pump out enough blood and the blood pressure decreases. The person may say that they ‘feel faint’ because they experience dizziness due to the decreased blood pressure and the resulting inadequate blood flow to the brain. If the blood pressure is not restored by, for example, lying down or sitting with the head between the knees, they may lose consciousness. Generally, the heart rate will quickly return to normal and the person awakens with an anxious crowd looking down at them.

Fainting may result also from standing still particularly in very hot conditions. This can cause the blood to collect or ‘pool’ in the lower limbs. Less blood reaches the heart and so less can be pumped out. If the blood pressure decreases excessively, then the patient may faint. Fighter pilots can encounter a similar effect when they make their aircraft turn very tightly at high speed. The high ‘G’ forces cause blood to drain into their lower limbs. They may experience a ‘grey out’ and then a ‘black out’ before losing consciousness as the blood pressure gradually decreases. An anti-gravity suit is designed to maintain pressure on the blood vessels in the lower limbs and so prevent pooling of the blood with the resulting reduction in blood pressure.

A simple faint can be treated by laying the person flat. This will help to restore the blood pressure to normal; consciousness will then return within a short time. More serious causes of shock require appropriate medical treatment.

The clinical picture of shock which is more than a transient faint includes the signs and symptoms of both inadequate circulation and the body's attempt to compensate for this circulatory failure. Reflex responses to reductions in blood pressure act to prevent or minimize the decrease. The heart rate will increase in an attempt to pump out more blood and this can be felt as a rapid pulse rate (although in a simple faint it is initially slow). The blood vessels to the non-vital organs may constrict in an attempt to move blood away from these tissues towards the vital organs. This can be seen as extreme pallor of the skin and may be one of the first signs that a person is about to faint. The pulse is not only rapid but is described as ‘thin’ or ‘thready’. Hormones, such as adrenaline, are released into the blood stream, and sweating, due to autonomic nervous activity can make the patient's skin feel moist to the touch.

Types of shock and their treatment

Hypovolaemic shock

follows major blood loss which may be caused by trauma or during surgery. The blood loss can be visible and obvious or may be hidden as occurs in some types of fractures of leg bones or in internal bleeding from abdominal organs. The signs are similar to those in a patient who has fainted, with a low volume pulse and a pale, moist skin. However, the pulse rate will always be rapid in a patient suffering from hypovolaemic shock as the heart attempts to compensate for the low blood pressure.

resuscitation from haemorrhagic shock following blood loss or a major burn requires rapid blood transfusion and/or administration of other intravenous fluids to replace the circulating volume and to ensure the circulation of well oxygenated blood from the lungs to the brain and other vital organs. An intravenous cannula is inserted and fluids are administered directly into the circulation. As well as whole blood, fluid replacement may involve administration of salt fluids, plasma, or artificial substitutes, and concentrated red blood cells. Blood contains many different components including antibodies which require the blood to be ‘cross-matched’ before administration to the patient. There is considerable research directed at producing artificial blood substitutes which can be used without the need for cross-matching. These are designed to be able to transport oxygen efficiently to the tissues with less risk of producing a transfusion reaction and of transferring infection from the donor to the recipient.

Cardiogenic shock

A heart attack (myocardial infarction) is usually the result of a blockage of one of the coronary arteries which supply the heart muscle. If the artery is relatively large or supplies a particularly vital part of the heart, the damaged tissue may reduce the ability of the heart to pump blood around the body. If blood supply to the tissues is decreased because of the reduced output from the heart, then the patient may be in cardiogenic shock.

Cardiogenic shock is usually treated in a specialist intensive care unit where the patient's condition can be monitored closely and the appropriate drugs administered. Anaphylactic shock also requires rapid and skilled medical treatment using intravenous fluids and powerful drugs to restore the circulation to normal. Drugs which cause constriction of the blood vessels may be required. However, the major complication of this therapy is that these drugs produce an increase in the work performed by the heart and an increase in the heart's oxygen consumption, with the result that any primary heart disease may be worsened.

Anaphylactic shock

can develop as a result of a serious allergy. The allergen causes the release of chemicals within the body which act to make the small arterial blood vessels dilate and to leak fluid from the capillaries into the surrounding tissues. The dilation of the blood vessels has the effect of expanding the capacity of the circulation, whilst leakage of fluid into the tissues reduces the volume of the blood. The net effect is that there is insufficient filling of the circulation and the blood pressure falls. This can cause a major decrease of blood pressure but the person's skin may be flushed and reddened rather than pale. Leakage of fluid into the tissues may cause swelling which may be seen most clearly around the face. Swelling of the vocal cords can reduce or completely block the patient's airway and so prevent them breathing. In a severe case, the condition can be life threatening and immediate medical treatment is required to combat the allergic response and to assist breathing.

Septic shock

Some types of very severe infections (sepsis), can release toxins which also cause the small blood vessels to dilate and to leak fluid into the tissues. This septic shock has the same effect as an anaphylactic shock but is preceded by signs of a severe infection and develops much more gradually. The lungs may be badly affected and the leakage of fluid into the lungs can greatly reduce the ability to transfer oxygen into the blood stream.

The mainstay of therapy for any infection and particularly for septic shock is eradication of the infection. Powerful antibiotics are administered according to the specific clinical situation, and any focus of infection or abscess must be drained surgically.

Consequences of severe shock of any type

When the patient's blood pressure decreases, sensors in the blood vessels (baroreceptors) send signals which lead to an increase in heart rate, more powerful beating of the heart, and constriction of the blood vessels supplying the less vital organs. Breathing becomes less effective because blood flow to parts of the lung, and therefore the uptake of oxygen, is inadequate, and the patient may be seen to breathe more heavily. Fluid begins to move into the circulation from the tissues to restore the balance, but this is a relatively slow process.

Some tissues are particularly sensitive to severe shock if it is prolonged. The kidneys can be damaged by insufficient blood flow and the patient may develop acute kidney failure. This can require treatment with an artificial kidney until normal function recovers.

After major shock, respiratory failure can occur and this may require the patient to receive artificial ventilation to maintain sufficient delivery of oxygen to the tissues.

A shock liver syndrome can occur in patients in whom marked reduction in blood pressure has persisted for some hours. This can lead to many complications. For example, the liver is responsible for making components necessary for clotting of the blood and lack of these components causes bleeding spontaneously or from relatively minor injury.

Shock, defined as a failure of the circulation, can therefore range from a simple faint which requires minimal treatment to more serious conditions which require skilled medical and nursing care to treat successfully. The essential feature of shock is an inadequate blood supply to the tissues to meet the requirements for oxygen supply and the removal of waste products of metabolism. Treatment must be rapid to protect organs such as the kidneys, lungs, and liver from damage. The challenge for medical staff is to prevent death from irreversible shock by perfecting in-hospital treatment of the seriously ill. Such optimum care will give these patients a chance to return to functional life.

Gavin Kenny

See also allergy; autonomic nervous system; blood pressure; blood circulation; fainting; G and G suits; haemorrhage; injury; stress.