Shock is a life-threatening condition in which blood pressure is too low to sustain life.
In the United States, hospital emergency departments report more than 1 million cases of shock each year. People go into shock when their blood pressure becomes so low that the body's cells do not receive enough blood and therefore do not receive enough oxygen. As a result, cells in numerous organs, including the brain, kidneys, liver, and heart, stop functioning normally. If blood flow (perfusion) to these cells is not quickly restored, they become irreversibly damaged and die. If enough cells are damaged or dead, the organ they are in may fail and the person may die. People in shock require immediate emergency treatment. The medical disorder of shock has nothing to do with the “shock” that people feel from a sudden emotional stress.
Shock has several causes: a low blood volume, which causes hypovolemic shock; inadequate pumping action of the heart, which causes cardiogenic shock; or excessive widening of blood vessels, which causes distributive shock.
Low blood volume results in less-than-normal amounts of blood entering the heart with every heartbeat and therefore less-than-normal amounts of blood being pumped out to the body and its cells.
Blood volume may be low because of severe bleeding, an excessive loss of body fluids, or, less commonly, inadequate fluid intake. Blood may be rapidly lost because of external bleeding, such as that caused by an accident, or internal bleeding, such as that caused by an ulcer in the stomach or intestine, a ruptured blood vessel, or a ruptured ectopic pregnancy (a pregnancy outside the uterus). An excessive loss of body fluids other than blood can result from major burns, inflammation of the pancreas (pancreatitis), perforation of the intestinal wall, severe diarrhea, kidney disease, or excessive use of loop diuretics, which increase the output of urine. Fluid intake may be inadequate because a physical disability (such as severe joint disease) or a mental disability (such as Alzheimer's disease) may prevent people from obtaining enough fluids even though they feel thirsty.
Inadequate pumping action of the heart can also result in less-than-normal amounts of blood being pumped out with every heartbeat. The inadequate pumping action usually results from complications of a heart attack (see Coronary Artery Disease: Complications of a Heart Attack) or a blood clot in the lungs (pulmonary embolism). Other causes include malfunction of a heart valve (particularly an artificial valve), rupture of the wall between the two sides of the heart (septum), an abnormal heart rhythm (arrhythmia), and inability of the heart to fill (cardiac tamponade).
Excessive dilation of blood vessels (vasodilation) increases the capacity of blood vessels, so that blood meets with less resistance as it flows through them. Blood pressure in the dilated vessels is lower, so the cells fed by those vessels get less blood.
Blood vessels may be excessively dilated because of a serious allergic reaction (anaphylaxis—see Allergic Reactions and Other Hypersensitivity Disorders: Anaphylactic Reactions), a severe bacterial infection (shock caused by such an infection is called septic shock—see Bacteremia, Sepsis, and Septic Shock: Introduction), overdose of drugs or poisons that dilate blood vessels, and injuries to the spinal cord and rarely the brain. The mechanisms by which these conditions cause vasodilation vary. For example, a spinal cord injury interrupts the nerves that maintain the tone of arteries; poisons or toxins released by bacteria can cause the blood vessels to dilate.
Symptoms and Diagnosis
Symptoms of shock are similar when the cause is low blood volume or inadequate pumping action of the heart. The condition may begin with lethargy, sleepiness, and confusion. The skin becomes cold and sweaty and often bluish and pale. If the skin is pressed, color returns much more slowly than normal. Blood vessels may become more visible as a bluish network of lines under the skin. The pulse is weak and rapid, unless a slow heartbeat is causing the shock. Usually, the person cannot sit up without feeling light-headed or passing out. Breathing is rapid, but breathing and the pulse may both slow down if death is imminent. Blood pressure drops so low that it often cannot be measured with a blood pressure cuff. Eventually, the person may die.
When shock results from excessive dilation of blood vessels, the symptoms are somewhat different. The skin may be warm and flushed, and the pulse may be strong and forceful (bounding) rather than weak, particularly at first. However, later on, shock due to excessive dilation of blood vessels also produces cold, clammy skin and lethargy.
In the earliest stages of shock, especially septic shock, many symptoms may be absent or may be undetected unless they are specifically looked for. In older people, the only symptom may be confusion. The blood pressure is very low. Urine flow is significantly reduced (because blood supply to the kidneys is reduced), and waste products build up in the blood.
Prognosis and Treatment
If untreated, shock is usually fatal. If shock is treated, the outlook depends on the cause, the other disorders the person has, the presence and severity of any organ failure, the amount of time that passes before treatment begins, and the type of treatment given. Regardless of treatment, the likelihood of death due to shock after a massive heart attack or due to septic shock, especially in older people, is great.
The first person to arrive on the scene can take several measures that help, including calling for additional help. A person who is in shock should be laid down and kept warm, with the legs elevated about 12 to 24 inches (about 30 to 60 centimeters) to facilitate the return of blood to the heart. Any bleeding should be stopped, and breathing should be checked. The head should be turned to the side to prevent inhalation of vomit. Nothing should be given by mouth.
When emergency medical personnel arrive, they may provide oxygen through a face mask or provide a mechanical device to assist breathing. Fluids are given intravenously at a fast rate and in large volumes to raise blood pressure. For shock caused by bleeding, a blood transfusion may be given. Usually, blood is cross-matched before transfusion, but in an emergency when there is no time for crossmatching, type O negative blood can be given to anyone. Drugs, if needed, are given intravenously. Opioids and sedatives are usually not used because they tend to decrease blood pressure.
Shock caused by excessive dilation of the blood vessels also may require drugs that constrict the vessels, such as epinephrine for people with anaphylaxis or low-dose dopamine for people with other forms of shock. The cause of the excessive dilation is also treated. For example, a bacterial infection is treated with antibiotics.
The intravenous fluid and blood transfusion may not be enough to counteract the shock if bleeding or fluid loss continues or if the shock is caused by a heart attack or another problem unrelated to blood volume. Drugs that constrict the blood vessels may be given to boost blood flow to the brain or heart. However, such drugs should be used as briefly as possible because they can reduce blood flow to other tissues in the body.
When shock is caused by an inadequate pumping action of the heart, efforts are made to improve the heart's performance. The rate and rhythm abnormalities of the heartbeat are corrected, and blood volume is increased if necessary. Atropine may be used to increase a slow heart rate, and other drugs may be given to improve the ability of the heart muscle to contract.
If the cause is a heart attack and shock persists after emergency treatment, a balloon pump may be inserted into the aorta to reverse shock temporarily. After this procedure, emergency percutaneous transluminal coronary angioplasty (PTCA—see Coronary Artery Disease: Percutaneous Coronary Intervention) or coronary artery bypass surgery (see Coronary Artery Disease: Coronary Artery Bypass Grafting) may be needed. By opening a blocked coronary artery (one of the arteries supplying the heart muscle), emergency PTCA can improve the heart's pumping action and can reverse the shock. If emergency PTCA or bypass surgery is not performed, a drug that helps break up clots (thrombolytic drug) is given as soon as possible, unless it could worsen problems in people who have another disorder, such as a bleeding ulcer, or who have had a stroke recently.
If the cause is a malfunctioning heart valve or rupture of the septum, surgery may also be needed. If the heart is unable to fill because of blood or fluid in the pericardium (the sac surrounding the heart), the fluid can be removed through a needle inserted into the pericardium.
Last full review/revision March 2007 by Scott Manaker, MD, PhD