What is shock?

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A life-threatening condition that may occur in response to a variety of circumstances, in which the heart is unable to pump enough blood to supply the vital organs.
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Causes and Symptoms

The primary goal of the cardiovascular system is to provide blood flow, carrying oxygen and other nutrients to all tissues to meet their requirements. The cardiovascular system performs this function by maintaining a blood pressure high enough to push sufficient blood flow throughout the body, especially the vital organs. To keep the blood pressure up, the heart must pump sufficient amounts of blood even when the demand for increased blood flow to some tissues occurs. The blood vessels also play an important role in maintaining blood pressure. The heart and the blood vessels work in a coordinated manner to maintain blood pressure and blood flow.

A healthy heart is capable of adjusting the strength of its beats and the rate of its beats (the heart rate) to produce enough flow to match the demands placed on it by the tissues of the body. For example, during exercise, the exercising muscles require greater blood flow. If the heart does not pump the increased amount of blood that is necessary, then the blood pressure will fall. Hormones such as adrenaline help the heart beat faster and harder to meet the increased demand for blood by the muscles.

The blood vessels (vasculature) have a special structure and function to help maintain blood pressure. The arteries and veins are elastic in nature and squeeze on the blood like an inflated balloon does to the air inside it. In addition, the walls of blood vessels have special muscle tissue, called smooth muscle, that can contract to make the vessels’ internal diameter smaller, which helps keep pressure up. If the vessels’ internal diameter becomes too small, however, then the blood flow through them will decrease. The concept of blood vessels getting narrow and making it more difficult to push blood through is termed resistance to flow or vascular resistance. The balance of blood flow produced by the heart (cardiac output) and vascular resistance keeps blood pressure at the proper level. When one or both of these components falter, cardiac output and blood pressure fall, which, if untreated, leads to shock.

When the cardiovascular system cannot supply adequate blood flow to the essential organs to sustain their function, the body is said to be in shock. A reduction in cardiac output is the primary problem in shock. There are two major ways in which cardiac output can decrease enough to cause shock. When the ability of the heart to pump falls 40 percent from its normal capacity, it is termed cardiogenic shock. Cardiogenic shock may occur after a heart attack, heart valve disease, lung collapse, and other disorders.

Cardiogenic shock may occur in several ways. The most common cause is a myocardial infarction (heart attack). During a heart attack, the heart is damaged, and like any other muscle when injured, it does not have the strength to pump much blood. Thus, cardiac output goes down and a fall in blood pressure will follow. Cardiogenic shock will progress to death if medical treatment is not rapidly obtained. After a myocardial infarction, while the heart is still healing, it has a reduced ability to pump blood. Exercise, even light exercise such as walking, must be resumed gradually. If it is not, the heart may not be able to pump enough blood to supply muscles even though demand for more flow is only slightly increased. This inability to meet the oxygen demand of the heart will cause further damage to the heart.

In cardiac tamponade, a type of obstructive circulatory shock, the stiff but pliable sac surrounding the heart (pericardium) fills with fluid or swells. This takes up room in the sac, squeezing the heart and prohibiting it from filling adequately from beat to beat. Therefore, the amount of blood pumped decreases, and a drop in blood pressure occurs. Cardiac tamponade can occur for several reasons. It can occur rapidly after trauma or heart surgery if the heart is punctured and bleeds into the pericardial sac. Cardiac tamponade occurs much more slowly when excess fluid is produced by the pericardium or when the pericardium becomes swollen. Both of these conditions can be caused by an infection.

A less common form of cardiogenic shock is caused by an extremely high heart rate. Normally, the heart beats at a rate between sixty and one hundred beats per minute. When the heart rate exceeds one hundred, the resulting condition is called tachycardia. Occasionally, in some people, the heart rate can go rapidly up to near two hundred beats per minute. The time between beats becomes so short that the heart does not have enough time to refill and cardiac output falls. If this condition persists, the blood pressure may fall, causing shock. When this occurs, the combination of the rapid heart rate and low blood pressure may cause a myocardial infarction.

Shock caused by a problem in the vascular system, not by a primary decrease in heart function, is generally termed hypovolemic shock. It is characterized by a lack of sufficient blood volume returned to the heart by the vascular system. Hypovolemic shock can be caused by a decrease in the body’s total blood volume.

Excessive bleeding (hemorrhage) is the most common form of hypovolemic shock. The blood vessels are elastic in nature, and they must remain filled with blood for arterial pressure to be maintained. In addition, enough blood must be in the veins to push it back to the heart, to be pumped through the lungs and back out into the arteries. When blood loss is slight, the body attempts to compensate by contracting the veins and arteries, thereby maintaining enough pressure and sufficient cardiac output. When enough circulating blood volume is lost, approximately 15 percent to 20 percent, hypovolemic shock occurs.

There are other ways in which blood volume may decrease. When a person is burned severely, plasma (the fluid in which blood cells are suspended) is lost through the burn sites. Enough can be lost to cause hypovolemic shock. Different forms of dehydration can also result in shock. Prolonged diarrhea, vomiting, and sweating can ultimately result in shock if the person does not drink enough liquids to replace fluid lost. All of these conditions lead to a loss in the circulating blood volume and subnormal return of blood to the heart, and thus reduced cardiac output.

A virtual loss in blood volume may also occur, resulting in neurogenic shock. Sometimes anesthesia, hypoxia (inadequate oxygen), low blood sugar, spinal cord injuries, or damage to the brain stem can cause the vascular smooth muscle around the arteries and veins to relax. This results in a loss of arterial and venous pressure. Blood tends to accumulate in the veins and is not returned to the heart, and cardiac output falls. A systemic (entire body) allergic reaction can cause a similar response, called anaphylactic shock. Severe infections, usually bacterial, can cause septic shock, which has a death rate of approximately 40 percent. In this type of shock, the immune system of the body responds to the infection. However, this response can cause damage to blood vessels or cause a release of chemicals that cause the vessels to dilate (expand). All types of shock can be deadly if they are not promptly treated.

Applications

In spite of the different causes of circulatory shock, the symptoms are quite common in nearly all cases of shock. The pulse (heart rate) is usually rapid and feeble. Breathing is generally rapid and shallow, and the person may feel dizzy. The skin is pale, cool, and sometimes moist. The mouth is dry, and thirst is intense. Blood pressure is decreased. Some of these signs are attributable to the body’s attempt to alleviate the problem.

The body has several defense mechanisms to help avoid circulatory shock. Several reflex systems function to maintain cardiac output and blood pressure. The body has sensors in the cardiovascular system that tell the brain what the pressure is in the arteries and the veins. When the brain senses a change in either or both of these pressures, it calls on its defenses.

When the arterial pressure sensors tell the brain that blood pressure is falling, the brain produces several responses. Through the nerves, the brain can make the heart beat faster and with greater force. In addition, the nerves can cause vascular smooth muscle to contract, making the vessels squeeze against the blood and increasing pressure. When the smooth muscle contracts, the veins squeeze blood back to the heart to enable it to pump more. The brain can also cause the release of adrenaline into the blood. This hormone can also cause the heart to beat harder and faster. The combined actions of this reflex mechanism attempt to compensate for the decreased cardiac output and blood pressure; however, these responses are only temporary before the person progresses to decompensation and death.

The sensors in the large veins and atria of the heart can cause a different response. Since most (75 percent) of the blood in the body is in the veins at any point in time, a 10 percent to 15 percent decrease in volume triggers the release of a hormone called vasopressin into the blood, which constricts the blood vessels to increase both arterial and venous pressure. The increase in venous pressure squeezes more blood back to the heart to improve cardiac output. The squeeze on the arteries raises blood pressure. Vasopressin also causes the kidneys to retain water. This fluid retained by the kidney is returned to the blood to keep up the vascular volume.

Specialized blood vessels in the kidneys can also initiate a reflex response to a decrease in blood pressure. The kidneys release a hormone called renin into the blood. Renin activates another hormone, angiotensin, which is a powerful constrictor of blood vessels. Angiotensin increases the release of yet another hormone, aldosterone, which helps the kidneys reabsorb more fluid. All of the above reflexes work together to increase blood volume, cardiac output, and blood pressure, in an attempt to alleviate shock. Despite these mechanisms to prevent shock, however, the by-products of these reflexes actually produce negative effects in the body that eventually lead to more damage.

When average arterial blood pressure falls below 60 millimeters of mercury (mmHg), the blood flow to the blood vessels supplying the heart (coronary vessels) cannot be maintained. When this occurs in shock, it happens at a time when the heart needs its critical supply of oxygen. In fact, the heart is trying to beat harder and faster, which increases its need for oxygen. As a result, the heart can weaken. When weakened, it pumps less and thus cannot bring the pressure back to normal. The heart becomes weaker and weaker. This condition is termed cardiac failure. In addition to cardiac failure caused by reduced coronary blood flow, the body can produce a hormone called myocardial depressant factor (MDF). This hormone directly causes a weakening of the heart that is independent of coronary blood flow. MDF also causes the body’s bacterial defense system to function poorly. The maintenance of heart function is important to defend against shock.

Because the blood vessels contract in most of the body during shock, blood flow to nonessential tissues such as skin, muscle, bone, and the intestines is reduced, depriving these tissues of oxygen. These tissues can tolerate short periods of low oxygen supply, but if shock persists for more than a few minutes, these tissues revert to other energy sources. The end products of these alternative energy sources are acids, which can begin a process of tissue damage. If this process is not controlled or reversed, tissues can die. If a critical amount of tissue in an organ dies, the organ cannot function and may fail. Acid produced by other tissues gets into the blood and can directly decrease the function of the heart and its ability to respond to beneficial reflex signals. Acid production makes it more difficult for the body to fight shock.

Derangements in blood clotting can occur during shock. Blood clots can form in the early stages of shock, blocking small vessels. This causes a loss of oxygen that results in acid production. Increased acid in the blood can increase the rate of formation of blood clots. Thus, the clotting system can start a vicious cycle that increases the severity of shock.

Even when shock is not bacterial in origin, the body needs its bacteria-fighting systems. During shock, the bacterial defenses are weakened. Normally, bacteria from the intestines constantly enter the blood and are rapidly neutralized. If this does not occur, endotoxic shock can intensify the already existing shock. Therefore, a capable bacteria-fighting system is important in defending against shock.

In shock, the blood vessels of the heart and brain are spared the constriction experienced by all other tissue blood vessels. In fact, arteries in these organs relax to permit as much blood flow as possible, maintaining oxygen supply to these vital organs. Even so, when very low blood pressure persists (less than 50 mmHg), the brain’s function decreases. At this point, the brain sends fewer of the beneficial reflex signals to the heart and blood vessels. The final result is a continuous decline in blood pressure and death. Maintenance of brain blood flow is a very important factor in surviving shock.

Treatment and Therapy

Emergency procedures in response to circulatory shock entail contacting emergency service providers such as paramedics and keeping the victim warm and flat on his or her back, with slightly elevated legs. The victim must get medical attention immediately.

Treatment of shock can vary depending on the cause. In many cases, shock can be effectively treated with intravenous (IV) fluids and medication. Some cases require surgical intervention. In all cases, the status of body fluids must be monitored and treated. The body’s blood volume is one of the most important things to maintain.

It is particularly important to optimize blood volume in forms of hypovolemic shock. With hemorrhagic shock, whole blood is given intravenously to replace lost blood. In other forms of hypovolemic shock, different intravenous fluids are usually given. In burn shock, when the blood’s plasma weeps from the burn sites, blood plasma is the medicine of choice to restore lost volume. IV fluids are immediately started to restore lost volume until the needed blood products can be acquired. When hypovolemia is caused by excessive diarrhea, vomiting, or sweating, IV fluids are given to replenish lost volume.

In other cases of shock, special drugs are needed to alleviate the symptoms. Shock caused rapidly by a myocardial infarction, with cardiac arrest, can be immediately supported by cardiopulmonary resuscitation (CPR), provided by a trained individual. After resumption of the heartbeat, the heart can be helped with several types of drugs. In the case of sustained tachycardia, a drug such as amiodarone can lower the very rapid heart rate to normal, allowing the heart to fill properly and pump adequate blood. Cardiac tamponade must be corrected to allow the heart to pump usual amounts. If the onset is rapid, as when it is caused by chest trauma, then the fluid in the pericardial sac may need to be removed immediately. A needle is placed into the sac, and the excess fluid is removed to alleviate the pressure around the heart. If fluid accumulates slowly as with an infection and is recognized early, appropriate drug treatment for the infection may resolve the problem. In cases of shock in which acidosis is a complication or even a potential complication, sodium bicarbonate, a chemical that can reduce the acidity of blood, may be given. There is no universal treatment for the complex process of shock. For example, hemorrhage may become complicated by heart failure and/or septic shock. Each case must be treated in accordance with the patient’s existing conditions.

Perspective and Prospects

Chinese writings of more than three thousand years ago indicate that a connection exists between the heart and blood. Until the second century CE, it was thought that arteries carry air, not blood. Through the Middle Ages, it was believed that spirits are the essence of life or vitality. This belief encouraged bloodletting as a treatment for many ailments, including shock. Leeches were applied to remove the evil spirit causing the sickness. This was not a very successful mode of therapy.

It was not until the seventeenth century that blood transfusions were tried, with the first successful experiments conducted by Richard Lower. In the 1660s, Jean-Baptiste Denis administered lamb’s blood to a sixteen-year-old boy who was very weak and who had a high fever. The condition of the boy, who had been bled several times, improved for a short period of time. Others continued to experiment with transfusion as the remedy for loss of blood, but until the discovery of blood typing at the turn of the twentieth century, most attempts were of limited success.

The practice of giving transfusions greatly increased after the discovery of blood types in 1901 by Karl Landsteiner, who won the 1930 Nobel Prize in Physiology or Medicine for his work. By 1920, blood could be transfused from a bottle thanks to the work of Luis Agote, who in 1914 discovered that citrated blood will not clot after being removed from the body. Blood banking was established in the 1930s after Andre Bagdasarovin discovered in 1932 that citrated blood can be stored at forty degrees Fahrenheit (four degrees Celsius).

All types of shock are treated by determining and correcting the underlying cause. Oxygen is given during shock to improve the amount of circulating oxygen to the tissues. In hypovolemic shock, the goal is to improve the amount of circulating volume. This volume is replaced by intravenous fluids until blood products are available. Potent intravenous cardiac drugs that improve the contractions of the heart, improve blood pressure, or cause vasoconstriction may be used in cardiogenic and other types of shock. Mechanical devices have been developed (an intra-aortic balloon pump and a ventricular assist device) to use in severe cardiogenic shock that has not responded to traditional therapy. Synthetic epinephrine (adrenalin) is the first-line treatment for anaphylactic (allergic reaction) shock. It causes the smooth muscle in the bronchioles (tubes into the lungs) to relax and constricts blood vessels. Other treatments may include oxygen, antihistamines, and corticosteroids. Septic shock is treated with IV antibiotics and fluids. Shock is a dangerous process that can occur for a variety of reasons. Quick detection and medical treatment are required to prevent negative outcomes.

Bibliography

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