Shock - Specific Types

Hypovolemic Shock

There needs to be enough red blood cells and water in the blood for the heart to push the fluids around within the blood vessels. When the body gets dehydrated, there may be enough red blood cells, but the total volume of fluid is decreased, and pressure within the system falls. Cardiac output is the amount of blood that the heart can pump out in one minute. It is calculated as the stroke volume (how much blood each heart beat can push out) multiplied by the heart rate (how fast the heart beats each minute). If there is less blood in the system to be pumped, the heart speeds up to try to keep its output steady.

Water makes up 90% of blood. If the body becomes dehydrated because water is lost or fluid intake is inadequate, the body tries to maintain cardiac output by making the heart beat faster. But as the fluid losses mount, the body's compensation mechanisms fail, and shock will occur.

Hypovolemic (hypo=low + volemic=volume) shock due to water loss can be the endpoint of many illnesses, but the common element is the lack of fluid within the body.

Gastroenteritis can cause significant water loss from vomiting and diarrhea, and is a common cause of death in third world countries. Heat exhaustion and heat stroke is caused by excessive water loss through sweating as the body tries to cool itself. Patients with infections can lose significant amounts of water from sweating. People in diabetic ketoacidosis lose significant water because of because of elevated blood sugars.

Ultimately in hypovolemic shock, the patient cannot replace the amount of fluid that was lost by taking in enough water, and the body is unable to maintain blood pressure and cardiac output. In all shock states, when cells start to malfunction waste products build up, a downward spiral of cell death begins, increased acidosis occurs, and a worsening body environment leads to further cell death - and ultimately organ failure.

Hemorrhagic Shock

A subset of hypovolemic shock occurs when there is significant bleeding that occurs relatively quickly. Trauma is the most common example of bleeding or hemorrhage, but bleeding can occur from medical conditions as well including:

• Bleeding from the gastrointestinal tract is common; examples include stomach or duodenal ulcers, colon cancers or diverticulitis.
  
  
• In women, excessive bleeding can occur from the uterus.
  
  
• People with cancers or leukemia have the potential to bleed spontaneously from a variety of sources if their bone marrow does not make enough clotting factors.
  
  
• Patients who are taking blood thinners (anticoagulant medications) can bleed excessively as well.

Blood loss has two effects on the body. First, there is a loss of volume within blood vessels to be pumped (see hypovolemic shock) and second, a reduced oxygen carrying capacity occurs because of the loss of red blood cells. Otherwise healthy people can lose up to 10% of their blood volume (about the amount that a person donates at a blood drive) without becoming symptomatic with weakness, lightheadedness, or shortness of breath.

The treatment of hemorrhagic shock depends on the cause. Finding and controlling the source of bleeding is of paramount importance. Intravenous fluids are used to help with resuscitation to increase the fluid volume within the blood vessel space, but blood transfusion is not always mandatory. If the bleeding is controlled and the patient becomes more stable, the bone marrow may be allowed to replenish the red blood cells lost.

If the red blood cell count in the blood decreases gradually over time, either because of bleeding or the inability of the body to make enough new red cells, the body can adjust to the lower levels to maintain adequate cell perfusion but the individual's exercise tolerance may decrease. This means that they may do well in normal daily activities but find that routine exercise or household activities bring on weakness or shortness of breath. The treatment depends on the underlying diagnosis, since it isn't a total fluid problem like in hypovolemic shock.

Cardiogenic Shock

When the heart loses its ability to pump blood to the rest of the body, blood pressure falls. Although there may be enough red blood cells and oxygen, they can't get to the cells that need them.

The heart is a muscle itself and needs blood supply to work. When a heart attack occurs, the blood supply to part of the heart is lost, and that can stun and irritate the heart muscle so that it isn't able to beat with an appropriate squeeze to push blood to the rest of the body. This decreases stroke volume, and cardiac output falls.

Treatment includes trying to restore blood supply and the use of medications to support blood pressure. In more dire circumstances, machines can be used to assist the heart to support blood pressure.

Neurogenic shock

There are involuntary muscles within blood vessel walls that maintain the squeeze so that the volume within the vessels stays constant even if the body changes position against gravity. Think of when you get up out of bed in the morning. If your blood vessels didn't squeeze a little tighter, gravity would make the blood flow to your feet, the lowest part of your body, away from your brain, and you might pass out. The squeeze is maintained by signals from nerves in the sympathetic trunk, a long bundle of fibers running from the skull to the tailbone alongside the vertebral column.

In brain or spinal injury, the sympathetic trunk stops working and blood vessels dilate and result in blood pooling away from the heart. Since there isn't enough blood returning to the heart, the heart has a hard time pumping blood around the body.

This situation looks like hypovolemic shock, since effectively there isn't enough fluid to fill the blood vessels, but because there isn't enough sympathetic tone or adrenaline release, the heart rate does not show a compensatory increase to increase cardiac output.

Treatment includes fluids and medications to increase the tone in the blood vessel walls.

Hypoglycemic Shock and Hyperglycemia

High or low blood sugars are almost associated with diabetes. In people with diabetes, the body does not make enough insulin to permit glucose to enter the cells for aerobic metabolism. As treatment, insulin needs to be injected, or medication needs to be taken to boost the body's lower insulin production. There must be a balance between how much medication is taken and how much food is eaten.

If not enough food is ingested, then the blood sugar drops (hypoglycemia) and no glucose is available to enter the cells, even if there is enough insulin to permit glucose to enter the cells. The brain is very susceptible to low blood sugars, and coma has a very quick onset. Treatment is providing sugar. If the person is awake enough to swallow, a sugar solution by mouth is used, otherwise, intravenous fluids containing glucose are provided. If the lack of sugar was of short duration, the person will awaken almost immediately after treatment. If blood sugars remain low for prolonged periods of time, the brain's ability to recover is potentially lost.

When blood sugar levels spiral high out of control, there is risk of significant dehydration (see below) and shock. If there is not enough insulin in the blood stream, cells cannot use the glucose that is present and instead turn to an alternative anaerobic metabolism to generate energy. Since glucose can't enter cells to be used, hyperglycemia (hyper= high + gly=sugar = emia) occurs as the glucose level build in the blood stream. The kidneys try to excrete excess sugar, but because of chemical concentration gradients between blood and urine, significant amounts of water are lost as well. The body quickly becomes dehydrated and blood pressure drops, decreasing blood flow to cells. Cells which are now lacking glucose inside them are now starved of oxygen and turn to anaerobic metabolism, causing acid waste product build up. Excess acid in the body changes the metabolism for all organs, making it more difficult for oxygen to be used. This downward spiral will continue until insulin and significant fluids are given.