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Hypovolemic and Hemorrhagic 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 becomes dehydrated, there may be enough red blood cells, but the total volume of fluid is decreased, and pressure within the system decreases. 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 may ensue.
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 with diabetes who have diabetic ketoacidosis lose significant water because of because of elevated blood sugar that cause excess water to be excreted in the urine.
Ultimately in hypovolemic shock, the patient cannot replace the amount of fluid that was lost by drinking 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.
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 such as:
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 20% of their blood volume (about twice the amount that a person donates at a blood drive) without becoming symptomatic with weakness, lightheadedness, low blood pressure 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 able to replenish the red blood cells that were 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 as in hypovolemic shock.
Medically Reviewed by a Doctor on 5/24/2016
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