Shock is often a misunderstood medical condition. It is far more complex than the average lay person and sometimes medical person understands. However it can be made simpler to understand. In the following pages shock will be discussed in depth. Shock will be discussed in its generalized syndrome with explanations of its etiology and pathophysiology. Following the generalized explanation, the five different types of shock will be reviewed in much the same manner. Let us first look at shock in general terms.
Shock is complex by its very nature. The pathophysiology of shock many times involves the multiple failures of organs and even death. Shock at its most basic level is the lack of equilibrium between the cellís need for oxygen and the bodyís inability to provide that cellular oxygen. This imbalance can have dire effects as mentioned above. The bodyís response to shock usually occurs in stages. These stages are initial, compensatory, progressive and refractory. The stages are a process that is very individualized. A patient may advance in different speeds through the stages depending on pre-existing conditions/disease processes as well as concomitant illnesses or injuries that could also complicate and exacerbate the process. The etiology of shock will be explained next.
The etiology of shock can be classified in three categories with one category having three separate subcategories.
The three main categories are hypovolemic, cardiogenic and distributive shock. Distributive shock can be further delineated with septic, neurogenic and anaphylactic shock. All five of these types of shock will be reviewed in the following pages. Next the pathophysiology of shock will be covered.
The pathophysiology of shock can be easily explained using the stages of shock as a starting point. The first stage of shock is called the initial stage. Here the cardiac output is seen to decrease, causing an impairment of the tissue perfusion. This action trips the cell to convert, because of lack of sufficient oxygen, from aerobic to anaerobic metabolism. The mechanism of anaerobic metabolism causes a sudden increase in waste products called lactic acid that further injures the cells which causes further cell death in a cyclical fashion. After the body enters the initial stage if the problem is not solved than shock will progress to the compensatory stage.
The compensatory stage is the stage where the body tries to compensate for the lack of equilibrium. In this stage the body will alter its hemodynamic functions to compensate for poor tissue perfusion. The heart rate will increase, the vessels of the body will begin constricting and the body will begin to retain sodium and water. Additionally as the stage progresses the blood glucose levels will begin to rise and the respiratory rate will increase (hyperventilation) in an attempt to blow off the effects of lactic acidosis which we found in the initial stage when the body switched over from aerobic to anaerobic metabolism. These changes set the stage for the progressive stage.
The progressive stage is characterized by the beginning of failure of the compensatory stage to bring the body back to equilibrium. The failure of the compensatory stage signals the beginning of the shock cycle being perpetuated. Basically, the cycle begins to self-perpetuate a downward spiral which can end in death. During this stage the cells are functioning on anaerobic metabolism which is causing a buildup of lactic acid. Anaerobic metabolism does not produce enough energy to sustain cellular life and cells begin to die.
Those cells that survive begin to see a failure of the sodium-potassium pump which causes the cells to begin to swell which if left unchecked will cause the cells to rupture and die. At this point even if you could give the cells more oxygen, it would be futile because they couldnít process it and use it. This affects every organ of the body and all of the bodyís processes. If left unchecked the progressive stage will cycle into the refractory stage.
The refractory stage is the stage where the patient will reach a point of no return. This point is where no heroic measures will save the patient. This is because every organ needs its basic building block, the cell, to survive. If the cells of an organ die then the organ dies. Once one of the organs die then others will follow suit and MODS or multiple organ failure will occur ending in death. In addition to the stages of shock, there are five types of shock.
Stages of shock reflect a process. Types of shock refer to different initial mechanisms of action that trip those stages. The first type is hypovolemic shock.
Hypovolemic shock is the most common type of shock. It is the type of shock that occurs from a lack of sufficient fluid in the intravascular space. The etiology of this is that the hypovolemic shock can occur one of two ways.
The first way is an external loss of body fluid such as blood or plasma. An example of this is a laceration to an artery that is not stopped or does not stop on its own. Another form of hypovolemic shock is when fluid in the body is moved to an area where it is not used such as what is called third spacing. An example of extracellular fluid loss is severe sodium deficiency. The pathophysiology of hypovolemic shock is that when fluid volume goes down a decrease in the circulating volume of blood is seen. When the circulating volume of blood occurs the preload to the heart is decreased. A decrease in preload causes a decrease in stroke volume which will cause a decrease in the cardiac output. With reduced cardiac output you will see decreased cellular oxygen perfusion. When cells donít receive enough oxygen they die. In addition to hypovolemic shock there is cardiogenic shock.
Cardiogenic shock is where the heart is unable to pump forward the amount of blood in one stroke to support life.
This can occur for several reasons. The etiology of cardiogenic shock is shown to be one of several problems that ultimately effect cardiac output. These examples include ischemia of the left ventricle, structural problems and dysrhythmias. Anything that hinders the flow of blood out of the heart can cause cardiogenic shock. When blood flow out of the heart is decreased, you will see also a decrease in oxygen availability to the cells. This decrease in oxygen available to the cell will cause the cells to switch over to anaerobic metabolism and the whole cycle of shock is started. In addition to hypovolemic and cardiogenic shock is distributive shock.
Distributive shock is a condition where the flow of blood is not evenly distributed. It is actually an umbrella for three other forms of shock. They are septic, anaphylactic and neurogenic shock. All of these are forms of distributive shock. Anaphylactic shock will be discussed first.
Anaphylactic shock is one of the more known types of shock because we hear horror stories all the time about how somebodyís child was given peanuts by some well meaning adult and come to find out the child is deathly allergic to nuts. Anaphylactic shock is when the bodyís antibody-antigen response is triggered by something the person is allergic to. It can happen upon the first exposure or after several exposures to a substance. The pathophysiology of anaphylactic shock is that when the antibody-antigen response is triggered they in turn trigger the release of biochemical mediators. These mediators have a two-fold effect. First vasoconstriction of the airways occurs making it hard to breath. A decrease in available oxygen to the lungs causes a decrease in available oxygen to the rest of the body. Once the bodyís cells start to receive not enough oxygen the cellular death cycle will begin. Additionally vasodilatation of the peripheral vessels occurs. This causes a decrease in preload volume to the heart. The decreased preload to the heart causes a decreased stroke volume which causes a decreased cardiac output. A decrease in cardiac out put ultimately causes a decrease in cellular oxygenation which will also begin the cellular death cycle. In addition to anaphylactic shock another form of distributive shock is neurogenic shock.
Neurogenic shock is the type of distributive shock that is caused by the suppression or outright loss of sympathetic tone. Neurogenic shock is the rarest form of shock. The etiology is anything which causes any disruption of the sympathetic nervous system. Some examples of this are spinal injury, spinal anesthesia, drugs and emotional stress to name a few.
The pathophysiology or how this occurs is that with sympathetic nervous system depression or loss it creates a vasodilatation of the extremities secondary to loss of muscle tone. This vasodilatation causes a decrease in cardiac preload and as explained before, this will ultimately cause the less than adequate oxygenation of cells which will trigger shock. In addition to neurogenic shock there is yet another form of distributive shock.
Sepsis is another form of distributive shock. Sepsis is, as most people know, a really bad infection that can kill you. However, most people donít understand how sepsis can kill you. Sepsis is an invasion of the body by some form of bacteria. Sepsis can be further divided into bacteremia or the presence of bacteria in the blood, sepsis which is the bodyís response to bacteria in the blood such as fever and increased respiratory rate, and lastly septic shock which is sepsis plus hypotension in spite of fluid resuscitation. In general terms, sepsis is the invasion of the body by bacteria which causes an immune response. The resulting fallout is that tissue perfusion is impaired and the cycle of shock is begun once again at the cellular level. The classic example of septic shock is TSS or toxic shock syndrome which is an invasion of the body by a toxin producing gram-positive bacteria. Once shock is understood, treatment is the next order of business.
The treatment of shock is determined by several factors. One factor to consider is which form of shock the patient is suffering and the other factor is in what stage this particular form of shock is occurring. The treatment of shock can be spoken of in general terms.
The basic idea for the treatment of shock is to improve and maintain tissue perfusion. This can be accomplished by several avenues. The first avenue is increase oxygenation. This can be accomplished by several ways.
The patient can be placed on oxygen which will increase the availability of oxygen to the lungs or the portion of the lungs still functioning. The second idea is to place the patient on a ventilator with oxygen. This will again increase the availability of oxygen as well as reduce the patientís workload to get oxygen. The third way is to do chest physiotherapy. The cupped hand pounding on the patientís thoracic walls stimulates the movement of fluid, clearing a path for the oxygen to get to the alveolar sacs. Besides increasing oxygenation helping the patient maintain cardiac output is also important.
It doesnít matter how much oxygen gets into the blood stream if the blood stream doesnít get to the tissues. There are available now a multitude of cardiac drugs that help with the heart cycle. Whether it is a low cardiac output, a low stroke volume or low pre-load there is usually some drug or combination of drugs that can be utilized to help improve it. In addition to helping with cardiac output, fluid volume can also be a source of resuscitation.
Fluid administration can increase cardiac pre-load as well as help with overall perfusion. There are two types of fluids. One is crystalloids such as normal saline, D5W, and lactated ringers. The other is colloids such as whole blood, plasma and hetastarch. While colloids might be preferred in some situations because of affinity to intravascular restoration, the down side is cost, potential for allergic reaction and speed or lack of speed in type and cross matching in an emergency.
In conclusion, while shock as a condition might be at first confusing it can be broken down and explained in simple terms. There are five basic forms of shock and they include neurogenic, septic, anaphylactic, hypovolemic, and cardiogenic. The basic treatment is oxygen and respiratory support, fluid support, and pharmacological (drugs) support.
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