circulatory shock

Physiology

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Circulatory Shock
Circulatory shock:- means generalized inadequate blood flow through the body, to the extent that the body tissues are damaged because of too little flow, too little oxygen and other nutrients delivered to the tissue cells. .
Physiologic Causes of Shock
Circulatory Shock Caused by Decreased Cardiac Output
Two types of factors can severely reduce cardiac output:

1. Cardiac abnormalities that decrease the ability of the heart to pump blood. These include especially myocardial infarction but also toxic states of the heart, severe heart valve dysfunction, heart arrhythmias, and other conditions. The circulatory shock that results from diminished cardiac pumping ability is called cardiogenic shock. .

2. Factors that decrease venous return also decrease cardiac output . The most common cause of decreased venous return is diminished blood volume, but venous return can also be reduced as a result of decreased vascular tone, especially of the venous blood reservoirs, or obstruction to blood flow at some point in the circulation, especially in the venous return pathway to the heart.

shock is divided into the following three major stages:

1. A non progressive stage (sometimes called the compensated stage), in which the normal circulatory compensatory mechanisms eventually
cause full recovery without help from outside therapy.
2. A progressive stage, in which, without therapy, the shock becomes steadily worse until death.
3. An irreversible stage, in which the shock has progressed to such an extent that all forms of known therapy are inadequate to save the persons life.


Nonprogressive ShockCompensated Shock
If shock is not severe enough to cause its own progression, the person eventually recovers. It is also called compensated shock, meaning that the sympathetic reflexes and other factors compensate enough to prevent further deterioration of the circulation.
They include the following:
1. Baroreceptor reflexes, which elicit powerful sympathetic stimulation of the circulation.
2. Central nervous system ischemic response,
which elicits even more powerful sympathetic stimulation throughout the body but is not activated significantly until the arterial pressure falls below 50 mm Hg.
3. Reverse stress-relaxation of the circulatory system,
which causes the blood vessels to contract around the diminished blood volume, so that the blood volume that is available more adequately fills the circulation.
4. Formation of angiotensin by the kidneys, which constricts the peripheral arteries and also causes decreased output of water and salt by the kidneys, both of which help prevent progression of shock.
5. Formation of vasopressin (antidiuretic hormone) by the posterior pituitary gland, which constricts the peripheral arteries and veins and greatly increases water retention by the kidneys.
6. Compensatory mechanisms that return the blood volume back toward normal, including absorption of large quantities of fluid from the intestinal tract, absorption of fluid into the blood capillaries from the interstitial spaces of the body, conservation of water and salt by the kidneys, and increased thirst and increased appetite for salt, which make the person drink water and eat salty foods if able. The sympathetic reflexes provide immediate help toward bringing about recovery because they become maximally activated within 30 seconds to a minute after hemorrhage. The angiotensin and vasopressin mechanisms, as well as the reverse stress-relaxation that causes contraction of the blood vessels and venous reservoirs, all require 10 minutes to 1 hour to respond completely,
but they aid greatly in increasing the arterial pressure or increasing the circulatory filling pressure and thereby increasing the return of blood to the heart. Finally, readjustment of blood volume by absorption of fluid from the interstitial spaces and intestinal tract, as well as oral ingestion and absorption of additional quantities of water and salt, may require from 1 to 48 hours, but recovery eventually takes place, provided the shock does not become severe enough to enter the progressive stage.

Shock Caused by HypovolemiaHemorrhagic Shock

Hypovolemia means diminished blood volume. Hemorrhage is the most common cause of hypovolemic shock. Hemorrhage decreases the filling pressure of the circulation and, as a consequence, decreases venous return. As a result, the cardiac output falls below normal, and shock may ensue

Hypovolemic Shock Caused by Plasma Loss

Loss of plasma from the circulatory system, even without loss of red blood cells, can sometimes be severe enough to reduce the total blood volume markedly, causing typical hypovolemic shock similar in almost all details to that caused by hemorrhage. Severe plasma loss occurs in the following conditions:
1. Intestinal obstruction is often a cause of severely reduced plasma volume. Distention of the intestine in intestinal obstruction partly blocks
venous blood flow in the intestinal walls, which increases intestinal capillary pressure. This in turn causes fluid to leak from the capillaries into the intestinal walls and also into the intestinal lumen. Because the lost fluid has a high protein content, the result is reduced total blood plasma protein as well as reduced plasma volume.
2. In almost all patients who have severe burns or other denuding conditions of the skin, so much plasma is lost through the denuded skin areas that the plasma volume becomes markedly reduced. The hypovolemic shock that results from plasma loss has almost the same characteristics as the shock caused by hemorrhage, except for one additional complicating factor: the blood viscosity increases greatly as
a result of increased red blood cell concentration in the remaining blood, and this exacerbates the sluggishness of blood flow. Loss of fluid from all fluid compartments of the body is called dehydration; this, too, can reduce the blood volume and cause hypovolemic shock similar to that resulting from hemorrhage. Some of the causes of this type of shock are (1) excessive sweating, (2) fluid loss in sever diarrhea or vomiting (3)excess loss of fluid by nephrotic kidneys, (4) inadequate intake of fluid and electrolytes, or (5) destruction of the adrenal cortices, with loss of aldosterone secretion and consequent failure of the kidneys to reabsorb sodium, chloride, and water, which occurs in the absence of the adrenocortical hormone aldosterone.


Neurogenic ShockIncreased Vascular Capacity

Shock occasionally results without any loss of blood volume. Instead, the vascular capacity increases so much that even the normal amount of blood becomes incapable of filling the circulatory system adequately.
One of the major causes of this is sudden loss of vasomotor tone throughout the body, resulting especially in massive dilation of the veins. The resulting condition is known as neurogenic shock.

Causes of Neurogenic Shock

1. Deep general anesthesia often depresses the vasomotor center enough to cause vasomotor paralysis, with resulting neurogenic shock.
2. Spinal anesthesia, especially when this extends all the way up the spinal cord, blocks the sympathetic nervous outflow from the nervous system and can be a potent cause of neurogenic shock.
3. Brain damage is often a cause of vasomotor paralysis. Many patients who have had brain concussion or contusion of the basal regions of the brain develop profound neurogenic shock. Also, even though brain ischemia for a few minutes almost always causes extreme vasomotor stimulation, prolonged ischemia (lasting longer than 5 to 10 minutes) can cause the opposite effect total inactivation of the vasomotor neurons in the brain stem, with consequent development of severe neurogenic shock.

Anaphylactic Shock and Histamine Shock

Anaphylaxis is an allergic condition in which the cardiac output and arterial pressure often decrease drastically. It results primarily from an antigen-antibody reaction that takes place immediately after an antigen to which the person is sensitive enters the circulation. One of the principal effects is to cause the basophils in the blood and mast cells in the per capillary tissues to release histamine or a histamine-like substance.

The histamine causes

(1) an increase in vascular capacity because of venous dilation, thus causing a marked decrease in venous return;
(2) dilation of the arterioles, resulting in greatly reduced arterial pressure;
and
(3) greatly increased capillary permeability, with rapid loss of fluid and protein into the tissue spaces. The net effect is a great reduction in venous return and sometimes such serious shock that the person dies within minutes. Intravenous injection of large amounts of histamine
causes histamine shock, which has characteristics almost identical to those of anaphylactic shock.


Septic Shock
A condition that was formerly known by the popular name blood poisoning is now called septic shock by most clinicians. This refers to a bacterial infection widely disseminated to many areas of the body, with the infection being borne through the blood from one tissue to another and causing extensive damage. There are many varieties of septic shock because of the many types of bacterial infections that can cause it and
because infection in different parts of the body produces different effects.
Septic shock is extremely important to the clinician, because other than cardiogenic shock, septic shock is the most frequent cause of shock-related death in the modern hospital.
Some of the typical causes of septic shock include
the following:
1. Peritonitis caused by spread of infection from the uterus and fallopian tubes, sometimes resulting from instrumental abortion performed under
unsterile conditions.
2. Peritonitis resulting from rupture of the gastrointestinal system, sometimes caused by intestinal disease and sometimes by wounds.
3. Generalized bodily infection resulting from spread of a skin infection such as streptococcal or staphylococcal infection.
4. Generalized gangrenous infection resulting specifically from gas gangrene bacilli, spreading first through peripheral tissues and finally by way of the blood to the internal organs, especially the liver.
5. Infection spreading into the blood from the kidney or urinary tract, often caused by colon.








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