Physiology
Lec.(5) Dr.Rafah Sami--------------------------------------------------------------------
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 eventuallycause 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 ensueHypovolemic 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.
SYNCOPE is a medical term for a blackout that is caused by a sudden lack of blood supply to the brain. This is very common, and occurs in roughly half of all individuals during their lives. There are many causes of Syncope, but the most common by far is 'Vasovagal Syncope' This is also known as the 'common faint'. Fainting may be simple, with a typical warning, going pale with a gentle collapse to the ground, a brief period of unconsciousness, and a prompt recovery. However, fainting may also be complex. There may be no warning, there may be jerking of the limbs and even incontinence, (accidental wetting), and some people may take quite a while to come around. To a patient, it may be very traumatic, and can be triggered by having blood drawn, Complex fainting may appear just like a "seizure" or "fit". However, this can occur when there is a sudden fall in blood flow to the brain, it does not have to be due to epilepsy. Blackouts due to epilepsy occur when brain cells suddenly activate chaotically, but usually the blood flow remains normal. Furthermore, while Syncope affects 50% of people, epilepsy affects about 1%. It is very important to make sure that Syncope and epilepsy are distinguished from each other.
" In Vasovagal Syncope, the heart ratedrops or may even stop for several seconds. (bradycardia) and the blood pressure drops sharply (hypotension). This reflex mechanism is present in all of us, but in a more exaggerated form in some individuals. In effect, the reflex has shut down the body's blood circulation. When this happens insufficient oxygenated blood is pumped to the brain leading to dizziness and fainting. This odd reflex can be triggered by many different stimuli. Seeing blood is a common one, but simply standing immobile may also trigger a Syncope. Fainting can also be triggered in situations that make people anxious, stressed or emotional. Being in a overly warm environment, not having eaten or not having drunk sufficient liquids are all factors that increase the chances of setting the reflex in motion. Some patients do not take enough salt in their food
Typical triggers for vasovagal episodes include:
Prolonged standing or upright sitting, particularly when standing with legs in a locked position for long periods of timeavoidance of long-term locking of one's legs in the standing position is taught in the military
Standing up very quickly
HYPERLINK "http://en.wikipedia.org/wiki/Stress_%28medicine%29" \o "Stress (medicine)" Stress
Any painful or unpleasant stimuli, such as:
HYPERLINK "http://en.wikipedia.org/wiki/Venepuncture" \o "Venepuncture" Venepuncture
Experiencing medical procedures with local anesthesia
Giving or receiving a needle immunization
Watching someone give blood
Watching someone experience pain
Watching or experiencing medical procedures
Sight of blood
Occasions of slight discomfort, such as dental and eye examinations
HYPERLINK "http://en.wikipedia.org/wiki/Hyperthermia" \o "Hyperthermia" Hyperthermia, a prolonged exposure to heat
High temperature, either in the environment or due to exercise
High pressure on or around the chest area after heavy exercise
Arousal or stimulants e.g. sex
Sudden onset of extreme emotions
Hunger
Nausea or vomiting
HYPERLINK "http://en.wikipedia.org/wiki/Dehydration" \o "Dehydration" Dehydration
HYPERLINK "http://en.wikipedia.org/wiki/Urination" \o "Urination" Urination (' HYPERLINK "http://en.wikipedia.org/wiki/Micturition_syncope" \o "Micturition syncope" micturition syncope') or HYPERLINK "http://en.wikipedia.org/wiki/Defecation" \o "Defecation" defecation, having a bowel movement ('defecation syncope')
Abdominal straining or 'bearing down'
Swallowing ('swallowing syncope') or coughing ('cough syncope')
Random onsets due to nerve malfunctions
Pressing upon certain places on the throat, sinuses, and eyes, also known as vagal reflex stimulation when performed clinically
Water colder than 10 Celsius (50 F), or ice that comes in contact with the face.
High altitude
Use of certain drugs that affect blood pressure, such as HYPERLINK "http://en.wikipedia.org/wiki/Amphetamine" \o "Amphetamine" amphetamine
Intense laughter .
Pathophysiology
Regardless of the trigger, the mechanism of syncope is similar in the various vasovagal syncope syndromes. In it, the HYPERLINK "http://en.wikipedia.org/wiki/Nucleus_tractus_solitarius" \o "Nucleus tractus solitarius" nucleus tractus solitarius of the HYPERLINK "http://en.wikipedia.org/wiki/Brainstem" \o "Brainstem" brainstem is activated directly or indirectly by the triggering stimulus, resulting in simultaneous enhancement of HYPERLINK "http://en.wikipedia.org/wiki/Parasympathetic_nervous_system" \o "Parasympathetic nervous system" parasympathetic nervous system ( HYPERLINK "http://en.wikipedia.org/wiki/Vagal" \o "Vagal" vagal) tone and withdrawal of HYPERLINK "http://en.wikipedia.org/wiki/Sympathetic_nervous_system" \o "Sympathetic nervous system" sympathetic nervous system tone.
This results in a spectrum of hemodynamic responses:
On one end of the spectrum is the cardioinhibitory response, characterized by a drop in heart rate (negative chronotropic effect) and in contractility (negative inotropic effect) leading to a decrease in cardiac output that is significant enough to result in a loss of consciousness. It is thought that this response results primarily from enhancement in HYPERLINK "http://en.wikipedia.org/wiki/Parasympathetic_tone" \o "Parasympathetic tone" parasympathetic tone.
On the other end of the spectrum is the vasodepressor response, caused by a drop in blood pressure without much change in heart rate. This phenomenon occurs due to HYPERLINK "http://en.wikipedia.org/wiki/Vasodilation" \o "Vasodilation" vasodilation, probably as a result of withdrawal of HYPERLINK "http://en.wikipedia.org/wiki/Sympathetic_nervous_system" \o "Sympathetic nervous system" sympathetic nervous system tone.
The majority of people with vasovagal syncope have a mixed response somewhere between these two ends of the spectrum..
Treatment
Treatment for vasovagal syncope focuses on avoidance of triggers, restoring blood flow to the brain during an impending episode, and measures that interrupt or prevent the HYPERLINK "file:///C:\\Documents%20and%20Settings\\MAF%20SYSTEM\\Desktop\\Vasovagal_response.htm" \l "Pathophysiology_and_mechanism" pathophysiologic mechanism described above.
The cornerstone of treatment is avoidance of triggers known to cause syncope in that person. However, new development in psychological research has shown that patients show great reductions in vasovagal syncope through exposure-based exercises with therapists
Because vasovagal syncope causes a decrease in blood pressure, relaxing the entire body as a mode of avoidance isn't favorableA patient can cross his/her legs and tighten leg muscles to keep blood pressure from dropping so drastically before an injection.
Before known triggering events, the patient may increase consumption of salt and fluids to increase blood volume. Sports and energy drinks may be particularly helpful.
Discontinuation of medications known to lower blood pressure may be helpful, but stopping HYPERLINK "http://en.wikipedia.org/wiki/Antihypertensive_drugs" \o "Antihypertensive drugs" antihypertensive drugs can also be dangerous. This process should be managed by an expert.
Patients should be educated on how to respond to further episodes of syncope, especially if they experience HYPERLINK "http://en.wikipedia.org/wiki/Prodrome" \o "Prodrome" prodromal warning signs: they should lie down and raise their legs; or at least lower their head to increase blood flow to the brain. If the individual has lost consciousness, he or she should be laid down with his or her head turned to the side. Tight clothing should be loosened. If the inciting factor is known, it should be removed if possible (for instance, the cause of pain).
Wearing graded compression stockings may be helpful.
There are certain orthostatic training exercises which have been proven to improve symptoms in people with recurrent vasovagal syncope.
Certain HYPERLINK "http://en.wikipedia.org/wiki/Medications" \o "Medications" medications may also be helpful:
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