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Shock (circulatory)

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Shock (circulatory)

Shock
Classification and external resources
Specialty Cardiology, critical care medicine
ICD-10 R57
ICD-9-CM 785.50
DiseasesDB 12013
MedlinePlus 000039
eMedicine emerg/531 med/285 emerg/533
MeSH D012769

Circulatory shock, commonly known as shock, is a life-threatening perfusion (i.e.: low urine output, confusion, or loss of consciousness).

The shock index (SI), defined as heart rate divided by systolic blood pressure, is an accurate diagnostic measure that is more useful than hypotension and tachycardia in isolation.[3] Under normal conditions, a number between 0.5 and 0.8 is typically seen. Should that number increase, so does suspicion of an underlying state of shock. Blood pressure alone may not be a reliable sign for shock, as there are times when a person is in circulatory shock but has a stable blood pressure.[4]

Circulatory shock is not related to the emotional state of shock. Circulatory shock is a life-threatening medical emergency and one of the most common causes of death for critically ill people. Shock can have a variety of effects, all with similar outcomes, but all relate to a problem with the body's circulatory system. For example, shock may lead to hypoxemia (a lack of oxygen in arterial blood) or cardiac and/or respiratory arrest.[5]

One of the key dangers of shock is that it progresses by a positive feedback mechanism. Poor blood supply leads to cellular damage, which in turn triggers tissues around the body to become inflamed and inhibit perfusion around the body. Because of this, immediate treatment of shock is critical to the survival of the sufferer.[4]

Signs and symptoms

The presentation of shock is variable with some people having only minimal symptoms such as confusion and weakness.[2] While the general signs for all types of shock are low blood pressure, decreased urine output, and confusion, these may not always be present.[2] While a fast heart rate is common, those on β-blockers, those who are athletic and in 30% of cases those with shock due to intra abdominal bleeding may have a normal or slow heart rate.[6] Specific subtypes of shock may have additional symptoms.

Hypovolemic

Hemorrhage classes[7]
Class Blood loss Response Treatment
I <15 %(0.75 l) min. fast heart rate, normal blood pressure minimal
II 15-30 %(0.75-1.5 l) fast heart rate, min. low blood pressure intravenous fluids
III 30-40 %(1.5-2 l) very fast heart rate, low blood pressure, confusion fluids and packed RBCs
IV >40 %(>2 l) critical blood pressure and heart rate aggressive interventions

Hypovolemia is a direct loss of effective circulating blood volume leading to:

  • A rapid, weak, thready pulse due to decreased blood flow combined with tachycardia
  • Cool, clammy skin due to vasoconstriction and stimulation of vasoconstriction
  • Rapid and shallow breathing due to sympathetic nervous system stimulation and acidosis
  • Hypothermia due to decreased perfusion and evaporation of sweat
  • Thirst and dry mouth, due to fluid depletion
  • Cold and mottled skin (Livedo reticularis), especially extremities, due to insufficient perfusion of the skin

The severity of hemorrhagic shock can be graded on a 1-4 scale on the physical signs. This approximates to the effective loss of blood volume. The shock index (heart rate divided by systolic blood pressure) is a stronger predictor of the impact of blood loss than heart rate and blood pressure alone.[3] This relationship has not been well established in pregnancy-related bleeding.[8]

Cardiogenic

Symptoms of cardiogenic shock include:

Distributive

Systemic inflammatory response syndrome[9]
Finding Value
Temperature <36 °C (96.8 °F) or >38 °C (100.4 °F)
Heart rate >90/min
Respiratory rate >20/min or PaCO2<32 mmHg (4.3 kPa)
WBC <4x109/L (<4000/mm³), >12x109/L (>12,000/mm³), or 10% bands

Distributive shock includes infectious, anaphylactic, endocrine and neurogenic causes. The SIRS features typically occur in early septic shock.[2]

Obstructive

Obstructive shock includes Cardiac Tamponade, Pulmonary Embolism, Aortic Stenosis

Septic shock

Main manifestations are produced due to massive release of histamine which causes intense vasodilation. Patients with septic shock will also likely be positive for the SIRS Criteria. The most generally accepted treatment for these patients is early recognition of symptoms, and early administration of broad spectrum and organism specific antibiotics.[11]

Pathophysiology

Effects of inadequate perfusion on cell function.

There are four stages of shock. As it is a complex and continuous condition there is no sudden transition from one stage to the next.[12] At a cellular level shock is the process of oxygen demand becoming greater than oxygen supply.[2]

Initial

During this stage, the state of hypoperfusion causes hypoxia. Due to the lack of oxygen, the cells perform lactic acid fermentation. Since oxygen, the terminal electron acceptor in the electron transport chain is not abundant, this slows down entry of pyruvate into the Krebs cycle, resulting in its accumulation. Accumulating pyruvate is converted to lactate by lactate dehydrogenase and hence lactate accumulates (causing lactic acidosis).

Compensatory

This stage is characterised by the body employing physiological mechanisms, including neural, hormonal and bio-chemical mechanisms in an attempt to reverse the condition. As a result of the lungs and brain. The lack of blood to the renal system causes the characteristic low urine production. However the effects of the renin–angiotensin axis take time and are of little importance to the immediate homeostatic mediation of shock.

Progressive

Should the cause of the crisis not be successfully treated, the shock will proceed to the progressive stage and the compensatory mechanisms begin to fail. Due to the decreased perfusion of the cells, [10] If the bowel becomes sufficiently ischemic, bacteria may enter the blood stream, resulting in the increased complication of endotoxic shock.[4][10]

Refractory

At this stage, the vital organs have failed and the shock can no longer be reversed. Brain damage and cell death are occurring, and death will occur imminently. One of the primary reasons that shock is irreversible at this point is that much cellular ATP has been degraded into adenosine in the absence of oxygen as an electron receptor in the mitochondrial matrix. Adenosine easily perfuses out of cellular membranes into extracellular fluid, furthering capillary vasodilation, and then is transformed into uric acid. Because cells can only produce adenosine at a rate of about 2% of the cell's total need per hour, even restoring oxygen is futile at this point because there is no adenosine to phosphorylate into ATP.[4]

Diagnosis

The first changes seen in shock is an increased cardiac output followed by a decrease in mixed venous oxygen saturation (SmvO2) as measured in the pulmonary artery via a pulmonary artery catheter. Central venous oxygen saturation (ScvO2) as measured via a central line correlates well with SmvO2 and are easier to acquire. If shock progresses anaerobic metabolism will begin to occur with an increased blood lactic acid as the result. While many laboratory tests are typically performed there is no test that either makes or excludes the diagnosis. A chest X-ray or emergency department ultrasound may be useful to determine volume state.[2][6]

Differential diagnosis

Shock is a common end point of many medical conditions.[1] It has been divided into four main types based on the underlying cause: hypovolemic, distributive, cardiogenic and obstructive.[13] A few additional classifications are occasionally used including: endocrinologic shock.[1]

Hypovolemic

Hypovolemic shock is the most common type of shock and is caused by insufficient circulating volume.[2] Its primary cause is haemorrhage (internal and/or external), or loss of fluid from the circulation. Vomiting and diarrhea are the most common cause in children.[1] With other causes including burns, environmental exposure and excess urine loss due to diabetic ketoacidosis and diabetes insipidus.[1]

Cardiogenic

Cardiogenic shock is caused by the failure of the heart to pump effectively.[2] This can be due to damage to the heart muscle, most often from a large myocardial infarction. Other causes of cardiogenic shock include dysrhythmias, cardiomyopathy/myocarditis, congestive heart failure (CHF), contusio cordis, or cardiac valve problems.[1]

Obstructive

Obstructive shock is due to obstruction of blood flow outside of the heart.[2] Several conditions can result in this form of shock.

Distributive

Distributive shock is due to impaired utilization of oxygen and thus production of energy by the cell.[2] Examples of this form of shock are:

  • Septic shock is the most common cause of distributive shock.[1] Caused by an overwhelming systemic infection resulting in vasodilation leading to hypotension. Septic shock can be caused by Gram negative bacteria such as (among others) Escherichia coli, Proteus species, Klebsiella pneumoniae which release an endotoxin which produces adverse biochemical, immunological and occasionally neurological effects which are harmful to the body, and other Gram-positive cocci, such as pneumococci and streptococci, and certain fungi as well as Gram-positive bacterial toxins. Septic shock also includes some elements of cardiogenic shock. In 1992, the ACCP/SCCM Consensus Conference Committee defined septic shock: ". . .sepsis-induced hypotension (systolic blood pressure < 90 mmHg or a reduction of 40 mmHg from baseline) despite adequate fluid resuscitation along with the presence of perfusion abnormalities that may include, but are not limited to, lactic acidosis, oliguria, or an acute alteration in mental status. Patients who are receiving inotropic or vasopressor agents may have a normalized blood pressure at the time that perfusion abnormalities are identified."
  • Anaphylactic shock Caused by a severe anaphylactic reaction to an allergen, antigen, drug or foreign protein causing the release of histamine which causes widespread vasodilation, leading to hypotension and increased capillary permeability.
  • High spinal injuries may cause neurogenic shock.[14] The classic symptoms include a slow heartrate due to loss of cardiac sympathetic tone and warm skin due to dilation of the peripheral blood vessels.[14] (This term can be confused with spinal shock which is a recoverable loss of function of the spinal cord after injury and does not refer to the haemodynamic instability per se.)

Endocrine

Based on endocrine disturbances such as:

Management

The best evidence exists for the treatment of septic shock in adults and as the pathophysiology appears similar in children and other types of shock treatment this has been extrapolated to these areas.[1] Management may include securing the airway via intubation if necessary to decrease the work of breathing and for guarding against respiratory arrest. Oxygen supplementation, intravenous fluids, passive leg raising (not Trendelenburg position) should be started and blood transfusions added if blood loss is severe.[2] It is important to keep the person warm as well as adequately manage pain and anxiety as these can increase oxygen consumption.[2]

Fluids

Aggressive intravenous fluids are recommended in most types of shock (e.g. 1-2 liter normal saline bolus over 10 minutes or 20ml/kg in a child) which is usually instituted as the person is being further evaluated.[15] Which intravenous fluid is superior, colloids or crystalloids, remains undetermined.[2] Thus as crystalloids are less expensive they are recommended.[16] If the person remains in shock after initial resuscitation packed red blood cells should be administered to keep the hemoglobin greater than 100 gms/l.[2]

For those with haemorrhagic shock the current evidence supports limiting the use of fluids for penetrating thorax and abdominal injuries allowing mild hypotension to persist (known as permissive hypotension).[17] Targets include a mean arterial pressure of 60 mmHg, a systolic blood pressure of 70-90 mmHg,[2][18] or until their adequate mentation and peripheral pulses.[18]

Medications

Vasopressors may be used if blood pressure does not improve with fluids. There is no evidence of superiority of one vasopressor over another.[19] Vasopressors have not been found to improve outcomes when used for hemorrhagic shock from trauma[20] but may be of use in neurogenic shock.[14] Activated protein C (Xigris) while once aggressively promoted for the management of septic shock has been found not to improve survival and is associated with a number of complications.[21] The use of sodium bicarbonate is controversial as it has not been shown to improve outcomes.[22] If used at all it should only be considered if the pH is less than 7.0.[22]

Treatment goals

The goal of treatment is to achieve a urine output of greater than 0.5 ml/kg/h, a central venous pressure of 8-12 mmHg and a mean arterial pressure of 65-95 mmHg. In trauma the goal is to stop the bleeding which in many cases requires surgical interventions.

Epidemiology

Haemorrhagic shock occurs in about 1-2% of trauma cases[18] and affects about 1/3 patients in the intensive care unit (ICU).[23]

Prognosis

The prognosis of shock depends on the underlying cause and the nature and extent of concurrent problems. Hypovolemic, anaphylactic and neurogenic shock are readily treatable and respond well to medical therapy. Septic shock however, is a grave condition with a mortality rate between 30% and 50%. The prognosis of cardiogenic shock is even worse.[24]

History

In 1972 Hinshaw and Cox suggested the classification system for shock which is still used today.[24]

References

  1. ^ a b c d e f g h i j
  2. ^ a b c d e f g h i j k l m n o
  3. ^ a b
  4. ^ a b c d
  5. ^
  6. ^ a b
  7. ^
  8. ^
  9. ^
  10. ^ a b c d e f g Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson; & Mitchell, Richard N. (2007). Robbins Basic Pathology (8th ed.). Saunders Elsevier. pp. 102-103 ISBN 978-1-4160-2973-1
  11. ^
  12. ^
  13. ^
  14. ^ a b c
  15. ^
  16. ^
  17. ^
  18. ^ a b c
  19. ^
  20. ^
  21. ^
  22. ^ a b
  23. ^
  24. ^ a b

External links

  • SIRS, Sepsis and Septic Shock Criteria
  • ProCESS Trial information
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