Shock Shock is a clinical syndrome of circulatory failure characterized by: • Low cardiac output. • Hypotension. • inadequate tissue perfusion Causes and types



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Shock

Shock is a clinical syndrome of circulatory failure characterized by:

• Low cardiac output.

• Hypotension.

inadequate tissue perfusion

Causes and types:

Shock occurs as a result of many disorders, and according to the underlying cause, four main types of shock can be identified.



1-Hypovolsemic shock

This occurs as a result of severe reduction of the blood volume. Its manifestations are those of severe haemontage It is called cold shock because the skin of the patient is cold due to severe cutaneous vasoconstriction.



Causes and types:

1. Hemorrhagic shock (severe haernorrhage)

2. Traumatic shock (severe trauma): This may be complicated by precp1tation of myoglobin (from the crushed muscles) in the renal tubules resulting in renal shutdown, kidney damage and anuria (crush syndrome)

3. Burn shock (extensive bums): It is a hypovolemic shock which follow extensive burns it is mainly due to loss of large amounts of plasma from the burned areas.

4. Surgical shock (major surgery): It is a hypovolemic shock which follows surgical operation (it is due to external and or internal hemorrhage)

5. Dehydration (e g in severe diarrhea) it is due to lass of large amount of Na+ in the urine or feces with the loss of water leading again to hypovolemic shock.



2-flistribntfre shock

This type of shock is caused by widespread vasodilation (VD) which markedly increases the capacity of the vascular system. The blood volume is normal but the cardiac output is maldistributed to different organs where more blood goes to the inactive abdominal viscera, skeletal muscles and skin and less blood goes to the active, vital organs especially the heart and brain which become under-perfused its manifestations are generally similar to those of hypovolemic shock, but the skin is warm due to VD (hence the name warm shock). Examples:



Neurogenic shock

This is a distributive shock which occurs on receiving sudden, shocking news strong emotions as extreme fear, grief or se ere pan in these cases, shocking news may cause failure of the sympathetic tone i e there is sudden withdrawal of the sympathetic vasomotor tone which results in widespread vasodilation. The decrease in cerebral blood flow leads to fainting (a= a prolonged syncope loss of consciousness due to cerebral ischemia).

A rapid fall in the blood pressure (B.P) also occurs in neurogenic shock in which the sympathetic tone is decreased, usually because of upper spinal card damage or spinal anesthesia.

Anaphylactic shock:

This is a distributive shock caused by a severe allergic reaction to an antigen to which the subject was previously exposed, and sensitized, eg. an injection of penicillin. The resulting antigen-antibody reaction causes the release of large amounts of histamine which produces massive vasodilation.



Septic shock:

This is a distributive shock produced by the invasion of the blood stream by bacteria or their toxins; especially the gram-negative bacteria which release an endotoxin that stimulates the polymorph nuclear leucocytes and tissue macrophages to secrete many VD cytokines (especially interleukin-i. tumor necrosis factor (TNT)). These substances produce massive vasodilation leading to shock.



3-Cardiogenic shock (Congested shock)

This occurs as a result of inadequate pumping action of the heart (severe depression of myocardial contractility) which leads to reduction of the cardiac output and arterial BP (systolic pressure fails below 80 mmHg, the central venous pressure (right atrial pressure) is elevated above 18 mmHg (congestion).

Its manifestations are similar to those of hypovolemic shock plus congestion of the lungs and viscera due to failure of’ the heart to pump all the venous blood returned to it (hence the name, congested shock).

Causes:

1- Extensive myocardial infarction involving the left ventricle.

2- Acute myocarditis.

3. Heart failure.

4. Severe ventricular arrhythmia.

4-Obstructive shock

It is caused by marked restriction of the diastolic filling of the ventricles, e.g. by pericardial tamponed (compression of. the heart by accumulation of excess fluid or blood in the pericardial sac) or massive pulmonary embolism. The cardiac output is markedly reduced leading to hypotension and shock.



Causes:

(a) A large pneumothorax.

(b) Massive pulmonary embolism.

(c) A cardiac tumor.



Danger of shock.. How does it cause death?

Serve shock may be fatal if not rapidly and properly treated, especially if becoming irreversible. Death occurs as a result of development of multiple positive feedback cycles for death cycles), for example:

• Hypotension → cerebral ischemia depression of the VCC → VU and bradycardia → more hypotension (and so on till death occurs).

• Hypotension → myocardial ischemia → low cardiac output → more hypotension. and so on till death occurs (cardiac damage may be so severe that the cardiac output is not restored to normal even if the blood volume is increased):

• A late cause of death is pulmonary damage due to pulmonary micro embolism by thrombi formed by coagulant agents released from the damaged cells (acute or adult respiratory distress syndrome, ARDS).

Fainting (syncope)

It is a sudden transient loss of consciousness. it usually results from cerebral ischemia; it is often due to cardiovascular abnormalities including either venous pooling or reduced cardiac output. A person ho has fainted typically exhibits shallow breathing, a weak pulse, and low blood pressure.



Etiology or Types:

1) Vasovagal syncope: due to hypotension produced as a result of sudden

vasodilation associated with bradcardia.

• Psychophysiological; this commonly occurs in strong emotions.

Reduction in venous return; by the increased intrathoracic pressure, this reduces the cardiac output leading to hypotension, like;

1. Micturition syncope: hypotension; in addition to reflex bradycardia induced by. voiding urine.

2. Defecation syncope.

3. Cough syncope.

4. Valsalva maneuver.

2) Orthostatic syncope or hypotension: it means a rapid hypotension that occurs on sudden standing. It is accompanied by dimness of vision, dizziness arid even fainting. It results on failure of the baroreflex to compensate for the sudden downward gravitational pull on the blood. On standing from supine position, the effect of gravity leads to pooling of blood in the lower part of the body → decrease in venous return → decrease in cardiac output → fall in arterial blood pressure (Orthostatic hypotension) → decreased cerebral blood flow → brain ischemia- syncope (Orthostatic syncope or fainting).. it may occur in normal persons, in hypovolemic, idiopathic, and in patients with diseases that damage the sympathetic nervous system e.g., diabetes. and syphilis, and in primary autonomic insufficiency (diabetes; decreased production of catecholamines due to Dopamine B-hydroxylase enzyme deficiency).

3) Carotide sinus syncope. carotide sinus syndrome.

4) Neurocardiogenic syncope; reduced cardiac output;

• syncope due to heart block or sinus arrest.

• syncope may also occur in severe arrhythmias (tachycardia more than 160 beat/minute or in bradycardia less than 40 beat/minute).

• Myocardial infarction with pump failure.

• Valve diseases (aortic stenosis. mitral stenosis).

• long QT syndrome.

• cardiomyopathy (HOCM),

• Congestive heart failure.

5) Deglutition syncope: due to VD and bradycardia induced by swallowing.



6) Effort syncope: common in patients having aortic or pulmonary stenosis.


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