Lect. 16
Shock
Objectives:
1. Define the term hypotensive shock and give three possible causes.
2. Summarize the cardiovascular reflexes elicited following the
sudden loss of 1 liter of blood.(appearance, cvexam (pulse, Bp )
PCV
3. Explain the mechanisms that help compensate for circulatory
shock. …( inc.h.r., stroke v . vasoconstriction samky =digestive t.,
aldosterone, ADH
Shock is a clinical syndrome of circulatory failure characterized by:
Low cardiac output.
Hypotension.
Inadequate tissue perfusion.
Causes and types:
According to the underlying cause, four main types of shock can be
identified.
1-Hypovolaemic shock
This occurs as a result of severe reduction of the blood volume. Its
manifestations are those of severe hemorrhage. It is called cold shock
because the skin of the patient is cold due to severe cutaneous
vasoconstriction.
Causes and types:
1. Hemorrhagic shock: could occur in severe hemorrhage; bleeding.
2. Traumatic shock (severe trauma): This may be complicated by
precipitation of myoglobin (from the crushed muscles) in the renal
tubules resulting in renal shutdown; kidney damage and anuria
(crush syndrome).
3. Burn shock (extensive burns): It is a hypovolemic shock which
follow extensive burns. It is mainly due to loss of large amounts of
plasma from the burned areas (in burn there is loss of fluid and
protein).
4. Surgical shock (major surgery): It is a hypovolemic shock which
follows surgical operation (it is due to external and or internal
bleeding).
5. Dehydration (e.g. in severe diarrhea, and Addison's disease). It is
due to loss of large amount of Na
+
in the urine or feces with the
loss of water leading again to hypovolemic shock.
2-Distributive shock
This type of shock is caused by widespread vasodilation (VD) leading
to state of hypovolemia despite that the blood volume is normal. In
other words, 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 manif-
estations are generally similar to those of hypovolemic shock, but the
skin is warm due to VD (hence the name, warm shock). Examples:
a- Neurogenic shock
This is a distributive shock which occurs on receiving sudden,
shocking news or with strong emotions as extreme fear, grief or severe
pain. Shocking news may cause failure of the sympathetic tone, i.e.,
there is loss (sudden withdrawal) of the sympathetic vasoconstrictor
tone which results in widespread arterial and venous vasodilation.
Venous return is reduced as blood pools in the venous system, leading
to a drop in cardiac output and hypotension with resultant reduced
cerebral blood flow that leads to fainting (a prolonged syncope; loss of
consciousness due to cerebral ischemia). Neurogenic shock also could
occur in case of upper spinal cord damage or spinal anesthesia.
b- 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, e.g.
reexposure to an injection of penicillin. The resulting antigen-antibody
reaction causes the release of large amounts of histamine from mast and
basophiles which produces massive vasodilation and increased vascular
permeability with edema that lead to the development of shock in
addition to bronchospsm and laryngeal edema.
c- 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 polymorphonuclear leucocytes
and tissue macrophages to secrete many vasodilator cytokines (especially
interlukin-1, tumor necrosis factor (TNF)). These substances produce
massive vasodilation that lead 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. pericardial effusion (tamponade; 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) Large pneumothorax.
(b) Massive pulmonary embolism.
(c) Cardiac tumor.
(d) Cardiac tamponade.
Danger of shock… How does it cause death?
Severe 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), for example:
Hypotension→ cerebral ischemia→ depression of the VCC→ VD
and bradycardia → more hypotension (and so on till death).
Hypotension→ myocardial ischemia→ low cardiac output→ more
hypotension, and so on till death (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
microembolism by thrombi formed by coagulant agents released
from the damaged cells (acute or adult respiratory distress
syndrome, ARDS).
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