Definition
Circulatory shock is a state of inadequate tissue perfusion needed for the normal cellular respiration. In other words
shock results from inadequate perfusion of the body cells with oxygenated
blood.
Pathophysiology of Shock
When the delivery of oxygen and glucose is low, cellular metabolism
changes from aerobic to anaerobic type. The product of aerobic respiration is carbon
dioxide and that of anaerobic respiration is lactic acid. Accumulation of lactic acid due to anaerobic respiration results
in metabolic
acidosis.
Whenever there
is metabolic acidosis, body tries to compensate it by hyperventilation
producing respiratory alkalosis. Inadequate perfusion results in baroreceptor
stimulation in the cardiovascular system. This causes release of catecholamines and
thereby sympathetic over activity. It is the cause of rapid pulse in shock.
Decreased
renal perfusion leads to decreased glomerular filtration and low urine output.Anti Diuretic
Hormone production is also increased due
to baroreceptor stimulation . ADH causes increased sodium and water re absorption
which further reduces urine output.
Activation of
the Renin–angiotensin–aldosterone axis causes peripheral vaso- constriction. This is the reason for the clinical
manifestation of cold extremities in shock.
Ischemia of the
intestine results in destruction of the mucosal barrier leading to bacterial translocation from the
lumen of the intestine to the systemic circulation.
Hypoxia and acidosis in turn activates the inflammatory and complement
cascade in the body resulting in the generation of oxygen free radicals and inflammatory
mediators. Due to the action of these mediators vascular endothelial damage
occurs and tissue oedema forms. Tissue oedema in turn causes tissue hypoxia.
Cessation of anaerobic metabolism causes failure of the Na/ K+
pumps and cell lysis. As a result intracellular K+ is released into the
bloodstream producing the electrolyte abnormality called hyperkalemia.
When the normal circulation is regained the inflammatory
mediators will be flushed away and can cause endothelial injury to lungs and kidneys.
Ultimately this can result in multi organ failure. Such an injury is known as Re-perfusion Injury.
Types of Shock
¨ Hypovolaemic
¨ Cardiogenic
¨ Obstructive
¨ Distributive
¨ Endocrine
Hypovolaemic
Shock
Caused by a reduced circulating volume and is the most common form of Shock. Two types of
Hypovolaemic Shock are Haemorrhagic or Non-haemorrhagic. Haemorrhagic shock is
due to the excessive blood loss.
Non-haemorrhagic causes of Hypovolaemic Shock
Dehydration
–
Poor fluid intake, Vomiting,
Diarrhoea
Urinary loss
(Polyuria -diabetes)
Evaporation- Heat
exhaustion
Third-space loss
Third space compartment is extra cellular space which is not normally
perfused with fluids. E.g. Peritoneal cavity, Pleural space etc.The fluid is re
distributed from ECF and is not available to the intravascular space. Hence
fluid sequestration in Third Space can cause Hypovolemia. Third space loss
occurs in –Peritonitis, Intestinal Obstruction, Pancreatitis and Burns.
Cardiogenic
Shock
Is
due to primary failure of the heart to pump blood
Causes of cardiogenic shock –
Myocardial infarction
Cardiac dysrhythmias
Valvular heart disease
Blunt myocardial injury
Cardiomyopathy
Obstructive
Shock
Due to mechanical
obstruction of cardiac filling
Resulting in a fall in
Cardiac Output
Common causes
Cardiac
tamponade
Tension
pneumothorax
Massive pulmonary embolus
Distributive
shock/ Septic Shock
Is due to generalized vasodilatation and low systemic vascular
resistance
Can occur in
Anaphylaxis-histamine release
Septic
shock-endotoxins
High spinal
cord injury
Endocrine
Shock
Is a combination of hypovolaemic, cardiogenic and distributive
shock.
Causes are
Hypothyroidism – due to
decreased responsiveness to catecholamines
Hyperthyroidism - due
to highoutput cardiac failure.
Adrenal insufficiency
- due to hypovolaemia
Phases of
Shock
Phase of Compensation: Body tries to preserve blood supply to brain, kidney and lungs
by reducing the blood flow to skin,
muscles and gastro intestinal tract. Body
can compensate a sudden loss of up to 15% of total blood volume. Blood pressure
is also maintained within the normal limits even up to the loss of 40% of
circulatory blood volume.
Phase of Decomposition: If the total loss is more than 15% of total blood volume, compensatory
measures fails and there will be progressive reduction in the function of lungs, heart and brain.
Severity
of Shock
Based on
the severity, decompensated shock is further classified in to mild, moderate
and severe.
Mild
Shock: Mild tachycardia, tachypnoea and
mild reduction in urine output. Blood pressure is maintained.
Moderate
Shock: More tachycardia and tachypnoea. Urine
output falls less than 0.5ml/ kg/ hr. Blood pressure falls and patient becomes drowsy.
Severe Shock: Severe tachycardia and laboured breathing with profound
hypotension. Urine output becomes nil and patient becomes unconscious.
Monitoring
of a patient in Shock
1.
Pulse rate
2.
Blood Pressure/
Central Venous Pressure Monitoing
3.
Respiratory Rate
4.
Level of Consciousness
5.
Urine Output
6.
Pulse Oxymetry for
Oxygen Saturation
7.
Arterial Blood Gas Analysis
8.
Serum Lactate
Treatment
of Shock
1.
Maintain Airway Patency
2.
Ensure proper Breathing
3.
Regain Circulation and control of
the bleeding
4.
Intravenous Fluids – Crystalloids/
Colloids
5.
Grouped and Cross
Matched Blood Transfusion
6.
Ionotropic agents
like Dopamine/ Dobutamine
7.
Nor adrenaline (Vasopressor
for peripheral vaso dilatation)
8.
Antibiotics in septic
shock
