Shock

Chapter 13 Shock




Questions


image What is meant in medical terms by the word ‘shock’?

image What are the causes of shock?

image How may shock be life threatening?

Shock can mean anything from a frightening or unexpected event to a fatal collapse of the body’s circulation. Physiological shock is a dramatic reduction in blood flow that, if left untreated, can lead to collapse, coma and even death. Our favourite definition is ‘systemic hypoperfusion owing to a reduction in either cardiac output or the effective circulating blood volume’. It is associated with hypotension. The clinical features of shock are caused by the cellular hypoxia resulting from reduced tissue perfusion. If relatively mild, the hypoxia causes reversible tissue injury. If severe or prolonged, irreversible cell damage occurs.




Pathogenesis


Shock can be classified according to pathogenesis into a number of types (Table 3.13.1). Hypovolaemic, cardiogenic and septic shock are the most common types.



Table 3.13.1COMPARISON OF THE DIFFERENT TYPES OF SHOCK

image


Hypovolaemic shock


The responses of the body to hypovolaemia can be understood as evolutionary adaptations to preserve life in the face of severe haemorrhage from traumatic injuries. When blood is lost, for example through severing of a major artery, the baroreceptor reflexes are stimulated, catecholamines are secreted, the renin–angiotensin–aldosterone system is activated, and antidiuretic hormone is released. The consequences are:


image an increase in heart rate and cardiac output

image generalized peripheral vasoconstriction to maintain blood pressure despite the diminished blood volume

image diversion of blood from less vital organs to the brain and heart (which occurs because the vessels of the brain and heart are less sensitive to sympathetic vasoconstriction and, therefore, maintain their calibre)

image renal retention of salt and water to increase plasma volume.

Unless the shock is corrected, anaerobic respiration in underperfused tissues causes accumulation of lactic acid. This metabolic lactic acidosis adversely affects the vasoconstrictor response of vessels and blood begins to pool in the peripheral circulation. Blood pressure and cardiac output diminish further, and hypoxic damage worsens. Organ failure becomes apparent.



Cardiogenic shock


Cardiogenic shock occurs when the heart is severely damaged—by a major heart attack, for example—and is no longer able to pump blood around the body properly (acute pump failure), causing very low blood pressure. The term is usually taken to include extrinsic compression of the myocardium caused by tamponade and outflow obstruction owing to pulmonary embolism. The responses of the body are similar to those observed in hypovolaemic shock.



Septic shock


The pathogenesis of septic shock is complicated. It occurs when an overwhelming bacterial infection causes blood pressure to drop. Typically, it results from serious infection with Gram-negative bacteria such as Escherichia coli, Klebsiella or Pseudomonas spp. Toxic shock syndrome is a rare but severe illness caused by certain strains of Staphylococcus aureus. The bacteria release bacterial wall lipopolysaccharides, also known as endotoxins, which cause inflammation by activating macrophages and neutrophils and initiating the alternative complement pathway and the extrinsic coagulation pathway. Sometimes, similar lipopolysaccharides molecules are released from Gram-positive bacteria and fungi. If large amounts of lipopolysaccharides are present, there will be massive activation of inflammatory cells with release of large quantities of cytokines into the circulation. Although these cytokines at low levels have beneficial effects by playing their part in host defence (Ch. 8), at high levels they cause:


image systemic vasodilatation such that the circulating blood volume cannot fill the greatly increased volume of the vascular bed, resulting in hypotension
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Jul 3, 2016 | Posted by in MUSCULOSKELETAL MEDICINE | Comments Off on Shock

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