Shock

Shock is when the circulation is unable to perfuse and oxygenate the tissues adequately. Hypoperfusion can occur at "normal" blood pressures especially in patients who are usually hypertensive. The markers of inadequate perfusion are discussed in Chapter 5. Traditionally, shock is divided into five types:

• haemorrhagic (see Chapter 5)

• cardiogenic

• anaphylactic

• neurogenic (after high spinal cord injury due to loss of sympathetic tone).

The characteristic haemodynamic variables in the different types of shock are shown in Table 6.1.

Compensatory mechanisms in shock first aim to restore blood pressure and then intravascular volume. Falling blood

Table 6.1 Characteristic haemodynamic variables in different types of shock

Shock type

CO

SVR

PAOP/CVP

Cardiogenic

uu

t

Any

Hypovolaemic

u

t t

u u

Septic

tt

uuu

uu

Anaphylactic

u

u u

u u

Neurogenic

u

uuu

uu

CO, cardiac output; CVP, central venous pressure; PAOP, pulmonary artery occlusion pressure; SVR, systemic vascular resistance

CO, cardiac output; CVP, central venous pressure; PAOP, pulmonary artery occlusion pressure; SVR, systemic vascular resistance pressure reduces the activity of pressure receptors in the arterial system, which leads to stimulation of the sympathetic nervous system, both centrally and via the adrenal glands. In cardiogenic or hypovolaemic shock, arterial vasoconstriction occurs and blood is diverted to vital organs (for example, the coronary arteries and brain). Tachycardia and vasoconstriction raise the cardiac output. Slower mechanisms aim to restore volume - the lower filtration pressure in the glomerulus activates the renin-angiotensin-aldosterone axis, thirst results, ADH is released because of reduced atrial pressure, and erythropoietin is manufactured.

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