A syndrome in which tissue perfusion is inadequate for the tissue’s metabolic requirement

Aetiology

Hypovolaemic

• Caused by loss of plasma or blood volume
• Decrease can either be due to haemorrhage (trauma, surgery, GI haemorrhage) or non-haemorrhage (vomiting, diarrhoea, excessive sweating)
• Decrease in blood volume → decreased venous return → decreased EDV → decreased SV (Frank-Starling) → decreased CO and BP → inadequate tissue perfusion

Cardiogenic

• ‘Pump failure’ causes reduced CO
• Primarily caused by ischaemia induced myocardial dysfunction
• Other causes include cardiomyopathies, valvular problems, dysrhythmias
• Decreased cardiac contractility (e.g. due to acute MI) → decreased stroke volume → decreased CO and BP → inadequate tissue perfusion
• If due to MI suggests that >40% of LV is involved
• Unless correctable pathology (e.g. valvular), mortality >75%

Distributive

• Caused by disruption of normal vascular autoregulation, and profound vasodilation, resulting in poor perfusion (despite increased CO)
• Loss of sympathetic tone to blood vessels and heart → massive venous and arterial dilation and heart rate slows → decreased venous return and SVR → decreased CO and BP → inadequate tissue perfusion
• Regional perfusion differences
• Alteration of oxygen extraction
• 3 main subtypes:

Septic shock