Systemic and wound oedema develops after a burn due to a number of factors:
- increased capillary permeability:
- triggered by inflammatory mediators
- increased diffusion, filtration and large molecular transport
- the latter is the most pronounced and there is a great increase in albumin transit
- the basement membrane may remain a selective filter for large molecules despite the disruption of endothelium
- decreased plasma oncotic pressure due to loss of albumin
- increased capillary hydrostatic pressure:
- increased flow to the area due to vasodilatation stimulated by the stress response
- increased resistance to outflow from post-capillary erythrocyte sludging and venular constriction
- decreased tissue hydrostatic pressure due to unfolding of complex macromolecules eg collagen
- increased tissue oncotic pressure due to:
- increased leak of plasma albumin into this space
- breakdown of intercellular ground substance into more osmotically-active units
Overall, the interaction of Starling Forces tend to favour transit of fluid into the interstitial space and hence oedema.