Capillary leak and the concept of the third space

Oedema is excessive fluid accumulation in the interstitial space and can be localised or generalised. Third space losses occur when fluid accumulates in a potential space such as the peritoneum, pleura, or bowel. The concept of the third space was developed to explain the phenomenon that some postoperative patients required more replacement fluid than their measurable losses. Studies showed that the patients' weight did not reduce but that they were intravascularly volume depleted. This was due to movement rather than loss of fluid.

"Capillary leak syndrome" has been defined as a syndrome characterised by a prolonged and severe increase in capillary permeability, the clinical features of which include hypoalbuminaemia, microalbuminuria, severe positive fluid balance, clinical oedema, and decreased oxygen delivery and uptake.

Capillary leak or microvascular failure is part of the inflammatory response to a range of insults (Figure 5.3). Studies have shown rapid and profound transcapillary loss of radiolabelled albumin occurring within 6 hours of surgery and similar changes occur in patients with severe inflammatory conditions (for example, sepsis, trauma, and ischaemia). The kidneys are uniquely placed to mirror changes in capillary permeability because they receive 25% of the cardiac output, which is filtered over a wide surface area. The unique nature of the renal-concentrating mechanism allows low level albumin loss (microalbuminuria) to be used as a sensitive monitor of changes in systemic capillary permeability. In uncomplicated cases capillary permeability returns to normal within 24 hours of major surgery. In extreme cases the result is SIRS - systemic inflammatory response syndrome - an unregulated inflammatory response that results in significant end-organ dysfunction and later multiple organ dysfunction syndrome (MODS) (Figure 5.4).

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