Increased extracellular volume occurs in numerous disease states. The physical signs depend on the distribution of excess volume and on whether the increase is local or systemic. According to Starling principles, distribution depends on:

• venous tone, which determines the capacitance of the blood compartment and thus hydrostatic pressure
• capillary permeability
• oncotic pressure – mainly dependent on serum albumin
• lymphatic drainage.

Depending on these factors, fluid accumulation may result in expansion of interstitial volume, blood volume, or both.

Clinical features

Peripheral oedema is caused by expansion of the extracellular volume by at least 2 L (15%). The ankles are normally the first part of the body to be affected, although they may be spared in patients with lipodermatosclerosis (where the skin is tethered and cannot expand to accommodate the oedema). Oedema may be noted in the face, particularly in the morning. In a patient in bed, oedema may accumulate in the sacral area. Expansion of the interstitial volume also causes pulmonary oedema, pleural effusion, pericardial effusion and ascites. Expansion of the blood volume (overload) causes a raised jugular venous pressure, cardiomegaly, added heart sounds, basal crackles as well as a raised arterial blood pressure in certain circumstances.

Causes

Extracellular volume expansion is due to sodium chloride retention. Increased salt intake does not normally cause volume expansion because of rapid homeostatic mechanisms which increase salt excretion. However, a rapid intravenous infusion of a large volume of saline will cause volume expansion. Thus, most causes of extracellular volume expansion are associated with renal sodium chloride retention.

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