When hypotension complicates hypovolemia, the intravascular fluid volume is depressed sufficiently for compensatory fluid mobilization from the extravascular to the intravascular space to be unable to maintain venous return, ventricular preload, and hence cardiac output. Causes of hypovolemia are categorized as external, due to fluid loss, and internal, due to a redistribution of intravascular volume such that it is functionally unavailable to support cardiac preload ( Table S).

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Table 3 Hypovolemic causes of hypotension

Hemorrhage, for example due to trauma or gastrointestinal bleeding, is the most common cause of hypovolemic hypotension due to external fluid loss. In this situation, acute losses up to 30 per cent of total blood volume (about 1600 ml blood in a 70-kg male) may be tolerated without marked decreases in blood pressure provided that circulatory compensation (sympathoadrenal mechanisms) is complete. Greater blood loss or blood loss in patients with inadequate circulatory compensation (i.e. in the presence of underlying ischemic heart disease which limits increases in cardiac output, or cord injury which limits augmented venous return and arterial vasoconstriction) is associated with hypotension and shock (Tabled). In the case of chronic dehydration, a loss of 6 to 10 per cent of the extracellular fluid volume is followed by a redistribution of blood volume to vital organs and an altered level of consciousness, while a fluid loss of 20 per cent or more of total body weight cannot be survived without immediate and aggressive intervention.

Table 4 Classification of hemorrhage

It is also important to categorize the cause of hypovolemia as due to either erythrocyte or salt/water loss. This categorization directs replacement of the right 'type' of fluid for emergency treatment and during further diagnostic procedures.

In patients with chronic hypovolemia, clinical findings other than hypotension typically include dry skin and tongue, loss of skin elasticity, and oliguria. Acute hypovolemia is accompanied by typical signs of shock such as tachycardia, vasoconstriction, cool peripheries, and confusion or loss of consciousness. Postoperative hypovolemia may often be masked by sympathoadrenal discharge due to postoperative pain. In these cases, the initiation of analgesia may be followed by a sudden decrease in blood pressure which reveals the need for immediate fluid therapy.

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