Trauma and illness alter the volume and composition of the intracellular and extracellular spaces as well as the kinetics of fluid distribution and excretion. Patients may have increased fluid losses from fever, dehydration, polyuria, bleeding, or breathlessness. An extra 500 ml/day is required for every degree Celsius above 37. Gastrointestinal losses also need to be considered - a patient with diarrhoea or gastric outflow obstruction could be litres behind in terms of fluid balance.
Certain patients are at increased risk of pulmonary oedema. Patients with heart failure, renal failure and the elderly are the most common examples. Patients with capillary leak, for example, in severe sepsis, are also at risk of interstitial oedema, yet can require large amounts of intravenous fluid as they are volume depleted. Monitoring and management in these cases is more complex (see Chapter 7).
To preserve plasma volume after trauma, surgery, haemorrhagic shock, or severe sepsis, the kidney reabsorbs filtered water and sodium avidly via ADH. Intravascular volume loss stimulates the renin-angiotensin-aldosterone axis and inhibits natriuretic peptide. This leads to sodium retention (Figure 5.2). Sodium loading occurs with sodium containing crystalloids or colloids. This results in a hyperosmolar state which in turn stimulates further ADH release and fluid retention. This tends to reduce urine output. In addition, high chloride levels from sodium chloride infusions cause renal vasoconstriction and a hyperchloraemic metabolic acidosis. The critically ill have a reduced renal concentrating ability and need to excrete more water to get rid of their solute load. However, these patients often become fluid overloaded and are subsequently fluid restricted, making the problem of sodium retention worse.
To get round these problems of sodium overload in critical illness one should always aim to give specific fluids for specific reasons:
• for electrolyte losses or maintenance - Hartmann's
• for water losses - 5% dextrose or water via nasogastric feeding
• for volume expansion - blood or another appropriate colloid.
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