Acute care: Volume resuscitation

Doctors learn about average daily fluid needs at medical school, but often little is taught about fluid needs in illness. The average 70 kg person needs about 3 litres day to cover insensible losses and the necessary intake to maintain normal fluid balance. But the patients we deal with are physiologically stressed through illness, and this dramatically alters 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 through fever, dehydration, bleeding, or breathlessness. An extra 500 ml of fluid a day is needed for every degree above 37°C. Gastrointestinal losses are often underestimated, particularly in patients with diarrhoea or bowel obstruction. Some conditions cause capillary leak syndrome, in which serious intravascular volume depletion occurs with peripheral and pulmonary oedema--severe pancreatitis and sepsis are examples. It can get complicated.

To preserve plasma volume after trauma, surgery, or in sepsis, the kidney reabsorbs water avidly when stimulated by antidiuretic hormone. Hyponatraemia is common in hospital due to this stress response and after giving hypotonic fluids (5% dextrose or 4% dextrose and 0.18% sodium chloride). Secretion of antidiuretic hormone is increased by a variety of stimuli in hospitalised patients, including pain, anxiety, opioid and anaesthetic agents, and positive pressure ventilation. Critically ill patients tend to retain sodium yet are given litres of sodium containing fluids to expand their intravascular volume. Although colloid is available in the United States suspended in 5% dextrose, all colloids in the UK are suspended in 0.9% sodium chloride. This tends to cause oedema. In general, you should aim to …