Regulation of salt and water homeostasis is frequently abnormal in hospitalised patients. Hyponatraemia (< 130 mmol litre-1) is the most common electrolyte abnormality
ADH release and thirst
ADH release and thirst
Figure 5.2 Sodium/water retention following surgery or hypovolaemia with an incidence of 25% in the general hospital population. Except in rare circumstances (for example, hyperglycaemia) a reduced serum sodium usually indicates a low serum osmolality and an expanded ICF.
Acute hyponatraemia, defined as a fall in serum Na below 130 mmol litre-1 in 48 hours, can result in cerebral oedema. This phenomenon has been described in both adults and children and has been frequently associated with the administration of hypotonic intravenous fluids in the perioperative period. In an adult series of patients having minor surgery, otherwise fit and healthy women were given an average of 8 8 litres in 48 hours - with a net positive balance of 7 5 litres. These patients failed to mount a normal physiological response to expansion of the ECF, which is to produce dilute urine. This is because ADH secretion is increased by a variety of stimuli in hospitalised patients including pain, anxiety, opiate and anaesthetic agents, surgery, and positive pressure ventilation.
The use of hypotonic solutions has been standard in paediatrics for many years and is based on assumptions extrapolated from studies of water requirement and energy expenditure done on normal children almost 50 years ago. The formulae are used for calculating what are commonly referred to as maintenance fluid requirements and this can result in the administration of large amounts of electrolyte-free water (EFW), which then requires the patient to inhibit ADH secretion and produce a dilute urine. Failure to do so would result in a fall in serum sodium with occasionally catastrophic results. The flaw in the application of these formulae is that hospitalised patients do not necessarily follow the rules that govern normal physiology. Different rules apply to sick patients. The assumption should be that when serum sodium is < 140 mmol litre-1 in the absence of sodium loss, ADH is acting. These patients, whatever their age, should not be given hypotonic fluids; 4% dextrose/0-18% saline is isotonic before administration but is effectively hypotonic in the patient once the glucose has been metabolised.
A study of over 100 children admitted to hospital with the usual spectrum of medical illnesses showed elevated ADH levels and low serum sodium levels at the time of admission, compared with a cohort of elective surgical admissions who had normal ADH and sodium levels. Other groups particularly at risk for the development of acute hyponatraemia are elderly females treated with thiazide diuretics for hypertension who undergo hip replacement surgery and patients undergoing colonoscopy.
At present the standard of care in many institutions is to use only isotonic fluids during surgery and in the postoperative period. This reduces the risk of a fall in serum sodium. In a study where only Ringer's lactate was used during elective surgery and in the first postoperative day, sodium fell from 140 to 136 mmol litre-1. This was associated with a total intravenous intake of 5 litres and a positive balance of 3 litres. The explanation for this fall despite isotonic fluids was demonstrated when urine sodium was measured. Most of the patients were producing a hypertonic urine (sodium >150 mmol litre-1). The hypothesis proposed to explain this was that large volumes of fluid were infused during surgery to maintain blood pressure, which falls due to the vasodilatory effect of anaesthetic drugs. When vasomotor tone was restored to normal at the end of surgery, the ECF was overfilled and the kidneys responded by eliminating sodium. In summary, hypotonic fluids may be used when the plasma sodium is > 145 mmol litre-1 and there is a need for EFW. Patients with sodium levels < 140 mmol litre-1 should receive isotonic fluids.
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