Fluid intake is derived from two sources: (1) exogenous; and (2) endogenous.
Exogenous water is either drunk or ingested in solid food. The quantities vary within wide limits, but average 2—3 litres per 24 hours, of which nearly half is contained in solid food.
Taking into consideration their body weight, the water requirements of infants and children are relatively greater than those of adults because of: (1) the larger surface area per unit of body weight; (2) the greater metabolic activity due to growth; and (3) the comparatively poor concentrating ability of the immature kidney.
Endogenous water is released during the oxidation of ingested food; the amount is normally less than 500 ml124 hours. However, during starvation, this amount is supplemented by water released from the breakdown of body tissues.
Water is lost from the body by four routes.
• By the lungs.About 400 ml of water is lost in expired air each 24 hours. In a dry atmosphere, and when the respiratory rate is increased, the loss is correspondingly greater (this also applies to the patient who has their trachea intubated).
•By the skin.When the body becomes overheated, there is visible perspiration, but throughout life invisible perspiration is always occurring. The cutaneous fluid loss varies within wide limits in accordance with the atmospheric temperature and humidity, muscular activity and body temperature. In a temperate climate the average loss is between 600 and 1000 m1124 hours.
• Faeces. Between 60 and 150 ml of water are lost by this route daily. In diarrhoea this amount is greatly multiplied.
• Urine. The output of urine is under the control of multiple influences, such as blood volume, hormonal and nervous influences, among which the antidiuretic hormone plays a major role controlling tonicity of the body fluids, a function that it performs by stimulating the reabsorption of water from the renal tubules, thus varying the amount excreted after the requirements of the first three routes have been met. The normal urinary output is approximately 1500 ml124 hours, and provided that the kidneys are healthy, the specific gravity of the urine bears a direct relationship to the volume. A minimum urinary output of approximately 400 m1124 hours is required to excrete the end products of protein metabolism.
Pure water depletion is usually due to diminished intake. This may be due to lack of availability, difficulty or inability to swallow because of painful conditions of the mouth and pharynx, or obstruction in the oesophagus. Exhaustion and paresis of the pharyngeal muscles will produce a similar picture. Pure water depletion may also follow the increased loss from the lungs after tracheostomy. This loss may be as much as 500 ml in excess of the normal insensible loss. After tracheostomy, humidification of the inspired air is an important preventive measure.
The main symptoms are weakness and intense thirst. The urinary output is diminished and its specific gravity increased. The increased serum osmotic pressure causes water to leave the cells (intracellular dehydration), and thus delays the onset of overt compensated hypovolaemia (see below).
This can occur when excessive amounts of water, low sodium or hypotonic solutions are taken or given by any route. The commonest cause on surgical wards is the overprescribing of intravenous 5per cent glucose solutions to postoperative patients. Colorectal washouts with plain water, instead of saline, have caused water intoxication during total bowel wash-through prior to colonic surgery. A major component of the TURP (transurethral resection of the prostate) syndrome is the water intoxication caused by excessive uptake of water (and glycine) from irrigation fluid.
Similarly, water intoxication can occur if the body retains water in excess to plasma solutes. This can be seen in the syndrome of inappropriate antidiuretic hormone (SIADH) secretion which is most commonly associated with lung conditions such as lobar pneumonia, empyema and oat-cell carcinoma of bronchus, as well as head injury.
These include drowsiness, weakness, sometimes convulsions and coma. Nausea and vomiting of clear fluid are common, and, with the notable exception of the SIADH, usually the patient passes a considerable amount of dilute urine. Laboratory investigations may show a falling haematocrit, serum sodium and other electrolyte concentrations.
The intake of water having been stopped, the best course is water restriction. If the patient fails to improve, transfer to an intensive care or high dependency unit will be necessary for more invasive monitoring and controlled manipulation of fluids and electrolytes. The administration of diuretics or hypertonic saline should not be undertaken lightly as rapid changes in serum sodium concentration may result in neuronal demyelination and a fatal outcome.