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.
Fluid output
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.
Water depletion
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.
Clinical features
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).
Water
intoxication
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 5
per 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.
Clinical features
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.
Treatment
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.