Dehydration

Dehydration is the result of abnormal fluid losses from the body which are greater than the amount for which the kidneys can compensate. The natural mechanisms for compensation have the primary aim of maintaining circulating volume and blood pressure at all cost. Thus the majority of patients with dehydration maintain their central circulation satisfactorily. Loss of central circulatory homeostasis constitutes hypovolaemic shock and is dealt with in Chapter 10.

The major causes of dehydration in children are gastrointestinal disorders and diabetic ketoacidosis. Some renal disorders (polyuric tubulopathy with urinary tract infection, polyuric chronic renal failure and diabetes insipidus) might also present in this way. Depending on the source of fluid losses and the quantities of electrolytes lost dehydration can be divided into three types:

1. Isotonic dehydration - sodium and water lost in proportion to each other.

2. Hyponatraemic dehydration - proportionately more sodium lost than water.

3. Hypernatraemic dehydration proportionately more water lost than sodium.

In all three types there is usually a total body deficit of salt and water. Between the three types the relative amounts of salt and water loss vary. Table B.5 shows the symptoms and signs of dehydration and gives a guide towards the assessment of the degree of dehydration. On the whole, the more severe the dehydration the more likely that hypovolaemia will be a problem; most patients with more than 10% dehydration are hypovolaemic at presentation. However, speed of fluid loss is important. Slow, prolonged losses can give rise to massive dehydration without hypovolaemia, similarly acute, severe loss can present as hypovolaemia without apparent significant dehydration. The latter is not infrequently the case in acute gastroenteritis in infants where acute fluid loss into the bowel causes hypovolaemia and the patient can present even before any diarrhoea has occurred.

Table B.5. Symptoms and signs of dehydration

Mild

Moderate

Severe

Sign/symptoms

<5%

5-10%

> 10%

Notes/caveats

Decreased urine output

+

+

+

Beware watery diarrhoea making nappies appear "wet"

Dry mouth

+ / —

+

+

Mouth breathers are always dry

Decreased skin turgor

+/ —

+

Beware the thin, use several sites

Tachypnoea

+/ —

+

Metabolic acidosis and pyrexia worsen this

Tachycardia

+/ —

+

Hypovolaemia, pyrexia and irritability cause this

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