Homeostasis

Volume depletion and dehydration

An important, common, easily missed clinical condition, especially in older people. Highly prevalent among acutely unwell older people admitted to hospital due to a combination of increased fluid loss (fever, gastrointestinal loss), and decreased intake (nausea, anorexia, weakness).

Causes

Often multifactorial, and include:

Blood loss
Diuretics
Gastrointestinal losses (eg diarrhoea, NG drainage)
Sequestration of fluid (eg ileus, burns, peritonitis)
Poor oral intake
Fever

Symptoms and signs

Thirst is uncommon in older people
Malaise, apathy, weakness
Orthostatic symptoms (lightheadedness or syncope) and/or postural hypotension
Nausea, anorexia, vomiting, and oliguria in severe uraemia
Tachycardia, supine hypotension (late signs, also in fluid overload)
Decreased skin turgor, sunken facies, absence of dependent oedema

The symptoms and signs of clinically important dehydration may be subtle and confusing. It is therefore under-recognized. Continual clinical assessment, assisted by basic tests (urinalysis; U,C+E) is essential. Invasive monitoring or other tests are rarely needed.

Older patients commonly become dehydrated because:

They are ‘run dry’ on the wards, as medical (and nursing) staff fear precipitating acute pulmonary oedema through excessive iv fluid administration
Iv infusions often run more slowly than prescribed, or cannot run for periods if iv access is lost
Moderate leg oedema is very poorly specific for heart failure—don’t treat this sign alone, in the absence of supporting evidence, with diuretic

▶Poor urine output on the surgical (or medical) wards is more often a sign of dehydration than of heart failure. Improving urine volume with diuretics is the wrong treatment.

▶There is no sensitive biochemical marker of dehydration—urea and creatinine are commonly in the normal range, and may be abnormal when normally hydrated (eg in CKD).

Challenges to volume status assessment in elderly patients

This can be difficult and requires care.

There is no gold standard in routine clinical examination, although a capillary wedge pressure will give a reliable estimate in ITU/HDU settings.

Most symptoms and signs can occur in both fluid overload and dehydration—use multiple indicators to make an overall decision about volume status.

Use serial assessments and if the response to treatment is not as anticipated, review your judgement.

Symptoms

Thirst is often absent in dehydration
Confusion can occur in dehydration and fluid overload
Breathlessness may occur in fluid overload, but also in, eg, chest sepsis with dehydration

Signs

Tachycardia may occur in dehydration (may be absent if there is β-blockade) but also occurs in cardiac failure
Hypotension similarly occurs both in dehydration and cardiac failure. A postural drop is more likely to indicate dehydration, but may also be induced by medication or autonomic dysfunction
Look at the skin turgor—choose a site away from peripheral oedema, eg the forehead. Pinch the skin gently and see how quickly it returns to normal. A sluggish response indicates dehydration
Check for peripheral oedema—remember that in a bedbound patient this may collect in the sacral region. It is possible to have peripheral oedema with intravascular depletion (eg in hypoalbuminaemia) so this is not a reliable indicator of fluid state
Check the JVP, which is elevated in cardiac failure and also in tricuspid regurgitation

Investigations

Urine specific gravity may be high in dehydration, and also in heart failure, and is less helpful when diuretics have been used
Elevated U,C+E often indicate dehydration—check for the patient’s baseline if possible. CKD will also elevate urea and creatinine, but is likely to be chronic. Remember that frail older people will have a lower creatinine (perhaps even in the normal range) because of low muscle bulk, but this may still represent a marked abnormality for them
Elevated Hb may occur in dehydration and in chronic hypoxia

Dehydration: management

Treat the underlying cause(s)
Suspend diuretic and ACE inhibitors
Continually reassess clinically, assisted by urinalysis/U,C+E. Measure and document intake, output, BP, and weight
If mild: oral rehydration may suffice. A ‘homemade’ oral rehydration mixture can be made by adding a level teaspoon of salt and eight level teaspoons of sugar to a litre of water with a touch of fruit juice. Old frail people need time, encouragement, and physical assistance with drinking. Enlist relatives and friends to help
More severe dehydration, or mild dehydration not responding to conservative measures, will require other measures—usually parenteral treatment, either s/c (see ‘HOW TO … Administer subcutaneous fluid’, p.406) or iv
The speed of parenteral fluid administration should be tailored to the individual patient, based on volume of fluid deficit, degree of physiological compromise and perceived risks of fluid overload. For example, a hypotensive patient who is clinically volume depleted with evidence of end-organ failure should be fluid resuscitated briskly, even if there is a history of heart failure. In the absence of end-organ dysfunction, rehydration may proceed more cautiously, but continual reassessment is essential, to confirm that the clinical situation remains benign, and that progress (input > output) is being made

HOW TO … Administer subcutaneous fluid

This method (hypodermoclysis) was widely used in the 1950s, but fell into disrepute following reports of adverse effects associated with very hypo/hypertonic fluid. Fluids that are close to isotonic delivered by competent staff are a safe and effective substitute for iv therapy.

A simple, widely accessible method for parenteral fluid/electrolytes
Fluid is administered via a standard giving set and fine (21-23G) butterfly needle into s/c tissue, then draining centrally via lymphatics and veins
S/c fluid administration should be considered when insertion or maintenance of iv access presents problems, eg difficult venous access, persistent extravasation, or lack of staff skills
Iv access is preferred if rapid fluid administration is needed (eg gastrointestinal bleed), or if precise control of fluid volume is essential

Sites of administration

Preferred sites: abdomen, chest (avoid breast), thigh, and scapula. In agitated patients who can tear out iv (or s/c) lines, sites close to the scapulae may foil their attempts.

Fluid type

Any crystalloid solution that is approximately isotonic can be used, including normal (0.9%) saline, 5% glucose and any isotonic combination of dextrose-saline. Potassium chloride can be added to the infusion, in concentrations of 20-40mM/L. If local irritation occurs, change site and/or reduce the concentration of added potassium.

Infusion rate

Typical flow (and absorption) rate: 1mL/min or 1.5L/day. Infusion pumps may be used. If flow or absorption is slow (leading to lumpy, oedematous areas):

Change site
Use hyaluronidase (a ‘digester’ of connective tissue). Add 150-300 units to each litre fluid and/or pre-treat site with 150 units
Use two separate infusion sites at the same time
Using these techniques, up to 3L of fluid daily may be given. For smaller volumes, consider an overnight ‘top-up’ of 500-1000mL, or two daily boluses of 500mL each (run in over 2-3hr) leaving the patient free of infusion lines during the daily rehabilitation/activity. Some patients need only 1L/alternate nights to maintain hydration

Monitoring

Patients should be monitored clinically (hydration state, input/output, weight) and biochemically as they would if they were receiving iv fluid.

▶ Be responsive and creative in your prescriptions of fluid and electrolytes. One size does not fit all.

Potential complications

Rare and usually mild. They include local infection, and local adverse reactions to hypertonic fluid (eg with added potassium) or to hyaluronidase.

Contraindications