Fluids and electrolyte management

After reading this chapter you should:

be able to assess, diagnose and manage fluid and electrolyte disturbances
be able to assess, diagnose and manage disorders of acid-base balance (included elsewhere)

An understanding of fluid and electrolyte requirements in children and young people is of major clinical importance. Patients who are well may need little intervention with their fluid requirements, but an assessment should always be made of their fluid status and reassurance obtained that the normal expected intake and output is appropriate for their age. Patients who are unwell will usually require a detailed review of fluid balance and close monitoring of electrolyte levels. Dehydration and severe electrolyte disturbance need careful management as there is a significant risk of complications if not appropriately addressed. The proposed management of fluids in paediatric practice has to take into account the body changes present in the growing child and the differing impact of any pathological process on differing ages.

Fluid requirements

All children admitted to an inpatient unit should have their fluid status assessed and their requirements and mode of fluid administration determined. The term ‘maintenance fluids’ is used to describe the volume of daily fluid required to replace the insensible losses (from breathing, perspiration and in the stool) and allow excretion of the excess solute load (urea, creatinine, electrolytes etc.) in a volume of urine that is of an osmolarity similar to plasma.

The standard rates of fluid administration are well established in clinical practice and are calculated from the weight of the patient using the Holliday-Segar formula in the following way ( Table 10.1 ):

Table 10.1

The Holliday-Segar formula for maintenance fluid replacement

Weight	Proposed fluid volume
1–10 kg of weight	100 mls/kg/day
11–20 kg of weight	50 mls/kg/day
each kg over 20 kg	20 mls/kg/day

Practice Point—Calculation of maintenance fluid requirements

A 23 kg child will require:

100 mls/kg for the first 10 kg = 1000 mls

50 mls/kg for the second 10 kg = 500 mls

20 mls/kg for all additional kgs = 60 mls

Total = 1560 ml

Hourly rate = 65 ml/hr

Young adult males rarely need more than 2500 mls and young adult females more than 2000 mls of maintenance fluids in a 24-hour period.

The basis for the Holliday-Segar formula is a proposed correlation between energy requirements and the associated fluid requirements in healthy, growing children. Children who are unwell and admitted to hospital are more likely to be catabolic, inactive and have altered organ function, and there are concerns that the standard formula shown above may overestimate the actual fluid requirements of the ill child. Although the Holliday-Segar formula should be used in the first instance when fluids are required, the potential for overhydration should be borne in mind during ongoing review.

If the weight of the patient is above the 91 ^st centile then it may be advisable to use the body surface area value to calculate IV fluid requirements. In these situations, intravenous maintenance fluid requirements should be given using an estimate of insensible loss of 400 ml/m ² /24 hours plus urine output.

There is little strong evidence for the fluid requirements in a newborn child, but NICE guidelines recommend the following volumes for babies who are given formula feeds ( Table 10.2 ).

Table 10.2

Maintenance fluid amounts for a newborn baby

Day 1	50–60 ml/kg/day
Day 2	70–80 ml/kg/day
Day 3	80–100 ml/kg/day
Day 4	100–120 ml/kg/day
Days 5–28	120–150 ml/kg/day

While most children will tolerate standard fluid requirements, some acutely ill children with inappropriately increased antidiuretic hormone secretion (SIADH) may benefit from their maintenance fluid requirement being restricted to two-thirds of the normal recommended volume. These children include those with:

pulmonary disorders (e.g. pneumonia, bronchiolitis)
CNS disorders (e.g. brain injury and infections, CNS tumours)

Appropriate intravenous fluid

If intravenous fluids are necessary, then isotonic solutions should be used in almost all circumstances to avoid iatrogenic hyponatraemia. There is currently little evidence to recommend a particular strength of glucose. Hypotonic fluids—0.18% and 0.45% sodium chloride with added glucose—should NOT be used as routine maintenance fluids in otherwise healthy children.

A commonly used standard solution for maintenance fluids is 0.9% sodium chloride with 5% dextrose, with or without added potassium. The use of 0.9% sodium chloride solutions will provide more than the required sodium maintenance for most children but, in a well child with normal renal function, this additional sodium will be excreted. In the example given, the 23 kg child given their fluid requirements as 0.9% saline would receive over 10 mmol/kg of sodium in the 24 hours ( Table 10.3 ).

Table 10.3

Composition of commonly used fluids

Fluid type	Osmolality mOsmol/l	Tonicity	Sodium (mmol/l)	Chloride (mmol/l)	Potassium (mmol/l)	Glucose (gm/l)
0.9% saline	308	Isotonic	154	154	0	0
0.45% NaCl with 5% dextrose with 20 mmol/l K+	432	Hypotonic	77	77	20	50
Hartmann’s solution	278	Isotonic	131	111	5	0
Plasma-Lyte	294	Isotonic	140	98	5	0
5% glucose	278	Hypotonic	0	0	0	50

Neonates (0–28 days of life) may have higher glucose requirements and lower sodium requirements, particularly in the first week of life, than these standard fluid preparations provide. Caution and senior supervision is required in prescribing intravenous fluids in this age group.

Ongoing losses

Ongoing losses should be assessed every four hours and the fluids chosen as replacement should reflect the electrolyte composition of the fluid being lost. In most circumstances this will be sodium chloride 0.9% with or without the addition of potassium.

Monitoring

Hyponatraemia can develop within a short timescale, and a robust monitoring regime is essential. Weight should be measured, if possible, prior to commencing fluid therapy and daily thereafter whilst fluid balance, including oral intake and ongoing losses, should be recorded and the balance calculated. Plasma sodium, potassium, urea, creatinine and glucose should be measured at baseline and at least once a day in any child receiving intravenous fluids with further electrolyte measurements every four to six hours if an abnormal reading is found.

Glucose monitoring is particularly important as plasma levels may rise during treatment with glucose containing solutions. Analysis of the urine chemistry may be useful in a small number of patients with high-risk conditions or when the cause behind an abnormal sodium result is unclear. Fluid balance and the ongoing need for intravenous fluids along with the details of the fluid prescription should be reviewed twice daily.

Assessment of hydration status

The clinical assessment of hydration is difficult and often inaccurate. In children who are dehydrated the accepted gold standard of assessment is a calculation of an acute weight loss but this is often not possible due to lack of accurate pre-illness weight. A weight should, however, be recorded at presentation and compared to any subsequent weight measurements ( Table 10.4 ).

Table 10.4

Assessment of hydration status. Items in capitals are recognised RED FLAG findings.

	No clinically detectable dehydration (< 3% weight loss)	Clinical dehydration (3%–10% weight loss)	Clinical shock (>10% weight loss)
Symptoms	appears well	appears to be unwell or deteriorating
	alert and responsive	irritable and lethargic	decreased level of consciousness
	normal urine output	reduced urine output
	skin colour unchanged	skin colour changed	pale or mottled skin
	warm extremities	warm extremities	cold extremities
Signs	eyes not sunken	sunken eyes
	moist mucous membranes	dry mucous membranes
	normal heart rate	tachycardia	tachycardia
	normal breathing pattern	tachypnoea	tachypnoea
	normal peripheral pulses	normal peripheral pulses	weak peripheral pulses
	normal capillary refill time	normal capillary refill time	prolonged capillary refill time
	normal skin turgor	reduced skin turgor
	normal blood pressure	normal blood pressure	hypotension

Prolonged capillary refill time, abnormal skin turgor, dry mucous membranes and absent tears have been shown to be the best individual examination measures. If two out of four of these parameters are present the child has a high chance of being more than 5% dehydrated.

Management of dehydration

Oral rehydration therapy

Gastroenteritis is one of the major causes of morbidity and mortality in children worldwide, and in those under 5 years of age, death is the more likely outcome. There has, however, been a decline in mortality over the last few decades due to the introduction and availability of oral rehydration solutions (ORS).

Management of children with significant diarrhoea is usually divided into two phases for management:

repletion phase —any calculated fluid deficit is replaced over 2–3 hours with frequent, small amounts of the oral rehydration solution. Ongoing fluid losses are added to the calculated requirement.
maintenance phase —continued use of rehydration solutions until child able to reestablish normal feeding pattern. Ongoing fluid losses are added to the calculated requirement.

The fluid deficit from dehydration needs to be calculated and introduced to the rehydration management plan ( Table 10.5 ).

Practice Point—Fluid deficit

Fluid deficit in ml = % dehydration x weight (kg) x 10

$Fluid deficit in ml = % dehydration x weight (kg) x 10$

Table 10.5

Fluid requirement in response to assessed dehydration

	No clinically detectable dehydration (< 3% weight loss)	Clinical dehydration (3%–10% weight loss)	Clinical shock (>10% weight loss)
Action	Oral rehydration solutions (ORS) given following maintenance phase	repletion—ORS at 50–100 ml/kg over 4 hours plus ongoing losses maintenance—required amounts plus ongoing losses	repletion—emergency IV fluids at 20 ml/kg isotonic solution.

This estimate is calculated from the child’s weight and the clinically assessed degree of dehydration.

Practice Point—Calculation of fluid deficit

A 23 kg child is assessed as moderately dehydrated at an estimated 5% dehydrated. 23 kg is equivalent to 23 litres and if he is 5% dehydrated, then his deficit is 5% of 23 litres:

5 / 100 x 23 x 1000 = 1150 mls

$5 / 100 x 23 x 1000 = 1150 mls$

The deficit is usually replaced over 24 hours and so should be added to the total daily maintenance volume before determining the hourly rate:

One day maintenance (1560 ml) plus 5 % deficit (1150 ml) = 2710 ml over 24 hours = 112 mls/hr

$One day maintenance (1560 ml) plus 5 % deficit (1150 ml) = 2710 ml over 24 hours = 112 mls/hr$

If the fluid deficit is to be replaced over a longer period, as in hypernatraemic dehydration, then the deficit has to be added to twice the daily maintenance and divide by 48 hours. It will also be important to also add any ongoing losses. In these situations, very close monitoring and accurate fluid balance checks are vital.

Resuscitation

If signs of circulatory collapse are present (prolonged capillary refill time, tachycardia or hypotension), then immediate resuscitation of intravascular volume must occur. This should take place through intravenous or intraosseous access lines, and an initial bolus of 20 ml/kg of isotonic 0.9% sodium chloride should be used. Reassessment of volume status and consideration of the cause of circulatory collapse is crucial.

More details are presented in Chapter 8 Emergency medicine.

Risk of acute kidney injury

Certain children and young people are particularly at risk of developing acute kidney injury from dehydration if any of the following are present or likely:

young age
hypovolaemia or hypotension
chronic kidney disease/transplant
oliguria (urine output less than 0.5 ml/kg/hr)
sepsis
a deteriorating paediatric Early Warning Score

Serum creatinine must be checked against any known baseline values for that patient, and if it is 1.5 times or more above that baseline then the risk of acute kidney injury is high. An estimated GFR (eGFR) of less than 90 ml/min per 1.73 m ² would also indicate that a patient may be at risk of developing acute kidney injury.

Electrolyte disturbance

Hyponatraemia

Hyponatraemia is defined as a plasma sodium of less than 135 mmol/l and severe hyponatraemia when the plasma sodium falls below 130 mmol/l. It is a common electrolyte abnormality in hospitalised children, and the most common cause is the result of an expanded extracellular fluid volume rather than by sodium depletion—the child with severe gastroenteritis who then receives hypotonic fluid replacement is then likely to develop hyponatraemic dehydration. Consequently, it is important to assess the fluid volume status of a child in order to understand the cause of the hyponatraemia and the required management. A major consequence of hyponatraemia is an influx of water into the intracellular space resulting in cellular swelling.

Causes of hyponatraemia

iatrogenic
- intravenous fluid administration (hypotonic solutions)
- diuretic medication
- diluted formula feeds (including factitious illness)
- desmopressin use
SIADH
- CNS infections
- head injury
- bronchiolitis, pneumonia
- surgery
extrarenal sodium losses
- gastroenteritis
- skin (sweating, burns)
- third space losses
renal sodium losses
- polyuric phase of acute tubular necrosis
- interstitial nephritis
- cerebral salt wasting
- absence of aldosterone or lack of effect
other
- glucocorticoid deficiency
- hypothyroidism
- nephrotic syndrome
- diabetic ketoacidosis
- psychogenic polydipsia

Clinical presentation

The symptoms and signs of severe hyponatraemia are predominantly neurological and the result of cerebral oedema and include:

headache
nausea, vomiting
lethargy or irritability
hyporeflexia
confusion and disorientation
seizures
decreased conscious state

Hyponatraemic encephalopathy is a serious complication and children are particularly susceptible to developing neurological complications. This is due to the reduced space for brain swelling in the skull and impaired ability of the paediatric brain to adapt to hyponatraemia.

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