After reading this chapter you should:
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be able to assess, diagnose and manage fluid and electrolyte disturbances
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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 ):
| 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 |
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 ).
| 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:
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pulmonary disorders (e.g. pneumonia, bronchiolitis)
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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 ).
| 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 ).
| 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:
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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.
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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 ).
| 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. |
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