Maintenance fluids (Holliday-Segar)

Clinical background

When a patient cannot maintain an adequate oral intake, clinicians need a structured way of estimating how much intravenous fluid is required to cover basal losses through urine, stool, perspiration and respiration. Without a systematic method there is a real risk of either over- or underdosing, leading to volume overload or dehydration respectively. The Holliday–Segar formula, often called the 4-2-1 rule, is the most widely used method for calculating the maintenance dose and has been in clinical use since the late 1950s. It gives an hourly rate that can then be multiplied by 24 to obtain the daily requirement.

Calculating maintenance fluids (Holliday–Segar)

The formula rests on the fact that the water requirement in mL per day corresponds approximately to energy expenditure in kcal per day, since approximately 1 mL of water is required per kcal expended. Holliday and Segar showed that energy expenditure scales with body weight in a predictable but non-linear way: the first kilograms require more energy per kg than those that follow [1]. This led to a stepwise formula in which the water requirement per kg falls as body weight rises. The hourly variant, the 4-2-1 rule, is a simplification of the original daily formula:

$$\text{mL/h} = 4 \times \min(w, 10) + 2 \times \min(\max(w - 10, 0), 10) + 1 \times \max(w - 20, 0)$$

where $w$ is body weight in kg. For a child weighing 25 kg the result is $4 \times 10 + 2 \times 10 + 1 \times 5 = 65$ mL/h, corresponding to 1,560 mL per day.

The derivation cohort consisted of hospitalised children in the USA, and the original article was published in 1957 [1]. The formula has since come to be used much more widely, including in adults, even though the original derivation was based on the physiology and energy expenditure of children.

Interpretation in practice

The output of the calculator is the basal maintenance rate in mL/h. It states what a patient needs to maintain volume status with normal losses, provided the patient is euvolaemic at the outset. In clinical practice, three components must be added or adjusted separately:

A patient who arrives dehydrated therefore needs more than the calculator indicates. Conversely, a patient with heart failure, renal failure or SIADH may need a reduced rate, often 50 to 70 per cent of the calculated maintenance, since antidiuretic hormone is raised and water retention is active.

Validation and performance

Unlike predictive instruments, the 4-2-1 rule has no c-statistic or calibration measure in the traditional sense. It is a physiological derivation rather than a statistical model, and its "performance" lies in how well it estimates the true water requirement in a given patient. Over more than six decades the formula has been questioned and modified, particularly in two respects: the volume and the tonicity of the fluid.

As regards volume, later studies of energy expenditure in hospitalised children have shown that the actual energy requirement is often lower than Holliday–Segar indicates, particularly in critically ill children in intensive care, in whom metabolic activity is downregulated. This means that the formula may overestimate the maintenance requirement in this population.

As regards tonicity, a substantial body of evidence has developed. The original approach paired the 4-2-1 rule with hypotonic solutions, often 0.2 to 0.45 per cent saline with glucose. A 2014 Cochrane review of 10 RCTs and 1,106 children found that isotonic solutions halved the risk of hyponatraemia compared with hypotonic ones (RR 0.48, 95 per cent CI 0.38 to 0.60, high quality) [2]. An updated meta-analysis from 2024 of 33 RCTs and 5,049 children confirmed this: isotonic maintenance fluids reduced the risk of mild hyponatraemia both within 24 hours (RR 0.38, 95 per cent CI 0.30 to 0.48) and after 24 hours (RR 0.47, 95 per cent CI 0.37 to 0.62), with high-quality evidence [3]. At the same time an increased risk of hypernatraemia was noted in neonates given isotonic solutions (RR 3.74, 95 per cent CI 1.42 to 9.85), and the authors therefore recommend balanced isotonic solutions rather than 0.9 per cent saline [3].

In 2018 the American Academy of Pediatrics published a guideline strongly recommending isotonic solutions with appropriate potassium and glucose as maintenance fluid for children from 28 days to 18 years of age (evidence quality A, strength of recommendation strong) [4].

Limitations

The 4-2-1 rule applies to euvolaemic patients with normal losses. The following patient groups fall outside its assumptions:

• Patients with increased antidiuretic hormone secretion: surgery, pain, breathlessness, sepsis and CNS involvement all stimulate ADH, which reduces free water excretion. Giving the full maintenance dose according to the formula can lead to hyponatraemia and cerebral oedema. In these patients the rate should be reduced to 50 to 70 per cent of the calculated maintenance.
• Patients with large ongoing losses: burns, an ileostomy, high drain output or voluminous diarrhoea require the losses to be measured and added separately. The formula does not cover this.
• Neonates under 28 days: the AAP guideline excludes this group, and both water and electrolyte requirements differ from what the formula indicates.
• Patients with renal dysfunction, heart failure or liver cirrhosis: volume tolerance is reduced and the maintenance rate must be individualised.
• Adults: the formula was derived for children but is often used for adults. Although it gives a reasonable approximation in healthy adults of normal weight, it is not validated for adults with obesity, and overweight patients should be calculated on ideal body weight rather than actual weight to avoid overestimation.

The commonest error in clinical practice is to give a hypotonic solution at maintenance rate to a child with ADH stimulation, combining two risk factors for hyponatraemia: a high volume and a low sodium concentration. A second common error is to confuse maintenance with rehydration, and thereby to give too little to a dehydrated patient.

References

1. Holliday MA, Segar WE. The maintenance need for water in parenteral fluid therapy. Pediatrics. 1957;19(5):823–32. PMID: 13431307
2. McNab S, Ware RS, Neville KA et al. Isotonic versus hypotonic solutions for maintenance intravenous fluid administration in children. Cochrane Database Syst Rev. 2014. PMID: 25519949
3. Amer BE, Abdelwahab OA, Abdelaziz A et al. Efficacy and safety of isotonic versus hypotonic intravenous maintenance fluids in hospitalized children: an updated systematic review and meta-analysis of randomized controlled trials. Pediatr Nephrol. 2024;39(1):57–84. PMID: 37365423
4. Feld LG, Neuspiel DR, Foster BA et al. Clinical Practice Guideline: Maintenance Intravenous Fluids in Children. Pediatrics. 2018;142(6):e20183083. PMID: 30478247