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Arch Dis Child. 2007 October; 92(10): 937.
PMCID: PMC2083218

Underestimation potentially better than overestimation

Luscombe and Owens1 propose a new formula (weight(kg) = 3(age)+7) for accurately estimating weight in the 21st century child. They point out that in most cases the classical formula (weight(kg) = 2(age+4)) underestimates actual weight and that this has implications for potential underdosing of resuscitation drugs and fluids. One of the factors implicated by the authors in this increase in weight is the rising prevalence of childhood obesity.

Changing the prescription of intravenous fluid volumes on the basis of what is essentially an increase in adiposity has wider implications extending beyond the resuscitation period. Current maintenance intravenous fluid requirements in children are based on the supposition that energy expenditure is linked to body weight, assuming 1 ml of water loss is associated with the consumption of 1 kilocalorie.2 In actual fact, resting energy expenditure is more closely related to fat‐free mass.3 In addition, it is well recognised that energy expenditure in sick children is significantly lower than in the healthy children studied in the original work on maintenance fluid requirements.4 This leads to the suggestion that current weight‐based fluid calculation formulae overestimate the maintenance intravenous fluid requirements of children. In addition, blood volume in children is most closely related to lean body mass,5 suggesting that this should be the basis for resuscitation fluid volumes.

A change in the weight estimation formula as described might therefore lead to significant, and potentially dangerous, excess fluid being administered to children. It would be interesting to know what proportion of the rise in weight over the last 50 years is related to height increase and therefore might be attributed to an increase in lean body mass. Until there is good evidence of harm, however, the current weight estimation formula should remain in place. It may confer significant benefit, rather than detriment, by virtue of its underestimation.

Footnotes

Competing interests: None declared.

References

1. Luscombe M, Owens B. Weight estimation in resuscitation: is the current formula still valid? Arch Dis Child 2007. 92412–415.415 [PMC free article] [PubMed]
2. Holliday M A, Segar W E. The maintenance need for water in parenteral fluid therapy. Pediatrics 1957. 19823–832.832 [PubMed]
3. Illner K, Brinkmann G, Heller M. et al Metabolically active components of fat free mass and resting energy expenditure in nonobese adults. Am J Physiol Endocrinol Metab 2000. 278E308–E315.E315 [PubMed]
4. Hatherill M. Rubbing salt in the wound. Arch Dis Child 2004. 89414–418.418 [PMC free article] [PubMed]
5. Raes A, Van Aken S, Craen M. et al A reference frame for blood volume in children and adolescents. BMC Pediatr 2006. 63

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