Dear Editor,

We read with interest the article by Luscombe and Owens (1) who present a potentially more accurate formula (Weight (kg) = (age x 3) + 7) for estimating a child’s weight based on their age derived from the weights of children attending an emergency department at Nottingham. The most familiar method to estimate the weight is based on the formula: Weight = (Age + 4) x 2, as widely taught in the Advanced Paediatric Life- Support (APLS) manual for children aged 1 to 12 years.(2) However this method is known to underestimate the weight, with the difference increasing with age.(3) It is not clear from the literature where this formula originated. The equation crudely describes the relationship between age and the mean weight of children based on the 1990 growth standards.(4) Luscombe and Owens (1) demonstrate that the current formula underestimates the child’s weight by a mean of 18.8% and recommend the implementation of the new formula in children aged 1 – 10 years of age, which underestimates the weight by just 2.48%.

Their conclusions are similar to our findings derived independently in 301 (167 males, 134 females) children, aged 1 – 12 years attending for a general paediatric outpatient appointment at the Sheffield Children’s Hospital. We chose children from an outpatient clinic, as children attending an emergency department are not routinely weighed in their undergarments (children in outpatients are), thus possibly confounding the data.

The mean and median weights for the subgroups were similar (table 1). The traditional formula of Weight = (Age + 4) x 2, underestimates the weight of the children in our sample group at every age, with the percentage difference between actual and estimated weights becoming progressively larger with advancing age. The overall mean difference is 17.3% (median 14.8%) across all the measurements (table 1). The median weights are displayed in Figure 1. The simplest formula which most accurately described the relationship between age and weight (based on the regression equation of the relationship between age and median weight = (3.27 x Age) + 5.26; r2 = 0.96) was similar to Luscombe and Owens(1), Weight = (Age x 3) + 7. This formula tends to overestimate the weight slightly in children aged 3 - 10 years old (2.2% to 8.6%), however the overall mean and median differences between the whole age subgroups was similar at 1.0% and -3.0% respectively (table 1). During puberty the variation in weights increases, probably because of variation in pubertal onset and a reduction in age of pubertal onset over the last decades. Although there is greater variation in weight after puberty, the new formula still gives a better estimate than the current formula.

Paediatric drug and fluid dosing is based on the weight, thus the new formula should optimise therapy in the ill child when a weight is not readily available. While caution is advised in proposing an alternative formula, it may be an appropriate time to consider revising the current formula. Although the authors alluded to the new formula being used for children older than 10 years, they do not present their data. Based on our data we also propose that the new formula be considered for use up to the age of 12 years to ensure consistency with current APLS guidelines. In view of the increase in the population weights and reduction in age of onset of puberty it may be time for considered debate on the adoption of a new formula to estimate a child’s weight.

References:

1. Luscombe M and Owens B. Weight estimation in resuscitation: is the current formula still valid? Arch Dis Child 2007;92:412-415.

2. Advanced Paediatric Life Support. Fourth Edition. 2006; Advanced Life Support Group, BMJ Books, Blackwell publishing.

3. Argall J, Wright N, Macway-Jones K, Jackson R. A comparison of two commonly used methods of weight estimation. Arch Dis Child 2003 88:789- 790.

4. Freeman JV, Cole TJ, Chinn S, Jones PR, White EM, Preece MA. Cross sectional stature and weight reference curves for the UK. 1990. Arch Dis Child 1995;73:17-24.

Table 1 Demographic details, mean and median weights and percent difference between the conventional and new formulas. The actual weight compared to the formula weight (age + 4) x 2; is consistently higher in all age groups. A minus (-) symbol indicates an actual weight that is less than the calculated formula weight.

Figure 1 Median weight for each age group (closed squares) and line of best fit (linear regression plot - broken line) r 2=0.96. (age + 4) x 2 * open squares; (age x 3) + 7 * open triangles.

Dear Editor,

Drs Coulthard & Skinner are correct to highlight the potential dangers of clot embolisation from paediatric central lines. There is however an equally strong argument for aspiration the lines to remove heparin and prevent inadvertent systemic anticoagulation [1,2].

Haemodialysis lines in particular have relatively large volumes in comparison to patient size and it is not uncommon to have a line with a lumen volume of 0.8ml in a small (e.g 10 kg) child. If the line is locked with 5000 unit per ml heparin as the authors describe this means the line contains 4000 units of heparin or 400 units/kg bwt. This dosage of heparin, flushed into the child is more than five times the bolus dose for systemic heparinisation [3]. There are reported intances of children & adults comuing to harm from the flushing of central lines containing heparin.

Central lines should certainly be aspirated before use and in addition, the concentration of heparin reduced to the minimum necessary to prevent clotting (in our unit we now use 100 units/ml). Alternatively a different drug should be used such as Urokinase or Alteplase, or Citrate, where the difference between line locking dose & systemic dose is far greater.

Perhaps most importantly, paediatricians should recognise that the drugs we use to flush and lock vascular access are indeed drugs capable of inducing harm.

References:

1.Polaschegg H, Shah C. Overspill of catheter locking solution: Safety & efficacy aspects. ASAIO Journal (2003) vol 49 (6); pp 713- 715.

2.Moritz M, Vats A, Ellis D. Systemic anticoagulation and bleeding in children with hemodialysis catheters. Pediatr Nephrol (2003). Vol 18; pp 68-70.

3. BNF-C

Dear Editor,

The study by Jackson et al(1) attempts to fill a gap in our knowledge in a very vital area. Paediatricians in the United Kingdom have traditionally not included a blood pressure measurement as part of routine clinical assessment, as hypertension is not generally considered to be a common paediatric problem. Those who did check BP had to rely on normal values derived from European and North American studies. Blood pressures centiles for GB are therefore a big step in the right direction and warmly welcomed. The data have been pooled from large representative samples and the methodology appears to be robust. The authors have chosen the well tested traditional nine-centile system, which all British health professionals are familiar with. It does however raise a number of issues.

Firstly the observed blood pressure appears to be remarkably high in a significant proportion of paediatric population. This is most obvious in the pubertal boys, nearly quarter of whom would be labeled as hypertensive as per the definition suggested by the British Hypertension Society(BHS). In fact the BHS classification of blood pressure level states that the optimal BP for adults is a value of < 120 mm Hg systolic and < 80 mm Hg diastolic. Although < 130 mm and < 85 may be accepted as normal, any value above 130/85 is at least high normal if not hypertensive(2). This is not concordant with the international definition of high blood pressure as suggested by World Health Organisation and International Society of Hypertension. In our own cardiology practice we struggle to see such high blood pressure values even in operated patients with coarctation of the aorta! More over if the author’s suggested definition of hypertension (BP above 98th centile) is applied, many children currently labeled as hypertensive would fall in the category of high normal/normal blood pressure. For any clinician this is a challenging conundrum. One has to ask if it is wise to label these children as normotensive when clearly a few years down the line they would be classified as hypertensive by our adult physician colleagues. Does accepting this new definition of hypertension inevitably mean that we are choosing to ignore an opportunity to identify and influence an important risk factor for future coronary heart disease? There is a growing body of evidence to suggest that future risk for coronary artery disease may start as early as in fetal life. Tireless efforts by professional bodies to prevent risk factors for ischaemic heart disease have lent its support to achieve even lower values of BP in adults. On the contrary, adopting higher normal blood pressure values in adolescence is going to be difficult to justify and is likely to lead to confusion let alone reduction of future risk of coronary artery disease.

Secondly the BHS guidelines for management of hypertension recommend that younger patients (age<20 years) should not be presumed to have essential hypertension and should be investigated for an underlying cause. In the light of the current data set this would mean that a quarter of British pubertal males need investigations for an underlying problem, and if not are we choosing to ignore a potential renal/ reno-vascular condition.

Thirdly by adapting a new centile system for defining normal and high blood pressure we are choosing to differ from both our American and European counterparts. This is at a time when there is universal agreement on the definition of hypertension in adults. The blood pressure centiles in the North American population are based on more recent data (1999-2000 National Health and Nutrition Examination Survey) and in view of the ongoing obesity epidemic much lower cut off for defining hypertension was recommended(3). They also recommend re-labeling high normal blood pressure as Pre-hypertension in order to promote preventive measures such as healthy diet and activity, which is an indication for life style changes. Admittedly these centiles are somewhat labour intensive and time consuming to use in routine clinical practice. In fact for the busy clinician the formula suggested by Somu et al(4), may prove to be an easier and quicker tool to identify children with hypertension without compromising from scientific normalcy.

Incorporating the new British blood pressure centiles in clinical practice effectively translates into ignoring a substantial number of children who would otherwise be a target for lifestyle and perhaps medical interventions. This is contrary to the recommendations made by British Hypertension Society and endorsed by National Institute for Clinical Excellence(5). We do not therefore feel comfortable in adopting the new Blood pressure centiles and definition of normal and high blood pressure values in children. We call for an open debate regarding the right way forward.

References:

1. Jackson LV, Thalange NKS, Cole TJ. Blood pressure centiles for Great Britain. Arch Dis Child 2007;92:298-303.

2. Wlliams B, Poulter NR, Brown MJ et al. British Hypertension Society guidelines for hypertension management 2004 (BHS_IV): summary. BMJ 2004;328:634-40.

3. The fourth report on the Diagnosis, Evaluation and Treatment of High Blood Pressure in Children and Adolescents. Pediatrics 2004;114:555-576.

4. Somu S, Sundaram B, Kamalanatham AN. Early detection of hypertension in general practice. Arch Dis Child 2003;88:302.

5. Hypertension: management of hypertension in adults in primary care. NICE clinical guideline 34. Issue date June 2006.

Dear Editor,

Luscombe and Owens (1) propose a new formula (weight(kg)=3(age)+7) for accurately estimating weight in the 21st century child. They point out that the classical formula (weight(kg)=2(age+4)) underestimates actual weight in the majority of cases and that this has implications for potentially 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 1ml 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 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 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 fifty 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.

References:

1. Luscombe M, Owens B. Weight estimation in resuscitation: is the current formula still valid? Arch Dis Child 2007;92:412-5.

2. Holliday MA, Segar WE. The maintenance need for water in parenteral fluid therapy. Pediatrics 1957;19:823-32.

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;278:E308-15.

4. Hatherill M. Rubbing salt in the wound. Arc Dis Child 2004;89:414-8.

5. Raes A, Van Aken S, Craen M et al. A reference frame for blood volume in children and adolescents. BMC Pediatr 2006;6:3.