The team decide that some pain relief would be appropriate for Toby. Imagine that you are in the pre hospital setting and you need to give drugs to a child. You may be giving something simple like oral pain relief or more complicated like IV pain relief or resuscitation drugs. Whatever the drugs, in children we give a dose based on weight. Sometimes the parents will have a good idea of what the child weighs (particularly in small babies) but many will have no idea. So you need to have a way of estimating the weight in order to prescribe drugs.
Up until fairly recently, most people dealing with children used the formula ([age+4] x 2)kg to estimate the weight of a child (for children up to the age of 10). However, we are becoming increasingly aware that in the developed world children are getting heavier and this formula tends to underestimate weights, particularly in older children. In response to this, the advice from ALSG (advanced life support group) is to use the old formula for children from 1-5 years and a new formula ([agex3] +7) for older children. For the under ones, ALSG recommends ([agex0.5]+4). In reality, as mentioned previously, parents will often have a good idea of the weight of children in this group (although in the UK they will often give that weight in pounds which will mean a conversion as drugs are prescribed in dose/kg).
There continues to be a lot of debate about the use of formulae for the estimate of weight in overweight children in which the formula will underestimate their weight. The use of population growth charts (which allows estimation of medians for any age, as well as 10th and 90th centiles) is common in hospital practice but is not practical in the pre hospital setting. Tapes such as Broselow are widely used and provide a quick estimate of weight along with corresponding doses for resuscitation drugs etc but many people will stick with formulae as they are quick and simple. Whilst the jury is out over whether we should use lean weights for drug dosing in obviously overweight children, no one would be criticised for working on a formula calculation of average weight in the pre hospital setting. Leave the debates over which weight should be used to the clinicians in the Emergency Department.
For children over 12, ALSG has never advocated a formula – the onset of puberty from 10 years onwards means that there is an even greater range of possible weights at any one age. A formula would therefore have a high likelihood of error. In addition, at this age, weights approach the maximum weight used to calculate drug doses and equipment sizes, ie weights of 40kg or 50kg. For inexperienced practitioners facing the difficulty of estimating the weight of older children, growth charts would be ideal, but if not available, an educated guess based on how close they are to the size of an adult, would make sense. Children who are clearly small for their age can have their weight estimated by the ([agex3] +7) was we know this will underestimate in this age group. Personally, I also take a look at older kids and compare them to myself. If you know what you weigh, this can be a pretty effective way of estimating weights in older children.
Have a look at these questions and discuss your answers below:
- Calculate an estimated weight for Toby who is a 5 and a half year old (nearly 5 and three quarters!) boy.
- Using this weight what dose of IV morphine would be appropriate?
- Does anyone have any concerns using morphine in the setting of a child with a head injury?