Saturday, 4 July 2015

More?!? The Salbutamol Paradox



Dosing in children is quite frankly a nightmare.  While formularies such as the BNFc are excellent and easy to follow, there are always pitfalls.  There are maximum doses and there are age related doses, weight related doses and doses that are the same at any age or weight.

Despite all the potential to get it wrong, I am pleased to report that with very few exceptions, the children presenting to the Paediatric Emergency Department where I work are prescribed the correct doses of their medicines.  This is less often the case for inhalers.  It is not uncommon for me to send a 2 year old child home with instructions to continue salbutamol (via a spacer of course) four puffs four hourly and then have another health care professional advise the parent that this is too large a dose.  If the next healthcare professional that they see applies the 'smaller children need smaller doses' principle, the parent may be advised to reduce the amount of bronchodilators to two or even one puff every four hours.

I can see the logic there.  Why would you give more puffs of inhaler to a 2 year old than a 20 year old?  Inhaled bronchodilators are a perfect example of why therapeutics in children is not governed by one or two simple principles.  In the case of treating wheeze in children it is not the size of the child or the age of the age of the child.  Once more we must turn to physics for the answer.

Delivery of the bronchodilator to the airways of the lung works best when you have good inspiration, without turbulence, through open tubes.  In a two year old, you have none of these things.  Crying looks as though it should do the job but studies have shown this to be wrong.  Aerosol deposition during crying is poor.  Even if the child is complying, they are still a victim of physics.  The flow through any tube is inversely proportional to the diameter to the power four.  This means that if the wheeze has halved the diameter of a bronchus, one sixteenth of the air will get through.  If there is turbulence of that air flow then the flow is at least halved again.  So, assuming mucous is rattling around, it doesn't take much bronchospasm to reduce the air getting to your alveoli substantially.  When you are two years old, your trachea is about 6-7mm internal diameter and your bronchi even smaller, so any reduction from bronchospasma and mucous is going to have a huge impact.



Suddenly four puffs doesn’t seem like a lot to give.  I wonder if we studied the salbutamol paradox properly if we would find that there is an argument for age banding salbutamol reliever puffs so that we give even more to the under 5 year olds as a standard reliever dose.


Edward Snelson
Using Real Science to save small lives
@sailordoctor

Disclaimer:   None of the children in medical research have ever consented to being a point mass in a vacuum. 

The Salbutamol Paradox is based on the ladder paradox, a physics thought experiment that involves getting something that seems too big into what appears to be too small a space.  Just like the ladder paradox, the salbutamol needed for a child's wheeze goes up substantially as the pace of the clinical scenario increases.


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