At an excellent Anaphylaxis Campaign conference last week (to be reported on in the next Foodsmatter newsletter) I spent a good part of the lunch break ‘listening in’ on a fascinating discussion about needle length in anaphylaxis treatment.
As most of you will know, the treatment for anaphylaxis is an immediate injection of adrenaline/epinephrine given in the thigh. However, why the thigh and is the length of the needle important?
Very briefly, with apologies to those who already know all this, when you go into anaphylactic shock the allergen ‘provokes’ a series of chemical reactions which result in a massive release of histamine. This causes angioedema or swelling of the face, the throat and the airways, creating wheeze and, in those who also suffer from asthma, often triggering a major asthma attack. (It is believed that deaths from anaphylaxis over the years may have been seriously under-reported as they were mistaken for straight asthma attacks, rather than anaphylactic reactions.)
However, histamine also causes the small blood vessels, or capillaries, to become leaky. So much fluid seeps out of the blood vessels into the cells that there is no longer enough fluid to keep the main vascular blood system flowing, so blood pressure plummets, depriving the major organs of sufficient blood to keep them operational.
Adrenaline reverses the symptoms of anaphylaxis by acting on alpha and beta adrenergic receptors in the body.
Alpha receptors are found on the walls of blood vessels. When adrenaline stimulates these receptors this causes the blood vessels to narrow, which stops the blood pressure from falling too low. It also redirects blood to vital organs like the heart and brain.
Beta receptors are found in the heart and lungs. When adrenaline stimulates these receptors this relaxes and opens the airways, making breathing easier. It also stimulates the heart, making it beat faster and stronger. The adrenaline also relieves itching, hives and swelling.
(Thanks for NetDoctor for this helpful explanation.)
Fine – but why the thigh?
Well, injecting adrenaline into the muscle of the outer mid thigh means that you are very unlikely to hit any nerves, tendons, arteries or veins by mistake and it is one of the least painful places to inject. But, more importantly, the adrenaline will get absorbed into the system much quicker if it is injected into a major muscle such as the thigh muscle which will propel it round the body than if it is injected into a relatively small muscle in the arm which may only distribute it locally.
However, injecting into the thigh has relevance also as far as needle length is concerned.
Adrenaline injections were originally, apparently, devised for the treatment of soldiers who would normally be fit young men with muscular thighs. No problem about the needle delivering the adrenaline/epinephrine directly into the muscle there. But what about these days when many of those suffering from serious allergies may be rather unfit, well covered young men, with a thick layer of adipose tissue between the skin and the muscle? And what about girls and women? Even relatively slim women have a much thicker layer of flesh covering the muscles on their thighs than young men – that is the way they are built. So are the standard length needles, designed for fit male thighs, long enough too reach the muscle in girls or heavier men and deliver the life saving adrenaline/epinephrine to where it is needed?
This, it would seem, is a hotly argued topic in the injector pen world although it would seem a bit of a no-brainer to me, especially since other treatments and therapies already use differing length needles. Diabetics, for example, get different length of needles for their injector pens depending on their body weight, while acupuncturists use longer needles to access points on the meridians of heavier patients.
But this discussion may be of more than academic importance. The death of 13-year-old Natalie Giorgi from peanut anaphylaxis in July this year after her father, a physician, had injected her with no less than three doses of epinephrine via three Epipens, could have resulted from her father waiting too long to administer the first dose. Or could it be that because Natalie was a girl, even though quite a slim girl, the epinephrine was not getting through the flesh of her thigh properly to reach the muscle? And Natalie is not the first girl to have died from anaphylaxis even though more than one Epipen had been administered.
Would, therefore, three or four different injector pens with differing lengths of needles, depending on the sex and weight of the patient, not seem a way to go?
18th October. Alex has just forwarded me a link to the following research just published: Predictors of epinephrine autoinjector needle length inadequacy. American Journal of Emergency Medicine.
2nd May 2014.
The European Medicine Agency has just set up a review of autoinjectors looking at, among other things, needle length. For more see here.
11th May 2014
Ruth Holroyd suggests that all women who carry autoinjectors should get an ultrasound check of their thigh to find out how deep their muscle is seated and what is the optimum point for injection. See her blog here.