by Rafael Ferrandiz, Ph.D. on September 16, 2014
Summary: Intramuscular injection is recommended for fast adrenaline effect. The needles of some currently available auto-injectors are too short to reach the thigh muscle in many patients. Adrenaline auto-injectors with longer needles are needed.
Intramuscular injection recommended
The intramuscular (IM) injection of adrenaline into the outer thigh is recommended . A significantly higher adrenaline peak in plasma is achieved, both in adults and children, when the injection is performed IM compared to the subcutaneous (SC) route. Moreover, the time to peak adrenaline is also reached four times quicker via IM injection [2, 3].
The needles are too short in many AAI
An often discussed problem has been the absence of an AAI with a needle sufficiently long enough to reach the thigh muscle, and the impact of this on treatment outcome [4-8]. Needles in most of the currently available auto-injectors are too short to penetrate the subcutaneous fat and connective tissue. This compromises IM injection achievement in many children, teens and adults.
Several studies have shown that the skin to muscle distance (STMD) in many patients is larger than the needle length of their auto-injectors. The exposed needle lengths for Anapen, EpiPen and Jext are 7.5 to 15 mm  with the subsequent risk of subcutaneous injection with those AAIs well documented [8, 10-16].
Women represent a particularly high risk group. Computed tomography of the STMD in the thigh showed up to 42% of women risk have a SC injection with EpiPen due to insufficient needle length . This failure is also reflected in an American study where 54% of women were also found to be at risk of not receiving an intramuscular injection with existing auto-injectors . In a recent study from the UK, Hobbins et al concluded that ”Jext and EpiPens have insufficient needle length to administer IM adrenaline to vastus lateralis in 68 % of our patient group” .
This problem is also reflected in children. In a paediatric study, 12% of children weighing less than 30 kg had STMD greater than 16 mm. These children would, therefore, not receive adrenaline intramuscularly with some currently available AAIs. Furthermore, in children weighing more than 30 kg, 30% of the children would not receive intramuscular adrenaline. The authors concluded that the AAI needles are too short to reach the muscle in significant numbers of children. They also point out that increasing the needle length of auto-injectors would increase the likelihood that more children receive adrenaline by the recommended intramuscular route .
In a similar study, carried out at Manchester Childrens’ Hospital, 82% of overweight and 25% of normal weight children were at risk of receiving a SC injection through using an AAI with a short needle. Bewick et al postulated that “A new epinephrine auto-injector […] with longer needle lengths […], will improve the chance of intramuscular injection in patients who are obese“ .
The needle length is crucial
It has been assumed that AAIs may increase injection depth through compression of the muscle and by the expulsion forces . This theory is misleading. An impermeable connective tissue (e.g. fascia lata) separates the subcutaneous fat and muscle compartments. Adrenaline can not pass through this fascia by expulsion force. To ensure an IM injection, the needle tip must pass through the connective tissue and reach the muscle. Moreover, Pumphrey et al showed a longer needle gives a deeper injection .
The recommended needle: 25 mm or more
In the U.S. the optimal needle for IM penetration in children has been studied by magnetic resonance imaging and computed tomography scans . In order to ensure IM administration into thigh muscle, the U.S. Centers for Disease Control and Prevention recommend needles of 16 mm in newborn children, 25 mm for children 1 to 12 months old, and needles from 25 to 31 mm for children up to 18 years .
For an IM injection the UK Resuscitation Council guidelines for healthcare providers recommend a 25 mm needle for all ages. For small infants, a 16 mm needle is suitable. In some cases a 38 mm is needed .
Large groups of patients, particularly women and children, are at risk of receiving a subcutaneous injection when using AAIs with short needles. A longer needle increases the chance of intramuscular injection in more patients.
Emerade adrenaline auto-injector has a longer needle, 25 mm for 500 and 300 micrograms, 16 mm for 150 microgram .
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