Are adrenaline autoinjectors fit for purpose? A pilot study of the mechanical and injection performance characteristics of a cartridge- versus a syringe-based autoinjector

Background: Adrenaline autoinjectors (AAIs) are prescribed to facilitate the intramuscular administration of adrenaline in patients diagnosed with life-threatening anaphylaxis. This pilot study investigated the injection and functional properties of two AAIs (deploying different delivery systems) under standard conditions, after dynamic and mechanical stresses, and in the presence of denim. Methods: The differences between a cartridge-based AAI (EpiPen ® Junior) and a syringe-based AAI (Anapen ® Junior) were assessed using three sets of tests. Test 1: under standard conditions, the injection depth and dose were measured in ballistic gelatine (a validated tissue simulant). Test 2: before the safety cap removal and activation forces were measured, AAIs were subjected to either of two preconditioning tests: 1) free-fall drop test; or 2) static load (ie, 400 N, equivalent to 40 kg weight) test; or 3) no preconditioning. Test 3: under standard conditions, injection properties into ballistic gelatine in the presence and absence of denim were investigated. Statistical analyses were performed using the Student’s t -test or Welch’s test. Results: The maximum depth of delivery was significantly greater with cartridge AAI (n = 4, mean 21.09 ± 2.54 mm) than with syringe AAI (n = 5; mean 11.64 ± 0.80 mm; P = 0.003). After 2.5 seconds, cartridge AAI (n = 4) discharged significantly more dose than syringe AAI (n = 3; 74.3% versus 25.7% of total dose; P = 0.001). Both cartridge and syringe AAI withstood the free-fall drop test, but almost all devices failed to activate following the static load test. Under standard conditions, significantly less force was required to remove the safety cap of cartridge AAI than syringe AAI (both n = 15; mean 9.56 ± 2.36 N versus 20.23 ± 6.61 N, respectively; P < 0.001), but a significantly greater activation force was required for cartridge AAI than syringe AAI (mean 23.01 ± 3.96 N versus 8.06 ± 0.51 N, respectively; P < 0.001). The presence of denim did not alter the activation force or effective needle length of either of the AAIs. Conclusion: Cartridge AAI appears significantly more capable of consistently and rapidly delivering a clinically relevant dose of intramuscular adrenaline than syringe AAI. However, both devices showed shortcomings in their ability to sustain mechanical stress similar to that which is likely over their shelf life, and as such, may not be fit for life-saving purpose. Keywords: anaphylaxis, intramuscular, delivery system, device