The deep fascia of the thigh forms an impenetrable barrier to fluid injected subcutaneously by autoinjectors.

Needle projection was measured from where the needle protruded from the plastic casing to the proximal edge of the needle lumen. Fluid jet velocity through the EAI at 21 C was determined by calculating the [distance moved by the plunger]/[injection time] [plunger cross-sectional area]/[needle lumen area]. Injection time is from start to finish of the plunger movement. n 1⁄4 3. *Mann-Whitney U test P value <.05 compared with EpiPen 300. Fluid injected from epinephrine autoinjectors into subcutaneous tissue is prevented from penetrating into the muscle by the deep fascia of the thigh. Intramuscular injection will not occur during firing of an autoinjector if the needle tip is in the subcutaneous tissue.