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Significance of Opening Pressure


Jeff Gadsden, MD. Published by B. Braun Medical Inc.

The Two Phases of Injection

Implications for objective injection pressure monitoring.

Isostatic Phase (opening pressure to start the flow)

There is no flow at the tip of the needle. A certain (opening) pressure must be reached within the syringe-tubing-needle system in order to INITIATE the injection into a tissue compartment. During this phase the pressure is equal throughout the entire closed system (including the needle opening; (Pascal’s law)32 and is therefore not impacted by needle or syringe size or the speed of injection.

Aborting injection when opening pressure is above  (>15 psi) can prevent the injection against the nerve, into a fascicle, or into other, undesired low compliance tissues (e.g., tendons, tissue fascicle) before it occurs. In this case, a slight withdrawal of the needle and reconfirmation of the needle tip position is suggested before initiating injection.

Dynamic Phase (during the flow)

Once the opening pressure is overcome the injection is initiated, the injection pressure becomes affected by the rate of injection and the flow characteristics of the fluid passages including needle or syringe size and speed of injection (Bernoulli’s principle)33

Regardless, avoidance of high pressures using an objective monitoring device can prevent too rapid, forceful injection.
Monitoring for opening pressure becomes relevant any time the operator repositions the needle for additional injection.

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