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Objective Injection Pressure Monitoring

   

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

Advantages

Whereas US is highly user, technique, equipment and anatomy-dependent and nerve stimulation is specific but relatively insensitive, opening injection pressure monitoring reliably detects abnormal resistance to injection, regardless of who performs the injection or number of needle repositions.

When injection pressure matters, an objective and quantifiable method of monitoring and documenting injection pressure is superior to the subjective syringe-hand-feel technique. Resistance to injection is a part of the standard documentation procedure during nerve blockade. The documentation typically refers to whether the resistance to injection was “normal” or “high” and the course of action if it was abnormal. In the past, documentation of the resistance was merely subjective and relied on the “learned feel” and experience of the provider.

Studies of experienced practitioners blinded to the injection pressure and asked to perform a mock injection using standard equipment reveals wide variations in applied pressure, some grossly exceeding the established thresholds for safety.29 Similarly, providers perform poorly when asked to distinguish between intraneural injection and injection into other tissues such as muscle or tendon in an animal model.30 It is therefore important to use an objective and quantifiable method of gauging injection pressure.

Monitoring a clinically relevant and objective pressure in real-time can help prevent an injection into a fascicle before it occurs. In a study of intraneural injections in canine sciatic nerves, a slow injection of local anesthetic while the neede tip was placed intra-fascicular was associated with an immediate and substantial rise in the pressure in the syringe-tubing-needle system (>20 psi), followed by return to low pressure (<5 psi) as the fascicle ruptured and local anesthetic leaked in the neighbouring tissue. In contrast, perineural and extrafascicular injections were associated with low opening injection pressure < 15 psi.31 Moreover, those limbs in which the nerves were exposed to high injection pressures developed clinical signs of nerve injury as well as histological evidence (inflammation, disruption of the nerve architecture). 

The implication is that injection into a low compliance compartment, such as within fascicles is associated with a high opening injection pressure, which if not stopped can either directly damage axons and/ or rupture the tough protective barrier- perrineurium leading to a nerve injury.

Can help prevent injections against the epineurium. Injection pressure monitoring reliably detects needle-nerve contact. In a clinical study of patients undergoing interscalene brachial plexus block using an automated injection pump, the observation of high (>15 psi) pressures prior to initiation of flow predicted the apposition of needle tip against the nerve trunk in 97% of cases as confirmed by ultrasound.1 In other words, flow could not be initiated below the threshold level of 15 psi in the vast majority of instances where the needle was in contact with the nerve, and the injection was aborted. This is important, as initiation of flow against the epineurium may result in an intraneural injection or nerve inflamation, and consequent nerve damage.

Limitations

It’s important to know what injection pressure provides and the limitations.

Although highly sensitive, injection pressure is not specific. As such, opening injection pressure can not distinguish between intrafascicular needle position or needle-nerve contact and other causes, such as needle obstruction by blood clot, tissue fascia, tendon, etc. Regardless, perineural injection normally requires low opening pressure (<15 psi) since it occurs in the loose connective tissue around nerves. Therefore, presence of high injection pressure (>15 psi) is rarely normal and should be avoided.

The accuracy of the dynamic injection, once injection has begun is impacted by numerous factors. It is important for practioners to understand that injection pressure monitoring is most clinically relevant for monitoring opening pressures, before the injection has begun.