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Injection line pressure - inaccurate but still useful?

Last post 19 Feb 2019, 10:35 AM by Saporito Andrea. 1 replies.
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  •  05 Feb 2019, 12:01 PM 2729

    Injection line pressure - inaccurate but still useful?

    Barrington and Lirk [1] and Saporito et al [2] make many valid arguments with regard to pressure monitoring during regional anaesthesia. However, I would like to dispel the implicit assumption that because injection line pressure is not an accurate reflection of needle-tip pressure, it is not useful. Being a positive pressure system, in-line pressure is equal or higher than the needle-tip pressure. If we are guided by in-line pressure, we err on the side of lower and therefore, safer needle-tip pressure.

    There is one scenario where in-line pressure is equal to needle-tip pressure and that is when there is no flow, assuming a patent fluid system. The in-line pressure at which flow starts, that is, the "opening pressure", will then be a reasonably accurate (if safer) reflection of needle-tip pressure.  

    As one of the authors of a paper [3] cited in the editorial [1], I regularly use the clinically improvised pressure gauge we described, which is a three-way tap with an air-filled 1 ml syringe inserted between the needle and the syringe, in my practice as an orthopaedic anaesthetist. I apply the principles described above, with the added bonuses of high availability, low cost and quick construction.  

    M. Lim

    Cardiff and Vale University Health Board,

    Cardiff, Wales.


    No external funding and no conflicts of interest declared.


    1. Barrington MJ, Lirk P. Reducing the risk of neurological complications after peripheral nerve block: what is the role of pressure monitoring? Anaesthesia 2019; 74: 9-12.  

    2. Saporito A, Quadri C, Kloth N, Capdevila X. The effect of rate of injection on injection pressure profiles measured using in‐line and needle‐tip sensors: an in‐vitro study. Anaesthesia 2019; 74: 64–8.

    3. Patil J, Ankireddy H, Wilkes A, Williams D, Lim M. An improvised pressure gauge for regional nerve blockade/anesthesia injections: an initial studyJournal of Clinical Monitoring and Computing 201529: 673–9.

    Dr Michael Lim
    Consultant Anaesthetist
    Cardiff and Vale University Health Board
  •  19 Feb 2019, 10:35 AM 2738 in reply to 2729

    Re: Injection line pressure - inaccurate but still useful?

    We agree with Lim that routinely monitoring the injection pressure during regional anaesthesia should be a safety standard. The monitoring system adopted must however be as reliable as possible, reproducible and unaffected by speed of injection. As shown in our previous articles, this can be achieved only by measuring injection pressure at the needle tip [1, 2].

    At first sight, the fact that measuring injection pressure along the injection line overestimate real pressure values can lead to erroneously assume that it does not jeopardise safety. In fact, as noted by Quadri et al. [1], threshold values supposed be able to discern between intra- and extraneural injections values measured along the injection line are valid only assuming those variables (speed of injection, needle and tube diameter) by which they were determined. Therefore, intraneural injection could pass unrecognised when injecting at lower speeds.

    As correctly noted by Lim the in-line pressure equals the tip pressure only in case of no flow through the needle. In this respect, Hadzic et al. [3] proposed a definition of the opening injection pressure as the peak pressure at the beginning of injection (during the first 10–15 seconds), stating that a certain pressure must be overcome (opening pressure) to initiate injection. However, the reality is that fluid starts flowing out of the needle and into tissues since the very beginning of the injection procedure and peak pressure occurs during the injection. Vermeylen et al. [4] have recently demonstrated this phenomenon during simulated nerve blocks of the lower limb in fresh human cadavers. They redefined the opening injection pressure as the peak pressure in the 60 seconds interval, during which the injection is initiated. As such, opening injection pressure has an unclear definition and is confusing. We would suggest to simply refer to peak injection pressure in order to identify the maximum pressure reached during injection.

    For all these reasons we are convinced that injection pressure monitoring shall be measured at the needle tip. This may lead to the necessity of redefining new standards in peripheral nerve blocks and new reproducible and objective threshold values in injection pressure monitoring.




    A. Saporito

    Bellinzona Regional Hospital,

    Bellinzona, Switzerland.

    C. Quadri

    Lugano Regional Hospital,

    Lugano, Switzerland.

    N. Kloth

    See‐Spital Horgen and Kilchberg,

    Horgen, Switzerland.

    X. Capdevila

    Montpellier University Hospital,

    University Montpellier, France.

    E-mail address: andrea.saporito@eoc.ch



    1. Quadri C, Saporito A, Capdevila X. Real-time continuous monitoring of injection pressure at the needle tip for peripheral nerve blocks: description of a new method. Anaesthesia 2018; 73: 187–94.

    2. Saporito A, Quadri C, Kloth N, et al. The effect of rate injection on injection pressure profiles measured using in-line and needle-tip sensors: an in vitro study. Anaesthesia 2019; 74: 64-8.

    3. Hadzic A, Dilberovic F, Shah S, et al. Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs. Regional Anesthesia and Pain Medicine 2004; 29: 417–23.

    4. Vermeylen K, Hermans M, Soetens F, et al. Opening injection pressure is higher in intraneural compared with perineural injections during simulated nerve blocks of the lower limb in fresh human cadavers. Regional Anesthesia and Pain Medicine 2017; 42: 362–7.

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