Injection line pressure - inaccurate but still useful?

Abstract

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.