Background: Nerve damage can occur after ultrasound-guided peripheral nerve block (PNB). Injection pressure monitoring could improve the safety of PNB. The aim was to analyse parameters affecting pressure measurements during PNB.Methods: The flow characteristics of needles connected to a pressure-sensing device were evaluated. Needles were placed under ultrasound guidance extra or epineurally in nerves/plexus of fresh cadavers. Using three flow rates, 4 mL of saline was injected and plateau pressure was measured. Finally, orthopaedic surgery patients receiving PNB were enrolled for an observational real-time pressure monitoring study. During PNB, periods with pressure > 50 mmHg were noted (high pressure ≥ 750 mmHg). A blinded investigator recorded injection pressure curves and peak pressure.Results: The needle diameter influenced the injection pressure (β = 66.8; P < 0.0001). Non-echogenic needles increased the injection pressure (β = 82; P = 0.0009) compared with echogenic needles. Cadaver injection pressure was higher for intraneural (255 [122.5-555] mmHg) versus extraneural needle tip location (90 [50-158] mmHg; P < 0001); for high flow (9.6 mL/min; 470 [265; 900] mmHg) versus low flow (1.2 mL/min; 120 [71-250] mmHg) (P < 0.001) and for cervical roots (900 mmHg, intraneurally) compared with nerves (300 mmHg, intraneurally). In 37 patients and 61 procedures, there were 7 [1-18] peaks of injection pressure per procedure. Pressure was noted > 750 mmHg during 13.80% of the procedural time.Conclusions: Needle diameter, needle tip location, type of nerve/plexus, flow rates, and the anaesthetist can have a significant effect on injection pressure values and monitoring.