Introduction Consistency in needle tip positioning within interfascial planes while performing infiltrative blocks under ultrasound guidance can be difficult. The exact determination of such planes may beyond the physical limits of common ultrasound machines. Aim of this pilot study was to understand if real-time continuous injection pressure monitoring at the needle tip, combined with ultrasound guidance, can help to immediately and consistently identify an interfascial plane needle tip placement.

Methods We performed four ultrasound-guided transversus abdominis plane (TAP) blocks on fresh cadaver using a modified conventional peripheral nerve block needle. The sensing needle contains Fabry-Perot miniaturized pressure sensor floating at the needle tip, connected to a measuring unit via an optical fiber. Injection-pressure measured at the needle tip was continuously recorded, while the needle was advanced toward the target and 0.9% saline was continuously injected via an electronic pump.

Results A recognizable, recurrent three-peaks injection pressure pattern was identified, while advancing the needle through the abdominal wall, the pressure peaks being identified with the needle to fasciae contact. In four different blocks, a total of 12 peaks and 12 troughs were identified. The mean injection pressure (95% CI) of the peaks varied substantially from the mean injection pressure of the troughs, from 119.55 kPa (95% CI 87.3 to 151 kPa) to 30.99 kPa (95% CI 12.5 to 47.5 kPa), respectively. The peaks (troughs) arose from reproducible pressure curves and were related to the needle tip encountering the muscle fasciae.

Conclusion The identified injection pressure pattern, together with ultrasound image, may help in determine real-time the needle tip position, while performing a TAP block.