Detection of intraneural needle-placement with multiple frequency bioimpedance monitoring: a novel method

Abstract

Electrical impedance measurements have been used to detect intraneural needle placement, but there is still a lack of precision with this method. The purpose of the study was to develop a method for the discrimination of nerve tissue from other tissue types based on multiple frequency impedance measurements. Impedance measurements with 25 different frequencies between 1.26 and 398 kHz were obtained in eight pigs while placing the tip of a stimulation needle within the sciatic nerve and in other tissues. Various impedance variables and measurement frequencies were tested for tissue discrimination. Best tissue discrimination was obtained by using three different impedance parameters with optimal measurement frequencies: Modulus (126 kHz), Phase angle (40 kHz) and the Delta of the phase angle (between 126 and 158 kHz). These variables were combined in a Compound variable C. The area under the curve in a receiver operating characteristic was consecutively increased for the Modulus (78 %), Phase angle (86 %), Delta of the phase angle (94 %), and the Compound variable C (97 %), indicating highest specificity and sensitivity for C. An algorithm based on C was implemented in a real-time feasibility test and used in an additional test animal to demonstrate our new method. Discrimination between nerve tissue and other tissue types was improved by combining several impedance variables at multiple measurement frequencies.

Keywords: Bioimpedance; Monitoring; Needle; Nerve; Nerve stimulation; Regional anaesthesia.

Figures

Fig. 1

Study setup: The tip of a stimulation needle was placed under ultrasound guidance within the sciatic nerve and other tissue types (a and b). An impedance measurement system (Solartron 1260 and SI 1294) was connected for 3-electrode impedance measurements (c). Impedance as function of frequency was obtained by sweeping the excitation frequency in 25 logarithmically distributed steps from 1.26 to 398 kHz

Fig. 2

Measurements of the impedance Modulus in a frequency range from 1.26 to 398 kHz when the needle tip was positioned in intraneural tissue (a), paraneural tissue (b), muscle (c) and subcutaneous fat (d). Each color represents repeated measurements from one test animal. The y axis gives the Modulus in ohms; the x axis gives the measurement frequencies in Hertz on a logarithmic scale

Fig. 3

Measurements of the impedance Phase angel in a frequency range from 1.26 to 398 kHz when the needle tip was positioned in intraneural tissue (a), paraneural tissue (b), muscle (c) and subcutaneous fat (d). Each color represents repeated measurements from one test animal. The y axis gives the Phase angle in degrees; the x axis gives the measurement frequencies in Hertz on a logarithmic scale

Fig. 4

Parameters used to discriminate intraneural needle positions from positions in other tissue types. Statistical analysis (PCA) showed best tissue discrimination at 126 kHz for the Modulus (a), at 40 kHz for the Phase angle (b), at a Delta phase angle between 126 and 158 kHz (c). Tissue discrimination was further improved by combining these measurements in a Compound variableC (d). The box plot denotes median, quartile, range and outliers. The quoted p values relates to the statistical differences of means versus intraneural needle positions. Not significant differences are denoted n.s

Fig. 5

Receiver operating characteristic curve (ROC) for the four parameters used to discriminate intraneural needle placement from other tissue types. True positive values (sensitivity) and False positive values (100 \%-specificity) are plotted while increasing cutoff in steps of 5 % from the lowest to the highest values. The area under the curve (AUC) is consecutively increasing for the Modulus (78 %), Phase angle (86 %), Delta (94 %) and the Compound variable C (97 %). Best specificity and sensitivity can be obtained by using the compound parameter C. The predefined cutoff value C equals 1 that was used for our prototype (See video 1, electronic supplementary material) is labeled with (1)