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"Difficulty in advancing the wire could be suggestive of looping or other source of malposition" Spiro et al (2022).
Guidewire looping during central line insertion

Abstract:

Central venous cannulation is not a risk-free undertaking, and multiple punctures can increase the risk of complications [2]. Dilation can be particularly dangerous over a malpositioned wire [3], but wire manipulation may lead to accidental knotting or tangling on other structures [4]. While imaging of a wire within a vessel can be useful, it does not rule out looping. Difficulty in advancing the wire could be suggestive of looping or other source of malposition.

Reference:

Spiro JA, Pearsall MF, Daly Guris RJ. A novel solution for guidewire looping during central line insertion. Anaesth Rep. 2022 Apr 13;10(1):e12162. doi: 10.1002/anr3.12162. PMID: 35434634; PMCID: PMC9006226.