Abstract: In one aspect, a method and system for detecting a change in fluid dynamics of a fluid flowing through an extra-corporeal circuit is disclosed, which includes establishing an acoustic wave resonance across a transverse dimension of at least a portion of a line associated with the extra-corporeal circuit through which the fluid flows, monitoring a phase signal of the resonant acoustic wave, and identifying occurrence of a change in fluid dynamics of the flowing fluid when the observed phase signal of the resonant acoustic wave indicates a deviation from the expected fluid flow signature. The change in fluid dynamics can be used to indicate a venous needle dislodgement event.