There is disclosed a portable electronic instrument including a case, at least one electronic unit housed inside the case, and at least a first input andor output terminal accessible from the exterior of the case and including an electrically conductive element which is mounted so as to be mobile with respect to the case. The respective optical path lengths of main and auxiliary optical paths differ from each other between the first and second optical couplers and between the second and third optical couplers. It is illuminated with linearly polarized light with the polarization direction at 45 degrees relative to the strain-induced fast and slow axes in the photoelastic material. A generally central binder attachment fitting is positioned atop the load, with each of the lines then being removably and adjustably attached to the fitting. Auto-capture of a cardiac pulse using pacing stimuli is further provided.