The present disclosure relates to structures that are adapted to facilitate the retention, storage, and transport of arrows. The disc fixing apparatus comprises a clamp engaging the hub and comprising a reference surface associated with the clamp axis of rotation. Due to the unique non-equilibrium response mechanism of such sensors, rotation of the polyion-sensitive membrane electrodes at 5000 rpm resulted in an enhancement in the detection limits toward heparin and protamine of at least 1 order of magnitude over that observed when the EMF responses of the same electrodes were assessed using a stir-bar to achieve convective mass transport. In this form of the invention, the AV interval is reduced by one millisecond per one bpm increase in heart rate but maintained at a value calculated from the end of the P-wave and the beginning of the QRS complex or delivery of a ventricular pacing stimulus or to the end of the end of the QRS complex. Also provided are compositions and methods for diagnostic and prognostic determination of such disorders.