Methods and systems for classifying cardiac responses to pacing stimulation andor preventing retrograde cardiac conduction are described. The stream table includes a valid field and a tag field. The second frame is moveable, with respect to the first frame, between a raised position, where the transverse element provides a back support, and a lowered position where the transverse element provides a rung of a boarding ladder. Conditional immortalisation provides a constant supply of clonal cells that produce microparticles such as exosomes.