Techniques are provided for estimating left atrial pressure or other cardiac performance parameters based on measured conduction delays. The medical system also comprises at least one invasive unit, wherein the invasive unit has at least one invasive consumable. The freeze-protected solution has a freeze point of 5\xb0 C. The stud may be biased by its weight or a spring so that its head is normally lowered against the mouth of a socket, but is shaped so that it can be wedged up by the hook member as that is offered up.