The contributions of Carl Ludwig to cardiology.

The basic instruments for measuring functional cardiovascular parameters and the most important discoveries made by Carl Ludwig and his disciples in cardiovascular physiology are described and put into perspective in regard to the further development of his methods and ideas. The most important apparatus was the kymograph, which, for the first time, made recording and documenting of functional parameters possible. This instrument was also used for the functional evaluation of the isolated perfused frog heart that was developed by Elias Cyon in Ludwig's Leipzig Physiological Institute. In the isolated frog heart, important phenomena were discovered such as the staircase ('Treppe'), the absolute refractory period and the all-or-none law of the heart. The isolated dog heart was used to determine the origin of the first heart sound, which was characterized as a muscle tone. To measure regional blood flow and eventually cardiac output, a flowmeter ('Stromuhr') was designed. Precise measurements of cardiac output became possible only when Adolf Fick had developed his principle, which served as the basis for the modern indicator methods. Cyon and Ludwig also discoverd the depressor nerve, which constitutes the basis of the baroreceptor reflex. Finally, the precise localization of the vasomotor centre in the ventrolateral medulla was achieved in Ludwig's Leipzig Physiological Institute. This was confirmed more than 100 years later with modern neuroanatomical methods making use of retrograde axonal transport. Thus, Ludwig and his scholars made major substantial contributions to cardiovascular knowledge that can be considered to constitute the basis of modern cardiology.