Chasing the wave. Unfashionable but important new concepts in arterial wave travel

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Abstract
The problem for intact system research is that elements are interdependent and are not easily held constant. There are many examples, e.g., cardiac output depends on the heart and the arteries but they are linked through afterload, coronary flow depends on aortic pressure and ventricular contraction and they interact, etc. In a system of tubes, the challenge frequently is how to resolve the directional basis of a perturbation, does it originate upstream or downstream, or both? How can interaction between constituent parts be considered as an interchange of energy? In the circulation, how do arteries serve as cables or transmission lines? It has long been recognized that the answers may be found in the mechanics of arterial wave travel (41), an area that has been mathematically redefined during the last few years (26). A wave-intensity analysis of forward and backward wave travel has the potential to quantify the amplitude and direction of energy transfer in the systemic (11, 12), pulmonary (9), or coronary arterial systems (32). This theoretical approach also has practical implications, because there is increasing evidence that upstream-downstream cardiovascular interactions have real prognostic (1) and therapeutic significance (24) for cardiology patients in the clinic. The disadvantageous development of left ventricular hypertrophy in hypertension and the advantageous use of vasodilators in heart failure serve as testimony to the importance of ventriculoarterial interaction.