Pump Function and its Interaction with the Systemic Load

Abstract

Many parameters have been used to categorize the mechanical function of the myocardium, such as the rate of rise of ventricular pressure (dP/dt) and the velocity of fibre shortening (dV/dt). Such factors attempt to relate the performance of the intact ventricle to that of isolated muscle strips and while a deeper understanding of ventricular function will eventually come from such isolated muscle studies there are still many unresolved difficulties in the way. An alternative approach is to investigate the function of the intact ventricle through measurements of pressure and flow which are the integrated external effects of tension and shortening velocity in the individual muscle segments. If the ventricle is regarded as a source of mechanical power coupled to the circulatory system which has as its chief characteristic an input impedance it is apparent that the simultaneous measurement of the aortic pressure and flow enables two further functions to be derived. Firstly, the product terms (pressure x flow) at each harmonic yield the power transferred into the circulation, and secondly, the quotient terms (pressure/flow), again at each harmonic and making due allowance for phase angle, give the input impedance.