Cardiac Efficiency by MR Spectroscopy and Elastography

Cardiac dysfunction is among the leading causes of death worldwide. In heart failure, pumping action is inadequate for the bodys needs. The heart is a thermodynamic engine, converting high- energy adenosine-triphosphate (ATP) into pressure-volume work for pumping blood. Like in all thermal engines, some of the provided energy is lost as heat. The ratio of work to supplied energy, in this case Gibbs free energy, is called thermodynamic efficiency. The aim is non-invasive assessment of myocardial efficiency using magnetic resonance (MR) spectroscopy and imaging techniques. Phosphorus magnetic spectra show resonances of the high energy phosphates from which Gibbs free energy can be derived. The hearts pumping action is a periodic process of contraction and relaxation. Mechanical work of the heart is the area circumscribed by pressure-volume loops, as in any thermal engine, concisely visualising the biomechanics of the ventricles. While ventricle volumes are readily available from cinematographic cardiac MR, ventricular pressure curves so far require invasive catheterisation. It has been shown that myocardial stiffness shows excellent proportionality to ventricular pressure, with the heart muscle stiffening during systole and relaxing during diastole. Special MR imaging techniques allow quantification of tissue stiffness. Cinematographic MR imaging provide ventricle volumes and stiffness over the cardiac cycle. Models will be developed to assess pressure-volume loops from these data. The area of the pressure-volume loop is the work performed by the heart muscle during one cardiac cycle. Finally, all these measures will be combined in a way that they can be done quasi-simultaneously (interleaved). Cardiac efficiency will be evaluated under varying heart rate and pressure using exercise in the magnet or pharmacological stress tests. This is common practice in cardiology to asses myocardial fitness and is of prognostic value in e.g. heart failure. Although the concept of myocardial efficiency is well understood for a long time, it is very challenging to quantify. The non-invasive simultaneous measurement of human cardiac energy supply along with external work has not been achieved so far. This limitation in clinical research and diagnostics in cardiovascular diseases will be overcome and will move the field several steps forward.