It is essential to understand the pathophysiology of cardiogenic shock and the possible deterioration of contractile function during high-risk PCI in order to select those patients who could benefit from mechanical support thus choosing the most suitable device in every situation. Percutaneous ventricular assist devices (pVAD) provide hemodynamic support by improving cardiac output and mean arterial pressure, but their specific features result in different hemodynamic effects and degrees of myocardial ischemic protection and left ventricle unloading. These features, together with ease of use, specific contraindications and individual risk of complications, must be taken into account in the evaluation and selection of the device. The aim of this review is to illustrate the principles of left ventricular mechanic, including the pressure-volume loop analysis, in order to better understand and to quantify the different hemodynamic effects of pVAD supports and to explain the pathophysiological key aspects of assisted PCI.