Background: Mechanical circulatory support (MCS) devices are frequently used in high-risk percutaneous coronaryinterventions (HR-PCI) due to their improved effects onsystemic hemodynamics. Little is known on the effects ofsuch devices on coronary hemodynamics in patients withcritical coronary stenosis. Prior studies using intra-aortic balloon pump have failed to demonstrate significant improvements in coronary hemodynamic in patients withcritical coronary stenosis. We therefore sought to investigate if more robust forms of MCS would improve coronary hemodynamics in critical coronary stenosis. Methods: From November 2015 to November 2016 we examined coronary perfusion pressures and distal coronarypressures, using a pressure wire (Volcano Corp. San Diego, CA), in 7 patient undergoing HRPCI with the use of MCS devices. Baseline hemodynamic were obtained after delivery of MCS. MCS was adjusted to a low output state and a high output state according to device specific recommendations and measurements were made prior to PCI. Results: Mean patient age was 79±10 years, 71% were male, and 42% had a left ventricular ejection fractio