Objective-To assess stability of cardiac output, mean arterial pressure, and systemic vascular resistance during biventricular pacing optimization.Design-Substudy analysis of data collected as part of a randomized controlled study examining the effects of optimized temporary biventricular pacing after cardiopulmonary bypass.Setting-Single center study at a university affiliated tertiary care hospital. Participants-Cardiac surgery patients at risk of left ventricular failure following cardiopulmonary bypass (CPB).Interventions-Biventricular pacing was optimized immediately after CPB. Atrioventricular delay (7 unique settings) was optimized first, followed by left ventricular pacing site (3 unique settings), followed by interventricular delay (9 unique settings) Each setting was tested twice for 10 seconds each time. Vasoactive medication and fluid infusion rates were held constant. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Measurements and Main Results-Aortic NIH Public Access