A venting catheter, system and method are provided for withdrawing blood and other fluids from a patient's heart to facilitate decompressing the heart during cardioplegic arrest and cardiopulmonary bypass, without the need for a thoracotomy and without puncturing the aorta, pulmonary artery, or heart itself. The venting catheter is configured to be introduced into a peripheral vein and intralumninally advanced through the right side of the heart and into the pulmonary artery. The venting catheter includes a lumen configured to withdraw blood at a rate of at least about 50 ml/min at a pressure of no less than about -350 mmHg. A flow-directing means is provided to facilitate guiding the catheter into the pulmonary artery by being carried by blood flow through the heart. The cardiac venting system may include, in addition to the cardiac venting catheter, a cardiopulmonary bypass system to maintain circulation of oxygenated blood, and means for arresting the patient's heart.