A cardioplegia system for delivering cardioplegic solution to the heart during open heart surgery in cooperation with an extracorporeal blood circuit employing a heart/lung machine, includes a conduit diverting a portion of the blood flow from the heart/lung machine to a cardioplegia delivery line. A heat exchanger assembly including a heat exchanger for controlling fluid temperature is provided in the cardioplegia delivery line. In addition to a heat exchanger, the heat exchanger assembly includes an integral bubble trap, filtration means and means to enable the measurement of certain physical characteristics of the circulating cardioplegia fluid and/or certain events occurring within the heat exchanger assembly.