Blood clotting monitor being used during heart surgery. Anti-clotting drugs (such as heparin) are used during heart surgery to prevent the blood from clotting, both in areas where blood flow is stopped, and also in the heart-lung machine that takes over the function of the heart and lungs. It is necessary to carefully monitor blood clotting times during the operation and adjust or reduce the amount of heparin accordingly. Two results are shown here, of tests that took 413 and 621 seconds respectively for the blood to clot. During heart surgery, these times should be kept above 300 seconds.