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A gallop, or gallop rhythm, refers to the rhythm of the heart. Ordinarily, the rhythm of the heart is a steady lup-dup, lup-dup, lup-dup caused by the noise of normal valve closure in the heart as it pumps. There are actually four thumps in the cardiac cycle, S1 (sound number one), S2, S3, and S4. S1 and S2 are "lup" and "dup." S3 is the sound of the blood rushing into the ventricle and striking the muscular wall as the ventricle fills passively in relaxation after the pumping stroke is finished. S4 is the thump made by the contraction of the atria. If these S3 or S4 sounds are particularly easy to hear - about as loud as the normal S1 and S2 sounds for example - it is abnormal and we say the patient has a gallop rhythm.