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Simultaneous recording of electrocardiogram (EKG) and heart sounds used in assessing cardiac time intervals. Q, start of QRS complex; QS1, electromechanical activation time; QS2, electromechanical systole; S1, first heart sound; S2, second heart sound; S1S2, interval between the first and second heart sounds; S2S1, interval between second and first heart sounds.
