First Heart Sounds Fourth Heart Sound Plus First Heart Sound Lesson Waveform Anatomy play pause Table of Contents 1First Heart Sound - Loud 2First Heart Sound (Minimally Split) 3First Heart Sound (Markedly Split) 4First Heart Sound - Decreased Intensity 5Fourth Heart Sound Plus First Heart Sound 6First Heart Sound plus Aortic Ejection Click Practice Quiz This video is an animation of Fourth Heart Sound Plus First Heart Sound A#23 (lesson# 5) within course number 5. Certain heart sound configurations mimic a split first heart sound.One of these conditions is a fourth sound gallop (S4) plus a first heart sound (S1). Since the S4 occurs just before the S1 it is easy to confuse a S4/S1 with a split S1.An S4 is lower frequency than an S1. To differentiate between a S4/S1 and a split S1, listen carefully to the frequency of the initial sound of the pair. If it is lower in frequency than the second sound it is an S4. If the two sounds are the same it is a split S1.