At rapid heart rates, S3 and S4 may merge to produce a summation gallop, sometimes referred to as S7.
2.
If the heart rate is also very fast ( tachycardia ), it can become difficult to distinguish between S 3 and S 4 thus producing a single sound called a "'summation gallop " '.
3.
It can also contain both of these sounds forming a "'quadruple gallop "', and in situations of very fast heart rate can produce a "'summation gallop "'where S 3 and S 4 occur so close as to be indistinguishable.
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