1. Why can a first-degree AV block augment the S4?
ANS: A first-degree AV block, i.e., in which the P waves come relatively early in diastole, may cause the atrium to contract early enough to coincide with rapid ventricular filling. Atrial contraction occurring at this time squeezes blood into the ventricle at the same time that rapid ventricular expansion is also drawing blood into the ventricle. Thus, a soft S4 can become very loud. Unless the P-R interval is extremely prolonged, this contraction of the atrium at the time of rapid ventricular filling will occur only with a tachycardia.
2. What is the gallop rhythm called when the high flow of the early rapid filling phase of the LV is augmented by atrial contraction, as with first-degree AV block and tachycardia?
ANS: A summation gallop, i.e., it is the summation of the mechanism for the production of an S3 with the mechanism for the production of an S4, to make an audible sound.
Si S2 f
Rapid filling and atrial contraction here
Si S2 f
Simultaneous occurrence of atrial contraction and early rapid filling produces a summation sound facetiously called the "S7" (S3 + S4). This usually requires a prolonged P-R interval.
Note: A summation gallop is physiological if neither an S3 nor an S4 would be present without the first-degree AV block and tachycardia.
3. When are summation gallops pathological?
ANS: When a pathological S3 is augmented by atrial contraction occurring very early in diastole or when a pathological S4 is augmented by occurring during the rapid filling phase. These are then called "augmented gallops." This implies that a pathological S3 or an S4 was augmented by the fortuitous assistance of a marked tachycardia or prolonged P-R interval.
Note: a. Summation sounds are very loud even if not pathological.
b. If the heart rate of a patient with a physiological summation gallop is slowed by carotid sinus pressure, you may hear nothing but an S, and S2. (The carotid sinus is level with the upper border of the thyroid cartilage.)
c. The summation gallop often has some duration and can mimic a mid-diastolic murmur.
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