Which of the following is true about fouh hea sound 'S4'?

Precordial vibrations resulting from atrial contraction are normally neither palpable nor audible. Under pathologic conditions, forceful atrial contraction generates a low-frequency sound (S4) just before the first hea sound (S1).S4 is also termed the atrial diastolic gallop or the presystolic gallop.Atrial contraction must be present for the production of S4. It is absent in atrial fibrillation.The S4 follows the onset of the P wave of the ECG by approximately70 ms.S 4 has a very low frequency--usually 20 cycles/sec or less. (Guyton and Hall)Audibility of the S4 depends not only on its intensity and frequency but also on its separation from S1. A loud S1 also can mask the audibility of a preceding softer S4.The S4 is best heard at the apex impulse with the patient turned in the left lateral position. It varies considerably with respiration, usually being heard best during expiration.Both the intensity and timing of the S4 are closely related to the end-diastolic volume of the ventricle.Audible fouh hea sounds are usually accompanied by a palpable presystolic apical impulse in the absence of obesity, emphysema, and so foh.The S4 generated by a forceful right atrial contraction is usually heard best at the lower left sternal border. Unlike the left-sided S4, it tends to be accentuated with inspiration. It is also accompanied by prominent 'a waves' in the JVP.An audible S4 with a palpable presystolic impulse is common in patients with ischemic hea disease during an acute episode of angina and in the early phases of transmural MI.In patients with LV an aneurysm or idiopathic or ischemic cardiomyopathy, abnormal fouh hea sounds are commonly present and are often associated with an S3, producing a quadruple rhythm.Ref: Hurst's The Hea 13/e p288, Guyton & Hall's Textbook of Medical Physiology 12/e p266