The fourth heart sound (S4), also known as the "atrial gallop", occurs just before S1 when the atria contract to force blood into the LV. If the LV is non-compliant and atrial contraction forces blood through the AV valves, an S4 is produced by the blood striking the LV.
CLINICAL PEARL: A S4 heart sound occurs during active LV filling when atrial contraction forces blood into a non-compliant LV.
Therefore any condition that creates a non-compliant LV will produce a S4, while any condition that creates an overly compliant LV will produce a S3 (as described above).
A S4 heart sound can be an important sign of diastolic heart failure or active ischemia and is rarely a normal finding. Diastolic heart failure frequently results from severe left ventricular hypertrophy (LVH) resulting in impaired relaxation (compliance) of the LV. In this setting, a S4 is often heard. Also, if a person is actively having myocardial ischemia, adequate ATP can't be synthesized to allow for the release of myosin from actin, thus the myocardium is not able to relax and a S4 will be present.
CLINICAL PEARL: A S4 heart sound is often a sign of diastolic heart failure and it is rarely a normal finding (unlike a S3).
Normal LV Dilated LV - S3 Present
CLINICAL PEARL: If the patient is in atrial fibrillation, the atria are not contracting and it is impossible to have a S4 heart sound.
Like S3, the S4 sound is low pitched and best heard at the apex with the patient in the left lateral decubitus position.
Comparing the 3rd and 4th heart sounds
S3 - "ventricular gallop"
S4 - "atrial gallop"
Occurs in early diastole
Occurs during passive LV filling
May be normal at times
Requires a very compliant LV
Can be a sign of systolic CHF
Occurs in late diastole
Occurs during active LV filling
Almost always abnormal
Requires a non-compliant LV
Can be a sign of diastolic CHF
Introduction to Heart Sounds
S1 Heart Sound
S2 Heart Sound
S3 Heart Sound
S4 Heart Sound
Extra Heart Sounds
Heart Sounds Multiple Choice Questions