The fourth heart sound (S4), also known as the “atrial gallop,” occurs just before S1 when the atria contract to force blood into the left ventricle. If the left ventricle is noncompliant, and atrial contraction forces blood through the atrioventricular valves, a S4 is produced by the blood striking the left ventricle.
CLINICAL PEARL: A S4 heart sound occurs during active LV filling when atrial contraction forces blood into a noncompliant LV.
Therefore, any condition that creates a noncompliant LV will produce a S4, whereas any condition that creates an overly compliant LV will produce a S3 heart sound.
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, or LVH, resulting in impaired relaxation (compliance) of the LV. In this setting, a S4 is often heard. Also, if an individual is actively having myocardial ischemia, adequate adenosine diphosphate cannot be synthesized to allow for the release of myosin from actin; therefore, the myocardium is not able to relax, and a S4 will be present.
It is important to note that if a patient is in atrial fibrillation, the atria are not contracting, and it is impossible to have a S4 heart sound.
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 vs. Dilated LV (S3 Present)
Like S3, the S4 sound is low pitched and best heard at the apex with the patient in the left lateral decubitus position.
Below is comparative information for S3 and S4.
S3 – “ventricular 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 congestive HF
S4 – “atrial gallop”
- Occurs in late diastole
- Occurs during active LV filling
- Almost always abnormal
- Requires a noncompliant LV
- Can be a sign of diastolic congestive HF