The fourth heart sound is due to atrial contraction inducing ventricular filling towards the end of diastole. They are never audible in normal subjects. A fourth heart sound is the result of powerful atrial contraction filling an abnormally stiff ventricle.
Left atrial heart sound is maximal at the apex, with possible causes including:
- left ventricular hypertrophy
- fibrotic left ventricle
- hypertrophic cardiomyopathy
Right atrial heart sound is maximal at the lower left sternal edge and on inspiration. This may occur in right ventricular hypertrophy.
Evidence base:
- in patients referred for non-emergency left sided heart catherisation, third and fourth heart sounds detected by phonocardiography (1)
- neither the phonocardiographic S3 nor the S4 is a sensitive marker of left ventricular dysfunction
- the phonocardiographic S3 is specific for left ventricular dysfunction and appears to be superior to the moderate specificity of the phonocardiographic S4
- thus the absence of a gallop rhythm should not be used to indicate the absence of left ventricular dysfunction and certainly cannot replace other diagnostic testing when screening for left ventricular dysfunction (2)
Reference:
- Marcus GM et al. Association between phonocardiographic third and fourth heart sounds and objective measures of left ventricular dysfunction. JAMA 2005; 293:2238-44.
- Commentary. Evidence-Based Medicine 2005;10:182.