S3
The third heart sound has good (as far as exam findings go) operating characteristics for determining if a patient has elevated left heart filling pressures.
In the JAMA rational clinical exam series, S3 has a reported pooled +LR of 11 and -LR of 0.9 [1] for discriminating CHF from other causes of dyspnea in the ED.
However, in Stephen McGee’s Evidenced Based Physical Diagnosis, S3 has a pooled +LR 3.9 and -LR 0.8 for suggesting elevated left heart filling pressures [2].
Limitation
Besides the variation in performance, S3’s seemingly major limitation is that examiners seem to be unable to agree if S3 is even present.
The κ for S3 inter-rater agreement has been reported anywhere from -0.17 (yes negative! <0 suggests no agreement) to as high as 0.84 (suggesting substantial to near perfect agreement) [2].
Our agreement or disagreement on S3 maybe because we are not assessing for S3 in a uniform way.
Performing S3
Reminder, S1 is atrio-ventricular valve closure. S2 is aortic and pulmonic valve closure.
S3 is caused by a rapid deceleration of flow across the mitral valve and suggests a high LV filling pressure.
S3 is a low frequency sound, which is better heard with the bell not the diaphragm. It is best heart at the apex in the left lateral decubitus position.
About the bell: If you press too firmly and stretch the skin over the bell it will act like a diaphragm and favor higher frequency sounds. Put just enough pressure to make a seal with the skin but no more.
LL decubitus? Really‽‽‽
- If you want a good Apical four chamber POCUS view (link), you will probably want to rotate your patient to LLD. So check for S3 at the same time.
Jump to the discussion of how to discriminate CHF from other causes of dyspnea in the ED.
- Wang et al. JAMA 2005
- McGee. Evidence Based Physical Diagnosis, 4th Ed
