“Rule of Thirds” on Bedside Echo Falls Short

Written by Chris Thom

Spoon Feed
Sensitivity of the commonly taught “rule of thirds” on point-of-care echocardiography was low, while specificity was high.

Rule of thirds should be employed with caution
This single retrospective study explored the accuracy of the long-taught “rule of thirds” (RoT). This refers to the idea that the RV, aortic root, and left atrium should be roughly the same size on the parasternal long axis (PSLA) echo view. If one of these structures is significantly dilated over the other two, it may indicate a potential abnormality without the need for any formal measurements. In this study, cardiology-performed echocardiograms were reviewed by two separate ED physicians with ultrasound training. These physicians would note if a chamber was dilated, and the test characteristics were compared to formal cardiology measurements. This included RVSP > 40 mmHg for RV size, aortic root dilation > 4 cm for aortic size, and the presence of systolic or diastolic LV dysfunction for LA size.

287 echo studies were reviewed. The sensitivity of abnormal RV on RoT was 12.9%, with specificity of 84%. Abnormal aortic root had a sensitivity of 22.7% and specificity of 95%, while abnormal LA had sensitivity of 44% and specificity of 84.3%. The agreement between the two physician rates was only 70%, which yielded a Cohen’s kappa of 0.36.

How will this change my practice?
The “rule of thirds” is a POCUS education standby for teaching novices the PSLA view. It passes the “sniff” test, in that one can quickly identify a significant anomaly by comparing the adjacent structures that generally share a similar size. With very low sensitivity compared to formal echo metrics and low interrater reliability, we should not be too reassured if the chamber sizes are roughly equal on PSLA echo. However, the high specificity does indicate there is likely true pathology if a chamber jumps out as significantly enlarged.

POCUS pro-tips and clips
The PSLA view is often the easiest and highest-quality view to obtain. To employ the “rule of thirds”, you’ll want to get a view where you can see the LA, mitral valve, LV, aortic valve, aortic root, and the RV at the top. A subtle rotation of the probe can sometimes help “elongate” the LV to the true long axis. And if the aortic root is poorly visualized, try one rib space higher on the chest wall to highlight that structure. 

Source
The diagnostic test characteristics of the “rule of thirds” on the parasternal long axis view: A cross-sectional study. Am J Emerg Med. 2025 Nov 27;100:133-137. doi: 10.1016/j.ajem.2025.11.027. Epub ahead of print. PMID: 41349275.