Am J Emerg Med. 2025 Nov 27;100:133-137. doi: 10.1016/j.ajem.2025.11.027. Online ahead of print.

OBJECTIVE: The Rule of Thirds (RoT) estimates the relationship between right ventricular outflow tract (RVOT), aortic outflow tract (AOT), and left atrium (LA) on parasternal long-axis echocardiography, proposing a 1:1:1 size ratio. Enlargement of any of the structures may reflect pathology. Despite common teaching, RoT’s diagnostic performance has not been evaluated.

METHODS: This retrospective cohort study involved two blinded emergency ultrasound fellows independently reviewing 272 consecutive comprehensive echocardiograms obtained by cardiology technicians. RoT assessment from parasternal long-axis view was recorded as normal (1:1:1 ratio) or abnormal (disproportionate structure size). This assessment of RoT-predicted pathology was compared in blinded fashion to cardiology-determined RVSP >40 mmHg, AOT > 4 cm, and the presence of LV systolic or diastolic dysfunction. Percent agreement and Cohen’s kappa assessed interobserver variability.

RESULTS: Abnormal RVOT on RoT showed 12.9 % sensitivity (95 % CI 9.2-17.3 %) and 84 % specificity (74.1-91.2 %). Abnormal AOT demonstrated 22.7 % sensitivity (11.5-37.8 %) and 94.9 % specificity (92.6-96.6 %). Abnormal LA had 43.9 % sensitivity (37.5-50.3 %) and 84.3 % specificity (79.8-88.2 %). Evaluator agreement was 69.9 % with Cohen’s kappa of 0.36 (95 % CI 0.25-0.47), indicating fair agreement.

CONCLUSION: The Rule of Thirds functions best as a rule-in tool but should not be used alone when evaluating critically ill patients. Interobserver agreement was only fair. Although broadly taught, caution should be exercised when utilizing RoT screening results.

PubMed:41349275 | DOI:10.1016/j.ajem.2025.11.027