The diagnostic test characteristics of the "rule of thirds" on the parasternal long axis view: A cross-sectional study

Document Type

Article

Publication Date

11-27-2025

Institution/Department

Emergency Medicine

Journal Title

The American journal of emergency medicine

Abstract

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.

First Page

133

Last Page

137

Link to Full Text

Share

COinS