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Updated: Jan 9, 2026
Ultrasonic Assessment of Myocardial Microstructure
Published on: January 14, 2014
The diagnostic test characteristics of the "rule of thirds" on the parasternal long axis view: A cross-sectional
Jordan Dow1, Margarita Popova2, Safinaz AlShiakh3
1Northeast Georgia Medical Center, Department of Emergency Medicine, Gainesville, GA, United States of America.
Insights
The Rule of Thirds (RoT) echocardiography method shows low sensitivity for detecting cardiac pathology but good specificity. RoT should be used cautiously as a rule-in tool, not alone, due to fair interobserver agreement.
Area of Science:
- Echocardiography
- Cardiac Ultrasound
- Diagnostic Imaging
Background:
- The Rule of Thirds (RoT) is a commonly taught echocardiographic technique.
- It estimates the size ratio of the right ventricular outflow tract (RVOT), aortic outflow tract (AOT), and left atrium (LA) on parasternal long-axis views.
- Abnormal ratios may indicate cardiac pathology, but RoT's diagnostic accuracy is largely unevaluated.
Purpose of the Study:
- To evaluate the diagnostic performance of the Rule of Thirds (RoT) in identifying cardiac abnormalities.
- To assess the interobserver variability of RoT assessments in a clinical setting.
Main Methods:
- A retrospective cohort study analyzed 272 comprehensive echocardiograms.
- Two blinded emergency ultrasound fellows independently assessed RoT from parasternal long-axis views.
- RoT findings were compared to established metrics for RVSP, AOT size, and left ventricular (LV) dysfunction.
Main Results:
- Abnormal RVOT on RoT had 12.9% sensitivity and 84% specificity.
- Abnormal AOT on RoT showed 22.7% sensitivity and 94.9% specificity.
- Abnormal LA on RoT had 43.9% sensitivity and 84.3% specificity.
- Interobserver agreement was fair (69.9% agreement, Cohen's kappa = 0.36).
Conclusions:
- The Rule of Thirds (RoT) demonstrates low sensitivity but high specificity, functioning best as a rule-in tool.
- RoT should not be used in isolation for evaluating critically ill patients.
- Fair interobserver agreement necessitates caution when interpreting RoT findings.
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.
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