What is the e/e' ratio in echocardiography (echo)?

E/e' Ratio in Echocardiography: A Key Measure of Left Ventricular Filling Pressure

The E/e' ratio in echocardiography is the ratio of early mitral inflow velocity (E) to early diastolic mitral annular velocity (e'), which serves as a noninvasive estimate of left ventricular filling pressures with values >14 indicating elevated pressures. 1

What is E/e' and How is it Measured?

The E/e' ratio combines two distinct echocardiographic measurements:

• E wave: The peak velocity of blood flow across the mitral valve during early diastole (rapid filling phase), measured at the level of the leaflet tips and reported in cm/s 1
• e' velocity: The peak velocity of mitral annular motion during early diastole, measured using tissue Doppler imaging (TDI) at either the septal or lateral mitral annulus (or averaged), reported in cm/s 1

The ratio is calculated by dividing the E wave velocity by the e' velocity, and is reported without units.

Clinical Significance and Interpretation

The E/e' ratio is clinically significant because:

• It corrects for the effect of LV relaxation on mitral E velocity 1
• Values <8 usually indicate normal LV filling pressures 1
• Values >14 have high specificity for increased LV filling pressures 1
• Values between 8-14 represent a "gray zone" where LV filling pressures are indeterminate 1

According to the American College of Cardiology/American Heart Association/American Society of Echocardiography guidelines, left ventricular filling pressure is considered:

• Normal when E/e' is <15
• Elevated when E/e' is >15 1

Diagnostic and Prognostic Value

The E/e' ratio has demonstrated important clinical utility:

• It has a pooled correlation coefficient of 0.56 with invasively measured filling pressures in heart failure with preserved ejection fraction (HFpEF) 1
• Each unit increase in E/e' is associated with a 17% increased risk of cardiac events in hypertensive patients 2
• It serves as a strong predictor of grade II-III diastolic dysfunction with a cut-off >10 having a sensitivity of 97.6% and negative predictive value of 98.2% in patients with decreased ejection fraction 3

Limitations and Special Considerations

The E/e' ratio has several important limitations:

• It is not accurate in normal subjects, patients with heavy annular calcification, mitral valve disease, and pericardial disease 1
• Different cutoff values apply depending on the sampling site (septal vs. lateral annulus) 1
• Accuracy is reduced in patients with coronary artery disease and regional dysfunction at the sampled segments 1
• In atrial fibrillation, simultaneous measurement of E and e' using dual-Doppler methods improves reliability 4
• The correlation with invasive measurements is only moderate (r=0.48-0.50) and is weaker in HFpEF patients 5

Special Populations

The utility of E/e' varies across different clinical scenarios:

• Atrial fibrillation: Septal E/e' ≥11 suggests elevated filling pressures 1
• Hypertrophic cardiomyopathy: Average E/e' >14 indicates elevated filling pressures 1
• Restrictive cardiomyopathy: Average E/e' >14 suggests elevated filling pressures 1
• Mitral regurgitation: E/e' >14 may be considered only in patients with depressed ejection fractions 1
• Noncardiac pulmonary hypertension: Lateral E/e' >13 suggests cardiac etiology 1

Alternative Measurements

When E/e' is not reliable, other parameters can be used:

• IVRT/TE-e' ratio (ratio of isovolumic relaxation time to time interval between onset of mitral E velocity and annular e' velocity) 6
• In mitral regurgitation, an IVRT/TE-e' ratio <3 predicts pulmonary capillary wedge pressure >15 mm Hg 6
• Comprehensive assessment should include multiple parameters including left atrial volume index and tricuspid regurgitation velocity 1

The E/e' ratio remains one of the most widely used and validated echocardiographic parameters for estimating left ventricular filling pressures, despite its limitations in certain clinical scenarios.