What is the significance of the E/e' (E over e prime) ratio in a patient with hypertension, diabetes, and symptoms of heart failure, particularly in relation to cardiac amyloidosis and hypertrophic cardiomyopathy?

What is the E/e' Ratio?

The E/e' ratio is a noninvasive echocardiographic parameter that estimates left ventricular filling pressures by dividing the early diastolic transmitral flow velocity (E) by the early diastolic mitral annular velocity (e'). 1

Physiological Components

E velocity represents the early diastolic transmitral flow measured by pulsed-wave Doppler, reflecting the pressure gradient between the left atrium and left ventricle during early diastole. 2 This measurement is influenced by left ventricular relaxation, restoring forces, and left atrial pressure at the time of mitral valve opening. 2

e' velocity is the early diastolic mitral annular velocity measured by tissue Doppler imaging (TDI), primarily reflecting left ventricular myocardial relaxation with minimal influence from loading conditions when relaxation is impaired. 2 The e' should be measured at both the septal and lateral mitral annulus positions, with proper sample volume positioning being critical for accurate measurement. 1, 2

Clinical Interpretation Thresholds

For average E/e' ratio, values <8 indicate normal left ventricular filling pressures, while values >14 have high specificity for elevated left ventricular filling pressures. 2 Values between 8-14 are considered indeterminate and require integration with additional echocardiographic parameters such as left atrial volume index >34 mL/m², tricuspid regurgitation peak velocity >2.8 m/s, or mitral inflow E/A ratio >2. 2

When using site-specific measurements, septal e' <7 cm/s or lateral e' <10 cm/s indicate delayed left ventricular relaxation. 2 An E/e' ratio >15 definitively indicates elevated filling pressures and persistent congestion. 2

Specific Context: Hypertension, Diabetes, and Heart Failure Symptoms

In your patient with hypertension, diabetes, and heart failure symptoms, the E/e' ratio serves as a critical tool for distinguishing heart failure with preserved ejection fraction (HFpEF) from other causes of dyspnea. 3 The presence of ≥2 abnormal diastolic measurements (including elevated E/e') increases diagnostic confidence for HFpEF. 3

For hypertensive patients specifically, an abnormally increased E/e' ratio is suggestive of primary cardiac events and helps identify hypertension-mediated cardiac damage. 4 The assessment should include left ventricular mass index calculation, as left ventricular hypertrophy is present in 36-41% of hypertensive patients and significantly impacts the interpretation of diastolic parameters. 5

Cardiac Amyloidosis Considerations

In cardiac amyloidosis, E/e' ratios are typically markedly elevated (>14), often accompanied by a restrictive mitral inflow pattern (E/A ratio >2.5), deceleration time <150 msec, and severely reduced e' velocities (3-4 cm/sec). 1 A distinctive feature is that lateral e' may be higher than septal e' (unlike constrictive pericarditis where the opposite occurs, termed "annulus reversus"). 1

Speckle tracking echocardiography showing apical sparing of longitudinal strain on polar plot is a characteristic phenotype that helps distinguish cardiac amyloidosis from other causes of left ventricular hypertrophy. 1, 4

Hypertrophic Cardiomyopathy Considerations

In hypertrophic cardiomyopathy (HCM), the predictive value of E/e' is significantly confounded by dynamic left ventricular outflow tract obstruction. 6 Higher E/e' predicts worse clinical outcomes specifically in non-obstructive HCM, but the correlation between E/e' and left ventricular outflow tract pressure gradient is lost in patients with labile or obstructive disease. 6

For HCM patients with obstruction who undergo myectomy, post-operative E/e' regains its prognostic value and correlates with outcomes. 6 This highlights that E/e' should be interpreted cautiously in obstructive HCM and may require reassessment after septal reduction therapy. 6

Critical Limitations and Pitfalls

E/e' has limited accuracy in several important clinical scenarios that must be recognized to avoid misinterpretation: 2

• Heavy mitral annular calcification significantly impairs the accuracy of e' measurement 2, 7
• Mitral valve disease (stenosis or regurgitation) invalidates the E/e' ratio 2
• Pericardial disease (constrictive or effusive-constrictive) alters the relationship between E/e' and filling pressures 2
• Regional wall motion abnormalities from coronary artery disease affect annular velocities 2
• Atrial fibrillation creates beat-to-beat variability that reduces reliability 2

The correlation between E/e' and invasively measured filling pressures is only modest (pooled correlation coefficient r=0.56) even in well-selected HFpEF populations. 8, 2 In patients with unexplained dyspnea, E/e' demonstrates poor agreement with pulmonary arterial wedge pressure (Bland-Altman limits of agreement -8.3 to 8.3 mm Hg), and an E/e' cutoff of 13 has only 6% sensitivity for identifying elevated filling pressures. 9

Practical Measurement Technique

Calculate the average E/e' by measuring e' at both septal and lateral positions and averaging the two values. 2 This approach provides optimal assessment of global diastolic function. 2 Normal reference values are septal e' ≥7 cm/s and lateral e' ≥10 cm/s. 2

When interpreting results, always integrate E/e' with other echocardiographic parameters rather than relying on this single measurement in isolation. 1, 3 The 2016 ASE/EACVI guidelines emphasize that no single parameter can reliably diagnose diastolic dysfunction or HFpEF. 8