When does the E/e' index not work? The pitfalls of oversimplifying diastolic function.

Since the E/e' ratio was first described in 1997 as a noninvasive surrogate marker of mean pulmonary capillary wedge pressure, it has gained a central role in diagnostic recommendations and a supremacy in clinical use that require critical reappraisal. We review technical factors, physiological influences, and pathophysiological processes that can complicate the interpretation of E/e'. The index has been validated in certain circumstances, but its use cannot be extrapolated to other situations-such as critically ill patients or children-in which it has either been shown not to work or it has not been well validated. Meta-analyses demonstrated that E/e' is not useful for the diagnosis of HFpEF and that changes in E/e' are uninformative during diastolic stress echocardiography. A similar ratio has been applied to estimate right heart filling pressure despite insufficient evidence. As a composite index, changes in E/e' should only be interpreted with knowledge of changes in its components. Sometimes, e' alone may be as informative. Using a scoring system for diastolic function that relies on E/e', as recommended in consensus documents, leaves some patients unclassified and others in an intermediate category. Alternative methods for estimating left heart filling pressures may be more accurate, including the duration of retrograde pulmonary venous flow, or contractile deformation during atrial pump function. Using all measurements as continuous variables may demonstrate abnormal diastolic function that is missed by using the reductive index E/e' alone. With developments in diagnostic methods and clinical decision support tools, this may become easier to implement.