2. Normal LV Filling Dynamics
During LV ejection, energy is stored as myocytes undergo compression, while the myocardial wall's elastic components are also compressed and twisted [
4]. Subsequently, at the end of systole, calcium ions are actively reabsorbed into the sarcoplasmic reticulum (SR) (
Figure 1). This process facilitates the detachment and repositioning of actin and myosin filament cross-bridges to their original configuration, thereby enabling muscle relaxation, a process known as uncoupling. This phase, consuming adenosine triphosphate (ATP), is known as "active relaxation," a process that does not occur instantaneously. Concurrently, during LV diastole, restoring forces come into play, further facilitating this relaxation phase. When contracted myocardium relaxes and untwists, this stored energy is released as the elastic elements recoil [
5], serving as the driving force in the early diastolic phase to aid myocardial fibers in extending from their minimum length (Lmin) during the contraction phase to their original length (L0). The elastic recoil of the base and apex from their previous systolic positions, resulting in the release of the contracted LV myocardium along the longitudinal axis, has been referred to in the literature by various names, including LV untwisting motion [
6], restoring force [
7], and elastic recoil [
8]. Elastic recoil is passive relaxation, during which the myocardium spontaneously relaxes without energy consumption.
These two forces result in a rapid decrease in LV pressure (LVP) during isovolumetric relaxation [
9]. During this phase, both the aortic valve and the mitral valve remain closed, and LVP rapidly declines until it equals the left atrial pressure (ie, LVP=LAP) (
Figure 2). But increasing the length of myocytes and the hermetically sealed ventricular chamber jointly generate potential energy for sucking the blood from the left atrium (LA) to the LV apex.
As more calcium ions are taken up by SR, an increased level of uncoupling occurs, resulting in a greater number of actin and myosin filaments returning to their initial positions. This facilitates muscle relaxation, allowing the cardiac muscle fibers to elongate and assume a longer state. This process contributes to the reduction of LVP, which further causes LVP to drop below LAP (LVP < LAP) (
Figure 2). This atrioventricular pressure gradient, which pulls blood toward the LV apex, can be considered a measure of LV suction and plays a crucial role in early LV filling. The pressure gradient for early LV filling (early rapid filling), represented by the E wave, depends on the difference in pressure between LA and the LV apex, expressed as ΔP = LAP − LVP. This gradient is influenced by changes in the rate of LV relaxation and filling pressure.
LV untwisting rate (derived from LV short-axis views) and e’ (the peak early diastolic mitral annular velocity) are used to evaluate LV early diastolic recoil capacity [
6,
10]. Additionally, e', LV longitudinal strain rate during isovolumic relaxation (SRIVR), and LV strain rate during early diastole (SR
E) are significantly associated with LV active relaxation [
11] (
Figure 3). Advancements in LV diastolic function assessment by strain and strain rate derived from 2D-speckle-tracking echocardiography (STE) are discussed in
Section 5.1.
With LV rapid filling, the pressure gradient between the LA and the LV apex decreases and briefly reverses (inflow deceleration). This reversed pressure gradient at the mitral valve decelerates and subsequently halts the rapid blood flow into the LV during early diastole (
Figure 2). The duration of inflow deceleration (deceleration time [DT]) and A wave (late diastolic filling flow, as well as atrial contraction) velocity transit time primarily influenced by the LV chamber's functional stiffness, serving as a noninvasive indicator of LV diastolic operational stiffness. During the mid-diastolic phase (ie, diastasis), LAP and LVP equalize, and mitral flow nearly ceases. In late diastole, atrial contraction generates a second LA-to-LV pressure gradient, propelling blood into the LV. Then, as the LA relaxes, the LAP falls below the LVP, initiating mitral valve closure. In short, LV diastolic function is characterized by early diastolic recoil, LV relaxation, and chamber stiffness—all of which, in turn, determine LV filling pressure (
Table 1).
As the preceding discussion shows, the contraction and relaxation functions of the heart are mutually dependent. A stronger systolic contraction results in more significant recoil, thereby increasing potential energy during diastole. Additionally, when more calcium is actively taken up and stored in SR by sarcoendoplasmic reticulum calcium (SERCA), it results in a greater release of calcium by the SR through ryanodine receptors (RyR, calcium-induced calcium release channels) during the subsequent systolic phase of the cardiac cycle (
Figure 1), ultimately leading to enhanced myocardial contractility [
12,
13]. In cases of heart failure with preserved ejection fraction (HFpEF), although LV ejection fraction (LVEF) is in the normal range, the LV's systolic performance is not quite normal [
14], as indicated by a reduced LV twist during exercise [
15]. The contraction and relaxation functions of the heart are tightly coupled, yet they are also influenced by independent factors. However, impaired LV relaxation is one of the initial signs of myocardial dysfunction.
2.1. Interpreting e’ and E/c’ in Diastolic Dynamics
The apex of the LV moves little throughout the cardiac cycle; therefore, septal or lateral mitral annular motion is a good surrogate measure of longitudinal LV contraction and relaxation [
16]. e' coincides with the mitral E wave, signifying the symmetrical expansion of the LV during early diastole as blood swiftly moves toward the LV apex due to a gradual pressure gradient from the LA. e′, referring to the early diastolic peak velocity of the mitral valve annulus, is highly feasible and reproducible and has a strong and consistent association with cardiovascular outcomes. These characteristics are influenced by three independent factors: restoring forces, LV relaxation, and lengthening load.
Restoring forces are responsible for passive elastic recoil, which occurs during LV relaxation and causes the ventricle to return to its resting position. These forces result from systolic contraction and, in the normal LV, generate a negative early diastolic pressure gradient that suctions blood into the ventricle. The restoring forces characterize the mechanical and elastic properties of the myocardium.
LV relaxation pertains to the rate at which active fiber force decays. It describes the active process of how quickly cardiac muscle cells return to their relaxed state after contracting during systole. LV relaxation reflects the heart's ability to actively relax in preparation for the next contraction cycle and, therefore, the heart's intrinsic ability to facilitate diastole.
Lengthening load refers to the pressure in the LA at mitral valve opening, which pushes blood into the LV and thus lengthens it. When the mitral valve is open, the values of lengthening load and filling pressure tend to be closely aligned.
Restoring forces and LV relaxation correspond to the passive relaxation observed during isovolumetric relaxation and the active relaxation seen during the rapid early diastolic filling phase mentioned earlier.
E wave is directly proportional to the ratio between filling pressure and the relaxation time constant (Tau, τ), while e' is inversely proportional to tau only [
17]. This relationship establishes a direct proportionality between the E/e' ratio and filling pressure [
18]. As a result, E/e' has consistently been strongly correlated with pulmonary capillary wedge pressure (PCWP) across a diverse patient population in research conducted by multiple laboratories. Consequently, E/e' is generally regarded as one of the most practical and reproducible estimates of filling pressure. One important tip is to sample at least two sites at the precise locations specified in the guidelines (between the tips of the mitral leaflets for E, and at the lateral and septal basal regions of the mitral annulus for e') and at adequate sample volume sizes. Furthermore, although the correlation between E/e' and LAP is most pronounced in cases of impaired LV systolic function, it remains accurate even in patients with preserved systolic function and variations in loading, such as those associated with aortic stenosis and exercise.
3. Abnormal LV Filling Patterns
LV diastolic dysfunction (LVDD), defined as impaired relaxation and potentially accompanied by reduced restoring forces, early diastolic suction [
19], and increased chamber stiffness [
20,
21], leads to symptomatic HF by causing elevated filling pressures at rest or with exertion [
22]. LVP and LAP interact significantly in patients with LVDD. Elevated LVP is a hallmark of LVDD. Concurrent with this elevation, inadequate LV filling leads to an accumulation of blood in the LA, thus increasing LAP. This rise in LAP directly stems from the LV's inability to relax adequately during diastole, causing disruptions in cardiac function and hemodynamics. Consequently, the dynamic relationship between LVP and LAP plays a pivotal role in assessing and understanding the severity of LVDD, reflecting the heart's filling status and the LA's burden. This comprehensive assessment is indispensable for the diagnosis, treatment, and evaluation of cardiovascular conditions, as well as patients’ overall cardiac function.
In the early stages of LVDD, particularly in mild cases with no significant alterations in LAP [
23], two noteworthy changes occur in the LV myocardium: a subtle reduction in relaxation, and a slight decrease in compliance. As in the previous discussion, the term "relaxation" primarily refers to the myocardium's active relaxation process, whereas "compliance" pertains to the myocardium's passive recoil (
Table 1). The small reduction in relaxation leads to a minor decrease in mitral annular velocity (e'). Simultaneously, delayed relaxation prolongs the E-wave DT and may be accompanied by a mid-diastolic peak in mitral flow (L wave) [
19]. Conversely, the decrease in compliance results in a minor increase in early LV filling pressure, subsequently causing a reduction in the E wave. The significance of atrial contraction as a compensatory mechanism heightens, culminating in an E/A ratio <1. This filling pattern is termed an "impaired relaxation pattern" or "grade 1 LVDD" (
Figure 4). In most patients with this impaired relaxation pattern, the mean LAP remains within the normal range despite an elevated LV end-diastolic pressure (LVEDP) maintained by robust atrial contraction.
In the subsequent stage of LVDD, a pseudonormal mitral inflow pattern becomes evident (
Figure 4). As LVDD continues to worsen, accompanied by an increase in LAP, the early diastolic LA-to-LV pressure gradient is reestablished despite elevated diastolic LVP. This reestablishment may return the E wave to the normal range. Additionally, during this phase, the E wave may increase slightly because the rise in LAP is relatively greater than the increase in LVP. The significantly slower relaxation rate that characterizes LVDD delays the e' wave so that it occurs after the E wave, which indicates that the LV does not expand symmetrically during diastole. The propagation of filling to the apex and longitudinal expansion occurs slowly after the LV is filled by the movement of blood from the LA into the LV inflow tract. In the presence of slow relaxation, the e’ wave does not coincide with the LA-to-LV pressure gradient, leading to a reduction in e' velocity, which becomes largely independent of LAP. Therefore, during this stage, a reduced and delayed e' wave may be observed [
24,
25].
In patients with more severe LVDD, characterized by significantly slowed relaxation and elevated LAP, several significant changes become evident, often indicative of a restricted filling pattern or grade 3 LVDD (
Figure 4). The E wave, reflecting the pressure gradient during diastasis, increases further, underscoring the extent of diastolic impairment. This heightened E wave is a consequence of the impaired LV relaxation and the sustained elevation of LAP. Simultaneously, LAP continues to rise during early diastole, surpassing the increase in LVP, leading to the enduring expansion of the pressure gradient between the LA and LV. These changes can be ascribed to the constraints on LV filling, with some blood potentially stagnating in the LA, impeding smooth flow through the mitral valve into the LV. This culminates in the accumulation of blood within the LA, augmenting both its volume and pressure. Over time, the prolonged LVDD may result in LA enlargement, indicating an increased capacity to accommodate blood, but it also brings about higher LAPs. With severe LVDD, the E-wave DT becomes notably short, and the e' wave is further reduced and delayed, collectively contributing to a marked elevation of the E/e' ratio. The peak late diastolic mitral annular velocity (a') may decrease, while the pulmonary venous systolic forward flow velocity is reduced and is lower than the diastolic forward flow velocity.
The presence of pseudonormalized (grade 2) and restricted (grade 3) filling patterns with elevated E/e' indicates the coexistence of LVDD and elevated LAP, resulting in blood being pushed out of the LA rather than being suctioned into the LV [
4,
26,
27]. As mentioned earlier, the E wave is enhanced when there is an elevated LA-to-LV pressure gradient. Conversely, the e' wave is reduced and delayed in cases of slow relaxation, potentially indicating impaired LV filling. Therefore, a high E wave and a low e' wave (resulting in an increased E/e' ratio) suggest that the increased E wave is primarily due to elevated LAP, rather than a decrease in LV diastolic pressure. The initial pathological changes in LV not only disrupt its own function but also extend their detrimental effects to the LA, setting the stage for a rapid escalation in LAP that soon exceeds the LA's adaptive capacity. The underlying mechanism for this phenomenon can be described as follows: Firstly, the LA, which typically has a thinner wall and smaller volume than the LV, is more susceptible to pressure fluctuations. As LVDD progresses beyond the LA's tolerance, the LA walls rapidly undergo dilation, decreased compliance, and remodeling, which are secondary to increased LV filling pressure. Ultimately, all these changes result in an increase in LAP. These factors collectively accelerate the deterioration rate of the LA, surpassing that of the LV. Secondly, during diastole, the LA acts as a reservoir for oxygenated blood returning from the pulmonary veins. Impaired LV relaxation and filling can lead to accumulation of blood in the LA. This filling constraint and blood buildup consequently trigger a rapid increase in LAP, as the LA must accommodate the augmented blood volume. In this scenario, the LA is caught between elevated pressures from both the upstream pulmonary artery (PA) and the downstream LV, exacerbating its deterioration. LA remodeling and dysfunction secondary to increased LV filling pressures are associated with worse symptoms, more pulmonary vascular disease, greater right ventricular dysfunction, depressed exercise capacity, and adverse outcomes [
28,
29]. Hence, some researchers propose that reduced LA strain (LAS) [
30,
31] and increased LA stiffness (E/e′/LAS) [
32] are the most accurate diagnostic criteria for diastolic HF.
3.1. Echocardiography Parameters and Evaluation Algorithms for Diastolic Dysfunction
The American Society of Echocardiography (ASE) guideline [
19] introduces two distinct algorithms for assessing diastolic function. Algorithm A is aimed at patients with unknown diastolic function, and its primary purpose is to distinguish between normal and abnormal diastolic function (
Figure 5). Algorithm B, conversely, is specifically designed for patients with known or suspected LVDD and focuses on estimating LV filling pressure and grading diastolic function. These two algorithms serve as valuable tools in the echocardiographic assessment of diastolic function across a spectrum of clinical scenarios, providing clinicians with the means to make informed diagnoses and treatment decisions. For algorithm A, abnormal diastolic function is defined as having ≥3 abnormal parameters, which comprise the following: annular e' velocity with septal e' <7 cm/s, lateral e' <10 cm/s, average E/e' ratio >14, LA volume index (LAVI) >34 mL/m
2, and peak tricuspid regurgitation (TR) Vmax >2.8 m/s. It is recognized that at times only the lateral e′ or septal e′ velocity is available and clinically valid, and in these circumstances a lateral E/e′ ratio >13 or a septal E/e′ >15 is considered abnormal. Algorithm B is detailed in
Figure 6.
LA volume is a crucial parameter for evaluating diastolic function and LV filling pressure [
23], as it directly reflects LA dilation and remodeling. Nonetheless, measuring LA volume alone is insufficient for identifying LA dysfunction. LA deformation analysis, particularly LA reservoir strain, appears to be robust for detecting LA dysfunction [
33,
34].
PA systolic pressure (PASP) and mean wedge pressure are correlated. In patients without pulmonary disease, an increase in PASP is indicative of elevated LAP [
3]. PASP is indirectly calculated using the Bernoulli principle from tricuspid regurgitation in systolic jet velocity (TR Vmax) [
19,
27]. A TR Vmax exceeding 2.8 m/s, corresponding to an estimated PASP of 32 mmHg, is associated with elevated LAP [
19,
35].
Differentiation between normal and abnormal diastolic function is complicated by overlap between Doppler indices in healthy individuals and those with LVDD [
19]. Individual parameters, including those discussed above, are no more than moderately associated with filling pressures and are notably insufficient when used independently, and this includes E/e' [
36]. Therefore, using an integrated approach with multiple parameters to evaluate diastolic function is necessary [
19]. If two of the three variables meet the cutoff values, this indicates an elevated LAP and grade II LVDD. If only one of the three available variables (
Figure 6, third row, center box) meets the cutoff value, LAP is considered normal, indicating grade I LVDD. If there is a 50% discordance between two or four available variables, findings are considered inconclusive for estimating LAP. Estimating LAP is not recommended if only one parameter provides a satisfactory signal.
The ESC guidelines for the diagnosis and treatment of HF evolved between their 2016 and 2021 iterations. The 2021 guidelines [
37] recommend specific criteria for the diagnosis of LVDD or elevated LV filling pressures. These criteria are an increased LV mass index (≥95 g/m
2 for women, ≥115 g/m
2 for men), an enlarged LA (LAVI >34 mL/m²), an E/e' ratio at rest >9, a relative wall thickness >0.42, PA systolic pressure >35 mmHg, and TR velocity at rest >2.8 m/s. The LA size and E/e’ criteria, plus mitral E velocity >0.9 m/s and septal e' velocity <9 cm/s, are critical thresholds, with values beyond these increasing the risk of cardiovascular mortality. [
1].
4. Stress Echocardiography Testing for Normal and Abnormal Diastolic Function
As discussed above, patients with LVDD may have a similar hemodynamic profile (in terms of cardiac output and filling pressure) at rest as healthy individuals who have normal diastolic function. The diastolic stress test refers to the use of exercise Doppler echocardiography (ie, SE) to detect impaired LV diastolic functional reserve and the resulting increase in LV filling pressures [
38,
39,
40,
41]. It is a noninvasive hemodynamic test used to assess patients with unexplained dyspnea. It also can improve the diagnosis of HFpEF or diastolic heart failure. Frequently, symptoms of LVDD occur only during exercise, because LV filling pressure is normal at rest but increases with exercise [
39], as stress-induced LVDD [
42]. The 2022 ACC HF guidelines state that exercise SE evaluation of diastolic parameters can be helpful if the diagnosis remains uncertain after standard clinical assessment and resting diagnostic tests have been performed [
43].
4.1. Stress Echocardiography during Relaxation in Healthy Individuals
Normal diastolic function enables the LV to adapt effectively to increased cardiac output during periods of stress or exertion. This adaptability is due to enhanced myocardial relaxation and more powerful early diastolic suction, neither of which significantly raises filling pressures. The E wave, representing early passive filling and relaxation rate, may increase slightly during stress due to elevated heart rate and increased cardiac output. Simultaneously, e', reflecting the longitudinal rate of myocardial relaxation, increases proportionally during exercise. The faster the myocardial relaxation is, the higher the patient’s stress/exercise capacity. Consequently, the E/e' ratio, which serves as an indicator of LV filling pressure, typically remains within the normal range [
44,
45,
46] because both mitral inflow and annular velocities increase in proportion [
47].
During exercise, the limited time available for diastolic LV filling due to tachycardia necessitates an acceleration in myocardial relaxation and an enhancement of LV suction to maintain or increase stroke volume while preserving normal filling pressure.
Distinct levels of e' elevation during exercise, indicative of longitudinal functional reserve, can serve as a parameter for evaluating LV diastolic reserve during exertion [
48]. Some studies test diastolic functional reserve to diagnose stress-induced LVDD, which is calculated as the product of Δe′ (the change of e’ from baseline to exercise) and baseline e′ (an early diastolic mitral annular velocity at rest) [
49]. Research has associated both exercise E/e' and diastolic reserve with exercise capacity [
42], particularly in patients with HFpEF [
48,
50]. The E/e′ ratio can also be utilized as a surrogate marker to estimate LAP or PCWP during both exercise and rest [
51] (
Table 2).
In healthy individuals under stress, cardiac output rises efficiently without a substantial increase in LVEDP, owing to enhanced myocardial relaxation. In contrast, patients with LVDD attain the necessary cardiac output only through an increase in LVEDP because these patients lack a sufficient early suction mechanism for normal LV filling during early diastole. E wave: mitral peak velocity of early filling; e’ wave: mitral annular velocity of early filling by tissue Doppler.
4.2. Stress Echocardiography in Patients with Left Ventricular Diastolic Dysfunction
In patients with LVDD, a different pattern emerges. The E wave increases significantly to augment stroke volume, highlighting the challenges posed by impaired relaxation and elevated filling pressures. Conversely, e' does not change as substantially as the E velocity in patients with abnormal myocardial relaxation [
52], which is also reflected in a reduction in diastolic functional reserve. This difference may be attributed to a pathological decline in the intrinsic relaxation capacity of the myocardium, affecting both active and passive relaxation. Consequently, even when stress increases the body’s demand for cardiac output, the heart of a patient with LVDD may not be able to augment myocardial relaxation to the necessary degree. This deficiency necessitates a higher filling pressure to maintain adequate blood filling and stroke volume. When E is elevated while e' either increases slightly or remains relatively unchanged, the E/e' ratio increases significantly (
Table 2). This observation aligns with the previously mentioned greater increase in LVP during stress conditions.
In summary, SE testing offers valuable insights into diastolic function and filling pressures, distinctly differentiating the parameter behaviors of individuals with normal diastolic function from those with LVDD. This differentiation assists clinicians in evaluating cardiac performance under various conditions and in identifying LVDD when it is present. According to published studies, an E/e' >15 (using septal e' velocity) can be used as a diagnostic criterion for stress-induced relaxation dysfunction [
39].
7. Future Perspectives
Diastolic function assessment through echocardiography holds immense promise, propelled by rapid advancements in technology and an enhanced understanding of cardiac physiology. AI and ML applications are on the cusp of reshaping the landscape, deploying algorithms to automate intricate measurements and deliver nuanced analyses. Ongoing studies are striving to harness AI's potential to refine diagnostic criteria and elevate risk stratification, signifying a pivotal shift toward precision medicine in the realm of LVDD. Simultaneously, strain imaging provides more comprehensive insight into cardiac dynamics, potentially heightening sensitivity to detect subtle changes. Researchers are actively exploring the seamless integration of these advanced imaging modalities into algorithms for diastolic function assessment, thereby enriching our diagnostic arsenal. In the most recent studies, efforts have been made to integrate STE measures of LAS with ML to enhance the classification of LVDD [
121].
In the inadequately explored domain of molecular imaging and biomarkers for LVDD, identifying novel markers could be the key to early detection and personalized management, aspects not covered in this review. Initiatives to unveil the molecular and cellular processes underlying LVDD could pave the way for targeted imaging agents and blood-based assays, equipping clinicians with tools to assess diastolic function at the molecular level. The future envisions a shift toward patient-centric approaches, integrating patient-reported outcomes and preferences into diagnostic strategies for a more personalized approach to care. Collaborative and multidisciplinary research initiatives are gaining momentum, addressing standardization challenges and translating research findings into clinical practice. This collective endeavor is poised to usher in a new era in cardiovascular medicine, where innovative technologies and collaborative endeavors converge to redefine the diagnosis and management of LVDD, ultimately enhancing patient outcomes.
8. Conclusion
Reduced myocardial relaxation is among the earliest indicators of LV mechanical dysfunction. The exploration of diastolic dynamics by echocardiography is crucial for understanding and addressing a spectrum of myocardial conditions, including myocardial ischemia, hypertensive heart disease, hypertrophic cardiomyopathy, and HFpEF. Echocardiography, as the primary imaging modality for assessing LVDD, provides valuable insights into its hemodynamic impact and improves prognostic accuracy. From detecting subtle changes to guiding personalized treatments, this approach encompasses rest echocardiography, SE, and STE, as well as the application of AI and ML. Its multifaceted role significantly contributes to improving patient outcomes. To enhance accuracy in estimating LV filling pressure and grading LV diastolic function, the ASE guideline strongly recommends a comprehensive approach that integrates clinical data with echocardiographic findings [
19].
Looking ahead, the future of echocardiography in diastolic function assessment is bright, with AI and ML poised to further refine diagnostic accuracy and enable personalized patient assessments. Research into molecular imaging and biomarkers is expected to unlock new possibilities for early detection and targeted management of LVDD. The integration of patient-reported outcomes and preferences into diagnostic strategies marks a shift toward more personalized care.
In conclusion, echocardiography remains an indispensable tool in the evaluation of diastolic function. The field is evolving rapidly, with technological advancements and collaborative research efforts driving significant improvements in cardiac diagnostics and therapy. This evolution is set to redefine the management of LVDD, ultimately enhancing patient outcomes and advancing the field of cardiovascular medicine.