In‐depth echocardiographic analysis of left atrial function in healthy adults using speckle tracking echocardiography and volumetric analysis

Purpose Left atrial (LA) dilatation is predictive for complications in a multitude of cardiac diseases; therefore, adequate assessment is essential. Technological advances have made it possible to quantify LA function with Speckle Tracking Echocardiography (STE); however, there are currently no recommendations for normal values with regard to LA function. We aimed to assess LA myocardial and volumetric function in a healthy cohort and investigate correlations with baseline characteristics. Methods This prospective cohort study included 147 (aged 20–72) healthy individuals and assessed LA volumetric function using maximum, minimum and pre‐a‐wave volumes and myocardial function using reservoir function using peak strain in LA relaxation (LA‐strain), conduit function using peak strain rate in early LA contraction (LA‐SRe) and pump function using peak strain rate in late LA contraction (LA‐SRa). Results Mean LA‐strain was 39.7 ± 6.2%, LA‐SRe −2.78 ± 0.62 s−1 and LA‐SRa −2.56 ± 0.62 s−1. Subjects were divided into 5 age decades (each 50% female). LA‐strain and LA‐SRe were lower in the oldest groups, whereas LA‐SRa was higher. LA‐SRa was higher in males(−2.69 ± 0.68 s−1 vs −2.42 ± 0.52 s−1). Age‐specific values are provided. Age proved to be an independent predictor for LA‐SRa after correction for blood pressure and heart rate. LA expansion index and passive emptying fraction decreased with age, while active emptying fraction increased with age. LA maximum volume did not increase with age. Conclusion This study provides normal values for the three phasic functions of the LA, assessed with STE and volumetric function. Our results suggest the need for age‐specific reference ranges, and normal values for this cohort have been calculated.


| INTRODUC TI ON
Assessment of the left atrium (LA) is gaining increased attention as it reflects the severity and chronicity of many different conditions and is associated with significant morbidity and mortality. 1 In the absence of valvular disease, LA volume reflects the presence of elevated left ventricular (LV) diastolic pressure and dysfunction. 2 LA maximum volume is the most often described parameter, but LA phasic function could be a more sensitive measure in patients with heart failure, valvular disease, and atrial fibrillation. LA function can be assessed by volumetric measurements and includes reservoir, conduit, and pump function which can be expressed as absolute volumes or fractions. Recently speckle tracking echocardiography (STE) has been validated for LA measurements 3 ; LA strain and strain rate can be measured which reflect LA myocardial function without the need for geometrical assumptions.
The clinical value of LA volumetric and myocardial function has not been translated into recommendations to be used in clinical practice. This is in part because solid reference ranges have not been established, neither for volumetric measurements 4-6 nor for strain measurements. [7][8][9][10][11][12] Therefore this study aims to provide reference ranges for LA myocardial and volumetric function in healthy adults and investigates the impact of age, sex, and BSA.
Details have been published earlier. 13 Briefly, subjects were recruited via advertisement and underwent a questionnaire regarding medical history and current health status, physical examination, venous blood sampling, 12-lead ECG, and an echocardiogram. Subjects were excluded if one or more of the following criteria were present: (prior) cardiovascular disease, systemic disease, the finding of cardiac abnormalities during the examination (including any valvular abnormalities) or risk factors including hypertension (cutoff values: 140/80 mm Hg), diabetes mellitus, impaired renal function or hypercholesterolemia. In case of elevated blood pressures, follow-up measurements were performed by the general practitioner to confirm this. If follow-up revealed normal blood pressures, the subject was included. Reasons for exclusion due to abnormalities on ECG were conduction disorders: Atrial fibrillation, right or left bundle branch block, prolonged PR interval, and prolonged QRS. Professional athletes, people who were morbidly obese (BMI > 40 kg/m 2 ), having breast implants or pregnant were excluded. This study was carried out according to the principles of the Declaration of Helsinki and approved by the local ethics committee.
Written informed consent was obtained from every participant.

| Echocardiographic image acquisition
Echocardiographic studies were performed by one of two experienced sonographers. Two-dimensional grayscale harmonic images were obtained in the left lateral decubitus position using a iE33 or EPIQ7 ultrasound system (Philips Medical Systems, Best, The Netherlands) equipped with a transthoracic broadband X5-1 matrix transducer (composed of 3040 elements with 1-5 MHz). The LA was acquired in dedicated apical four-and two-chamber views with frame rates ≥ 50 frames/s. 14 At least 2 consecutive heartbeats were recorded.

| Volumetric analysis
In order to assess LA maximum volume, the revised recommendations for cardiac chamber quantification were used. 1 LA minimum volume (measured at mitral valve closure) and pre-a-wave volume (one frame before atrial contraction starts) were measured using the biplane method-of-disk summation technique ( Figure 1) and the area-length method. All measurements were performed with Xcelera (Philips Medical Systems). Using the above volumes, LA function can be assessed as follows: Left atrial reservoir function: • Left atrial total emptying volume (TEV) = LA maximum volume-LA minimum volume.
• Left atrial total emptying fraction = TEV/LA maximum volume. • Left atrial active emptying fraction = AEV/LA pre-a-wave volume.
All reported volumes are indexed for BSA. Since the Dutch population is the tallest in the world, 15 we indexed for an allometric function of height 2.7 . 16 LV diastolic function was assessed according to the EAE-ASE recommendations for diastolic function. 17

| Speckle tracking analysis
Offline analysis was performed using QLAB10 (Philips Medical Systems). LA myocardial function was assessed according to an earlier published guideline and a recent validation study, 18,19 using the apical four-and two-chamber views and the R-wave as reference point.
LA reservoir function can be expressed as peak strain (LA-strain) and

| Statistical analysis
Normal distribution was checked using histograms and Shapiro-Wilk tests. Depending on data distribution, continuous data are presented as mean ± standard deviation (SD) or median with first-third quartile. Categorical data are presented as frequencies and percentages.
Student's t-test, the Mann-Whitney U test, chi-square test or Fisher's exact test was used when appropriate. Correlations between LA measurements and baseline characteristics were assessed using the Pearson correlation test. When a variable was statistically significant and did not show collinearity with another variable, they were included in a multivariable linear regression model. In case of collinearity, the one with the strongest correlation was selected. Statistical analysis was done with the Statistical Package for Social Sciences version 21 (IBM DPDD Statistics for Windows, Armonk, NY, USA). A P-value of ≤0.05 (twosided) was considered statistically significant.
Interobserver (RG, MS) agreement was assessed for LA volumetric and strain parameters using Bland-Altman plots in a sample of 30 random subjects. 20 Measurements were done while being blinded for the other measurement approximately 1 month later. Agreement between two measurements was determined as the mean of the difference ± 1.96 SD.

| RE SULTS
Out of the 155 eligible subjects, 147 subjects were included (median age 43.8 [32.7-56.2], 50% female) into 5 age groups (n ≥ 28 per group). In total, 8 subjects were excluded: 2 due to having breast implants, 2 subjects had valvular pathology, 1 had a surgically closed ductus, 1 had hypertension, 1 with morbid obesity, and 1 with a right bundle branch block. Table 1 shows the baseline characteristics of the study population.

| LA volumetric function
Feasibility for volumetric measurements was good, ranging from 92.5% to 95.9% (Table 2). LA volumes were indexed for BSA (Table 2), and an additional analysis was performed with height indexed parameters (Table 3). Changes in volumes can be seen between the age groups regardless of the indexation method. LA minimum and pre-a-wave volumes increased with each age decade. With regard to function, LA reservoir and conduit function decreased while pump function increased with age (Table 4).

| LA myocardial function
Left atrial-strain analysis results are shown in Table 4, including the feasibility, which ranged from 78.2% to 80.3%. LA-strain was lowest in the oldest age groups as was LA-Sre, but LA-Sra increased with age ( Figure 3). LA-Sra was significantly more negative in men than women, no sex-dependent differences were found in LA-strain and LA-Sre ( Figure 4). The limits of normal (mean ± 2 SD) were also calculated ( Table 5).

| Correlations
Besides age, LA reservoir function did not correlate with baseline characteristics (Table 6). Conduit function decreased slightly with increasing weight, BMI, and blood pressure, while pump function increased with BMI, heart rate, and blood pressure.

| Reproducibility
Interobserver agreement was assessed for volumetric and strain measurements: Mean difference for LA maximum volume was −5.2 ± 12.1 mL. For pre-a-wave and minimum volume, this was −0.9 ± 10.2 and −1.0 ± 8.4 mL, respectively. Regarding strain

| LA volumetric vs myocardial function
This study demonstrates that LA volumetric and myocardial assessment is highly feasible. We recognize that the BSA-indexed maximum volume in our study was large according to current guidelines.
However, with parameters such as LA expansion index, passive and active emptying fraction this is no longer relevant, since these measurements are relative. 22

| Factors influencing LA function
There are a lot of factors that could influence LA volume and consequently function. We have assessed the LA through volumetric function with total emptying fraction, a sort of ejection fraction of the LA. It is well known that this is divided into a passive and active phase and that a portion will flow back into the pulmonary veins.
Therefore, we also provided LA expansion index, which better describes reservoir function. Instances that influence LA volumes are age, sex, height, and weight. To address these, LA volumes are often TA B L E 2 Left atrial echocardiographic volumes indexed for BSA per age decade  indexed using BSA. In our study, we no longer found differences between men and women after correcting for BSA but we did find relatively high values; a quarter of these volunteers had a LA max volume above the upper limit of normal. 1 This might be explained by the fact that height and weight are not both as important for LA volume. The Dutch are the tallest people in the world 15 which is why an additional analysis was done correcting for height as done previously by Eshoo et al. 16 We found no differences when comparing these results with the BSA corrected volumes. The only exception was that LA maximum volume became significantly but weakly correlated with age (r: 0.202, P: 0.018).

| Effects of age and LV diastolic function on LA function
Several studies have looked at possible age-related effects on LA size and function, with mixed results. [3][4][5][6]23 The idea that age influences LA function is not new; Benjamin et al 24 stated that E-wave velocity decreases while A-wave velocity increases with advancing age. Our study demonstrates that age influences LA myocardial function. LA-strain and LA-Sre are lowest in older subjects while LA-Sra is higher, which is as expected. This is partly in line with the study of Morris et al, 19 who implicated as much for LA-strain measurements. In our study, LA-Sre and LA-Sra also changed with age, though the values that we found for LA-strain were slightly lower than reported earlier. 19 This may be due to age differences between studies or intervendor differences, as a recent study showed that QLAB10 reports slightly lower values for GLS than other software packages. 25 The study of Miglioranza et al, 26 which looked at influences due to age, showed similar effects, though the actual results cannot be compared as the P-wave was used as onset.
Currently, there is no consensus on how to assess LA phasic function with STE. In this study, we used R-wave as onset because that would allow extrapolation of our data to patients with atrial fibrillation.
There are other recent studies that used either the R-wave or the Pwave as onset showing that both these techniques are possible. 19,26,27 We choose for strain rate to assess LA booster pump function instead of peak strain, as this was found to be superior. 27,28 Pathan et al 29 performed a meta-analysis to formulate normal values for LA function.
Reservoir function was 39.4% which corresponds very well with our findings, unfortunately for conduit and pump function, strain instead of strain rate was used, which makes it impossible to compare our findings.
Left ventricular diastolic dysfunction is closely related with LA function, and our results reflect that as well. E-and A-wave velocity correlated well with LA conduit and pump function, regardless of the method used. An increase in LV stiffness leads to a reduction in LA conduit function, which is compensated by an increase in pump function. This can be witnessed by the E/A-ratio, which inverses with age. This was seen for the LA myocardial function parameters.

| Limitations
This was a single-center study including Dutch Caucasian subjects.
Extrapolation to other ethnicities should be done with caution. We used QLAB for the strain analysis, though a recent study found no differences between vendors for LA measurements, 29 comparison with other vendors should be done with caution. Also, subjects had no restrictions regarding food intake prior to the echocardiographic examination. This could influence tissue-and pulsed-Doppler measurements. 30

| Clinical implications
The results from this study may add to the foundation to formulate reference values regarding LA functional analysis, in preparation for studies to determine potential diagnostic and prognostic value which may eventually be used to assess patients in a clinical setting. In our experience, LA functional analysis, especially myocardial deformation, is easy and quick to perform. As expected, age plays an important role, which is why we propose age-dependent reference ranges. The fact that LA maximum volume did not correlate with age but LA-strain did indicates that functional assessment is a more sensitive marker.
Future studies should investigate the potential prognostic value of LA function and which technique, myocardial deformation or volumetric assessment, is most valuable.

| CON CLUS ION
Left atrial volumetric and myocardial function measurement is a viable option, and age-dependent reference ranges for LA phasic function are presented. LA myocardial and volumetric function parameters have proven to be age-but not sex-dependent. Considering the high feasibility and clinical relevance of LA myocardial function

CO N FLI C T O F I NTE R E S T
None declared.

R E FE R E N C E S
TA B L E 6 Bold mean statistically significant, *P-value < 0.05, **P-value < 0.001