In this CMR study, we systematically established age- and sex-specific reference values for LA, RA volumes and phasic function based on a large sample of validated healthy Chinese adults free of hypertension, diabetes, and obesity. Moreover, we further confirmed that both sex and age had a considerable impact on atrial volumes and phasic function.
Comparison with previous studies
At present, although there are a few CMR studies on normal biatrial reference values for Chinese adults, this study still presents certain advantages by providing more comprehensive atrial volume and function parameters based on a significantly larger sample size. Specifically, compared with the present study, a study on 200 healthy Chinese volunteers by Zhuang et al.  reported comparable normal values of LA Vmax, LA Vmin, and RA Vmax, but significantly greater RA Vmin (29.7 ± 10.9 mL versus 53.52 ± 16.12 mL). However, they did not provide atrial phasic function parameters including EF, EV, and EI. In addition, another CMR study by Li et al.  reported similar normal reference values of biatrial Vmax, Vmin, Vpac, EF total, EF passive, and EF booster based on 135 healthy Chinese adults. However, their provided parameters were not further stratified by age. Finally, as definitive indicators of adverse atrial remodeling in the clinical scenario such as acute coronary syndrome [23, 24], information regarding normal values of atrial EV and EI has scarcely been reported. To the best of our knowledge, this is the first CMR study to establish age- and sex-specific normal values of atrial EV and EI in Chinese adults.
Previous multi-ethnic studies have confirmed that there are significant racial differences in atrial volumes and function, as described by the smaller atrial sizes and higher atrial function parameters for Asians compared with those for whites [14, 15]. Compared with several studies in western populations [11, 13, 21, 25], LA and RA volumetric parameters (including Vmax, Vmin, and Vpac) in the present study were smaller, while the phasic function parameters (including EF, EV, and EI) were slightly higher, which may confirm the racial difference in arial volumes and function as well.
Gender differences in LA and RA volumes and phasic function
In general, men presented with higher absolute LA Vmax, LA Vmin, and LA Vpac than women, which was consistent with a few previous studies [26, 27]. Interestingly, we found that BSA-indexed LA Vmax and LA Vpac in women were significantly greater than those in men. Besides, we confirmed that LA phasic function was also affected by sex. Specifically, women showed higher LAEF total and LAEF booster than men, which was in accordance with the findings of Vasconcellos et al. . In contrast, some other studies, such as those by Maceira et al.  and Truong et al. , did not observe these sex differences. Currently, there are still few studies on gender difference in LAEV and LAEI. Maceira et al.  reported that women presented with significantly smaller LAPEV and higher LAEI than men. Different from their study, we demonstrated that there was no significant sex difference in LATEV, LAPEV, and LAAEV, while women had a significantly higher LAEI.
For the right atrium, in line with several previous studies [15, 28], we demonstrated that the absolute and indexed RA Vmax, RA Vmin, and RA Vpac for men were all significantly greater than those for women. However, there are still considerable controversies about gender differences in RAEFs. For example, several studies showed that RAEF booster was not associated with gender [13, 15, 16]. In comparison, our study demonstrated that female sex was associated with greater RAEFs including RAEF total, RAEF passive, and RAEF booster, which was consistent with the findings from Peluso et al. . We speculated that the differences in sample size, ethnicity, and baseline characteristics of subjects may together contribute to these inconsistencies among studies. Furthermore, we confirmed that men had significantly greater RATEV and RAAEV and a lower RAEI compared with women, while there was no significant sex difference in RAPEV, which was generally agree with the study by Maceira et al. .
Correlation between atrial volumes, phasic function, and age
It is controversial regarding the correlation between atrial volumes and age. For example, Petersen et al.  showed that aging was associated with decreased LA Vmax based on 804 Caucasian adults aged 45 to 74 years. A CMR study by Li et al.  in 135 healthy Chinese volunteers aged 49.9 ± 17.1 years showed that age was positively correlated with LA Vmax, LA Vmin, and LA Vpac. In line with the study by Van Grootel et al. , we found that age showed no significant correlation with LA Vmax, while was associated with increased LA Vmin and LA Vpac. Besides, with respect to the right atrium, most previous studies reported aging was associated with no change in or decreased RA Vmax [16, 21, 27]. In contrast, we demonstrated that age was positively correlated with all the RA volumetric parameters including RA Vmax, RA Vmin, and RA Vpac, even after normalized by BSA. These differences may be related to the narrow age range as well as the limited sample size in previous studies.
Our study demonstrated that age was a major determinant of biatrial phasic function, as found that the measurements of LA and RA reservoir and conduit function were decreased, whereas booter function was increased with advancing age. These findings further reinforced the previously described associations between age and atrial function [13, 15]. The observed age-related variations in atrial function were in accordance with physiologic knowledge. Normal aging is associated with loss of cardiomyocytes, myocardial fibrosis, and impaired ventricular relaxation, which may together contribute to the increased atrial booster function, decreased atrial reservoir and conduit function [30–32].