Abbreviations : BMI body mass index, BSA body surface area, RA
right atrial, RV Right ventricle.
Discussion
This is the first study to provide normative age-related data for RALS
in a Sub-Saharan African population. Additionally, we have also provided
complementary data regarding RAVI according to age and gender in this
population. RALS had tendency to decrease with age concurrent with a
decline in RV diastolic function despite no alteration in RAVI, and BMI
was an important independent predictor of RALS. Even though there was a
tendency of RAVI to be higher in males, RAVI and RALS were not
influenced significantly by gender.
Previous studies using 2DE by Y Wang et al. in 1984, and S. Kou et al.
from the Normal Reference Ranges for Echocardiography (NORRE) study
2013, were focused on Caucasians populations, and were in line with the
ASE and ESC 2015 chamber quantification guideline document, which states
that RAVI should be <33 mL/m2 (or <35 mL/m2 in men
and <31 mL/m2 in women)(23,24). In this
study we found that, while the lower limits were similar to those of ASE
and ESC guidelines, the upper margins were much lower (20.8 ± 6.3 mL/m2
in male and 18.7 ± 5.2 in females). Studies in African and Asian
populations by Nel et al. and Asian by Karki et al. respectively have
also observed the same pattern.(25,26) Soulat-Dufour
with WASE study 2,008 healthy adult individuals around the world, has
shown that generally, Asian subjects have lower BSA compared to subjects
in non-Asian countries.(16) Ethnicity may cause
variation in genetics and parameters of BSA and BMI and thus may
directly influence echocardiographic measures. Thus, correlation between
ethnicity and RA volumetric parameters is important.
In this study, there was a trend towards higher RAVI in males when
compared to females but no influence of age on RAVI was noted. The
aforementioned finding was akin to the recent study by Nel et al. and
the WASE pertaining to RAVI in a normal population. Grünig et al. and
D’Ascenzi et al. have shown that males have a larger RA area compared to
females, explaining volume differences between the
gender.(27,28) D’Oronzio et al ,Y. Wang et al. and
Peluso et al. using 2DE and 3DE respectively, did not find any
correlation between atrial volume and ageing for reasons that are not
fully understood.(23,29,30)In the current study the
mean RALS was 32.7 ± 10.5% , which was lower than the value reported by
Padeletti et al. and D’Ascenzi et al.(22,28) In the
aforementioned Italian comprising 84 and 74 subjects, RALS was
documented using the 2D STE and the mean RALS was 49 ± 13% and 48±12.68
% respectively .(22,28) The above-mentioned studies
were done in a group of subjects with overall lower BMI ( 22.4± 3.5
kg/m2) in contrast to the current study ( 24-34.9
kg/m2) and, is the likely explanation for higher RALS
in their cohort. The following paragraph describes in detail the
influence of obesity on RALS.
Obesity has been a health problem of growing significance all over the
world; its prevalence is increasing in both developed and developing
countries. According to WHO data, 39% of the global population above 18
years of age are overweight and of these ,13% are obese. In Africa
there is a significant obesity trends, with increments in BMI documented
in both gender.(36) Micklesfield et al. studied South
Africa population from Soweto and demonstrated a significant gender
difference with regard to BMI, with women being remarkably overweight
and obese.(37)
In this study 67% of normal volunteers had BMIs above 25
kg/m2 , which occurred more commonly in women (49 %).
BMI was the only independent predictor of RALS in this study. An inverse
relationship was noted between BMI and RALS in the current study. Recent
study by
Chirinos
et al. quantified left atrium (LA) strain and strain rate (SR) via STE
among 1,531 middle-aged community-based participants enrolled in the
Asklepios study. They demonstrated that longitudinal LA strain measured
using STE decreased with elevated BMI.(33)
Obesity has been shown to have many effects on cardiovascular structure
and function. Excess adiposity imposes an increased metabolic demand on
the body and both cardiac output and total blood volume are elevated in
obesity leading to a hyperdynamic circulation, which causes LV and RV
structural changes in obesity and subsequently leads to increased
ventricular mass and cavity dilatation. Obesity is the main lead in
tissues fibrosis formation.(38) Sokmen et al., and
Csige et al. showed that uncomplicated obesity was associated with RV
and RA dilatation, and increased thickness of the RV free wall. Also,
these structural indices were found to be positively correlated with
BMI. Myocardial fat accumulation as a consequence of obesity, may cause
atrial interstitial fibrosis and subsequently atrial dilatation and
stiffness.(39,40) This may be a possible mechanism
explaining the inverse relationship of BMI and RALS in this study, that
obesity may result in RA fibrosis, stiffness with a consequent reduction
in myocardial deformation and RALS.
Furthermore, this study demonstrates that RALS tended to decrease with
age and males tended to have higher values compared to females, though
this finding did not reach statistical significance.
Aging is associated with the development of myocardial fibrosis.
Fibrotic tissue is stiffer and less compliant, resulting in subsequent
cardiac dysfunction.(31,32) . We hypothesised that a
combination of stiff RA and RV diastolic dysfunction associated with age
related myocardial fibrosis will have a poor myocardial deformation and
this may result in age related decrease in RALS. However, we did not
perform biomarkers or CMR imaging to objectively assess for presence of
fibrosis in this study.
The trend towards gender differences of RALS (though not statistically
relevant) could be multifactorial in this study. In addition to a
difference in biology, a higher BMI in females compared to males may
explain the aforementioned finding. Male participants were also younger
and had higher RAVI and likely more compliant RA compared to females
which translated into higher RALS. Further, males had a lower heart rate
compared to females ( 65.0 (59.0-76.5) beats/min vs 76.0 (67.0-82.5)
beats/min, P=0.002) which allowed for a prolonged filling time of RA
chamber and thus increased stretch of the right atrial wall with
resultant higher RALS.
Padeletti et al. did not find differences in RALS gender and ageing.
This lack of association between RALS , gender and aging may be related
to the limited capacity of the software in identifying all the segments
of the RA due to the higher tricuspid annulus deformation compared to
the mitral valve .. Furthermore, this may be attributed to varying
sample sizes and racial differences in the two studies . With the recent
technologies Nemes et al., and Yang-Yang Qu et al. using
three-dimensional speckle tracking echocardiography (3DSTE) and CMR
respectively , have demonstrated obvious gender differences in RA
strain. In contrast to our findings, they noted RA strain to be higher
in females and showed an age related decline in both
genders.(34,35) The differences in analysis software
and technique used by the above-mentioned studies may explain the
discordances in results. Further studies are warranted to confirm our
findings in a larger African population and also to further assess RALS.
We have also confirmed the utility of RALS as a marker of subclinical
disease in this population as we did not see changes in RAVI with age
but noted a trend towards lower RALS with increasing age., Thus, RALS
may anticipate the RA impairment in diseases prior to changes in
traditional parameters such as RA size and volumes. This may aid in
earlier diagnosis of disease and prompt treatment strategies at a
subclinical stage of the disease.