Discussion
The main result of this study is that in CHF outpatients, the LA
reservoir, a 2D-STE measure of atrial function, is independently
associated with worsening heart failure. Moreover, the integration of
the LA reservoir with the ratio between mitral early diastolic peak
velocity at pulsed Doppler and TDI provides a measure reflecting the
interaction between atrial and ventricular function which is even more
accurate to stratifying prognosis.
LA function is often considered only a passive bystander of LV
dysfunction. However, several studies have shown that LA contractile
function is determined by intrinsic LA contractility, LV compliance, LV
filling pressures (LA afterload), and pulmonary vascular capacitance
[1]. Atrial function improves ventricular filling by storing blood
during ventricular systole while the atrioventricular valves are closed
(reservoir function), as a passive conduit and emptying during early
ventricular filling (conduit function), and providing additional
ventricular filling with active contraction before atrioventricular
valve closure (booster pump function). The interplay between LA and LV
functions throughout the cardiac cycle (LA–LV coupling) is crucial in
several pathophysiological conditions [3]. In this
pathophysiological setting 2D-STE could represent a useful tool not only
to evaluate the LV and RV functions [11-13] but also to provide
measures reflecting the atrial function [14]. In particular, it
allows the study of the complexity of the atrial deformation by
analysing three sequential phases, i.e. the reservoir, the conduit, and
the contraction.
The reservoir phase begins at the closure of the atrio-ventricular
valves and ends with their opening at the end of isovolumic ventricular
relaxation. In this phase, the atrial pressure quickly rises while
receiving blood from the veins, and the atria stretch to their maximum
capacity. The greater the atrial stretch, the greater the value of the
reservoir. The other two phases of atrial function are reciprocal to the
reservoir. The first is the conduit phase, which is related to the first
half of ventricular diastole, after the opening of the atrioventricular
valves. During this phase, the atria quickly unload in the ventricles
the blood accumulated in the reservoir phase (passive atrial emptying),
and the strain analysis will register a negative deflection of the
curve, followed by a plateau which corresponds to the diastasis, when
the blood directly flows from the veins to the ventricles. This phase is
influenced by the compliance of the ventricles and the diastolic
function [14]. The last phase is the contraction phase,
corresponding to the atrial systole. This phase reflects the contractile
capacity of the atria, along with the ventricular stiffness and the
ventricular end diastole filling pressures [14].
Previous studies have demonstrated the role of peak atrial strain in
predicting heart failure prognosis [5, 6]. However, different from
these studies, we attempted to evaluate the prognostic role of all the
phases of the atrial function. Interestingly, all the measures were
associated with heart failure worsening, but only LASr remained
independently associated with the occurrence of events during
multivariate regression analyses. This evidence could be related to the
ability of the reservoir to reflect a global atrial dysfunction.
Consequently, although the conduit and contraction phases could help to
define more clearly the mechanisms responsible for atrial dysfunction,
the reservoir, being the expression of both the other phases, is
associated with more important prognostic information.
This hypothesis is supported by the studies demonstrating the close
relationship between a reduced LASr and the pathophysiological
mechanisms underlying atrial dysfunction [14–16], i.e. the
increased LV filling pressure, the LA remodelling, and the loss of
contractile capacity. For this reason, the impairment of LASr could
precede anatomical alterations [15], thus allowing a more accurate
identification of diastolic dysfunction than conventional echo
variables, such as LA volume index or E/e’ [17-20]. Moreover, the
therapeutic strategies able to improve LV function can be associated
with the improvement of atrial function [21].
The relevance of our study is also related to the demonstration that the
combination of the strain measure of LA function, LASr, and that of LV
filling pressure based on pulsed Doppler and TDI, E/e’, is characterised
by even greater prognostic accuracy in predicting heart failure
worsening. In previous studies, the ratio between LASr and E/e’ showed
greater accuracy in detecting high filling pressure [7, 22], but
this measure should also be considered as a parameter that can better
reflect the relationship between LA and LV functions. In fact, LV
function worsening could cause an increase in LV end-diastolic
pressures, which can then cause LA wall stiffening, thereby resulting in
decreased LA relaxation and decreased LA strain peak during LV systole.
Conversely, the decreased LA function could further favour the increase
in LV filling pressure, thus leading to a vicious circle responsible for
heart failure progression. Consequently, lower ratio between LASr and
E/e’ corresponds to worse interaction between LA and LV functions, i.e.
greater LV filling pressure and worse LA function, thus allowing to
provide a significant parameter of prognostic stratification.
In conclusion, 2D-STE is an echocardiographic technique able to provide
measures of atrial function independently associated with heart failure
worsening. Among these measures, the reservoir shows greater accuracy in
predicting the events. Moreover, this accuracy is even greater when the
strain measure of the reservoir is combined with the Doppler measures
related to LV filling pressure. These data further strengthen the
possible usefulness of 2D-STE in providing information about both
ventricular and atrial functions.