Clinical Cardiovascular Research

Abstract

Left atrial stiffness index is a new method to evaluate left atrial function. It¨s calculated by the relation between E/E´ divided by left atrial reservory strain.  In this review we evaluate the relationship of these values to the concept of rigidity, the current clinical evidence and the prospects for their clinical use.

Left atrial function and its importance in clinical practice is well known. What is less commented is that since the left atrium (LA) is not free in the thorax but coupled to the rest of the cavities, especially to the left ventricle, its function indexes will be closely related to the intrinsic properties of the ventricular chamber (1,2).

Table 1 shows the dependence of the atrial function indexes.

Table 1. Dependence of atrial function

Another very little discussed but evident aspect of the LA pressure-volume curves (Figure 1) is the suction effect that the LA has in the initial reservoir phase where, although it begins to increase its volume, its pressure, instead of rising, falls, which is characteristic of suction and I suspect very dependent on the elastic properties of LA.

Image 1. Pressure curve volume of left atrium, in the initial phase of the reservoir despite an increase in volume (thick blue arrow), pressure drop is observed (thick red arrow), typical of suction

The atrioventricular coupling and its interdependence make it very difficult to obtain function indices that express purely atrial function. And yet the literature is very extensive on the value of LA volumes and deformity indices in a number of entities such as:

1.         Prediction of diastolic dysfunction (3,4).

2.         Diagnosis, mortality and events in CHF with preserved EF (5, 6, 7).

3.         Prediction of mortality and events in patients with reduced EF (8,9)

4.         Development of atrial fibrillation (10)

5.         Prediction of events in patients with atrial fibrillation (11).

6.         Maintenance of sinus rhythm after cardioversion or atrial isolation (12, 13).

7.         Prediction of survival after myocardial infarction (14) and prediction of events in ischemic heart disease and mortality (15).

8.         The size of the LA is related to postoperative mortality in mitral regurgitation (16).

In recent years, a new index of atrial function has appeared that we will review, the LA stiffness index.

LA Stiffness index

It is a simple index that is calculated by dividing the E/E ́ ratio by the reservoir strain.

It is a simplification of the real concept of stiffness, the stiffness of a chamber is evaluated by measuring the change in pressure that causes a change in volume. Thus, a very rigid chamber increases a lot of pressure with small changes in volume and a compliant the opposite.

If we look again at the pressure-volume curve of the LA (Figure 2), the stiffness would be the slope of the line that joins the zone of lowest pressure with the zone of highest pressure

Figure 2. Pressure/volume curve of LA, the blue dotted line represents the change in pressure with respect to the volume, its slope is the stiffness of the chamber

This simplification will work well as long as the E/E' ratio represents the pressure changes and the reservoir strain represents the volume changes.

It must be taken into account:

• There is dissociation of the pressure of the LA with E/E' in mitral stenosis or in the calcification of the annulus (17)
• E/E' represents the wedge pressure which is not the same as LA pressure

On the other hand, strain curves are very similar to those of LA volume (Figure 3), however in very dilated atria the strain may be reduced despite adequate reservoir volume (strain volume dissociation) (17)

Fig 3. Volume curve of LA over time (very similar to the strain curve)

Despite all these considerations, several publications have been appearing since 2009, which are worth discussing.

Kurt et al. (including Dr Nagueh, ref 18) using right heart catheterization and simultaneous echo found that the stiffness index of the LA using reservoir wedge/strain reservoir and using reservoir E/E'/strain reservory was different between the groups:

Table 2. Stiffness index of invasive and non-invasive LA in the different groups

(from J Am Coll Cardiol Img 2011; 4:1080–7.2023, 16(4): 446-449)

There was a significant difference between all groups, even patients with diastolic dysfunction despite not having CHF already had different stiffness. Non-invasive measured stiffness was able to diagnose patients with diastolic CHF

Interestingly, the two measures of stiffness (invasive and echo-only correlated) correlated very well (r=0.85 and 0.8 respectively) with systolic pulmonary pressure (Figure 4)

Appleton (19) in an Editorial in the same issue considers in a very skeptical commentary that the denominator of this relationship (reservoir strain) is very dependent on ventricular function, that new intrinsic properties of the LA need to be found considering the coupling of the atrium with the ventricle.

Shaikh (20) in a prospective study finds that the stiffness index is completely different in normal patients vs. patients with atrial fibrillation (AF) (0.23 vs 1.32) even after cardioversion (1.05), however the index was not different between those who maintained sinus rhythm vs. those with AF recurrence

Using cardio resonance imaging, Lamy et al. (21) found a relationship between gadolinium uptake by the LA and the stiffness index of the LA

Kim, in a recent publication (22), evaluated the prognostic implications of the LA stiffness index in 307 patients with heart failure with preserved ejection fraction (HFPEP) who had wedge pressure > 16 mm Hg and EF > 50%. Increased LA rigidity was associated with an increased risk of overall mortality and hospitalizations due to heart failure in patients with Heart failure with preserved ejection fraction (HFPEF) (Figure 5)

Fig 4. Correlation between lung pressure and the stiffness index of non-invasive IAs (modified from Cir Cardiovasc Imaging 2009; 2:10-15)

Fig 5. Survival according to the stiffness index in patients with HF PEP

In the same issue, Dr. Nagueh makes an editorial comment in which highlights the need to find the normal values of the stiffness index, its relationship with sex and age and in addition to its role in HFPEF and AF, what role it has in valvular disease, cardiomyopathies and systemic diseases that affect the IA and LV (23)

The LA stiffness index seems to add data to the conventional indices of the diastolic function study and requires, as Nagueh says, more studies to define its usefulness in daily clinical practice

Personally, it seems to me that it was born with a wrong name, we will see later how much clinical utility it can provide as suggested by preliminary studies

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