Quantitative analysis of left atrial function in asymptomatic patients with b-thalassemia major using real-time three-dimensional echocardiography

Cardiac involvement represents an important complication of b-thalassemia major, and results in increased mortality and morbidity rates. Disease mechanisms implicate iron infiltration of the heart structures due to frequent blood transfusions. Transfusion-dependent patients receive 20 times the normal intake of iron, which leads to iron accumulation and damage in the liver, heart, and endocrine organs [1]. Although iron chelating therapy has markedly improved outcomes, cardiac failure remains an important cause of death in thalassemic patients [2]. Therefore, early recognition of myocardial dysfunction is imperative, given the fact that majority of this patient group have preserved ejection fraction until late in disease process.

On the other hand, left atrial (LA) enlargement plays a pivotal role in the prognosis and management of various cardiovascular diseases, including coronary artery disease and heart failure [3, 4]. Recently real-time three-dimensional echocardiography (RT3DE) has been integrated into clinical practice, providing unique data on phasic changes of LA volume during the cardiac cycle [5]. Furthermore, it has been validated against cardiac magnetic resonance (CMR) and has been proven as a reliable and robust method for the evaluation of LA volume; being more accurate than conventional 2D echocardiography [6].

The aim of the current study is to investigate the performance of the left atrium in b-thalassemic patients with preserved left ventricular ejection fraction (EF) and no myocardial iron overload employing RT3DE.

RT3DE reported no differences with regards to the phasic changes of atrial volumes (Table 4). The mean left atrial maximum volume assessed by 3D-echo was 35.5 vs. 31.8 ml, while the mean left atrial minimum volume obtained by 3D-echo was 16.0 vs. 13.5 ml, for the patient and control groups respectively. Moreover, mean left atrial emptying volumes both passive (10.3 vs. 7.5 ml for patients and control respectively) and active (9.3 vs. 10.8 ml for patients and controls respectively), were not statistically different. On the contrary, the mean atrial active emptying fraction was significantly lower in the patient vs. the control group (34.2 vs. 48.3 ml respectively), while the mean passive emptive fraction was similar for both groups (patients 27.5 vs. controls 22.5 ml), as illustrated in Figure 4.

P	Patients	Controls
Vmax	35.5 ± 13.4	31.8 ± 9.8
0.2
Vmin	16.0 ± 6.0	13.5 ± 4.2
0.1
VpreA	25.4 ± 9.8	24.3 ± 7.2
0.7
AAEV	9.3 ± 5.1	10.8 ± 4.7
0.3
TAEV	19.7 ± 9.7	18.2 ± 8.3
0.5
PAEV	10.3 ± 7.6	7.5 ± 3.7
0.1

Values are expressed as mean and standard deviation. AAEV = active atrial emptying volume; TAEV = total atrial emptying volume; PAEV = passive atrial emptying volume; TAEV = total atrial emptying volume.

The agreement between the two readers was 0.918 ± 0.012 (0.894-0.942 95% C.I). Intra-observer agreement was 0.892 ± 0.017 (0.859-0.925, 95% C.I).

According to our knowledge, this is the first study that attempts to assess phasic changes of LA volumes in asymptomatic patients with b-thalassemia major by employing RT3D technology. As indicated by RT3D volumetric measurements, AAEF appears reduced in the patient population. This reflects abnormal systolic function of the LA, even in the absence of other affected echocardiographic parameters. This in turn could be explained by the fact that our patients were on intense chelating therapy, which maintained tissue iron levels within normal range (as illustrated by serum ferritin levels); therefore no iron accumulation was reported in the CMR evaluation. Indeed, studies have demonstrated that intense chelating therapy may prevent myocardial systolic function deterioration or even improve it [8]. Moreover, it should be noted that our patients did not have co-morbidities such as arterial hypertension or diabetes mellitus, which could potentially affect myocardial systolic and diastolic performance, and thus confound our results.

Iron toxicity is a major determinant of disease prognosis in b-thalassemic patients [9]. Although MRI is the gold standard in assessing early iron accumulation in the myocardium, echo is more widely available and cheaper. Monitoring ejection fraction can be useful, but its value is limited by the masking of ventricular dysfunction via the basal high cardiac output seen in chronic anaemia, and its late occurrence in the disease process [10]. Even modern contractile measures, such as tissue Doppler imaging, correlate poorly with cardiac iron. In a TDI study conducted by Silvilairat et al. [11], diastolic dysfunction was completely absent in patients who had ferritin levels below 2500 ng/ml. This observation was consistent with our results. Furthermore, data by Kremastinos [12] illustrated diastolic abnormalities only after the onset of cardiac failure. Margi et al. [13] have suggested strain indices as potential markers of early myocardial dysfunction in young asymptomatic patients, who nevertheless had myocardial iron as suggested by the reduced T2* values on MRI. Again in that study all conventional echo measurements failed to illustrate any significance between the patient and the healthy control groups.

On the other hand, left atrial volumetric assessment employing RT3D seems to offer an alternative approach as it can provide useful clinical information in early disease states, regardless of the left ventricular ejection fraction [5]. In our study, the impaired AAEF reflects early dysfunction of the left atrium, which could be attributed to the fact that the LA is formed by thin walls that are earlier affected as compared to the left ventricular wall. The early impairment of left atrial function in thalassemia major has also been implicated by studies in asymptomatic thalassemic individuals employing biochemical parameters such as atrial natriuretic peptides [14, 15]. In addition to that, Trikas et al. have illustrated that LAEF was reduced in well-preserved thalassemic patients, a finding that was verified by our results. According to the same authors, this reduction was associated with reduced exercise capacity [16].

The echocardiographic evaluation of LA dimensions and performance is a matter of intense research. LA diameters as measured with 2D and M-Mode from the parasternal view tend to underestimate the true atrial dimensions, although they are useful and widely available [17]. Enlargement of the LA is often asymmetrical and may occur in the medial-lateral as well as the superior-inferior axes, due to limited anteroposterior axis enlargement by the thoracic cavity. These limitations led to the establishment of atrial volumes as more accurate indices for the evaluation of LA dimensions [18, 19]. Although LA volumes are influenced by body type, this was not the case in our study since both groups had similar BMI and BSA. As suggested by Cameli [20] and other investigators [21], a relatively novel technique for the assessment of atrial function would be the utilization of strain rate imaging. As stated by the authors, this could provide a useful insight into the pathophysiology of various cardiovascular diseases, since it is a feasible and reproducible technique. It should be noted though that this method has major limitations, such as the inability of capturing a region of interest resembling the atrial shape or the confounding effects on the echo signal by structures surrounding the left atrium [20].

More recently, LA volume has been measured using real-time 3D echocardiography that has shown good agreement with other imaging modalities including magnetic resonance imaging [22, 23]. Additionally, 3DE direct volumetric and 3D speckle-tracking methods give comparable and reproducible quantification of LV and LA volumes and function; this renders interchangeable application a viable option in daily clinical practice [24]. Left atrial volume assessment by 3D echocardiography has the most favourable test-retest variation with the least intra- and inter-observer variability compared with other echocardiographic techniques, with good correlations between biplane 2D and 3D measurements [25]. Moreover, RT3D can overcome limitations due to geometric assumptions, which is a main drawback of the two-dimensional approach [26]. For all these reasons, RT3D was chosen as the method of reference for the more accurate evaluation of LA volumes.

However, studies on 3D echo measurement of LA volume have been limited, and there is no consensus on the methods or comparisons with established normal values. In addition, the low frame-rates achieved by the X3-1 transducer constitute a major limitation. Furthermore, we did not employ strain and strain rate measurements, which could provide information on early atrial involvement in our study population [13, 27]. Biochemical parameters such as natriuretic peptides have not been assessed, which could lead to correlations of clinical significance.

Real-time 3D seems to be a novel method for the accurate assessment of LA phasic changes in asymptomatic patients with b-thalassemia major. Additionally, AAEF may represent an early index of impaired left atrial performance in the specific population. In turn, left atrial dysfunction could be related to the increased incidence of arrhythmias and reduced functional capacity during exercise in thalassemic patients. Our results imply that left atrial function should be thoroughly investigated by clinicians.