Patients with single-ventricle (SV) physiology are among the highest-risk congenital heart disease patients with increased mortality and morbidity, frequently caused by ventricular dysfunction [1]. These patients undergo staged surgical management with the end result of the Fontan or total cavopulmonary circulation, where the dominant ventricle supports only the systemic circulation. In contrast, systemic venous return drains directly into the pulmonary circulation without mixing with the pulmonary venous return [2]. Depending on the underlying type of pulmonary and systemic blood flow, most newborns with single-ventricle will require intervention to secure systemic blood flow and control pulmonary circulation. This is commonly achieved with the Norwood stage 1 procedure [2]. This will be followed by superior bidirectional cavopulmonary anastomosis (BCPA) or Norwood stage II, performed at 4 to 6 months of age, and a Fontan completion (stage III) at 18 to 48 months of age [2]. Patients with single ventricle physiology present a progressive decline in myocardial performance due to combined structural and functional disturbances [3]. Abnormal direction of myofibers in the dominant ventricle, increased myocardial fibrosis, abnormal ventricular outflow morphology and non-contractile septum were shown to be among the potential contributing factors [3].

Cardiac magnetic resonance imaging (MRI) is the ‘gold’ standard for non-invasively assessing ventricular volume and function. As such, it is an additional tool for evaluating single ventricle patients, especially when deciding Fontan eligibility [4]. However, its use for frequent serial assessments presents limitations, including lack of availability in all centers, the need for sedation in young children, lengthy analysis times and higher costs [4]. On the other hand, transthoracic echocardiography can be used on children without sedation, is readily available, more cost-effective, faster and easier [4]. As such, it can be used for the routine serial assessment of these patients. However, the complex asymmetric geometry of the dominant ventricle and the potential presence of segmental wall motion abnormalities make the echocardiographic evaluation of myocardial performance in single-ventricle patients challenging [5]. Therefore, conventional 2-dimensional (2D) echo measurements of ventricle function, such as ejection fraction either by Simpson’s biplane method or by M-mode shortening fraction, cannot be used as in normal biventricular hearts [6]. Due to its angle- and geometry independence and good reproducibility, speckle-tracking echocardiography (STE) may have significant potential for assessing single-ventricle patients [7, 8, 9]. Moreover, STE indices are more sensitive than conventional echocardiography parameters, identifying early ventricular function changes [10, 11, 12].

This review aims to examine the role of speckle-tracking echocardiography in the pre-and post-operative assessment of single-ventricle patients and to explore its prognostic value.

During the last decade, several studies used STE to assess global and regional ventricular deformation and mechanical synchrony in patients with SV physiology, to understand the progress of intrinsic myocardial function during the various palliation surgical stages and to identify prognostic markers. However, most of these studies are retrospective and included patients with hypoplastic left heart syndrome (HLHS), whereas only a few included patients with other anatomic types of the single ventricle.

Multiple studies have investigated the potential use of ventricular strain and strain rate as prognostic indicators in patients with single ventricle physiology. Colquitt et al. [28] conducted a prospective study on 35 infants with HLHS, which revealed that post-Norwood, pre-Glenn, and post-Glenn STE parameters could predict adverse outcomes, including death, heart transplant, and persistent or moderate RV dysfunction (Table 2). Reduced longitudinal and circumferential strain and strain rate indices were identified as markers of adverse outcomes, with post-Norwood global longitudinal strain (GLS) $>$–16% and pre-Glenn GLS $>$–13% showing the highest sensitivity and specificity.

Lin et al. [29] examined the sensitivity and specificity of conventional and speckle-tracking echocardiography measures of RV function in predicting death or need for heart transplantation in a group of 64 HLHS patients before bidirectional cavopulmonary anastomosis (Table 2). The study found that patients with the endpoint had lower longitudinal strain and strain rate, circumferential strain rate, and RVFAC when compared to the rest of the patients. While both STE indices and abnormal RVFAC (35%) had reasonable specificity for predicting negative outcomes, their sensitivity was low. The addition of strain or strain rate to abnormal RVFAC did not significantly improve the prediction of death or HT need. However, STE parameters showed better reproducibility than RVFAC.

Borrelli et al. [30] showed that the decline in longitudinal strain ($\mathrm{\Delta }$ LS) between prior- and one month after Norwood operation was the strongest predictor of death or HT need (p = 0.01) in all patients irrespective of normal or abnormal RVFAC (Table 2). In this study, the reproducibility of STE was also better than RVFAC’s. One week before bidirectional cavopulmonary anastomosis (BCPA), $\mathrm{\Delta }$ LS was significantly reduced in the event group compared to the rest of the HLHS patients. However, earlier $\mathrm{\Delta }$ LS, one month after Norwood, was a better predictor than $\mathrm{\Delta }$ LS one week before BCPA.

Additionally, Park et al. [31] examined the potential prognostic value of pre-TCPC STE in 135 patients with right or left dominant ventricle (Table 2). In this study, invasively measured pulmonary vascular resistance (PVR) and circumferential strain rate (CSR) were the only variables independently associated with a prolonged length of hospital stay ($>$14 days), with CSR showing better prognostic ability. Receiver operating characteristic curve analysis showed an area under the curve of 0.70 for circumferential strain rate and 0.64 for pulmonary vascular resistance, suggesting a potential role of STE during pre-TCPC assessment in addition to invasively assessed hemodynamic parameters.

Overall, the findings from these studies indicate that ventricular strain and strain rate, particularly STE-derived indices, may serve as potential prognostic markers in patients with single ventricle physiology.

Circumferential ventricular strain has been shown to have prognostic value in post-Fontan patients. In Ghelani et al.’s study [32] of 127 Fontan patients, low circumferential ventricular strain was found to have the strongest association with mortality or need for heart transplant (hazard ratio 1.3 per unit change, 95% confidence interval 1.1 to 1.5, p = 0.001) among echocardiographic variables (Table 2). The study also explored the prognostic role of cardiac magnetic resonance (CMR) imaging, with indexed ventricular end-diastolic volume being the stronger predictor of adverse outcomes among CMR parameters. When comparing echocardiography and CMR, neither technique was found to be superior in prognostic ability in this specific cohort. However, both techniques identified important factors for risk stratification.

The assessment of diastolic ventricular function can be improved by using atrial strain measured by STE, which can detect abnormalities before atrial volume changes are observed [33]. The atrial strain has three components: the reservoir strain, conduit strain, and active/pumping strain. In single ventricle patients, effective atrial function is crucial in ensuring adequate preload as diastolic ventricular filling depends more on atrial contraction [34].

Studies have shown that single ventricle patients exhibit increased atrial size and active atrial strain but decreased reservoir and conduit atrial strain compared to participants with biventricular hearts [35, 36]. Moreover, decreased atrial compliance and early diastolic emptying suggest the increased dependency of ventricular filling on atrial contraction. Lower right atrial strain (RAS) after initial palliation surgery has also been associated with a longer duration of milrinone therapy and a higher chance of digoxin use in single RV patients [37].

In Fontan patients, a decrease in global atrial strain has been observed, with no correlation between global atrial strain and invasively measured systemic ventricular filling pressure [38, 39]. However, atrial reservoir strain has been shown to correlate positively with cardiac index, and its decreased function is associated with adverse clinical outcomes, such as death, HT, arrhythmia, heart failure symptoms, protein-losing enteropathy, plastic bronchitis, seizures, and pleural effusions [39]. Atrial strain indices have also been shown to correlate with oxygen uptake during exercise, with higher atrial conduit strain, higher reservoir strain, and lower active/reservoir strain ratio correlating with higher VO2 [40]. However, more studies are needed to understand the role of atrial mechanics in the risk stratification of Fontan patients, as clinical outcomes result from several factors.

Progressive decline in myocardial function and subsequent heart failure are among the complications which limit functional status and prognosis in patients with single ventricle physiology [41]. In these patients, heterogeneities in the cardiac morphology and the effects of loading conditions make a reliable assessment of myocardial function with conventional echocardiography difficult or even not feasible. On the other hand, speckle-tracking echocardiography does not depend on the geometry of the ventricles, is free of angle-dependency limitations and straightforward to employ [42, 43, 44, 45]. As a result, it is an appealing supplementary method for assessing single-ventricle patients.

There are several potential clinical implications for using STE imaging in the routine assessment of patients with single-ventricle physiology. Several echocardiography studies on single-ventricle patients have used STE indices to assess the longitudinal, radial and circumferential ventricular performance, mechanical dyssynchrony and atrial active, reservoir and conduit function [18, 20, 30, 31]. The results showed STE to be more sensitive than conventional echocardiography in detecting an early and progressive decline in the performance of the dominant ventricle [18, 20, 30, 31]. As such, serial interstage and post-Fontan completion STE assessment may help to early identify ventricular dysfunction and Fontan circulation failure and to allow prompt management of complications [18, 20, 22, 23, 26, 27]. Moreover, pre-and post-operative STE indices were associated with mortality, need for heart transplantation and prolonged hospitalization [13, 28, 29, 30, 31, 32]. Thus they serve as potential prognostic markers of adverse events and may play a role in optimizing risk stratification and informed patient counselling.

Despite the growing research in this field over the past years, most of the studies were single-center, retrospective, with small sample sizes of single ventricle patients. Even if STE is a handy and valuable tool for assessing single ventricle function, its definite diagnostic role must be proven through prospective, multi-center studies, including more patients with better representation of various single-ventricle anatomic types who will be followed up for a more extended period.

GK and DC designed the research study. PK and GK performed the research (acquisition, interpretation and analysis of the data). PG and DC provided advice during the performance of the study and critical appraisal. All authors contributed to the interpretation of the data, revised critically the final manuscript for important intellectual content and performed editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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This research received no external funding.

The authors declare no conflict of interest.