Managing early postoperative morbidity following a Fontan operation remains a significant clinical challenge. This surgery is the definitive palliation for single-ventricle physiology, yet its unique circulatory arrangement inherently leads to elevated systemic venous pressures [1, 2], predisposing patients to severe complications such as prolonged pleural effusions and protein-losing enteropathy [2, 3, 4]. To mitigate these adverse outcomes, surgical fenestration, a controlled communication between the Fontan circuit and the atrium, is widely employed as a decompression strategy [1]. While surgical fenestration has become standard practice, the optimal fenestration diameter remains poorly defined, and its net clinical benefit is a subject of ongoing controversy [5, 6, 7].

The existing literature presents inconsistent findings [7, 8]. While some evidence supports routine fenestration to reduce postoperative morbidity [5, 6, 9], other reports suggest that its advantages are confined to high-risk cohorts, prompting some centres to move away from its routine use [9, 10]. This clinical discrepancy is further highlighted by computational and translational models [11, 12, 13], which indicate that only a narrow range of fenestration sizes may optimise systemic oxygen delivery without causing excessive arterial desaturation, a critical haemodynamic trade-off [11]. In addition, recent evidence has suggested that preoperative NT-proBNP may serve as a useful biomarker for risk stratification when deciding on fenestration strategy [14].

For example, some computational studies suggest that while fenestration can relieve venous pressure, this comes at the expense of reduced oxygen saturation, even with commonly used fenestration sizes around 4 mm [1, 11]. These analyses suggest that the physiological trade-off becomes increasingly pronounced as fenestration size enlarges, with the greatest haemodynamic impact likely occurring at diameters exceeding 4 mm. Nevertheless, this size-dependent relationship remains insufficiently defined in clinical settings, emphasising the need for empirical studies to clarify the optimal balance between decompression and oxygenation.

Given this uncertainty, a significant knowledge gap persists in defining the precise relationship between fenestration dimensions and key clinical outcomes. The aim of this study was to rigorously evaluate the association between fenestration size and early postoperative outcomes following Fontan surgery, including major complications, changes in peripheral oxygen saturation and time-related recovery metrics.

The objective of this study was to evaluate the relationship between fenestration size and early postoperative outcomes in paediatric patients undergoing Fontan surgery, a relatively uncommon procedure, over a 5-year period at a specialised cardiac centre in southern Vietnam. In this cohort, fenestration diameters ranged from 4 to 8 mm and were not clearly associated with the overall incidence of postoperative complications. Smaller fenestrations were linked to higher postoperative oxygen saturation, whereas larger fenestrations were associated with more favourable recovery, including shorter pleural drainage and reduced hospital stay. These findings present the double-edged nature of fenestration strategy in children, where the gain in oxygenation must be weighed against the advantage of a smoother postoperative recovery. Our findings, while consistent with established physiology, provide clinical evidence in an underrepresented Southeast Asian population. Importantly, they underscore the need for a risk-stratified, individualised approach to fenestration sizing. In patients with borderline oxygen saturation and low risk of prolonged effusions, smaller fenestrations may be preferable to avoid severe hypoxemia. Conversely, in those with elevated systemic venous pressures or high risk of effusions, larger fenestrations may facilitate smoother early recovery, with a subsequent plan for transcatheter closure once haemodynamic stability is achieved. This framework may help guide practical decision-making beyond the traditional ‘one-size-fits-all’ strategy.

Our analysis did not demonstrate a dose-dependent relationship between fenestration diameter and the overall incidence of postoperative complications. This result is consistent with previous meta-analyses by Li et al. [7] and Bouhout et al. [8], which also found no significant difference in Fontan failure or mortality between fenestrated and non-fenestrated patients. The lack of association may reflect the multifactorial nature of major complications following Fontan surgery, where factors such as ventricular function, pulmonary vascular resistance and underlying cardiac anatomy likely play a greater role than fenestration size [16]. Although fenestration is a critical element of the modern Fontan operation, its diameter alone does not appear to be the key determinant of postoperative morbidity.

This study demonstrated a strong inverse correlation between fenestration diameter and perioperative oxygen saturation, an observation consistent with established physiology. Larger fenestrations permit greater right-to-left shunting, resulting in lower systemic SpO₂ [11], a relationship supported by both clinical and computational studies [12, 13]. This is consistent with transcatheter closure studies, where Greenleaf et al. [17] reported a 7.9% increase in mean SpO₂, compared with a 10.6% increase in our cohort. Likewise, Devanagondi and Leonard [18] observed an improvement from 87% to 95%, whereas we noted a rise from a median of 76%–86%. Although persistent cyanosis is associated with long-term risks such as collateral formation and thromboembolic events [19, 20, 21], studies indicate that patients with oxygen saturation levels of 85%–90%, similar to those observed in our cohort, may still achieve comparable exercise capacity to non-fenestrated patients [17, 22]. Collectively, these findings confirm that fenestration size strongly influences systemic oxygenation.

Our data demonstrated a significant inverse correlation between fenestration diameter and the durations of pleural drainage and hospitalisation. This observation is in strong agreement with prior literature that consistently highlights the decompressive benefits of fenestration. Multiple meta-analyses and comparative studies have shown that fenestrated patients experience reduced ICU stay, overall LOS and chest tube drainage duration [7, 9, 23]. Corno et al. [5] further reported that a prospective randomised trial confirmed shorter ICU stay and LOS in fenestrated patients, while Fu et al. [3] identified non-fenestration as an independent predictor of prolonged pleural drainage, with their cohort showing a median postoperative LOS of 14 days. The underlying mechanism is likely related to the role of the fenestration as a functional ‘pop-off valve’ that relieves systemic venous pressure and reduces effusion formation [6]. Larger fenestrations provide more effective decompression, thereby accelerating recovery and shortening both drainage duration and hospitalisation. Therefore, these findings also suggest that a larger fenestration may be a rational strategy for patients at increased risk of persistent effusions.

The clinical implications of our findings are clear: a ‘one-size-fits-all’ approach to fenestration sizing is suboptimal [5, 11]. The decision requires a careful, individualised balance between the decompressive benefits, recovery outcomes and hypoxemia risks. We recommend a risk-stratified approach to be incorporated into clinical protocols. For patients with borderline preoperative oxygenation or at low risk of effusions, a smaller fenestration may be prudent to avoid severe hypoxemia [10]. Conversely, in patients with elevated systemic venous pressures or other risk factors for prolonged effusions, a larger fenestration confers a net advantage by facilitating a smoother early recovery [5, 7]. We further recommend that clinicians view the fenestration as a temporary, two-stage intervention by leveraging the early benefits of a larger fenestration and having a clear plan for subsequent transcatheter closure to durably improve oxygenation once the patient is haemodynamically stable [24].

This study examines a rare clinical population, with only 20 fenestrated Fontan procedures performed over a 5-year period at a tertiary centre in Vietnam. The limited number of patients reflects both the uncommon nature of the Fontan operation in our country and the challenges of conducting research in a resource-constrained setting. Consequently, the small sample size reduces statistical power and limits the generalisability of our findings. These results should therefore be interpreted with caution. To validate and extend our observations, larger multicentre prospective studies are warranted. The median age at Fontan completion was 5 years, later than the international standard of 1.5–3 years. This reflects delayed referral and limited access to specialised surgery in Vietnam, which may reduce external validity and influence outcomes.

This study provides quantitative evidence of the dual effects of fenestration size in the Fontan procedure. Fenestration diameters of 4–8 mm were associated with significant perioperative improvement in oxygenation, whereas larger fenestrations facilitated better early recovery metrics but at the expense of lower oxygen saturation. These findings suggest that a uniform sizing strategy is inadequate and highlight the importance of an individualised approach that balances systemic venous decompression with acceptable arterial desaturation. Future multicentre studies with larger cohorts are needed to establish evidence-based guidelines for optimal fenestration sizing.

The datasets used in this study can be obtained from the corresponding author upon a reasonable request.

Conceptualization: LTKV. Data curation: LTKV, HNV. Investigation: LTKV, HNV. Methodology: HTB. Formal analysis: LTKV, HTB. Statistical analysis: HTB. Visualization: HTB. Validation: LTKV, HNV. Writing — original draft: LTKV, HNV. Writing — review & editing: HNV, HTB. Supervision: LTKV, HNV. Final approval of the manuscript: LTKV, HNV, HTB. 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.

The study was conducted in accordance with the ethical principles of the Declaration of Helsinki and was approved by the Institutional Review Board of Cho Ray Hospital (approval number: 67-27/CN HDDD on June 3rd 2025). In view of its retrospective design, the requirement for individual informed consent was waived by the board. All patient data were anonymised, and confidentiality was strictly maintained throughout the study.

The authors thank the nurses, physicians, and participants at Cho Ray Hospital, where the study was conducted.

This research received no external funding.

The authors declare no conflicts of interest.