Impaired lung function in children and adolescents with Fontan circulation may improve after endurance training

Cardiorespiratory fitness in children with surgically corrected congenital heart disease: A meta-analysis and meta-regression

Congenital heart disease (CHD) is the most common birth defect, and despite advancements in medical care, children with surgically corrected CHD often experience reduced cardiorespiratory fitness, which is associated with negative long-term health outcomes. This meta-analysis aimed to quantify peak oxygen consumption (̇VO2peak) impairments in children with surgically corrected CHD, examine isolated diagnosis-specific impairments, and explore the relationship between clinical variables and cardiorespiratory fitness. A total of 45 studies encompassing 2,536 children with CHD and 3,108 healthy controls were included in the meta-analysis, revealing that children with CHD had significantly lower ̇VO2peak (standardized mean difference = 1.13, 95 % CI 0.98 to 1.28), with those having univentricular hearts being most affected (standardized mean difference = 1.61, 95 % CI 1.34 to 1.87). Reduced saturation during exercise, chronotropic impairment and early onset of anaerobic threshold are likely to play a role in this impairment.

Retrograde Flow to Aortic Root Predicts Inferior Cardiopulmonary Performance and Restrictive Lung Physiology in Fontan Circulation

Background

Cardiac output in Fontan circulation depends on systemic venous pressure, pulmonary vascular resistance, and ventricular function. Because myocardial function is dependent on coronary perfusion, we studied whether retrograde flow to aortic root in the mitral/aortic atresia subgroup of hypoplastic left heart syndrome (HLHS) affects cardiopulmonary performance.

Methods

We studied 26 stable Fontan patients (14.4 ± 2.4 years) with right (RV, n = 17) and left (LV, n = 9) systemic ventricle morphology. All RV patients had HLHS and were subdivided according to postnatal flow to the hypoplastic ascending aorta being antegrade (HLHS-A) or retrograde (HLHS-R) due to valve atresia. Physical activity was assessed by questionnaire (LASERI, a questionnaire for Finnish children regarding physical activity), cardiopulmonary exercise test (1-minute ramp protocol), body composition (Biacorpus RX 4000), and muscle fitness (EUROFIT). These data were correlated with the postnatal aorta size and current branch pulmonary artery size index (McGoon index).

Results

Patients with HLHS-R seldom self-reported engagement in vigorous physical activity and had significantly lower cardiopulmonary performance (peak oxygen uptake [VO2peak]) than patients with LV morphology (P = 0.037), but not compared with patients with HLHS-A. Branch pulmonary artery size did not correlate with VO2peak. Patients with HLHS-R had most severe lung restrictions (forced vital capacity z-score -3 ± 0.9, P = 0.0073; forced expiratory volume in 1 second z-score -3.3 ± 1.1, P = 0.001).

Conclusions

Young Fontan patients with LV had better cardiopulmonary performance than patients with HLHS. Patients with HLHS-R were the least active and had the lowest VO2peak and most restrictive lungs. It is important to recognize postnatally single ventricle patients at high risk for inactivity to promote an active and healthy lifestyle.

Cardiac Rehabilitation for Fontan Circulation Patients: A Systematic Review, and Meta-Analysis

Background and Objectives: Despite advances in the surgical management of patients with Fontan circulation, their exercise capacity and quality of life remain significantly impaired. Exercise-based cardiac rehabilitation (CR) offers promising improvements in these areas, but the implementation and adherence to these programmes are often inconsistent. This systematic review and meta-analysis aimed to evaluate the safety, efficacy, and optimal exercise modalities for Fontan patients. Materials and Methods: A systematic search of PubMed, Scopus, Web of Science, and Cochrane Library was conducted on 24 August 2023. Studies were screened and assessed for quality using the Cochrane RoB Tool 2 and STROBE checklist. Meta-analysis was performed using a continuous random-effects model to determine the effectiveness of various CR interventions, including aerobic exercise training (AET), resistance training, and inspiratory muscle training (IMT). Results: A total of 26 studies (7 RCTs, 19 cohorts) comprising 22 distinct cohorts were included, with a total sample size of 428 Fontan patients. The interventions ranged from 4 weeks to 24 months and included AET (18 studies), resistance training (11 studies), and IMT (6 studies). The meta-analysis revealed significant improvements in exercise capacity, with a pooled mean difference in peak VO2 of 1.947 (95% CI: 1.491 to 2.402, p < 0.001). Subgroup analyses showed that combined AET and resistance training had the most robust effect, with a mean difference of 2.11 (95% CI: 1.57 to 2.65, p < 0.001). Home-based interventions showed significant benefits, while supervised and hybrid interventions did not show statistically significant differences. Publication bias was identified, particularly in home-based interventions, where smaller studies demonstrated larger effect sizes, as confirmed by Egger's test (Intercept = 2.417, 95% CI: 1.498 to 3.337, p = 0.001). However, no significant bias was detected in supervised or hybrid interventions, which displayed symmetrical distributions in funnel plots and non-significant Egger's test results. Conclusions: CR appears to be an effective intervention for improving exercise capacity in Fontan patients, particularly when combining AET with resistance training. Home-based programmes offer promising results, though the potential for publication bias, especially in smaller studies, warrants cautious interpretation of these findings. Further research is needed to refine protocols, explore long-term outcomes, and determine the underlying mechanisms, particularly for patients with more severe clinical presentations. The low incidence of adverse events across the studies reinforces the safety of these interventions.

Home-based long-term physical endurance and inspiratory muscle training in children and adults with Fontan circulation

Background and study aim

Regular physical activity is highly recommended for patients with Fontan hemodynamics. Our aim was to investigate the effects of a long-term individualized home-based endurance training (IHET) on a bicycle ergometer in combination with inspiratory muscle training (IMT) in pediatric and adult patients after Fontan palliation. Additionally, factors influencing the trainability of Fontan palliated patients were analyzed.

Methods

From 2018 to 2021 a single-center prospective study was performed initially including 25 Fontan palliated patients. During study period nine patients were excluded due to incompliance. A Magbike® bicycle ergometer (DKN Technology, France) was used for IHET and a POWERbreathe® Medic plus device (HaB GmbH, Germany) was utilized for the IMT. Over the study period, bike training was increased from 90 min of basic endurance training per week to additional 25 min of interval training per week. IMT consisted of 30 breaths per day for 6-7 days per week with pressure adaption over time. Patients underwent cardiopulmonary exercise testing (CPET) and body plethysmography including measurement of respiratory muscle strength at baseline and at follow-up examinations at 4, 10 and 22 months.

Results

Follow-up examinations were completed by 18/25 patients (72.0%) at 4 and 10 months and 16/25 patients (64.0%) at 22 months. Median exercise capacity slightly increased by 0.13 W/kg from baseline to last follow-up (p = 0.055, 95%CI: 0.0-0.36). However, a significant increase of oxygen pulse of 0.7 ml/beat (p = 0.006, 95%CI: 0.38-2.22) was detectable. IMT significantly improved respiratory function with an increase of inspiratory vital capacity (VCin/reference) by 4.0% (p = 0.016, 95%CI: 0.8-8). Median maximal inspiratory pressure increased by 1.2 kPa (p = 0.003, 95%CI: 0.64-3.19) and expiratory pressure by 1.5 kPa (p = 0.036, 95%CI: 0.08-2.29). No adverse events or unplanned interventions occurred during the study. Patients' subjective quality of life did not significantly change over the study period.

Conclusion

In Fontan palliated patients, IHET in combination with IMT leads to a significant increase in oxygen pulse, inspiratory vital capacity as well as median maximal inspiratory and expiratory pressure but not to significant improvement of quality of life. Fontan patients should be encouraged to perform regular home-based exercise training.

Pulmonary responses following cardiac rehabilitation and the relationship with functional outcomes in children and young adults with heart disease

Introduction

Patients with congenital heart disease (CHD) often have pulmonary abnormalities and exercise intolerance following cardiac surgery. Cardiac rehabilitation (CR) improves exercise capacity in patients with CHD, but minimal study has been performed to see if resting and dynamic pulmonary performance improves following CR in those with prior cardiac surgery.

Methods

This was a retrospective cohort study of all patients who completed ≥12 weeks of CR from 2018 through 2022. Demographic, cardiopulmonary exercise test (CPET), spirometry, 6-minute walk, functional strength measures, and outcomes data were collected. Data are presented as median[IQR]. A Student's t-test was used for comparisons between groups and serial measurements were measured with a paired t-test. A p < 0.05 was considered significant.

Results

There were a total of 37 patients [age 16.7 (14.2-20.1) years; 46% male] included. Patients with prior surgery (n = 26) were more likely to have abnormal spirometry data than those without heart disease (n = 11) (forced vital capacity [FVC] 76.7 [69.1-84.3]% vs. 96.4 [88.1-104.7]%, p = 0.002), but neither group experienced a significant change in spirometry. On CPET, peak oxygen consumption increased but there was no change in other pulmonary measures during exercise. Percent predicted FVC correlated with hand grip strength (r = 0.57, p = 0.0003) and percent predicted oxygen consumption (r = 0.43, p = 0.009). The number of prior sternotomies showed negative associations with both percent predicted FVC (r = -0.43, p = 0.04) and FEV1 (r = -0.47, p = 0.02).

Discussion

Youth and young adults with a prior history of cardiac surgery have resting and dynamic pulmonary abnormalities that do not improve following CR. Multiple sternotomies are associated with worse pulmonary function.

Lung Function in Fontan Patients Over a Ten-Year Period: Is the Fontan Circulation Impairing Lung Development?

Few studies have investigated how the Fontan circulation affects lung function, and no studies have investigated the development of lung function over longer time in these patients. We aimed to describe the development of lung function in Fontan patients over a 10-year period. Pulmonary function tests (PFT), including spirometry and diffusion capacity for Carbon Monoxide (DLCO) and Nitric Oxide (DLNO), were conducted in a Danish Fontan cohort in 2011 (PFT-I). In 2021, re-investigations were performed (PFT-II). We investigated changes in percent predicted (%pred) lung function from PFT-I to PFT-II. Patients were categorized into a pediatric group (age under 18 at PFT-I) and an adult group (age 18 or older at PFT-I). Out of the 81 patients completing PFT-I, 48 completed PFT-II. In the pediatric group (32 patients), there were significant declines in %pred forced expiratory volume in 1s (99.7 (92.4, 104.4)-89.3 (84.9, 97.2), p < 0,001), forced vital capacity (98.3 (87.8, 106.1)-96.7 (86.7, 100.6), p = 0.008), and alveolar volume (95.5 (89.5, 101.6)-89.5 (79.7, 93.2), p < 0.001). The corresponding measurements remained stable in the adult group. However, the median %pred DLNO significantly declined in the adult group (58.4 (53.3, 63.5)-53.7 (44.1, 57.3), p = 0.005). Over a 10-year period, several lung function parameters declined significantly in the younger Fontan patients, suggesting possible impairments in lung development during growth. The decline in %pred DLNO in the adult patient group indicates deterioration of the membrane component of diffusion capacity, implying that the Fontan circulation might negatively affect the alveolar membrane over time.

Possible effects of left pulmonary artery stenting in single ventricle patients on bronchial area, lung volume and lung function

Background

Left pulmonary artery (LPA) stenting is often required in single ventricle (SV) patients. Due to their close anatomical relationship an LPA stent could potentially compress the left main bronchus (LMB). We assessed the impact of LPA stenting on bronchial size, pulmonary volumes, and lung function in a cohort of SV patients.

Methods

Forty-nine patients underwent cardiovascular magnetic resonance (CMR) and 36 spirometry 11 (8-15) years after Fontan. All patients were free of respiratory symptoms. LPA stents were inserted in 17 (35%) patients at 8.8 (3.4-12.6) years. Area/shape of the main bronchi (n = 46) and lung volumes (n = 47) were calculated from CMR-ZTE images for each lung and transformed in right-to-left (r/l) ratio and indexed for BSA. The effect of early stent insertion (prior to stage III) was analyzed.

Results

Patients with LPA stent had larger r/l ratio for main bronchus area (p < 0.001) and r/l ratio difference for lung volumes was slightly larger in patients with early stenting. A trend toward a deformation of LMB shape in patients with LPA stent and toward a higher prevalence of abnormal spirometry in patients with early stent implantation was observed.

Conclusions

In this cohort of patients, early insertion of LPA stents seems to relate with smaller LMB sizes and a trend toward smaller left lung volume and higher prevalence of impaired lung function. Whether these findings are caused by the stent or, at least to a certain degree, present prior to the implantation needs to be verified.

Abnormalities in pulmonary function and volumes in patients with CHD: a systematic review

BACKGROUND

Lung function and cardiac function are naturally correlated by sharing the thoracic cage and handling the whole cardiac output sequentially. However, lung function studies are rare in patients with CHD, although results worthy of investigation could be expected. This review summarises existing studies with the lung function parameters (spirometry and body plethysmography) in CHD patients during the last decade.

METHODS

A systematic review was performed in the relevant database (PubMed, Cochrane, and Scopus) in studies including paediatric and adult patients with CHD where lung parameters (spirometry, body plethysmography) were investigated from January 2010 to December 2020. Two independent reviewers evaluated the studies according to the Study Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies of the National Heart, Lung, and Blood Institute.

RESULTS

Eight studies investigated patients with Fontan palliation including 704 patients (306 female). Four studies included patients after repaired tetralogy of Fallot examining 219 patients (103 female), with one study using double. Further six studies included 3208 (1324 female) children and adults with various CHDs. Overall, four studies were categorised as "good", ten as "fair", and four as "poor". While the measurements were consistently standardised, references to calculate %predicted differed substantially across all studies. All evaluated studies showed reduced forced vital capacity in the majority of CHD patients.

CONCLUSIONS

Many CHD patients have a reduced forced vital capacity independent of their underlying defect. Spirometry should not only follow a standardised measure according to ATS (update 2019) but also stick to the 2012 GLI reference values.

Fitter Fontans for future—Impact of physical exercise on cardiopulmonary function in Fontan patients

Background

In Fontans, exercise tolerance is poorer compared to their healthy peers. Higher V˙O2peak represents a strong predictor for mortality and morbidity in these patients. Cardiac rehabilitation programs have been shown to improve cardiopulmonary function in Fontans. More habitual physical activity should therefore lead to a better exercise tolerance.

Methods

We performed cardiopulmonary exercise testing in 24 Fontan patients who had engaged in physical activity for a minimum of 3 h per week over their lifetime. As a control we performed cardiopulmonary exercise testing in 20 Fontan patients who had undertaken no physical activity or &lt;3 h per week in the past.

Results

A total of 44 Fontan patients was included (mean age 18.1 years). The mean parameters measured at peak exercise differed significantly between the active and inactive group (peak oxygen uptake [V˙O2peak] of 34.0 vs. 25.0 ml/min/kg, peak heart rate (HR) of 169.8/min vs. 139.8/min). Even though the O2pulse and the EF did not differ significantly between both groups, N-Terminal-Pro-B-Type Natriuretic Peptide (NT-pro BNP) was significantly higher in the inactive group. The two groups did not differ with respect to their cardiac function determined by magnetic resonance imaging (MRI). V˙O2peak was positively correlated with hours of sports performed by Fontans.

Conclusions

V˙O2peak and maximum HR were significantly higher in Fontans who had been physically active compared to those who had been inactive. The values reported in this study were higher than in other studies and reached normal values for V˙O2peak for most Fontans in the physically active group. The positive correlation between V˙O2peak and physical activity is an indicator of the importance of incorporating physical exercise programs into the treatment of Fontan patients.

Sports participation, activity, and obesity in children who have undergone the Fontan procedure

BACKGROUND

Children with CHD are at risk for obesity and low levels of activity. These factors are associated with an increased risk of poor outcome. Participation in organised sports is an important avenue for children to maintain physical activity, though the relationship between sports participation and obesity has not been examined in the Fontan population.

METHODS

We performed a cross-sectional study of children aged 8-18 who had been evaluated between January 1, 2015 and October 1, 2019 at the Doernbecher Children's Hospital outpatient paediatric cardiology clinic and had previously undergone a Fontan. Patients were excluded if they were unable to ambulate independently or if they had undergone a heart transplant. Patient characteristics were recorded from the electronic medical record. Parents were interviewed via a telephone survey and asked to describe their child's activity levels and sports participation.

RESULTS

Our final cohort included 40 individuals, 74% were male. The overall prevalence of obesity (CDC BMI >95% for sex/age) in the cohort (23%) was significantly higher in non-athletes (33%) than athletes (0) (p = 0.02). There was no difference in cardiac complications or comorbidities between athletes and non-athletes. Athletes were more likely to meet daily activity recommendations (p = 0.05).

CONCLUSION

Fontan patients who do not participate in sports are significantly more likely to be obese and less likely to be active than those who do. This is the first study to demonstrate the association between competitive sports participation and decreased likelihood of obesity in the Fontan population.

Exercise Capacity and Training Programs in Paediatric Fontan Patients: A Systematic Review

Background

Exercise training programs can effectively enhance exercise capacity in adults with congenital heart disease, including Fontan patients. However, few studies have explored the impact of exercise training exclusively on paediatric Fontan cohorts. This study systematically reviews exercise capacity in paediatric Fontan patients and the impact of training programs on their cardiovascular health.

Methods

Medline and Embase were searched for articles published between January 1990 and November 2021. Studies were included in which data could be analyzed discretely for patients who had undergone the Fontan procedure and were ≤20 years old at the time of study. Cardiopulmonary exercise parameters were extracted from all studies, and training protocols were collected from training programs.

Results

The studies demonstrated that Fontan patients exhibit significantly diminished peak exercise capacity relative to healthy peers. We identified 9 training programs that exclusively studied Fontan patients ≤20 years. The programs ranged from 6 weeks to 12 months in duration, with 8 programs incorporating aerobic activity and 1 focused only on inspiratory muscle training. At least 1 measure of maximal or submaximal exercise capacity improved significantly within each program in which statistical analysis was performed, with no reported adverse events. There were 2 additional training programs in which the patients were predominantly (>65%), but not exclusively, Fontan patients.

Conclusions

Overall, the results indicate that exercise training programs can safely and effectively improve at least 1 measure of exercise capacity in paediatric Fontan patients.

Exercising with a Single Ventricle: Limitations and Therapies

Treatment for Hypoplastic Left Heart Syndrome (HLHS) and other single ventricle conditions requires a series of surgical interventions for long-term survival, typically culminating in the Fontan procedure. The result is an abnormal circulatory physiology with an absence of a sub-pulmonary ventricle. Exercise capacity in the Fontan circulation is often limited and is due to multiple factors, both central and peripheral. Multiple interventions, both pharmacologic and nonpharmacologic, have been studied to attempt to overcome these inherent limitations. This review will focus on the physiology of the exercising Fontan patient and on the interventions aimed at the enhancement of exercise capacity studied thus far.

Home-Based Long-Term Physical Endurance and Inspiratory Muscle Training for Children and Adults With Fontan Circulation—Initial Results From a Prospective Study

Background

Patients with congenital heart disease (CHD)—including those after Fontan operation—are encouraged to be physically active.

Aim

To prospectively determine the effects of an individually adapted, home-based cycle ergometer endurance training in combination with inspiratory muscle training (IMT) in pediatric and adult Fontan patients. We, herein, report the results of the initial 10-months follow-up (phase 1).

Methods

18 patients (median age 16.5 years; range 10-43 years) completed baseline check-ups, and 4 and 10 months follow-up visits, which each included cardiopulmonary exercise testing (CPET), bodyplethysmography (including measurement of respiratory muscle strength), and a quality of life questionnaire (PedsQL™). The training program consisted of a home-based cycle ergometer endurance training on a “Magbike® AM-5i/3i” (DKN Technology®, Clermont-Ferrand, France) and IMT with a handheld “POWERbreathe® Medic plus” device. Patients performed 90 min of endurance training per week in addition to IMT (30 breaths per day, 6-7 times per week). After the first 4 months, patients underwent additional interval training.

Results

After 10 months of training, we observed significant increases in maximum relative workload (W/kg, p = 0.003) and in maximum inspiratory (MIP, p = 0.002) and expiratory (MEP, p = 0.008) pressures. Peak VO2 values did not increase significantly as compared to baseline (p = 0.12) in the entire cohort (n = 18), but reached statistical significance in a subgroup analysis of teenage/adult patients (n = 14; p = 0.03). Patients' subjective quality of life did not show any significant changes after 10 months of training.

Discussion

In Fontan patients, an individually adapted home-based training is safe and associated with improvements in some CPET variables. However, these improvements did not translate into an improved QoL after 10 months. With an unclear, but most likely negative, impact of the COVID-19 pandemic, improvements in QoL may become evident during further follow-up (phase 2 of the study).

Endurance training may improve exercise capacity, lung function and quality of life in Fontan patients

Children born with univentricular hearts undergo staged surgical procedures to a Fontan circulation. Long-term experience with Fontan palliation has shown dramatically improved survival but also of a life-long burden of an abnormal circulation with significant morbidity. Many Fontan patients have reduced exercise capacity, oxygen uptake, lung function and quality of life. Endurance training may improve submaximal, but not maximal, exercise capacity, lung function and quality of life. Physical activity and endurance training is also positively correlated with sleep quality. Reviewing the literature and from our single-centre experience, we believe there is enough evidence to support structured individualised endurance training in most young Fontan patients.

Impairments in Pulmonary Function in Fontan Patients: Their Causes and Consequences

Patients with a Fontan circulation lack a sub-pulmonary ventricle with pulmonary blood flow passively redirected to the lungs. In the Fontan circulation, ventilation has a significant influence on pulmonary blood flow and cardiac output both at rest and with exercise. Children and adults with a Fontan circulation have abnormalities in lung function. In particular, restrictive ventilatory patterns, as measured by spirometry, and impaired gas transfer, as measured by the diffusing capacity of carbon monoxide, have been frequently observed. These abnormalities in lung function are associated with reduced exercise capacity and quality of life. Moderate to severe impairment in lung volumes is independently associated with reduced survival in adults with congenital heart disease. Skeletal and inspiratory muscle weakness has also been reported in patients with a Fontan circulation, with the prospect of improving respiratory muscle function through exercise training programs. In this review, we will present data on cardiopulmonary interactions in the Fontan circulation, the prevalence and severity of impaired lung function, and respiratory muscle function in this population. We will discuss potential causes for and consequence of respiratory impairments, and their impact on exercise capacity and longer-term Fontan outcome. We aim to shed light on possible strategies to reduce morbidity by improving respiratory function in this growing population of patients.

Use of a High-Flow Nasal Cannula in a Child With a Functional Single Ventricle and Difficult Airway

Inducing anesthesia and securing the airway without disrupting the patient's hemodynamic state are challenging in pediatric patients with a functional single ventricle (FSV). Here, the authors report effective use of a high-flow nasal cannula (HFNC) as a tool in providing oxygen supplementation for airway management in pediatric FSV patients with a history of a difficult airway. A female patient, aged 5 years 7 months, was admitted for extracardiac conduit Fontan procedure. The patient had a history of multiple failed attempts at endotracheal intubation and was diagnosed with retrognathia and severe oral trismus of less than 1 finger width. The patient had another event of mask ventilation failure after propofol sedation during the preoperative computed tomography scan a day before the surgery. The patient's preoperative cardiac catheterization report revealed that the Qp/Qs ratio of 0.82 at room air, and the patient's peripheral oxygen saturation (SpO₂) ranged from 70% to 80% at room air. On entering the operating room, the preoxygenation process began with HFNC at an oxygen flow of 16 L/min, with a fraction of inspired oxygen (F_IO₂) set at 95% after light sedation with an intravenous bolus of midazolam, 0.1 mg/kg. After 4 minutes of applying HFNC, with SpO₂ rising from 76% to 98%, anesthetic medications were administered intravenously without a neuromuscular blocking agent to preserve spontaneous breathing. The patient was not ventilated with a facemask but instead left with HFNC in place for continuous supplemental oxygenation. The patient's airway was secured in a single attempt in 80 seconds. HFNC is an ideal option for oxygen supplementation during airway management of pediatric FSV patients, as their balance of pulmonary and systemic flow is perturbed easily by subtle physiologic alteration and therapeutic maneuvers during the induction of general anesthesia and highly susceptible to rapid desaturation and cardiovascular collapse, and should be considered as having a physiologically difficult airway.

Appropriate heart rate during exercise in Fontan patients

OBJECTIVE

To evaluate heart rate against workload and oxygen consumption during exercise in Fontan patients.

METHOD

Fontan patients (n = 27) and healthy controls (n = 25) underwent cardiopulmonary exercise testing with linear increase of load. Heart rate and oxygen uptake were measured during tests. Heart rate recovery was recorded for 10 minutes.

RESULTS

Heart rate at midpoint (140 ± 14 versus 153 ± 11, p < 0.001) and at maximal effort (171 ± 14 versus 191 ± 10 beats per minute, p < 0.001) of test was lower for patients than controls. Heart rate recovery was similar between groups. Heart rate in relation to workload was higher for patients than controls both at midpoint and maximal effort. Heart rate in relation to oxygen uptake was similar between groups throughout test. Oxygen pulse, an indirect surrogate measure of stroke volume, was reduced at maximal effort in patients compared to controls (6.6 ± 1.1 versus 7.5 ± 1.4 ml·beat-1·m-2, p < 0.05) and increased significantly less from midpoint to maximal effort for patients than controls (p < 0.05).

CONCLUSIONS

Heart rate is increased in relation to workload in Fontan patients compared with controls. At higher loads, Fontan patients seem to have reduced heart rate and smaller increase in oxygen pulse, which may be explained by inability to further increase stroke volume and cardiac output. Reduced ability to increase or maintain stroke volume at higher heart rates may be an important limiting factor for maximal cardiac output, oxygen uptake, and physical performance.

A 4D flow MRI evaluation of the impact of shear-dependent fluid viscosity on in vitro Fontan circulation flow

The Fontan procedure for univentricular heart defects creates a nonphysiologic circulation where systemic venous blood drains directly into the pulmonary arteries, leading to multiorgan dysfunction secondary to chronic low-shear nonpulsatile pulmonary blood flow and central venous hypertension. Although blood viscosity increases exponentially in this low-shear environment, the role of shear-dependent ("non-Newtonian") blood viscosity in this pathophysiology is unclear. We studied three-dimensional (3D)-printed Fontan models in an in vitro flow loop with a Philips 3-T magnetic resonance imaging (MRI) scanner. A 4D flow phase-contrast sequence was used to acquire a time-varying 3D velocity field for each experimental condition. On the basis of blood viscosity of a cohort of patients who had undergone the Fontan procedure, it was decided to use 0.04% xanthan gum as a non-Newtonian blood analog; 45% glycerol was used as a Newtonian control fluid. MRI data were analyzed using GTFlow and MATLAB software. The primary outcome, power loss, was significantly higher with the Newtonian fluid [14.8 (13.3, 16.4) vs. 8.1 (6.4, 9.8)%, medians with 95% confidence interval, P < 0.0001]. The Newtonian fluid also demonstrated marginally higher right pulmonary artery flow, marginally lower shear stress, and a trend toward higher caval flow mixing. Outcomes were modulated by Fontan model complexity, cardiac output, and caval flow ratio. Vortexes, helical flow, and stagnant flow were more prevalent with the non-Newtonian fluid. Our data demonstrate that shear-dependent viscosity significantly alters qualitative flow patterns, power loss, pulmonary flow distribution, shear stress, and caval flow mixing in synthetic models of the Fontan circulation. Potential clinical implications include effects on exercise capacity, ventilation-perfusion matching, risk of pulmonary arteriovenous malformations, and risk of thromboembolism.NEW & NOTEWORTHY Although blood viscosity increases exponentially in low-shear environments, the role of shear-dependent ("non-Newtonian") blood viscosity in the pathophysiology of the low-shear Fontan circulation is unclear. We demonstrate that shear-dependent viscosity significantly alters qualitative flow patterns, power loss, pulmonary flow distribution, shear stress, and caval flow mixing in synthetic models of the Fontan circulation. Potential clinical implications include effects on exercise capacity, ventilation-perfusion matching, risk of pulmonary arteriovenous malformations, and risk of thromboembolism.