Heart beat but not respiration is the main driving force of the systemic venous return in the Fontan circulation

Augmented Biphasic Breathing Using Sniff and an Oral Positive Expiratory Pressure Device (Sniff-PEP) in Fontan Patients

The use of an oral positive expiratory pressure device (oPEP) with sniff breathing (Sniff-PEP) mimics biphasic ventilation. Biphasic ventilation increases pulmonary blood flow and cardiac output in Fontan patients. The aim of this study was to assess the effect of Sniff-PEP on Fontan flow velocities. A single-center, pilot, prospective study was carried out in 15 subjects with Fontan circulation enrolled to use the oPEP device for 1 month. Subjects were instructed on Sniff-PEP and to use the device for 10-15 min 3-4 times a day. Measurements of flow velocity and cardiac output were measured via echocardiogram and quality of life assessments were performed at baseline and 4-6 weeks later. The mean age at enrollment was 19.9 ± 8.7 years (age range of 10-37 years). 7 patients (47%) had dominant left ventricle and 8 (53%) had an open fenestration. There was a statistically significant increase in flow velocities in the hepatic vein from 27.5 ± 7.6 to 35.1 ± 11.3 cm/s (p = 0.003), left pulmonary artery from 51.6 ± 16.6 to 57.6 ± 21.1 cm/s (p = 0.01), and right pulmonary artery from 43.1 ± 14.2 to 45.8 ± 17.2 cm/s (p = 0.04). With chronic use, the mean fenestration gradient slightly decreased from 4.5 ± 1.6 to 4.1 ± 1.9 mmHg but the difference was not statistically significant (p = 0.14). oPEP device therapy increased flow velocity in several areas in the Fontan circulation with acute use. Further studies are needed to assess the effects long term.Clinical Trial Registration: URL: https://clinicaltrials.gov . Unique identifier: NCT03251742.

In a porcine model of implantable pacemakers for pediatric unilateral diaphragm paralysis, the phrenic nerve is the best target

BACKGROUND

A frequent complication of Fontan operations is unilateral diaphragmatic paresis, which leads to hemodynamic deterioration of the Fontan circulation. A potential new therapeutic option is the unilateral diaphragmatic pacemaker. In this study, we investigated the most effective stimulation location for a potential fully implantable system in a porcine model.

METHODS

Five pigs (20.8 ± 0.95 kg) underwent implantation of a customized cuff electrode placed around the right phrenic nerve. A bipolar myocardial pacing electrode was sutured adjacent to the motor point and peripherally at the costophrenic angle (peripheral diaphragmatic muscle). The electrodes were stimulated 30 times per minute with a pulse duration of 200 µs and a stimulation time of 300 ms. Current intensity was the only variable changed during the experiment.

RESULTS

Effective stimulation occurred at 0.26 ± 0.024 mA at the phrenic nerve and 7 ± 1.22 mA at the motor point, a significant difference in amperage (p = 0.005). Even with a maximum stimulation of 10 mA at the peripheral diaphragm muscle, however, no effective stimulation was observed.

CONCLUSION

The phrenic nerve seems to be the best location for direct stimulation by a unilateral thoracic diaphragm pacemaker in terms of the required amperage level in a porcine model.

Pulmonary blood flow in children with univentricular heart and unilateral diaphragmatic paralysis

OBJECTIVES

Spontaneous breathing has an important effect on pulmonary arterial blood flow in patients with Glenn/Fontan circulation. Unilateral diaphragmatic paralysis (DP) is a frequent complication after heart surgery in congenital heart disease. The aim of this study was to investigate the influence of unilateral DP on blood flow distribution in the pulmonary arteries with Glenn/Fontan circulation.

METHODS

Magnetic resonance phase-contrast imaging was used to evaluate stroke volume index (SVI) in the left and right pulmonary arteries in patients with Glenn/Fontan circulation with unilateral DP. Data for 18 patients with univentricular heart and unilateral DP were analysed, 8 in the Glenn stage and 10 in the Fontan stage. Ten patients had right-sided DP, and 8 had left-sided DP. A diaphragmatic plication was performed in 7 patients. The control group consisted of 36 patients with Glenn (n = 16)/Fontan (n = 20) circulation without DP.

RESULTS

In both left- and right-sided DP, the SVI to the ipsilateral side was significantly lower than in controls [2.81 (1.45-4.50) ml/m2 left vs 11.97 (7.36-16.37) ml/m2 in controls, P < 0.0002; 8.2 (4.49-12.64) ml/m2 with right vs 12.64 (9.66-16.61) ml/m2 in controls; P = 0.0284]. The SVI to the contralateral side showed a slight but non-significant increase in the presence of unilateral DP.

CONCLUSIONS

Unilateral DP in patients with Glenn/Fontan circulation has a negative impact on pulmonary arterial SVI on the side of the paralysis.

Haemodynamic performance of 16-20-mm extracardiac Goretex conduits in adolescent Fontan patients at rest and during simulated exercise

OBJECTIVES

To date, it is not known if 16-20-mm extracardiac conduits are outgrown during somatic growth from childhood to adolescence. This study aims to determine total cavopulmonary connection (TCPC) haemodynamics in adolescent Fontan patients at rest and during simulated exercise and to assess the relationship between conduit size and haemodynamics.

METHODS

Patient-specific, magnetic resonance imaging-based computational fluid dynamic models of the TCPC were performed in 51 extracardiac Fontan patients with 16-20-mm conduits. Power loss, pressure gradient and normalized resistance were quantified in rest and during simulated exercise. The cross-sectional area (CSA) (mean and minimum) of the vessels of the TCPC was determined and normalized for flow rate (mm2/l/min). Peak (predicted) oxygen uptake was assessed.

RESULTS

The median age was 16.2 years (Q1-Q3 14.0-18.2). The normalized mean conduit CSA was 35-73% smaller compared to the inferior and superior vena cava, hepatic veins and left/right pulmonary artery (all P < 0.001). The median TCPC pressure gradient was 0.7 mmHg (Q1-Q3 0.5-0.8) and 2.0 (Q1-Q3 1.4-2.6) during rest and simulated exercise, respectively. A moderate-strong inverse non-linear relationship was present between normalized mean conduit CSA and TCPC haemodynamics in rest and exercise. TCPC pressure gradients of ≥1.0 at rest and ≥3.0 mmHg during simulated exercise were observed in patients with a conduit CSA ≤ 45 mm2/l/min and favourable haemodynamics (<1 mmHg during both rest and exercise) in conduits ≥125 mm2/l/min. Normalized TCPC resistance correlated with (predicted) peak oxygen uptake.

CONCLUSIONS

Extracardiac conduits of 16-20 mm have become relatively undersized in most adolescent Fontan patients leading to suboptimal haemodynamics.

Real-Time Magnetic Resonance Imaging

Real-time magnetic resonance imaging (RT-MRI) allows for imaging dynamic processes as they occur, without relying on any repetition or synchronization. This is made possible by modern MRI technology such as fast-switching gradients and parallel imaging. It is compatible with many (but not all) MRI sequences, including spoiled gradient echo, balanced steady-state free precession, and single-shot rapid acquisition with relaxation enhancement. RT-MRI has earned an important role in both diagnostic imaging and image guidance of invasive procedures. Its unique diagnostic value is prominent in areas of the body that undergo substantial and often irregular motion, such as the heart, gastrointestinal system, upper airway vocal tract, and joints. Its value in interventional procedure guidance is prominent for procedures that require multiple forms of soft-tissue contrast, as well as flow information. In this review, we discuss the history of RT-MRI, fundamental tradeoffs, enabling technology, established applications, and current trends. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

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.

Frequency-Selective Manipulations of Spins allow Effective and Robust Transfer of Spin Order from Parahydrogen to Heteronuclei in Weakly-Coupled Spin Systems

We present a selectively pulsed (SP) generation of sequences to transfer the spin order of parahydrogen (pH₂) to heteronuclei in weakly coupled spin systems. We analyze and discuss the mechanism and efficiency of SP spin order transfer (SOT) and derive sequence parameters. These new sequences are most promising for the hyperpolarization of molecules at high magnetic fields. SP‐SOT is effective and robust despite the symmetry of the ¹H‐¹³C J‐couplings even when precursor molecules are not completely labeled with deuterium. As only one broadband ¹H pulse is needed per sequence, which can be replaced for instance by a frequency‐modulated pulse, lower radiofrequency (RF) power is required. This development will be useful to hyperpolarize (new) agents and to perform the hyperpolarization within the bore of an MRI system, where the limited RF power has been a persistent problem.

The Influence of Respiration on Blood Flow in the Fontan Circulation: Insights for Imaging-Based Clinical Evaluation of the Total Cavopulmonary Connection

Congenital heart disease is the most common birth defect and functionally univentricular heart defects represent the most severe end of this spectrum. The Fontan circulation provides an unique solution for single ventricle patients, by connecting both caval veins directly to the pulmonary arteries. As a result, the pulmonary circulation in Fontan palliated patients is characterized by a passive, low-energy circulation that depends on increased systemic venous pressure to drive blood toward the lungs. The absence of a subpulmonary ventricle led to the widely believed concept that respiration, by sucking blood to the pulmonary circulation during inspiration, is of great importance as a driving force for antegrade blood flow in Fontan patients. However, recent studies show that respiration influences pulsatility, but has a limited effect on net forward flow in the Fontan circulation. Importantly, since MRI examination is recommended every 2 years in Fontan patients, clinicians should be aware that most conventional MRI flow sequences do not capture the pulsatility of the blood flow as a result of the respiration. In this review, the unique flow dynamics influenced by the cardiac and respiratory cycle at multiple locations within the Fontan circulation is discussed. The impact of (not) incorporating respiration in different MRI flow sequences on the interpretation of clinical flow parameters will be covered. Finally, the influence of incorporating respiration in advanced computational fluid dynamic modeling will be outlined.

Segmental assessment of blood flow efficiency in the total cavopulmonary connection using four-dimensional flow magnetic resonance imaging: vortical flow is associated with increased viscous energy loss rate

Aims

To study flow-related energetics in multiple anatomical segments of the total cavopulmonary connection (TCPC) in Fontan patients from four-dimensional (4D) flow magnetic resonance imaging (MRI), and to study the relationship between adverse flow patterns and segment-specific energetics.

Methods and results

Twenty-six extracardiac Fontan patients underwent 4D flow MRI of the TCPC. A segmentation of the TCPC was automatically divided into five anatomical segments [conduit, superior vena cava (SVC), right/left pulmonary artery (LPA), and the Fontan confluence]. The presence of vortical flow in the pulmonary arteries or Fontan confluence was qualitatively scored. Kinetic energy (KE), viscous energy loss rate, and vorticity were calculated from the 4D flow MRI velocity field and normalized for segment length and/or inflow. Energetics were compared between segments and the relationship between vortical flow and segment cross-sectional area (CSA) with segment-specific energetics was determined. Vortical flow in the LPA (n = 6) and Fontan confluence (n = 12) were associated with significantly higher vorticity (P = 0.001 and P = 0.015, respectively) and viscous energy loss rate (P = 0.046 and P = 0.04, respectively) compared to patients without vortical flow. The LPA and conduit segments showed the highest KE and viscous energy loss rate, while most favourable energetics were observed in the SVC. Conduit CSA inversely correlated with KE (r = −0.614, P = 0.019) and viscous energy loss rate (r = −0.652, P = 0.011).

Conclusions

Vortical flow in the Fontan confluence and LPA associated with significantly increased viscous energy loss rate. Four-dimensional flow MRI-derived energetics may be used as a screening tool for direct, MRI-based assessment of flow efficiency in the TCPC.

Surveillance of Fontan Associated Liver Disease in Childhood and Adolescence

Fontan associated liver disease (FALD) has been recognized as a potentially serious sequela of the Fontan circulation. Prevalence of FALD among different age groups and risk factors for advanced changes were assessed. FALD screening included abdominal ultrasound and laboratory tests. A "liver disease score (LDS)" incorporating items from ultrasound and blood testing was calculated to grade FALD severity (5 items each, maximum score 10 points). 240 patients (male: n = 139, female: n = 101, systemic right ventricle: n = 160) underwent FALD screening 10 (IQR 7-15) years after Fontan surgery. Ultrasound was abnormal in 184 (76.6%) patients (surface nodularity / blunted liver edge: n = 133, 55.4%; heterogeneous parenchyma: n = 93, 38.8%; splenomegaly: n = 68, 28.3%; ascites: n = 23, 9.6%). At least one abnormal laboratory test was detected in 218 (90.8%) patients. Gamma-glutamyl-transpeptidase was elevated in the majority of patients (n = 206, 85.8%). Median LDS was 3 (2-4). Scores ≥5 were observed in 32 (13.3%) patients. Longer follow-up (15 (11-20) vs 9 (6-14) years, P <0.001), higher central venous (13 (11-15) vs 10 (9-12) mmHg, P <0.001) and end-diastolic pressure (8 (5-10) vs 6 (5-7) mmHg, P = 0.001), impaired ventricular function and absence of sinus rhythm were associated with LDS ≥5. Longer follow-up (OR 1.2 (1.1-1.3), P <0.001) and higher central venous pressure (OR 1.6 (1.3-2.1), p < 0.001) were the only independent predictors of advanced FALD. Abdominal ultrasound and laboratory abnormalities suggestive of FALD are common during routine follow-up already in childhood and adolescence irrespective of ventricular morphology. More advanced findings are associated with longer follow-up and higher central venous pressure.

5D Flow MRI: A Fully Self-gated, Free-running Framework for Cardiac and Respiratory Motion-resolved 3D Hemodynamics

PURPOSE

To implement, validate, and apply a self-gated free-running whole-heart five-dimensional (5D) flow MRI framework to evaluate respiration-driven effects on three-dimensional (3D) hemodynamics in a clinical setting.

MATERIALS AND METHODS

In this prospective study, a free-running five-dimensional (5D) flow sequence was implemented with 3D radial sampling, self-gating, and a compressed-sensing reconstruction. The 5D flow was evaluated in a pulsatile phantom and adult participants with aortic and/or valvular disease who were enrolled between May and August 2019. Conventional twofold-accelerated four-dimensional (4D) flow of the thoracic aorta with navigator gating was performed as a reference comparison. Continuous parameters were evaluated for parameter normality and were compared between conventional 4D flow and 5D flow using a signed-rank or two-tailed paired t test. Differences between respiratory states were evaluated using a repeated-measure analysis of variance or a nonparametric Friedman test.

RESULTS

A total of 20 adult participants (mean age, 49 years ± 17 [standard deviation]; 18 men and two women) were included. In vitro 5D flow results showed excellent agreement with conventional 4D flow-derived values (peak and net flow, <7% difference over all quantified planes). Whole-heart 5D flow data were collected in all participants in 7.65 minutes ± 0.35 (acceleration rate = 36.0-76.9) versus 9.88 minutes ± 3.17 for conventional aortic 4D flow. In vivo, 5D flow demonstrated moderate agreement with conventional 4D flow but demonstrated overestimation in net flow and peak velocity (up to 26% and 12%, respectively) in the ascending aorta and underestimation (<12%) in the arch and descending aorta. Respiratory-resolved analyses of caval veins showed significantly increased net and peak flow in the inferior vena cava in end inspiration compared with end expiration, and the opposite trend was shown in the superior vena cava.

CONCLUSION

A free-running 5D flow MRI framework consistently captured cardiac and respiratory motion-resolved 3D hemodynamics in less than 8 minutes. Supplemental material is available for this article. © RSNA, 2020.