Magnetic resonance imaging of constant total heart volume and center of mass in patients with functional single ventricle before and after staged Fontan procedure

In Memoriam: Paul Morris Weinberg, MD

On 15 October, 2020, Dr. Paul M. Weinberg, a true giant in the field of paediatric cardiology, succumbed to a prolonged illness. Dr. Weinberg had a 43-year career and was described as a pillar of The Children's Hospital of Philadelphia and the spirit of the Division of Cardiology, a cherished and beloved teacher, and an outstanding clinician. His impact on the field and on the careers of his students will be remembered for generations to come.In 2019, Dr. Weinberg wrote for Jefferson Medical School's 50th year reunion memory book: "In the true spirit of Hippocrates, I seek to mentor the next generation as I was mentored by the last, without expectation of reward. I am forever indebted to these educators for all the knowledge they imparted to me and for the wisdom that I acquired under their tutelage." These words are a true reflection of his unassuming dedication to teaching the next generation of paediatric cardiologists. His legacy will continue to live on through these trainees and impact the field for generations to come.

Exercise capacity in the Bidirectional Glenn physiology: Coupling cardiac index, ventricular function and oxygen extraction ratio

In Bi-directional Glenn (BDG) physiology, the superior systemic circulation and pulmonary circulation are in series. Consequently, only blood from the superior vena cava is oxygenated in the lungs. Oxygenated blood then travels to the ventricle where it is mixed with blood returning from the lower body. Therefore, incremental changes in oxygen extraction ratio (OER) could compromise exercise tolerance. In this study, the effect of exercise on the hemodynamic and ventricular performance of BDG physiology was investigated using clinical patient data as inputs for a lumped parameter model coupled with oxygenation equations. Changes in cardiac index, Qp/Qs, systemic pressure, oxygen extraction ratio and ventricular/vascular coupling ratio were calculated for three different exercise levels. The patient cohort (n=29) was sub-grouped by age and pulmonary vascular resistance (PVR) at rest. It was observed that the changes in exercise tolerance are significant in both comparisons, but most significant when sub-grouped by PVR at rest. Results showed that patients over 2 years old with high PVR are above or close to the upper tolerable limit of OER (0.32) at baseline. Patients with high PVR at rest had very poor exercise tolerance while patients with low PVR at rest could tolerate low exercise conditions. In general, ventricular function of SV patients is too poor to increase CI and fulfill exercise requirements. The presented mathematical model provides a framework to estimate the hemodynamic performance of BDG patients at different exercise levels according to patient specific data.

Effect of high altitude exposure on the hemodynamics of the bidirectional Glenn physiology: modeling incremented pulmonary vascular resistance and heart rate

The considerable blood mixing in the bidirectional Glenn (BDG) physiology further limits the capacity of the single working ventricle to pump enough oxygenated blood to the circulatory system. This condition is exacerbated under severe conditions such as physical activity or high altitude. In this study, the effect of high altitude exposure on hemodynamics and ventricular function of the BDG physiology is investigated. For this purpose, a mathematical approach based on a lumped parameter model was developed to model the BDG circulation. Catheterization data from 39 BDG patients at stabilized oxygen conditions was used to determine baseline flows and pressures for the model. The effect of high altitude exposure was modeled by increasing the pulmonary vascular resistance (PVR) and heart rate (HR) in increments up to 80% and 40%, respectively. The resulting differences in vascular flows, pressures and ventricular function parameters were analyzed. By simultaneously increasing PVR and HR, significant changes (p <0.05) were observed in cardiac index (11% increase at an 80% PVR and 40% HR increase) and pulmonary flow (26% decrease at an 80% PVR and 40% HR increase). Significant increase in mean systemic pressure (9%) was observed at 80% PVR (40% HR) increase. The results show that the poor ventricular function fails to overcome the increased preload and implied low oxygenation in BDG patients at higher altitudes, especially for those with high baseline PVRs. The presented mathematical model provides a framework to estimate the hemodynamic performance of BDG patients at different PVR increments.

The total cavopulmonary connection resistance: a significant impact on single ventricle hemodynamics at rest and exercise

Little is known about the impact of the total cavopulmonary connection (TCPC) on resting and exercise hemodynamics in a single ventricle (SV) circulation. The aim of this study was to elucidate this mechanism using a lumped parameter model of the SV circulation. Pulmonary vascular resistance (1.96+/-0.80 WU) and systemic vascular resistances (18.4+/-7.2 WU) were obtained from catheterization data on 40 patients with a TCPC. TCPC resistances (0.39+/-0.26 WU) were established using computational fluid dynamic simulations conducted on anatomically accurate three-dimensional models reconstructed from MRI (n=16). These parameters were used in a lumped parameter model of the SV circulation to investigate the impact of TCPC resistance on SV hemodynamics under resting and exercise conditions. A biventricular model was used for comparison. For a biventricular circulation, the cardiac output (CO) dependence on TCPC resistance was negligible (sensitivity=-0.064 l.min(-1).WU(-1)) but not for the SV circulation (sensitivity=-0.88 l.min(-1).WU(-1)). The capacity to increase CO with heart rate was also severely reduced for the SV. At a simulated heart rate of 150 beats/min, the SV patient with the highest resistance (1.08 WU) had a significantly lower increase in CO (20.5%) compared with the SV patient with the lowest resistance (50%) and normal circulation (119%). This was due to the increased afterload (+35%) and decreased preload (-12%) associated with the SV circulation. In conclusion, TCPC resistance has a significant impact on resting hemodynamics and the exercise capacity of patients with a SV physiology.

The quantitative relationship between longitudinal and radial function in left, right, and total heart pumping in humans

The total heart volume variation (THVV) during systole has been proposed to be caused by radial function of the ventricles, but definitive data for both ventricles have not been presented. Furthermore, the right ventricle (RV) has been suggested to have a greater longitudinal pumping component than the left ventricle (LV). Therefore, we aimed to compare the stroke volume (SV) generated by radial function to the volume variation of the left, right, and total heart. To do this, we also needed to develop a new method for measuring the contribution of the longitudinal atrioventricular plane displacement (AVPD) to the RVSV (RVSVAVPD). For our study, 11 volunteers underwent cine MRI in the short- and long-axis planes and MRI flow measurement in all vessels leading to and from the heart. The left, right, and total heart showed correlations between volume variation from flow measurements and radial function calculated as SV minus the longitudinal function ( r = 0.81, P < 0.01; r = 0.80, P < 0.01; and r = 0.92, P < 0.001, respectively). Compared with the LV, the RV had a greater AVPD (23.4 ± 0.8 vs. 16.4 ± 0.5 mm), center of volume movement (13.0 ± 0.7 vs. 7.8 ± 0.4 mm), and, RVSVAVPD(82 ± 2% vs. 60 ± 2%) ( P < 0.001 for all). We found that THVV is predominantly caused by radial function of the ventricles. Longitudinal AVPD accounts for ∼80% of the RVSV, compared with ∼60% for the LVSV. This difference explains the larger portion of THVV found on the left side of the heart.

Relationship of pulmonary vein flow to left ventricular short-axis epicardial displacement in diastole: model-based prediction with in vivo validation

Previous studies in healthy humans have established that the (approximately 850 ml) volume enclosed by the pericardial sac is nearly constant over the cardiac cycle, exhibiting a transient approximately 5% decrease (approximately 40 ml) from end diastole to end systole. This volume decrease manifests as a "crescent" at the ventricular free wall level when short-axis MRI images of the epicardial surface acquired at end systole and end diastole are superimposed. On the basis of the (near) constant-volume property of the four-chambered heart, the volume decrease ("crescent effect") must be restored during subsequent early diastolic filling via the left atrial conduit volume. Therefore, volume conservation-based modeling predicts that pulmonary venous (PV) Doppler D-wave volume must be causally related to the radial displacement of the epicardium (Delta) (i.e., magnitude of "crescent effect" in the radial direction). We measured Delta from M-mode echocardiographic images and measured D-wave velocity-time integral (VTI) from Doppler PV flow of the right superior PV in 11 subjects with catheterization-determined normal physiology. In accordance with model prediction, high correlation was observed between Delta and D-wave VTI (r=0.86) and early D-wave VTI measured to peak D-wave velocity (r=0.84). Furthermore, selected subjects with various pathological conditions had values of Delta that differed significantly. These observations demonstrate the volume conservation-based causal relationship between radial pericardial displacement of the left ventricle and the PV D-wave-generated filling volume in healthy subjects as well as the potential role of the M-mode echo-derived radial epicardial displacement index Delta as a regional (radial) parameter of diastolic function.

Center of volume and total heart volume variation in healthy subjects and patients before and after coronary bypass surgery

BACKGROUND

Total heart volume variation (THVV) and center of volume variation (COVV) likely affects the efficiency of cardiac pumping, but no study has determined COVV of the heart throughout the cardiac cycle or the effect of surgery on THVV in adults. Therefore, the purposes of this study were to determine COVV in healthy adults and patients with cardiac failure due to ischemic heart disease (IHD), identify any difference in THVV between these two groups, and determine how these parameters are affected by coronary bypass surgery.

METHODS

Six healthy volunteers and eight patients before and after surgery were investigated with cardiovascular magnetic resonance imaging. The atrioventricular plane movement (AVPM), THVV and time resolved three-dimensional coordinates of the center of the cardiac volume (COVV) were measured.

RESULTS

COVV followed a loop in 3D space that between the end-points was approximately 2 mm with no difference between healthy subjects and patients before surgery (P = 0.093), although AVPM was significantly lower in patients (P = 0.002). However, after surgery the COVV during the cardiac cycle doubled (P = 0.012) and the increase in THVV was significant (P = 0.050), although of very small magnitude, and the AVPM remained unchanged (P = 0.401).

CONCLUSION

COVV and THVV were similar in patients and healthy subjects even though AVPM was lower in the patient population. After surgery, however, COVV doubled despite a very small change in THVV and no change in AVPM. Taken together, the results of this study may provide new insights into the energy expenditure and efficiency of cardiac pumping.

Designing therapeutic strategies for patients with a dominant left ventricle, discordant ventriculo-arterial connections, and unobstructed flow of blood to the lungs

The palliation of the cyanotic child with a dominant morphologically left ventricle, discordant ventriculo-arterial connections, and obstruction to the pulmonary outflow tract has continued to evolve and mature. The evolution began in the early days of surgical palliation with the Blalock-Taussig shunt, extended to construction of cavopulmonary shunts, if required, and then to the Fontan procedure and its subsequent modifications. This journey took nearly 30 years to complete. There is increasing clinical data to document the beneficial effects of this approach, with ever-improving outcomes. Some aspects of the history of the cavopulmonary shunt have been previously reviewed in this journal and elsewhere, as have analysis of outcomes for some groups of patients considered for surgical completion of the Fontan circulation. While there has been some ongoing interest in ventricular septation since the early success of Sakakibara et al., this approach has largely been abandoned. Considerably more challenges and debate resonate in the surgical algorithms defined for patients whose hearts are characterized by a dominant left ventricle, discordant ventriculo-arterial connections, and unobstructed flow of blood to the lungs. This latter group will be the focus of this review, as will the aetiology of the myocardial hypertrophy that is particularly frequent in this group of patients, its clinical recognition, indeed its anticipation, and the multiple surgical strategies designed to prevent or treat it. All these manoeuvres are considered to optimise suitability for, and outcome from, creation of the Fontan circulation.

MRI-determined left ventricular "crescent effect": a consequence of the slight deviation of contents of the pericardial sack from the constant-volume state

During one cardiac cycle, the volume encompassed by the pericardial sack in healthy subjects remains nearly constant, with a transient +/-5% decrease in volume at end systole. This "constant-volume" attribute defines a constraint that the longitudinal versus radial pericardial contour dimension relationship must obey. Using cardiac MRI, we determined the extent to which the constant-volume attribute is valid from four-chamber slices (two-dimensional) compared with three-dimensional volumetric data. We also compared the relative percentage of longitudinal versus radial (short-axis) change in cross-sectional area (dimension) of the pericardial contour, thereby assessing the fate of the +/-5% end-systolic volume decrease. We analyzed images from 10 normal volunteers and 1 subject with congenital absence of the pericardium, obtained using a 1.5-T MR scanner. Short-axis cine loop stacks covering the entire heart were acquired, as were single four-chamber cine loops. In the short-axis and four-chamber slices, relative to midventricular end-diastolic location, end-systolic pericardial (left ventricular epicardial) displacement was observed to be radial and maximized at end systole. Longitudinal (apex to mediastinum) pericardial contour dimension change and pericardial area change on the four-chamber slice were negligible throughout the cardiac cycle. We conclude that the +/-5% end-systolic decrease in the volume encompassed by the pericardial sack is primarily accounted for by a "crescent effect" on short-axis views, manifesting as a nonisotropic radial diminution of the pericardial/epicardial contour of the left ventricle. This systolic drop in cardiac volume occurs primarily at the ventricular level and is made up during the subsequent diastole when blood crosses the pericardium in the pulmonary venous Doppler D wave during early rapid left ventricular filling.

Assessment and consequences of the constant-volume attribute of the four-chambered heart

The constant-volume hypothesis regarding the four-chambered heart states that total pericardial volume remains invariant throughout the cardiac cycle. Previous canine studies have indicated that the pericardial volume remains constant within 5%; however, this hypothesis has not been validated in humans using state-of-the-art technology. The constant-volume hypothesis has several predictable functional consequences, including a relationship between atrial ejection fraction and chamber equilibrium volumes. Using cardiac magnetic resonance (MR) imaging (MRI), we measured the extent to which the constant-volume attribute of the heart is valid, and we tested the accuracy of the predicted relationship between atrial ejection fraction and chamber equilibrium volumes. Eleven normal volunteers and one volunteer with congenital absence of the pericardium were imaged using a 1.5-T MR scanner. A short-axis cine-loop stack covering the entire heart was acquired. The cardiac cycle was divided into 20 intervals. For each slice and interval, pericardial volumes were measured. The slices were stacked and summed, and total pericardial volume as a function of time was determined for each subject. In the normal subjects, chamber volumes at ventricular end diastole, end systole, and diastasis were measured. Pericardial volume remained invariant within 5 +/- 1% in normal subjects; maximum variation occurred near end systole. In the subject with congenital absence of the pericardium, total heart volume, defined by the epicardial surface, varied by 12%. The predictions of the relationship between atrial ejection fraction and chamber equilibrium volumes were well fit by MRI data. In normal subjects, the four-chambered heart is a constant-volume pump within 5 +/- 1%, and constant-volume-based modeling accurately predicts previously unreported physiological relationships.

Hearts late after fontan operation have normal mass, normal volume, and reduced systolic function

OBJECTIVES

The purpose of this study was to assess ventricular mass, volume, and systolic function in patients late after Fontan operation by cardiac magnetic resonance imaging.

BACKGROUND

An assessment of determinants for ventricular function in post-Fontan patients was intended.

METHODS

Twenty-three unselected patients (9 female, 14 male) at a median age of 19.4 years (range, 7.8 to 31.3 years), at a median time of 10.5 years (range, 4.1 to 18 years) after Fontan operation were studied. A standard 1.5-T scanner was used, and analysis was performed using dedicated software. Ten healthy volunteers (median age 26.4, range 18 to 39.3 years) served as the control group.

RESULTS

Median end-systolic mass index was 72.2 g/m(2) (range, 43 to 138 g/m(2)) and 86.6 g/m(2) (range, 52 to 123 g/m(2)) in the control group (p = NS). Median end-diastolic ventricular volume was 64 ml/m(2) (range, 32 to 117 ml/m(2)) compared with 67.7 ml/m(2) (range, 59 to 75 ml/m(2)) in the control group (p = NS). Median ejection fraction was 49.3% (range, 20% to 63%) compared with 64.8% (range, 57% to 79%) in normals (p = 0.00001).

CONCLUSIONS

We conclude that long-term survivors of a Fontan operation have normal ventricular mass, normal volume, but reduced systolic ventricular function.

Ventricular motion during the ejection phase: a computational analysis

In the present paper, the study of the ventricular motion during systole was addressed by means of a computational model of ventricular ejection. In particular, the implications of ventricular motion on blood acceleration and velocity measurements at the valvular plane (VP) were evaluated. An algorithm was developed to assess the force exchange between the ventricle and the surrounding tissue, i.e., the inflow and outflow vessels of the heart. The algorithm, based on the momentum equation for a transitory flowing system, was used in a fluid-structure model of the ventricle that includes the contractile behavior of the fibers and the viscous and inertial forces of the intraventricular fluid. The model calculates the ventricular center of mass motion, the VP motion, and intraventricular pressure gradients. Results indicate that the motion of the ventricle affects the noninvasive estimation of the transvalvular pressure gradient using Doppler ultrasound. The VP motion can lead to an underestimation equal to 12.4 +/- 6.6%.

Right ventricular function in congenital heart disease: pressure and volume overload lesions

The right ventricle is often subject to both pressure and volume overload in congenital heart disease. Evaluating right ventricular function in both the native lesion and after surgery in light of these loading conditions, presents a unique challenge for investigators studying these misshapen hearts. The purpose of this article is to briefly delineate what is generally known about right ventricular function in congenital heart disease and to touch on some noninvasive imaging modalities which have helped shed some light on this matter.

Comparison of patterns of pulmonary venous blood flow in the functional single ventricle heart after operative aortopulmonary shunt versus superior cavopulmonary shunt

In this study we investigated the patterns of pulmonary venous flow in children with functional single ventricles to obtain a better understanding of the determinants of transpulmonary blood flow. Sixty-eight patients with functional single ventricles and aortopulmonary shunt (n = 34, group I), or superior cavopulmonary connection (n = 34, group II) underwent transesophageal Doppler echocardiographic assessment of flow in the left upper pulmonary vein before undergoing the next stage of surgery. Twelve patients from group II also underwent simultaneous evaluation of superior vena caval flow. Biphasic forward pulmonary venous flow was noted in 62 patients in sinus rhythm (S wave in systole, D wave in diastole); in 6 patients with junctional rhythm, significant early systolic reversal of flow was present. Both the S- and D-wave velocity-time integrals (VTI) were greater in group I than in group II (S(VTI) 9.9 +/- 4.2 vs 8.0 +/- 2.6, p = 0.02; D(VTI) 8.0 +/- 3.5 vs 4.2 +/- 2.6, p <0.001). In both groups, pulmonary venous flow was predominantly systolic; however, the proportion of flow during ventricular systole was significantly greater in group II than in group I (S(VTI)/D(VTI) group II: 2.4 +/- 1.5; group I 1.4 +/- 0.5, p = 0.001; percent systolic fraction of pulmonary venous flow group II = 67%, group I = 56%, p <0.001). Analysis of superior vena caval flow in group II revealed a single predominant wave with onset at early systole and peak in late systole at a mean of 150 ms after the pulmonary venous S-wave peak. Our data suggest that ventricular systole (i.e., atrial relaxation, atrioventricular valve descent) asserts great influence on transpulmonary blood flow in the functional single ventricle.

Late ventricular geometry and performance changes of functional single ventricle throughout staged Fontan reconstruction assessed by magnetic resonance imaging

OBJECTIVES

We sought to test the hypothesis that late ventricular geometry and performance changes occur in functional single ventricles as they progress through staged Fontan reconstruction.

BACKGROUND

Indexes of ventricular geometry and performance are important in evaluating the functional state of the heart. Magnetic resonance imaging determines these indexes in complex ventricular shapes with minimal geometric assumptions. Previous studies have shown that 1 week after hemiFontan, the mass/volume ratio markedly increases.

METHODS

Multiphase, multislice, spin echo (n = 5) and cine (n = 30) magnetic resonance imaging was performed in 35 patients with a functional single ventricle (1 week to 12 years old) at various stages of Fontan reconstruction (15 in the pre hemiFontan stage, 11 after [6 to 9 months] the hemiFontan procedure and 9 after [1 to 2 years] the Fontan procedure). Volume and mass were calculated at end-systole and end-diastole. Ventricular output was then obtained. Ventricular centroid motion was also calculated.

RESULTS

No difference was noted (power > 72%) from the pre hemiFontan stage to 6 to 9 months after the hemiFontan procedure in (mean +/- SD) end-diastolic volume (104 +/- 24 vs. 123 +/- 40 cc/m2), mass (171 +/- 46 vs. 202 +/- 61 g/m2), ventricular output (7.9 +/- 2.2 vs. 6.6 +/- 2.4 liters/min per m2) or centroid motion (6.9 +/- 2.8 vs. 6.7 +/- 2. mm/m2). Patients in the Fontan group demonstrated a marked decrease in all indexes, indicating significant volume unloading and decrease in mass and ventricular performance. Mass/volume ratio was not significantly different among all three groups.

CONCLUSIONS

No geometric and performance changes from the volume-loaded stage are noted 6 to 9 months after the hemiFontan procedure; however, major changes occur 1 to 2 years after the Fontan procedure. The dramatic changes in the mass/volume ratio seen early after the hemiFontan procedure were not detected at 6 to 9 months. Furthermore diminution of mass, volume and ventricular performance are present at least 2 years after the Fontan procedure.

MR imaging and heart function in patients pre- and post-Fontan surgery

MR imaging allows functional evaluation of the ventricles of the entire heart because it evaluates cardiac anatomy three-dimensionally. Such evaluation is independent of chamber size and shape, and virtually independent of mathematical assumptions. Applying standard measurements of volume and function in children who have undergone multi-staged Fontan operations, we have shown diminished ventricular volumes and mass, and decreased cardiac indices, in post-Fontan procedure. These variations may be predictive of ultimate outcome in these complex patients.

A study in ventricular-ventricular interaction. Single right ventricles compared with systemic right ventricles in a dual-chamber circulation

BACKGROUND

Ventricular-ventricular interaction is known to occur in normal human heart. To determine whether it plays a role in the function of single right ventricles, systemic right ventricles were compared with and without a left ventricle mechanically coupled to it.

METHODS AND RESULTS

A noninvasive magnetic resonance tagging technique (spatial modulation of magnetization [SPAMM]) that lays intersecting stripes down on the myocardium was used to examine 18 patients with systemic right ventricles: 7 with a single right ventricle who have undergone the Fontan procedure (age, 38.8 +/- 8.9 months) and 11 with transposition of the great arteries who have undergone an atrial inversion operation (age, 16.3 +/- 3.9 years). The motion of the intersection points was tracked through systole to determine regional twist and radial shortening. Shortening rates also were evaluated. Finite strain analysis was applied to the grid lines using Delaunay triangulation, and the two-dimensional strain tensor and principal E1 strains were derived for the various anatomic regions. Basal and apical short-axis planes through the ventricular wall were categorized into four distinct regions spaced equally around the circumference of the slice. We observed the following results. (1) Strain was greatest and heterogeneity of strain was least in patients with transposition of the great arteries who were status post atrial inversion operation (six of eight regions). Marked differences were noted in the distribution of strain within a given region, from endocardium to epicardium, and from atrioventricular valve to apical plane between patient subtypes and those with a normal left ventricle. (2) Contrary to the normal subject studied by the use of the same method, for both patient subtypes, there was counterclockwise twist in one region, clockwise twist in the posterior or inferior wall, and a transition zone of no twist at which the two regions of twist met. Normal human adult left ventricles studied in short-axis twist uniformly counterclockwise as viewed from apex to base. (3) Radial inward motion was greatest in the superior wall of both types of systemic right ventricle. The inferior walls of Fontan patients and the posterior (ie, septal) walls of patients with transposition of the great arteries, status post atrial inversion, moved paradoxically in systole. The shortening rate at the atrioventricular valve of patients with transposition of the great arteries, status post atrial inversion, was significantly lower than at the apex or in Fontan patients.

CONCLUSIONS

Marked differences in regional wall motion and strain were demonstrated in systemic right ventricles, depending on whether a left ventricle was present to augment its function. Ventricular-ventricular interaction appears to play an important role in affecting the biomechanics of systemic right ventricles. These observations were markedly different from those in the normal systemic left ventricle. These techniques demonstrate tools with which we can begin to evaluate surgical outcomes using regional myocardial mechanics and may provide a clue to single right ventricle failure.