The embryonic vertebrate heart tube is a dynamic suction pump

The embryonic vertebrate heart begins pumping blood long before the development of discernable chambers and valves. At these early stages, the heart tube has been described as a peristaltic pump. Recent advances in confocal laser scanning microscopy and four-dimensional visualization have warranted another look at early cardiac structure and function. We examined the movement of cells in the embryonic zebrafish heart tube and the flow of blood through the heart and obtained results that contradict peristalsis as a pumping mechanism in the embryonic heart. We propose a more likely explanation of early cardiac dynamics in which the pumping action results from suction due to elastic wave propagation in the heart tube.

Optimal vortex formation as an index of cardiac health

Heart disease remains a leading cause of death worldwide. Previous research has indicated that the dynamics of the cardiac left ventricle (LV) during diastolic filling may play a critical role in dictating overall cardiac health. Hence, numerous studies have aimed to predict and evaluate global cardiac health based on quantitative parameters describing LV function. However, the inherent complexity of LV diastole, in its electrical, muscular, and hemodynamic processes, has prevented the development of tools to accurately predict and diagnose heart failure at early stages, when corrective measures are most effective. In this work, it is demonstrated that major aspects of cardiac function are reflected uniquely and sensitively in the optimization of vortex formation in the blood flow during early diastole, as measured by a dimensionless numerical index. This index of optimal vortex formation correlates well with existing measures of cardiac health such as the LV ejection fraction. However, unlike existing measures, this previously undescribed index does not require patient-specific information to determine numerical index values corresponding to normal function. A study of normal and pathological cardiac health in human subjects demonstrates the ability of this global index to distinguish disease states by a straightforward analysis of noninvasive LV measurements.

Active myocyte shortening during the ‘isovolumetric relaxation’ phase of diastole is responsible for ventricular suction; ‘systolic ventricular filling’

OBJECTIVE

To study the 'isovolumetric relaxation' phase of rapid ventricular filling by analysis of the shortening of cardiac muscle in the endocardial and epicardial segments of the left ventricle in the dual helical model of the ventricular band, described by Torrent-Guasp.

METHODS

In 10 pigs (27-82 kg), temporal shortening by sonomicrometer crystals was recorded while recording ECG, and measuring intraventricular pressure and dP/dt with Millar pressure transducers.

RESULTS

The following sequence was observed; shortening began in descending or endocardial segment, and 82+/-23 ms later it was initiated in the epicardial or ascending segment of the band. The descending segment stops shortening during the rapid filling phase of fast descent of ventricular pressure, but the ascending segment shortening continues for 92+/-33 ms, so that active shortening continues during the period of isovolumetric relaxation. During the rapid filling phase, dopamine decreased the interval between completion of endocardial and termination of epicardial contraction from 92+/-20 to 33+/-8 ms. Conversely propranolol delayed the start of epicardial shortening from 82+/-23 to 121+/-20 ms, and prolonged the duration of endocardial contraction, causing a closer (21+/-5 ms vs 92+/-20 ms) interval between termination of contraction of endocardial and epicardial fibers. The resultant slope of the rapid descent of the left ventricular pressure curve became prolonged.

CONCLUSIONS

These time sequences show that ongoing unopposed ascending segment shortening occurs during the phase of rapid fall of ventricular pressure. These active shortening phases respond to positive and negative inotropic stimulation, and indicate the classic concept of 'isovolumetric relaxation', IVR, must be reconsidered, and the new term 'isovolumetric contraction', IVC, or systolic ventricular filing may be used.

Structure/function interface with sequential shortening of basal and apical components of the myocardial band

OBJECTIVE

To study the sequential shortening of Torrent-Guasp's 'rope-heart model' of the muscular band, and analyze the structure-function relationship of basal loop wrapping the outer right and left ventricles, around the inner helical apical loop containing reciprocal descending and ascending spiral segments.

METHODS

In 24 pigs (27-82 kg), temporal shortening by sonomicrometer crystals was recorded. The ECG evaluated rhythm, and Millar pressure transducers measured intraventricular pressure and dP/dt.

RESULTS

The predominant shortening sequence proceeded from right to left in basal loop, then down the descending and up the ascending apical loop segments. In muscle surrounded by the basal loop, epicardial muscle predominantly shortened before endocardial muscle. Crystal location defined underlying contractile trajectory; transverse in basal versus oblique in apical loop, subendocardial in descending and subepicardial in ascending segments. Mean shortening fraction average 18+/-3%, with endocardial exceeding epicardial shortening by 5+/-1%. Ascending segment crystal displacement followed descending shortening by 82+/-23 ms, and finished 92+/-33 ms after descending shortening stops, causing active systolic shortening to suction venous return; isovolumetric relaxation was absent.

CONCLUSIONS

Shortening sequence followed the rope-like myocardial band model to contradict traditional thinking. Epicardial muscle shortened before endocardial papillary muscle despite early endocardial activation, and suction filling follows active systolic unopposed ascending segment shortening during the 'isovolumetric relaxation' phase.

An In Vitro Study of Changing Profile Heights in Mitral Bioprostheses and Their Influence on Flow

In vitro assessment of different profiles for prosthetic mitral valves can result in better understanding of the physics of transmitral flow for each design. It has been postulated that decreasing the profile height of the mitral bioprosthetic valve has potential clinical benefit. In the present study, we compared the atrial and ventricular flow characteristics in different conditions using Carpentier-Edwards Perimount mitral valves with various profile heights. Each valve was placed at the intersection of the left ventricle, made of transparent silicone rubber, and the left atrium in Caltech's left heart pulsed flow simulator system. Digital particle image velocimetry has been used as the quantitative flow visualization technique. With the intention of studying the blood wash out around each valve, circulation and particle residence time were computed based on the vorticity and velocity fields around each valve, respectively. Results show that by increasing the profile's height at the atrial side of the valve, the magnitude of circulation near the atrial side of the valve decreases while particle residence time increases. However, extreme reduction of profile height in the ventricular side may increase the magnitude of circulation around the valve and decrease the particle residence time.

The role of optimal vortex formation in biological fluid transport

Animal phyla that require macro-scale fluid transport for functioning have repeatedly and often independently converged on the use of jet flows. During flow initiation these jets form fluid vortex rings, which facilitate mass transfer by stationary pumps (e.g. cardiac chambers) and momentum transfer by mobile systems (e.g. jet-propelled swimmers). Previous research has shown that vortex rings generated in the laboratory can be optimized for efficiency or thrust, based on the jet length-to-diameter ratio (L/D), with peak performance occurring at 3.5<L/D<4.5. Attempts to determine if biological jets achieve this optimization have been inconclusive, due to the inability to properly account for the diversity of jet kinematics found across animal phyla. We combine laboratory experiments, in situ observations and a framework that reduces the kinematics to a single parameter in order to quantitatively show that individual animal kinematics can be tuned in correlation with optimal vortex ring formation. This new approach identifies simple rules for effective fluid transport, facilitates comparative biological studies of jet flows across animal phyla irrespective of their specific functions and can be extended to unify theories of optimal jet-based and flapping-based vortex ring formation.

Four-dimensional cardiac imaging in living embryos via postacquisition synchronization of nongated slice sequences

Being able to acquire, visualize, and analyze 3D time series (4D data) from living embryos makes it possible to understand complex dynamic movements at early stages of embryonic development. Despite recent technological breakthroughs in 2D dynamic imaging, confocal microscopes remain quite slow at capturing optical sections at successive depths. However, when the studied motion is periodic--such as for a beating heart--a way to circumvent this problem is to acquire, successively, sets of 2D+time slice sequences at increasing depths over at least one time period and later rearrange them to recover a 3D+time sequence. In other imaging modalities at macroscopic scales, external gating signals, e.g., an electro-cardiogram, have been used to achieve proper synchronization. Since gating signals are either unavailable or cumbersome to acquire in microscopic organisms, we have developed a procedure to reconstruct volumes based solely on the information contained in the image sequences. The central part of the algorithm is a least-squares minimization of an objective criterion that depends on the similarity between the data from neighboring depths. Owing to a wavelet-based multiresolution approach, our method is robust to common confocal microscopy artifacts. We validate the procedure on both simulated data and in vivo measurements from living zebrafish embryos.

Structure function interface with sequential shortening of basal and apical components of the myocardial band

OBJECTIVE

To mechanically test the intact cardiac structure to determine the sequence of contraction within the myocardial mass to try to explain ejection and suction.

METHODS

In 24 pigs (30-85 kg), segment shortening at the site of sonomicrometer crystals was continuously recorded. The ECG evaluated rhythm, and Millar pressure transducers measured intraventricular pressure and dP/dt.

RESULTS

Study of segment shortening defined a sequence of contraction within the myocardial mass, starting at the free wall of the right ventricle and on the endocardial side of the antero-septal wall of the left. Crystal location defined underlying contractile trajectory; transverse in right ventricle followed by basal posterior left ventricle, and from the endocardial anterior wall to the posterior apical segment and finally to the epicardial side of the anterior wall. Mean shortening fraction averaged 18+/-3%, with endocardial exceeding epicardial shortening by 5+/-1%. Epicardial segment crystal displacement followed endocardial shortening by 82+/-23 ms in the anterior wall, and finished 92+/-33 ms after endocardial shortening stopped, time frame that matches the interval of fast drop of ventricular pressure and the start of suction.

CONCLUSIONS

Crystal shortening fraction sequence followed the rope-like myocardial band model to contradict traditional thinking, with two starting points of excitation-contraction, the right anterior free wall of the right ventricle, and the endocardial side of the anterior wall. Active suction may be due to active shortening of the epicardial fibers of the anterior wall, because relaxation was not detected when both mitral and aortic valves were closed during the interval previously termed 'isovolumetric relaxation'.

Can regurgitant flow damage the left atrial endothelium in patients with prosthetic mechanical heart valves?

BACKGROUND AND AIM OF THE STUDY

Previous in-vitro studies of mechanical heart valves (MHVs) in the closed position demonstrated the formation of regurgitant flows, with bubbles and jets forming vortices during each systole. The study aim was to determine whether the regurgitant flow observed in patients with MHVs can damage the left atrial endothelium, due to shear stresses exerted on the endothelial layers. This objective has been accomplished by appropriate in-vitro simulation experiments.

METHODS

In these experiments, leakage flow through several commercial MHVs was investigated. The geometry of the set-up closely resembled that of the left atrial anatomy. Water was forced through the slit of a closed MHV and directed toward the hemispherical cup coated with fluorescent paint. The flow field between the valve and the cup was photographed using high-speed videography, from which local velocities were measured, using digital particle imaging velocimetry. Qualitative damage to the surface of the cup was assessed from the amount of fluorescent paint removed from the cup.

RESULTS

The experimental results and calculations indicated that flows through the gaps of the closed valves were sufficient to generate strong vortices, with velocities near the atrial wall in the range of 0.5 to 4.0 m/s, depending on the valve. This led to high shear stresses on the left atrial wall, which far exceeded physiologically acceptable levels.

CONCLUSION

The calculated shear stresses exceeded by orders of magnitude the maximum physiologically tolerated stresses. This suggests that shear stresses associated with regurgitant jets in MHVs may damage the endothelial cells, leading to the activation of the inflammatory reaction, enhanced procoagulation, platelet activation and aggregation, and mechanical cell denudation.

Body-induced vortical flows: a common mechanism for self-corrective trimming control in boxfishes

Boxfishes (Teleostei: Ostraciidae) are marine fishes having rigid carapaces that vary significantly among taxa in their shapes and structural ornamentation. We showed previously that the keels of the carapace of one species of tropical boxfish, the smooth trunkfish, produce leading edge vortices (LEVs) capable of generating self-correcting trimming forces during swimming. In this paper we show that other tropical boxfishes with different carapace shapes have similar capabilities. We conducted a quantitative study of flows around the carapaces of three morphologically distinct boxfishes (spotted boxfish, scrawled cowfish and buffalo trunkfish) using stereolithographic models and three separate but interrelated analytical approaches: digital particle image velocimetry (DPIV), pressure distribution measurements, and force balance measurements. The ventral keels of all three forms produced LEVs that grew in circulation along the bodies, resembling the LEVs produced around delta-winged aircraft. These spiral vortices formed above the keels and increased in circulation as pitch angle became more positive, and formed below the keels and increased in circulation as pitch angle became more negative. Vortices also formed along the eye ridges of all boxfishes. In the spotted boxfish, which is largely trapezoidal in cross section, consistent dorsal vortex growth posterior to the eye ridge was also present. When all three boxfishes were positioned at various yaw angles, regions of strongest concentrated vorticity formed in far-field locations of the carapace compared with near-field areas, and vortex circulation was greatest posterior to the center of mass. In general, regions of localized low pressure correlated well with regions of attached, concentrated vorticity, especially around the ventral keels. Although other features of the carapace also affect flow patterns and pressure distributions in different ways, the integrated effects of the flows were consistent for all forms: they produce trimming self-correcting forces, which we measured directly using the force balance. These data together with previous work on smooth trunkfish indicate that body-induced vortical flows are a common mechanism that is probably significant for trim control in all species of tropical boxfishes.

Flow patterns generated by oblate medusan jellyfish: field measurements and laboratory analyses

Flow patterns generated by medusan swimmers such as jellyfish are known to differ according the morphology of the various animal species. Oblate medusae have been previously observed to generate vortex ring structures during the propulsive cycle. Owing to the inherent physical coupling between locomotor and feeding structures in these animals, the dynamics of vortex ring formation must be robustly tuned to facilitate effective functioning of both systems. To understand how this is achieved, we employed dye visualization techniques on scyphomedusae (Aurelia aurita) observed swimming in their natural marine habitat. The flow created during each propulsive cycle consists of a toroidal starting vortex formed during the power swimming stroke, followed by a stopping vortex of opposite rotational sense generated during the recovery stroke. These two vortices merge in a laterally oriented vortex superstructure that induces flow both toward the subumbrellar feeding surfaces and downstream. The lateral vortex motif discovered here appears to be critical to the dual function of the medusa bell as a flow source for feeding and propulsion. Furthermore, vortices in the animal wake have a greater volume and closer spacing than predicted by prevailing models of medusan swimming. These effects are shown to be advantageous for feeding and swimming performance, and are an important consequence of vortex interactions that have been previously neglected.

Sensitivity analysis of kinematic approximations in dynamic medusan swimming models

Models of medusan swimming typically rely on kinematic approximations to observed animal morphology to make such investigations tractable. The effect of these simplifications on the accuracy of predicted dynamics has not been examined in detail. We conduct a case study of the scyphozoan jellyfish Chrysaora fuscescens to isolate and quantify the sensitivity of dynamic models to common kinematic approximations. It is found that dynamic models exhibit strong dependence on the nature of some approximations and the context in which they are implemented. Therefore it is incorrect and potentially misleading to assume that achieving kinematic similarity in models of measured animal locomotion will necessarily provide dynamically correct models.

Mitral mechanical heart valves: in vitro studies of their closure, vortex and microbubble formation with possible medical implications

OBJECTIVE

The goal of the present work was to create the closest possible in vitro fluid dynamic environment in which prosthetic mitral valves in the patients' hearts function, in order to demonstrate whether microbubbles are generated, and if yes, under what conditions and at which stage of the cardiac cycle. Microbubbles were observed in the blood of patients with mitral mechanical heart valves (MHV) by means of echocardiography. The phenomenon, often referred to as high-intensity transient signals (HITS), appears as bright, intense, high-velocity and persistent echoes detected by Doppler echocardiography at the instant of valve closure. The question is no longer whether microbubbles are being formed in patients with MHV. as an inherent aspect of their design, but rather how they evolve and when. The answer to this question was the objective of the present paper.

METHODS

Hemodynamic conditions in which microbubbles were observed in patients with mitral MHV were simulated in our laboratory. We were able to describe the bubble formation process, as one consisting of nucleation and microbubble growth. While mild growth of nuclei is governed by diffusion, extensive growth of microbubbles is controlled by pressure drop during deceleration of the leaflets on the housing on the atrial side of the mitral MHV.

RESULTS

The present study has shown that bubbles form in a fluid at the instant of closure of mechanical valves. The formation of vortices after valve closure, although clinically not yet observed, was also demonstrated in the present in vitro studies. We believe that impact of such vortices on the endothelial layer of the left atrial wall may have clinical significance. These two phenomena were not observed in bioprosthetic valves.

CONCLUSIONS

As demonstrated, there exist two distinct phenomena characteristic of mechanical heart valves, which take place during valve closure, namely, that of vortex formation and that of microbubble growth. Both phenomena may have far reaching clinical implications.

Flow in the early embryonic human heart: a numerical study

Computational fluid dynamic (CFD) experimentation provides a unique medium for detailed examination of flow through complex embryonic heart structures. The purpose of this investigation was to demonstrate that streaming blood flow patterns exist in the early embryonic heart and that fluid surface stresses change significantly with anomalous alterations in fetal heart lumen shape. Stages 10 and 11 early human embryo hearts were digitized as calibrated two-dimensional (2D) cross-sectional sequential images. A 3D surface was constructed from the stacking of these 2D images. CFD flow solutions were obtained (steady and pulsatile flow). Particle traces were placed in the inlet and outlet portions of these two stages. Sections of the embryonic heart were artificially reshaped. CFD flow solutions were obtained and surface stress changes analyzed. Streaming was shown to exist, with particles released on one or the other side of the cardiac lumen tending not to cross over and mix with particles released from the opposite side of the cardiac lumen. Shear stress changes (stage 10) occur in the altered lumens. Streaming exists in steady and pulsatile flow scenarios in the embryonic heart models. There are differences in local shear stress distributions with surface shape anomalies of the fetal heart lumen. These observations may help shed light on the potential role of fluid dynamic factors in determining patterns of abnormal heart development.

Correlation between negative near-wall shear stress in human aorta and various stages of congestive heart failure

The critical effect of advanced congestive heart failure is reduced blood flow in descending aorta resulting from mild to severe reduction in cardiac output, usually accompanying low ejection fraction. In these patients the heart tries to compensate by beating faster, but reduced blood flow combined with increased heart rate can lead to retrograde flow and negative shear stress along the vessel walls during each cardiac cycle. Our studies show that near-wall negative shear stress can result from an entire-retrograde flow at normal heart rates or a Womersley-type phase delayed near-wall retrograde flow at high heart rate and low ejection fraction conditions. In our experiments, a compliant aortic loop with appropriate pressure and flow instrumentation was used, running on either various aqueous glycerin solutions or property filtered, anticoagulated diluted bovine blood. The flow field was mapped using a General Electric Vingmed System 5 platform. The resulting images were analyzed with Caltech's digital ultrasound speckle image velocimetry technique. We showed the occurrence of near-wall retrograde flow under certain aortic flow rates and frequencies, charted via an empirical relationship between Reynolds and Womersley numbers. Also, we demonstrated a strong correlation between retrograde flow level and transition from preliminary to advanced congestive heart failure patients.