Intracardiac Flow Patterns Research Articles

Study of intracardiac flow patterns of left ventricle in patients with premature ventricular complexes from the right ventricular outflow tract

Objective To evaluate the patterns of intracardiac flow of left ventricle (LV) in patients with premature ventricular complexes (PVCs) from the right ventricular outflow tract (RVOT) by analyzing the vortex during isovolumetric contraction phase and the distribution rules of flow-time curves in each layer of LV.Methods Twenty-seven patients with frequent isolated PVCs from RVOT were involved and 25 healthy subjects as control.The color Doppler image of LV at apical four-chamber view was acquired.Vector flow mapping (VFM) was performed to assess the parameters of vortex during isovolumic contraction phase, including diameter (transverse and vertical diameter), velocity (maximal positive and negative velocity) and the number of vortex rings.Positive flow during systole and negative flow during diastole of LV in each layer were measured by flow-time curve.All values of patients with PVCs were recorded during sinus beats (PVC-S) and PVC (PVC-V) respectively.Results Significant differences were demonstrated in all parameters of vortex between the PVC-V and control subjects.And the flow-time curves disarrayed in PVC-V.The velocity of vortex in PVC-S was lower than that in control subjects.And the distribution pattern of flow-time curves in LV of PVC-S differed from that of control subjects.Conclusions Alternation of intracardiac fluid pattern in LV was demonstrated in patients with PVCs from RVOT during both sinus beats and PVC.VFM can be used to analyze the intracardiac flow field in normal and pathological electrical activation. Key words: Echocardiography; Ventricular premature complexes; Ventricular function, left; Hemodynamic phenomena

Quantification of presystolic blood flow organization and energetics in the human left ventricle

Intracardiac blood flow patterns are potentially important to cardiac pumping efficiency. However, these complex flow patterns remain incompletely characterized both in health and disease. We hypothesized that normal left ventricular (LV) blood flow patterns would preferentially optimize a portion of the end-diastolic volume (LVEDV) for effective and rapid systolic ejection by virtue of location near and motion towards the LV outflow tract (LVOT). Three-dimensional cine velocity and morphological data were acquired in 12 healthy persons and 1 patient with dilated cardiomyopathy using MRI. A previously validated method was used for analysis in which the LVEDV was separated into four functional flow components based on the blood's locations at the beginning and end of the cardiac cycle. Each component's volume, kinetic energy (KE), site, direction, and linear momentum relative to the LVOT were calculated. Of the four components, the LV inflow that passes directly to outflow in a single cardiac cycle (Direct Flow) had the largest volume. At the time of isovolumic contraction, Direct Flow had the greatest amount of KE and the most favorable combination of distance, angle, and linear momentum relative to the LVOT. Atrial contraction boosted the late diastolic KE of the ejected components. We conclude that normal diastolic LV flow creates favorable conditions for ensuing ejection, defined by proximity and energetics, for the Direct Flow, and that atrial contraction augments the end-diastolic KE of the ejection volume. The correlation of Direct Flow characteristics with ejection efficiency might be a relevant investigative target in cardiac failure.

An Immersed Boundary Heart Model Coupled with a Multicompartment Lumped Model of the Circulatory System

A new computational model of the circulatory system is developed to investigate the intracardiac blood flow patterns and the motion of the mitral valve. In this work, we couple an existing two-dimensional model for the left heart with a multicompartment lumped model for the whole circulation to analyze the hemodynamics of the normal circulation in humans. The two-dimensional left-heart model is based on the immersed boundary method, and the lumped circulation model is governed by a system of ordinary differential equations. We investigate the intraventricular velocity field and the velocity curves over the mitral ring, between the valve leaflets, and across the aortic outflow tract. The flow and pressure curves are also measured in the left and right hearts and the systemic and pulmonary arteries. Our simulation results are in reasonably good agreement with ones in the literature and comparable to the existing magnetic resonance data, despite the inherent deficiency of a two-dimensional modeling of the left heart.

Dependence of Aortic Arch Morphogenesis on Intracardiac Blood Flow in the Left Atrial Ligated Chick Embryo

Partial left atrial ligation before cardiac septation redistributes intracardiac blood flow and produces left ventricular hypoplasia in the chick. We hypothesized that redistributed intracardiac blood flow adversely alters aortic arch development. We ligated the left atrial appendage with a 10-0 nylon suture at stage 21 chick embryos, then reincubated up to stage 34. Sham embryos had a suture tied adjacent to the atrial wall, and normal controls were unoperated. We measured simultaneous atrioventricular (AV) and dorsal aortic (DAo) blood velocities from stage 24 embryos with an ultrasound pulsed-Doppler flow meter; and the left and right third and fourth aortic arch blood flow with a laser-Doppler flow meter. Ventricular and atrial cross-sectional areas were measured from sequential video fields for planimetry. Intracardiac flow patterns were imaged on video by injecting India ink into the vitelline vein. In separate embryos, radiopaque microfil was injected into the cardiovascular system for micro-CT scanning. We analyzed the morphologic characteristics of the heart at stage 34. Active AV and DAo stroke volume (mm(3)), right third and fourth aortic arch blood flow (mm(3)/s) were all decreased in ligated embryos (P < 0.05) when compared with normal and sham embryos. Ventricular end-diastolic volume versus normal and sham embryos decreased by 45% and 46%, respectively (P < 0.05). India ink injection revealed altered right aortic arch flow patterns in the ligated embryos compared with normal embryos. micro-CT imaging confirmed altered arch morphogenesis. Alterations in intracardiac blood flow disrupt both early cardiac morphogenesis and aortic arch selection.

Following the Flow in Chambers

This article summarizes the current progress in imaging the sequence of blood flow inside the cardiac chambers. Observations from experimental models and computational fluid dynamics have been compared with in vivo findings from phase contrast MRI, high-resolution Doppler imaging, and echo particle imaging velocimetry. The discussion includes chamber and valve geometry and its role in asymmetric flow propagation with an emphasis on the cardiac mechanical sequence that helps stabilize the redirected blood streams. Mapping intracardiac flow patterns in health and disease provides a rich and challenging field of research that warrants careful application in future investigations.

Daniel Kalmanson, pioneer of intracardiac Doppler exploration in 1969: Emphasis on the first mitral and tricuspid flow velocity recordings

Intracardiac Doppler flow patterns remained unexplored until 1969, when Daniel Kalmanson could specify the features and physiological interpretation of intracardiac traces by mounting the new directional continuous-wave Doppler device on a catheter-tip. Within a year of time, he defined the continuum of changes from a three-waved venous inlet pattern (two positive S and D, one negative A) to a single systolic wave S at the arterial outlet. Moreover, the first descriptions of mitral and tricuspid Doppler flow traces were reported on man from 1969 (right heart) to 1972 (left heart). Pathophysiologic significance of their fundamental changes in pattern was specified in patients. Major clinical advances resulted from the integration of flow phenomena into physician’s medical reasoning. (E-mail: colette.veyrat.resedal@noos.fr)

Acutely altered hemodynamics following venous obstruction in the early chick embryo.

In the venous clip model specific cardiac malformations are induced in the chick embryo by obstructing the right lateral vitelline vein with a microclip. Clipping alters venous return and intracardiac laminar blood flow patterns, with secondary effects on the mechanical load of the embryonic myocardium. We investigated the instantaneous effects of clipping the right lateral vitelline vein on hemodynamics in the stage-17 chick embryo. 32 chick embryos HH 17 were subdivided into venous clipped (N=16) and matched control embryos (N=16). Dorsal aortic blood flow velocity was measured with a 20 MHz pulsed Doppler meter. A time series of eight successive measurements per embryo was made starting just before clipping and ending 5h after clipping. Heart rate, peak systolic velocity, time-averaged velocity, peak blood flow, mean blood flow, peak acceleration and stroke volume were determined. All hemodynamic parameters decreased acutely after venous clipping and only three out of seven parameters (heart rate, time-averaged velocity and mean blood flow) showed a recovery to baseline values during the 5h study period. We conclude that the experimental alteration of venous return has major acute effects on hemodynamics in the chick embryo. These effects may be responsible for the observed cardiac malformations after clipping.

Elective primary repair of acyanotic tetralogy of Fallot in early infancy: overall outcome and impact on the pulmonary valve

OBJECTIVESWe sought to determine if early primary repair of acyanotic tetralogy of Fallot (ToF) can be performed safely with low requirement for transannular patching (TAP) and thereafter allow normal right ventricular outflow tract (RVOT) growth.BACKGROUNDEarly primary repair of ToF normalizes intracardiac flow patterns, which may allow subsequent normal RVOT growth. Traditionally repair is deferred until symptoms occur or children are deemed of adequate size for operative risk to be acceptable because of a perceived increased requirement for TAP in small infants.METHODSBetween July 1992 and March 1999, 42 acyanotic infants aged 4 to 87 days (median 62) and weight 2.6 to 6.6 kg (median 4.55) underwent complete repair of ToF. Pulmonary annulus measured 4 to 10.5 mm (median 6.5) with “z-value” of −5.6 to +3.0 (median −1.9). RVOT reconstruction was tailored to each patient; pulmonary valvotomy was performed in 26, main pulmonary arterioplasty in 22, and infundibular patching in 2. Only 10 (24%) required TAP.RESULTSPostoperative RVOT gradient was 0 to 30 mm Hg (median 10) and pRV/pLV ratio 0.3 to 0.6 (median 0.44). Pulmonary insufficiency was trivial/mild. There were no deaths. Junctional ectopic tachycardia developed in seven; only one required treatment. ICU stay was 2 to 14 days (median 4) and hospital stay 4 to 22 days (median 7). At follow-up 12 to 64 months later (median 38) there were no deaths. One child required reoperation for recurrent RVOT obstruction and two required balloon pulmonary arterioplasty. Follow-up RVOT gradient was 0 to 36 mm Hg (median 12), unchanged from early postoperative condition, and median z-value was −1.2 (−2.8 to +2.5); pulmonary insufficiency remained trivial/mild.CONCLUSIONSComplete repair of acyanotic ToF can be performed in early infancy with low morbidity and mortality and low requirement for TAP. Though results are not statistically significant, early repair may allow normal RVOT growth thereafter.

Extraembryonic venous obstructions lead to cardiovascular malformations and can be embryolethal.

To expand our knowledge concerning the effect of placental blood flow on human heart development, we used an embryonic chicken model in which extraembryonic blood flow was manipulated. First, one of the three major vitelline veins was ligated, while blood flow was visualized with Indian ink. In this way, we could study the effect of different ligation positions on intracardiac flow patterns. Secondly, these vitelline veins were ligated permanently with a microclip until cardiac septation was completed, thereafter, the hearts were morphologically evaluated. In this way, we could study the impact of the ligation position on the severity and frequency of heart malformations. On combining the results, we were able to study the effect of different intracardiac flow patterns on heart development. Although ligation of each vein resulted in different intracardiac flow patterns, long-term ligation resulted in similar cardiovascular malformations in survivors. These consisted mainly of ventricular septum defects (VSDs), semilunar valve anomalies, and pharyngeal arch artery malformations. There was no significant difference (p > 0.05) between the ligation position and the incidence of cardiovascular malformations. However, the percentage mortality after clipping the left lateral vitelline vein was significantly higher (p < 0.05) than after ligation of either the right lateral or posterior vitelline vein. Early extraembryonic venous obstruction leads to altered flow patterns, which probably result in shear stress changes. In postseptation stages, these result in a spectrum of cardiovascular malformations irrespective of the ligation position. A diminished incidence of VSDs in the oldest stage was attributed to delayed closure of the interventricular foramen.

Foetal bypass: concepts and controversies

One of the most controversial and challenging surgical undertakings of the next century promises to be foetal cardiac surgery. Animal studies have been underway for several years to gain an understanding of the physiological mechanisms required to achieve this undertaking. Not since the days of crosscirculation has there been a maternal risk associated with open-heart surgery. The diagnosis of congenital heart defects with foetal ultrasound can now be made as early as 12 weeks gestation. Simple cardiac abnormalities, such as valvular stenosis or atresia, alter intracardiac flow patterns and affect normal cardiac chamber development. Without early intervention, these complex lesions often require major surgical reconstruction, beginning in the neonatal period. Foetal cardiac bypass techniques have evolved from the use of roller pumps and bubble oxygenators primed with maternal blood to the use of an axial flow pump incorporated in a right atrial to pulmonary artery or aortic shunt. Because the blood entering the right atrium is oxygenated by the placenta, an oxygenator in the bypass circuit is probably not needed. The low prime axial flow pump system avoids the dilution of the foetus with the maternal adult haemoglobin and improves the outcome. A major focus of research has concentrated on maintenance of placental blood flow with the use of vasodilators and cyclooxygenase inhibitors. Investigation with primates will be necessary to confirm the placental physiology before human operations can be performed. As the foetal bypass challenges are overcome, there is the potential for a reduction in the number of complex cardiac lesions requiring early surgical intervention in the twenty-first century.

A novel method for determining intracardiac flow patterns and pressure gradients using time-resolved 3D MRI data

A novel method for determining intracardiac flow patterns and pressure gradients using time-resolved 3D MRI data

Unilateral vitelline vein ligation alters intracardiac blood flow patterns and morphogenesis in the chick embryo.

To study the role of blood flow in normal and abnormal heart development, an embryonic chicken model was developed. The effect of altered venous inflow on normal intracardiac blood flow patterns was studied by visualization of blood flow with India ink. At stage 17, India ink was injected into a capillary or small venule within a specific yolk sac region. After determination of the normal intracardiac flow pattern, the right lateral vitelline vein was ligated, and the new intracardiac flow pattern was studied. Ligation resulted in disturbance of normal intracardiac flow patterns, which was most obvious in the conotruncus. The long-term effect of these abnormal intracardiac flow patterns on the development of the heart and pharyngeal arch arteries was investigated by permanent ligation in ovo with a microclip at stage 17 and subsequent evaluation at stages 34, 37, and 45. These experiments revealed anomalies of the vascular system in 58 of the 91 ligated embryos studied. We observed intracardiac malformations consisting of subaortic ventricular septal defects (n = 52), semilunar valve anomalies (n = 19), atrioventricular anomalies (n = 7), and pharyngeal arch artery malformations (n = 32). It is concluded that abnormal intracardiac blood flow, resulting from hampered venous inflow, may result in serious intracardiac and pharyngeal arch artery malformations comparable to defects observed in embryonic chicken models subjected to neural crest ablation, cervical flexure experiments, and excessive retinoic acid treatment.

Cervical flexion: its contribution to normal and abnormal cardiac morphogenesis.

Although His in 1881 and Patten in 1922 suggested that cervical flexion could play an important role in normal cardiac morphogenesis, until recently this hypothesis has been largely neglected. The purpose of this report is to present data indicating that prevention of cervical flexion leads to double outlet right ventricle (DORV), which is unrelated to any affect on neural crest cell migration. At stage 11-12, suture material was inserted into the neural tube to prevent cervical flexion. Six out of 22 experimental embryos survived until stage 38 and 5 out of 6 had DORV. Neither abnormalities of the aortico-pulmonary septum nor interruption of the aortic arch were observed at dissection. Hemodynamic studies performed at stage 18 revealed distinctive characteristics that are inconsistent with hemodynamic studies previously reported following neural crest ablation. With respect to immunohistochemical studies using neural crest associated antigen HNK-1 antibody, normal migration of neural crest cells was noted in the outflow tract in the experimental embryos at stage 22. These hemodynamic and immunohistochemical studies suggest that insertion of suture material into the neural tube at stage 11-12 does not jeopardize neural crest migration. We propose that reduction in cervical flexion increases the distance between the future aorta and the left ventricle, which prevents the transition of intracardiac flow pattern from a serial circulation to a parallel one, leaving persistence of a "double outlet" from the right ventricle.

Colour flow Doppler echocardiography in normal horses.

Colour flow Doppler echocardiography is a technique that is used with two-dimensional (2-D) echocardiography to study blood flow patterns in the heart and blood vessels. This method was used to define normal flow patterns and to evaluate valvular function in 40 clinically normal Thoroughbred and Thoroughbred cross horses. Flow patterns from 10 standardised echocardiographic images were described in relation to anatomic landmarks and timing during the cardiac cycle. Consistent intracardiac flow patterns were identified in the normal horses. High velocity flow signals or regurgitant jets were recorded at the tricuspid (77.5%), mitral (67.5%), aortic (47.5%) and pulmonary valves (40%) in clinically normal horses. Most of these signals were transient, and many were associated with valve closure. This study demonstrates that colour flow Doppler echocardiography is a sensitive technique for the detection of intracardiac flow in horses. It will provide a basis by which to compare studies in horses suspected of having valvular heart disease.

Intracardiac blood flow patterns related to the yolk sac circulation of the chick embryo.

Intracardiac flow patterns during heart development were studied by injection of india ink into the yolk sac circulation of chick embryos at Hamburger-Hamilton stages 10 to 17. We injected india ink into a small venule or capillary, carefully preventing application of overpressure to the vascular system, and recorded the intracardiac route by video. From stage 12 onward, blood flow was laminar, and separate intracardiac currents were visualized. The yolk sac was divided into a left and a right half. Blood coursed through each half in concentric loops, ranging from the marginal sinus to the sinus venosus. This parallel array persisted within the heart. Bilateral to the embryo, two lateral regions arose that extended wedge-like within each half, resulting in six equally sized yolk sac regions at stage 16. The process of heart looping was not accompanied by a change in flow pattern. However, developmental changes of the yolk sac circulation were reflected in alteration of the intracardiac flow pattern. From stage 16 onward, the intracardiac flow pattern was no longer determined by the left- or right-hand side of the yolk sac but by bilateral anterior, lateral, and posterior regions of the yolk sac. Blood from the lateral regions of the yolk sac was preferentially distributed to the head. The results show that in preseptation stages a relatively stable flow pattern is present. We suggest that alterations in blood flow could influence the process of normal heart development.

Transesophageal Echocardiographic Findings of Intracardiac Events During Cardiac Assist

Transesophageal echocardiography (TEE) is an easy, noninvasive and reproducible method for evaluation of left ventricular function, coronary flow patterns, and intracardiac blood flow patterns during cardial assist. TEE facilitated better management of the patients with LVAD and gave us the various information: (a) Observing the drawing catheter tip by TEE, it could be settled just above the mitral orifice to obtain the maximum bypass flow; in addition, dislocation of the catheter tip could be easily diagnosed. (b) The intracardiac thrombus could be clearly detected by TEE; it was removed without complications, weaning from left ventricular assist device (LVAD). (c) The left ventricular wall motion was periodically observed. The end systolic LV diameter (Ds) was decreasing and fractional shortening (%FS) was increasing as the natural heart recovery. Additionally, the preejection period (PEP) and the ejection time (ET) were measured. The ratio of PEP/ET was decreasing gradually. Ds did not alter with preload change during on-off testing unlike the other parameters. Ds seems to be the most reliable parameter for the natural heart recovery within certain amounts of bypass flow. (d) The flow velocity in the left anterior descending coronary artery could be measured by transesophageal Doppler method; it was clinically shown that counterpulsation produced a larger amount of coronary blood flow than copulsation.

Doppler Echocardiographic Detection of a Ruptured Acquired Aneurysm of the Sinus of Valsalva: Clinical-Morphologic Correlations

Two-dimensional, pulsed Doppler echocardiographic and pathologic features of an unusual form of ruptured aneurysm of the sinus of Valsalva are presented. The presence of an aneurysm of the left sinus of Valsalva protruding into the left atrium complicating acute aortic valvular endocarditis was detected by two-dimensional echocardiography. Rupture of the aneurysm of the sinus of Valsalva into the left atrial cavity was suggested by pulsed Doppler echocardiography. The size and location of the site of the rupture precluded recognition by two-dimensional echocardiography or contrast angiography. This report illustrates the unique value of pulsed Doppler echocardiography to define the location and direction of intracardiac flow patterns in evaluating patients with an aneurysm of the sinus of Valsalva and suspected rupture.

Advances in noninvasive cardiovascular imaging: implications for the anesthesiologist.

We have presented a review of recent advances in medical imaging which are relevant to the practice of anesthesia and associated research. The appropriate interpretation and use of the information derived from these noninvasive technologies can prevent unnecessary morbidity and mortality. Echocardiography remains the most advanced tool for noninvasive cardiac imaging because of its applicability for most cardiac disorders and its exquisite spatial resolution. Two-dimensional systems produce real time, dynamic, qualitative assessments of cardiac chamber morphology, size, thickness, and performance. The development of transesophageal echocardiography has brought this imaging power into the operating room for use by anesthesiologists. Recently developed quantitative and color-coded Doppler techniques will reveal intracardiac flow patterns and their alterations by anesthetics and surgery. These advantages are partially offset by inherent difficulties in quantifying echocardiographic data, and the need for highly trained operators for image reproduction. Nuclear cardiology and echocardiology are highly complementary. The scintigraphic methods identify myocardium at risk for infarction, confirm infarction when present, and produce quantitative, highly reproducible estimates of ventricular filling and performance. Time required to obtain data can be very brief for first-pass techniques, and these data are ideally suited for computer processing. Equilibrium studies require a larger dose of radioactive material, but provide excellent assessment of segmental wall motion. Preoperative studies with dipyridamole and Tl can indicate the patients truly at high risk for perioperative myocardial infarction. Monitoring and intensive care efforts may be better allocated with this information. No new technology in the past decade has stirred as much interest among clinicians as magnetic resonance imaging. Like echocardiography, it uses no ionizing radiation and is entirely noninvasive. But, unlike other imaging techniques, it utilizes multiple tissue characteristics to provide quick, highly resolved, tomographic images. Since bone is invisible to the magnetic resonance scanner, tissues inside bony structures are often best revealed with MRI. Nonimaging studies, i.e., spectroscopic data not spatially encoded, may prove to be the most important research currently underway in this field. In vivo estimates of intracellular functions, enzyme kinetics, and drug kinetics and metabolism are already in progress. The effects of anesthetic in the central nervous system and other organs may be explored in ways previously not possible.(ABSTRACT TRUNCATED AT 400 WORDS)

Two-dimensional contrast echocardiography with carbon dioxide in the detection of congenital cardiac shunts

Medically pure (100%) carbon dioxide directly injected into a peripheral vein was used for 2-dimensional contrast echocardiography in 134 patients with an arteriovenous shunt demonstrated by cardiac catheterization and cineangiography, Qp Qs ratios of 1.5 to 3.7, pulmonary-to-systemic peak systolic pressure ratios of 0.2 to 0.8 and no oximetrically demonstrable venoarterial shunt. Two patients with transposition of the great arteries, intact ventricular septum and a Senning operation as well as 30 normal subjects of comparable age also were studied. In patients with an atrial septal defect, the gas microbubbles opacified the left atrium. In patients with a ventricular septal defect, the gas microbubbles opacified the left ventricle, whereas the left atrium was free of contrast. In all patients with patent ductus arteriosus, the gas microbubbles opacified the abdominal aorta, whereas the left atrium, left ventricle, aortic root and aortic arch remained free of contrast. In 2 patients in whom an aneurysm of the sinus of Valsalva ruptured into the right ventricle, the “negative” contrast effect permitted localization of the shunt. In 2 patients with transposition of the great arteries, an intact ventricular septum and a Senning operation, the intracardiac flow pattern was clearly demonstrated. No complication was observed. We conclude that pure carbon dioxide directly injected into a peripheral vein is a safe and advantageous echocardiographic contrast material. Because of its greater diffusibility in comparison with oxygen and fluid contrast media, small venoarterial shunting can be detected in defects such as atrial septal defect, ventricular septal defect and patent ductus arteriosus, in which only an arteriovenous shunt can be demonstrated by oximetry.

Intracardiac flow patterns in early embryonic life. A reexamination.

Microangiography, using methylene blue injected at eight vitelline vein sites, was performed on 156 developing chick embryos at Hamburger-Hamilton stages 14-22. Two stream patterns were observed. Type A coursed sequentially through the dorsal portion of the sinus venosus, the cranial segments of the primitive atrium and atrioventricular canal, the ventral parts of the primitive ventricle and conus cordis, and, finally, the left branchial arches. Type B coursed through the ventral portion of the sinus venosus, the caudal segments of the primitive atrium and atrioventricular canal, the dorsal parts of the primitive ventricle and conus cordis, and, finally, the right branchial arches. Both streams flowed in parallel fashion in the conus cordis. At Hamburger-Hamilton stages 17-18, the dye stream from the right lateral vitelline vein was chiefly type A, whereas that from the left lateral vitelline vein was type B. At Hamburger-Hamilton stages 19-22, those patterns reversed, i.e., the right lateral vitelline vein stream ran as type B, whereas the left lateral vitelline vein stream assumed type A pattern. The cranial-caudal relationship of the two streams at the primitive atrium and atrioventricular canal is not consistent with the hypothesis that these streams separately expand the future right atrium and left atrium. Their parallel direction at the conus cordis does not support the theory that spiral septation is initiated by two spiral streams. The longitudinal separation of the two streams at and beyond the branchial arches also argues against aortico-pulmonary septation as a consequence of flow streaming. Our observations do not support the traditional flow-molding theory.