Y. Goose. Mount Senario College.
A second model generic doxycycline 200mg without prescription antibiotics for acne pregnancy, where operating teams pay a single visit to a developed country may again provide benefit for a limited number of children locally purchase doxycycline without prescription recommended antibiotics for acne, although it often does little to boost local infrastructure and skills and may degenerate into surgical tourism. It is likely that greater benefits will be derived from a third approach, which focuses on long-term support and education (80). It is essential that these projects are established in a thoughtful and structured manner. First, a site visit to the center requesting support will be useful to assess the local political situation, local demographics, hospital infrastructure, and allied health support. A careful assessment needs to be undertaken of all personnel already involved in the program and key individuals who are likely to be opinion-makers need to be identified. Second, it may be useful to provide key individuals from the requesting center with an opportunity to spend between 3 weeks and 3 months at the host institution, learning the culture and capabilities of a modern heart center. Third, there needs to be significant strengthening of local infrastructure, which may include biomedical engineering, consolidation of a pure water supply, electricity, etc. Fourth, a long-term philanthropic partner needs to be identified and a fundraising strategy needs to be developed. It is only when there is adequate infrastructure in place and any necessary capital equipment has been obtained through philanthropy, that the first active treating- team visit should occur. After this first treating visit, a long-term plan for continued support will need to be developed, with, minimally, teams visiting at least once every 6 months and educational fellowships to the supporting institution continuing (80). Essential factors for a successful partnership include (1) commitment to work together in a spirit of collaboration, (2) commitment to track and measure the results of the program, (3) commitment to establish an acceptable structure through which resources can be directed, (4) commitment to build local governmental and community support. It has been suggested that optimally, a center selected for development should have an existing cardiac program (performing approximately 100 cardiac cases per year), with active core personnel and related allied healthcare providers in place. It should be recognized as a center of excellence locally and should have the support of the wider local institution to enter into a multiple-year relationship (81). Careful attention must be paid to the development and selection of infrastructure and capital equipment in a resource-limited environment. Contracts may need to be renegotiated for consumables, including valves, sutures, etc. Many patients present late so that, in a study of patients undergoing surgery for large ventricular septal defects in South India, the average age at surgery was 7. About one in four of these patients had a preoperative lung infection, with some requiring preoperative ventilation, which resulted in a longer hospital stay and requirement for intensive care. Malnutrition is common and frequently those who with preoperative growth failure have suboptimal recovery of somatic growth even after successful surgery (83,84). Infectious complications before or after surgery are common and important determinants of outcome, with one study demonstrating that of 330 neonates who underwent cardiac surgery, 70 (21. Chagas Disease Chagas disease (American trypanosomiasis) is caused by infection with the protozoa Trypanosoma cruzi. Infection is most commonly transmitted to man by blood-feeding triatomine bugs, but nonvectorial modes of transmission are also common through, for example, mother-to-child transmission, blood transfusion, and oral transmission from eating contaminated food. Today the disability adjusted life years resulting from Chagas disease render this condition one of the leading tropical infections in the Western Hemisphere (86). It is now recognized that the condition occurs in three stages as a result of vector- associated transmission. The first, acute phase represents the entry of the parasite and invasion into the bloodstream, during which most patients are mildly symptomatic or asymptomatic. The second, indeterminate phase follows, during which a patient is asymptomatic for often many years, although serology for T. The third phase of chronic complications occurs in approximately 20% to 30% of patients often many years after the initial attack and is manifest by serious cardiac (Chagasic cardiomyopathy) and gastrointestinal (e. Chronic chasic cardiomyopathy is the main cause of death in patients infected with T. Indeed, Chagas disease is the most common cause of cardiomyopathy in South and Central America and the leading cause of cardiovascular death in endemic areas (88). Consequently, Chagas disease is the third leading indication for heart transplantation in Brazil (89). Because of the scarcity of parasites in the cardiomyopathic heart, it was long- considered that Chagasic cardiomyopathy represented an autoimmune disease directed against self-epitopes showing cross-reactivity with parasitic antigens. Together, these and other recent observations suggest that the myocardial inflammation in Chagas disease is more likely to represent a direct response to locally persisting parasites (in the amastigote form) within the myocardium. Such findings guide treatment approaches with anti-parasitic drugs that are cidal for T. The link between Chagas disease and poverty relates largely to housing of poor quality, which facilitates invasion by triatomines, lack of access to health care, as well as migration of humans into habitats where T. As a result of deforestation for agriculture in Latin America triatomines that were unable to feed because of displacement of wild animals started to colonize areas around and within human homes, adapting to feed on domestic animals and humans as a zoonosis. Among the first systematic investigations of the worldwide prevalence of Chagas disease were in the 1980s when it was determined that there were likely to be more than 18 million cases in 21 endemic countries with 100 million people at risk of infection. This reduction relates in part to reductions in new cases through vector control program, especially in affected areas of the so-called P. Both diseases are associated with health disparities, disproportionally affecting those living in poverty; both are chronic conditions requiring prolonged, expensive treatment; both can be associated with maternal-to-child-transmission and congenital infection. The recent spread of Chagas disease into previously uninfected areas, secondary to migration (often illegal) of infected individuals, adds the additional burden of stigma to this condition. There is increasing recognition of a profound change in the transmission and epidemiology of Chagas disease in recent years. While vectorial transmission has predominated in the past, other modes of transmission, particularly blood transfusion, are becoming increasingly important and are emerging as a most common mode of transmission in Brazil. Patients with advanced Chagas cardiomyopathy who undergo cardiac transplantation require specific treatment to prevent disease reactivation, which occurs in up to 20% (89). Maternal-to-child transmission is also increasingly recognized as an important route of infection. The large rural to urban migration has the effect that there are now large numbers of people with Chagas disease living in cities in Latin America not previously considered to be endemic for the parasite. Thus, for example in Santa Cruz, the largest city in Bolivia, infection is found in 60% of patients with heart disease and in up to 20% of women presenting for delivery (102). The increasing migration and travel of individuals from endemic to nonendemic countries has resulted in increasing reports of Chagas disease across the world (103). It is estimated that there are at least 300,000 (104) and possibly up to 1 million (99) people infected with T. As a result, several nonendemic countries have instituted comprehensive blood bank and organ screening for T. More recent information from studies in Texas indicate that a significant percentage of cases of Chagas disease in the United States results from autochthonous transmission (106,107). However, the percentage of cases of Chagas disease in the United States that result from autochthonous transmission versus important from immigration is not known.
Even in those patients who are normotensive at rest purchase generic doxycycline bacteria quizlet, a hypertensive response to exercise may be associated with left ventricular hypertrophy and abnormal vascular function (116 order 100 mg doxycycline with amex antibiotic resistance originates by,117,118). Endothelial dysfunction, reduced vessel elasticity, and enhanced baroreceptors may all play a role in the development of chronic systolic hypertension and the commonly found systolic hypertensive rise to graded dynamic or isometric exercise (119,120,121). Previous information regarding cardiac catheterizations is also important, particularly in patients who have had balloon dilation of native coarctation or dilation of recurrent/residual coarctation. The presence of an upper-to- lower extremity blood pressure gradient should alert the physician to the presence of a possible residual coarctation. Maximal exercise testing is useful to assess the blood pressure response to exercise in these patients. This may be related to residual abnormal vascular reactivity that may be seen in these patients as stated above. Leisure Activities and Activities of Daily Living Many studies have examined exercise performance in patients with repaired coarctation of the aorta but longitudinal data regarding the risk of intense exercise participation and training are sparse. Those patients with hypertension in the absence of residual coarctation should follow the recommendations listed later in this chapter for systemic hypertension. Patients with a bicuspid aortic valve should follow the recommendations for bicuspid valves in Table 10. Competitive Sports Patients with isolated coarctation of mild degree (<20 mm Hg systolic blood pressure gradient) may participate in all sports; however, activities that have a maximally strenuous isometric component should probably be discouraged. Patients with residual obstruction should be referred for either catheter-based or surgical intervention prior to participating in competitive sports (82). Resting or exercise-induced hypertension in the absence of a residual gradient should be treated as discussed in the section on systemic hypertension. As with recreational activities, competitive sports in patients with repaired coarctation and bicuspid aortic valve should defer to the section on bicuspid aortic valve. The degree of obstruction is variable, but is typically mild and may regress spontaneously. More advanced obstruction results in right ventricular hypertrophy and/or strain, and if left untreated, can result in exercise intolerance (122), and/or atrial arrhythmias secondary to right atrial dilation. Most patients with advanced obstruction benefit from intervention, typically balloon valvuloplasty. Freedom from reintervention and exercise capacity have been reported to be quite favorable; however, the long-term impact of chronic pulmonary regurgitation as a result of the intervention remains to be seen (123,124). Moderate (30 to 50 mm Hg peak gradient) stenosis may be well tolerated in children and adolescents and rarely effects performance. However, decreased exercise capacity may be seen in young and middle age adults even in the presence of preserved right ventricular systolic function. Performance in both moderate and severe stenosis typically improves after intervention (125,126,127). Further studies will depend on the severity of the stenosis and the extent of any associated additional cardiac abnormalities. Similar recommendations apply for patients with moderate stenosis and no more than moderate regurgitation. However, exercise testing is useful in this patient population, especially in older patients, for the reasons stated above. These patients may benefit from a formal exercise prescription to help optimize both their dynamic and static exercise performance (Table 10. Patients with severe stenosis should be restricted from exercise until they can undergo repair. Patients with severe stenosis should not engage in competitive sports but they can resume sports 3 to 6 months after successful intervention. Types of activities depend upon residual hemodynamic findings (see above) (82,130). Significant ventricular dilation can lead to arrhythmias as can scarring associated with ventriculotomies. Residual stenosis, regurgitation, and branch pulmonary artery stenosis have all been independently associated with diminished exercise performance, and inefficient ventilation during exercise. The latter is manifested as high ventilatory equivalents for carbon dioxide (minute ventilation is high when compared to carbon dioxide excretion) as well as a steep rise in the slope of minute ventilation relative to carbon dioxide production (131,132,133,134). This heterogeneity in exercise performance reflects both the heterogeneity of the defect itself as well as the broad spectrum of residual disease seen following operative repair. Those patients with significant pulmonary regurgitation accompanied by biventricular systolic dysfunction appear to have the lowest exercise capacity (132). These are often young adults with long-standing residual right-sided abnormalities. Patients with restrictive right ventricular mechanics may not develop significant right ventricular dilation despite severe pulmonary regurgitation and often appear to have more preserved exercise capacity at long-term followup (135,136). Premature atrial and ventricular ectopy is commonly observed on exercise testing and can be seen in as many as 50% of patients. However, fast atrial or ventricular couplets or runs of arrhythmias are not common during exercise testing and are likely a cause for concern. Of note is that in one such study, patients with documented ventricular arrhythmias were excluded from participation (141). All patients should have regular Holter monitoring and exercise testing to evaluate arrhythmias and assess cardiopulmonary capacity during exercise. Leisure Activities and Activities of Daily Living Because of the heterogeneity of this population, recommendations for activities and sports participation will vary widely depending on the state of the individual patient. There are some data that suggest maintenance of an active lifestyle in patients with ToF results in improved long-term aerobic capacity. This may be as a result of improved musculoskeletal conditioning as well as direct cardiac effects (142). Asymptomatic patients with significant pulmonary regurgitation who have at least moderate right ventricular dilation, but with preserved right ventricular function and no arrhythmias at rest or during exercise should follow recommendations as delineated in Table 10. Asymptomatic patients with significant regurgitation, significant right ventricular dilation, and abnormal function may engage in mild dynamic exercise assuming no arrhythmias at rest or during exercise. These patients as well as the symptomatic patients described in the following paragraph may benefit from a formal exercise prescription to better assess their individual limitations and to assure that they are performing activities that are safe and appropriate for their individual capacities. Symptomatic patients with residual right ventricular lesions and/or left ventricular dysfunction, patients with right ventricular to systemic systolic pressures ratios of two-thirds or more, patients with important residual intracardiac shunts, and patients with documented sustained atrial or ventricular arrhythmias that are refractory to treatment should engage in only low-dynamic, low-static activities (Table 10. The recent extraordinary performance of an American freestyle snow boarder is testament of the safety of the pursuit of athletic competition at very high level in repaired patients who do not have significant residual lesions. Ebstein Anomaly There is scant literature regarding exercise performance and the risk associated with exercise in patients with Ebstein anomaly. Heterogeneity in this patient population is great and will vary with the severity of the valvular abnormalities as well as with the presence and degree of atrial right-to-left shunting. Patients repaired at a younger age who have lower cardiothoracic ratios on chest x-ray at the time of intervention appear to have the best outcomes. Preoperative patients frequently have cyanosis at rest that worsens with exercise.
Following removal of Ascending Aortic Anastomosis the cannulas order doxycycline amex prescribed antibiotics for sinus infection, protamine is given order doxycycline cheap online infection lymph nodes. Hemostasis is assisted with The proximal truncus is anastomosed to the distal ascend- thrombin-soaked gelfoam. Chest tubes are placed and the chest is closed of the disparity in size, it is necessary to aggressively tailor in the routine fashion. It should rarely be necessary in the full- down the proximal truncus by taking wide bites on the trun- term neonate to leave the sternum open. If the disparity is greater than 2:1 which is not uncommon, it is preferable Management of the Regurgitant Truncal Valve to take a tuck on the rightward and posterior aspect in what The regurgitant truncal valve is almost always amenable to would usually be the noncoronary sinus. Replacement should rarely if ever in forming this dog ear, as well as in running the suture across be necessary in the neonatal period. One of the most useful the posterior wall to avoid any tension or distortion of the left techniques is to support a prolapsing leafet by suturing it to adjacent leafets (Fig. This is generally facilitated coronary ostium which should be carefully visualized. Prior by the fact that the prolapsing leafet is thickened and the to tying the suture anteriorly, the left heart should be allowed adjacent leafet edges are also relatively thickened and hold to fll with blood, and air should be vented through the suture sutures surprisingly well for a neonatal valve. Any remaining air is then vented through the original is often exacerbated by splaying of the tops of the commis- cardioplegia site. This can be improved by wedge excisions taken into the sinuses Proximal Homograft Anastomosis of Valsalva. It is even possible to completely excise leafets, The proximal anastomosis is simplifed if a femoral vein is including the adjacent sinus of Valsalva with reconstitution used. The autologous pericardium which was har- vested initially is used to roof the proximal anastomosis. Management of Associated Interrupted Toward the inferior end of the ventriculotomy there should Aortic Arch (Video 29. As described for the standard repair of truncus, depth to it rather than lying taut and fat. Before completion the child is cooled with a single arterial cannula in the dis- of this suture line, the left heart is once again de-aired and tal ascending aorta. The pulmonary arteries are occluded the aortic cross-clamp is released with gentle compression with tourniquets. Rewarming is begun and fow is grad- (see Chapter 32, Interrupted Aortic Arch) because fow to the ually increased. The open proximal homograft anastomosis descending aorta passes from the truncus through the ductus allows venting of blood from the right heart before cardiac arteriosus into the descending aorta. It is also possible to pass a sucker through perature is less than 15°C, the ascending aorta is clamped the homograft valve to vent the left heart if there is evidence and cardioplegia solution is infused into the truncal root, of left heart distention. Distention of the homograft is a use- while the ductus is controlled with forceps. Upon diffcult to distinguish ductal tissue, but if this is apparent it completion of the anastomosis, air is displaced by running should be excised. The remainder of the proce- the descending aorta to the site of excision of the pulmonary dure can be undertaken as for a standard repair. The arch anasto- is severely hypoplastic and will be inadequate to carry the mosis will be too proximal on the truncus and will interfere entire cardiac output. Instead, a longi- helpful to plasty the hypoplastic ascending aorta with a patch tudinal anastomosis should be made on the ascending aorta of autologous pericardium that extends from the proximal immediately proximal to the takeoff of the head vessels and perhaps extending a little on to the left common carotid artery truncal root to a point just beyond the anastomosis to the if this is a type B interruption (Fig. An alternative approach could aorta can be controlled with a C-clamp in order to reduce be to use a short segment of nonvalved femoral vein homo- tension on the anastomosis as this is fashioned. Tourniquets are no longer used on the pulmonary arteries to the proximal descending aorta. Because of the marked disparity between the proximal truncus and the distal ascending aorta, it is necessary to reduce the size of the proximal truncus by taking a tuck on the rightward and posterior aspect of the truncal root, thereby creating a dog ear. The ascending aorta is sutured to the left side of the truncal root which improves the lie of the homograft conduit. However, the patients who had more aggressive techniques were doing well at follow-up. The age who underwent primary repair of truncus arteriosus at median age at surgery was 2 weeks. Nine of the patients University of California, San Francisco between 1992 and had associated interrupted arch. A total of 23% of patients had moderate or severe trun- truncal valve regurgitation was diagnosed preoperatively, cal valve regurgitation and 12% had interrupted aortic arch. Five patients underwent months, there were two deaths resulting in a Kaplan–Meier truncal valve repair and one underwent homograft replace- estimate of survival at 1 year of 92%. The nifcantly associated with poorer survival over time were actuarial survival overall was 96% at 30 days, 1 and 3 years. The two deaths in the series occurred in patients replacement among early survivors was 57% at 3 years. None of the patients required with interrupted aortic arch and truncus at Royal Children’s reoperation because of truncal valve problems or aortic arch Hospital, Melbourne. Freedom from aortic reop- duit replacement was necessary in 17 patients after a mean eration was 76. Functional had an aortic homograft was 4 years and for those who had a status in all patients was good. Repeat surgical intervention truncal valvuloplasty methods has neutralized the traditional was rare and major complications related to root dilation did risk factor of truncal valve regurgitation. In this earlier timeframe, cal repair with a homograft conduit at Children’s Hospital truncal valve regurgitation, interrupted aortic arch, coronary Boston between 1990 and 1995. Although the early mortality artery anomalies, and age at repair greater than 100 days was satisfactory at 4. Pulmonary hyper- interrupted aortic arch or absent pulmonary valve syndrome, tensive episodes were fewer and duration of ventilator depen- the durability of homografts was disappointing. A total of dence, as well as absolute pulmonary artery pressure, were 47% of the patients underwent conduit replacement after a signifcantly less in patients undergoing correction before 30 mean follow-up of 34 months. Univariate analysis revealed that patients Other centers have confrmed the effectiveness of aggres- with a homograft with an internal diameter of 8 mm or less sive truncal valve repair, although initial truncal valve regur- were most likely to suffer early graft failure and reoperation. Simple cases had improved long- In 2001, Mavroudis and Backer26 described the results term survival relative to complex cases. Age, weight, pathol- of surgery for eight patients who underwent intervention for ogy, type of homograft (aortic versus pulmonary), length of severe truncal valve insuffciency between 1995 and 2000. Among the three report has suggested that failure of homografts in infants 582 Comprehensive Surgical Management of Congenital Heart Disease, Second Edition is not simply related to somatic outgrowth. The anatomy of common aor- reviewed imaging studies of 40 patients who had undergone ticopulmonary trunk (truncus arteriosus communis) and its embryonic implications. This is most commonly tion between Foxd3 and Pax3 in cardiac neural crest develop- observed at the annular area. Persistent truncus arteriosus: a clas- A number of authors have described the use of conduits sifcation according to anatomic types.
Thus purchase doxycycline 100mg with mastercard antimicrobial beer line, an essential function of the cardiovascular system is to generate sufficient flow of substrate buy doxycycline 100mg amex antibiotics for dogs skin, for example O , through the circulation to maintain normal tissue metabolism. We generally take the matching of systemic O2 2 consumption with adequate levels of delivery for granted in the healthy state, possibly with the exception of periods of intense exercise. There has been considerable interest, however, in the impact of inadequate oxygen delivery on metabolism during critical illness (50), for example in the setting of severe heart failure, and in the potential inability of the circulation to maintain adequate levels of systemic O delivery to match the increases in2 its consumption in the patient with, for example severe sepsis (51). Conversely, in nature, the hibernating animal is able to tolerate extreme reductions in cardiac output and systemic O delivery for months on end, because of a2 dramatic reduction in O requirements (2 52). The systemic delivery of O can be calculated from the product of2 the O content of the arterial blood and the cardiac output. The systemic O consumption is in turn calculated2 2 from the product of the arteriovenous O content difference and the cardiac output. Clearly circulatory physiology2 is central to maintaining the relationship between systemic O delivery and consumption. From first principles this ability to maintain a constant level of O consumption in the face of a reduction in delivery reflects2 potentially two phenomena; the first is the ability of some organs to increase their O extraction, in the face of2 reduced flow and the second, the ability of other organs to maintain flow locally, in the face of a global reduction in cardiac output (autoregulation). Nonetheless, if O delivery falls below a “critical” level, this is accompanied by2 a concomitant fall in consumption, as the ability of these essential homeostatic mechanisms is overwhelmed. The Relationship between Systemic O Delivery and Consumption after Cardiac Surgery2 The changes in systemic O consumption and delivery in the adult, early after cardiac surgery have been well2 described. Typically, O delivery is reduced, reflecting a diminished cardiac output, while paradoxically O2 2 consumption may be elevated, secondary to an elevation in temperature and possibly a systemic inflammatory response. In a study of children during the early hours after cardiac surgery, it was observed that the initial O2 consumption was unrelated to either the duration of cardiopulmonary bypass or to the duration of aortic cross- clamping. During subsequent hours, changes in O consumption closely followed changes in core temperature. Given the pivotal role of metabolism in the preservation of tissue function, one might expect that changes in the balance between systemic O consumption and delivery might provide a predictor of outcome after pediatric2 cardiac surgery. Indeed one study demonstrated that in infants undergoing cardiac surgery a systemic O2 extraction ratio of more than 0. Another study, while not demonstrating an association between either systemic O delivery or consumption and risk of adverse outcome2 showed that an elevated plasma lactate did appear to indicate a subsequent adverse event (55). Unfortunately there are2 additional complexities in the clinical measurement of these relationships imposed by the parallel circulations (56). Nonetheless, in a group of patients after the Norwood operation in whom the relationship between systemic O delivery and consumption was inferred from the venous O concentration, a saturation of less than 30% was2 2 predictive of the presence of anaerobic metabolism (57) and a saturation of 40% predicted impaired neurodevelopment in survivors (58). Changes in Systemic O Delivery during Catecholamine Infusions2 In the critically ill patient in whom the relationship between O delivery and consumption is altered, an important2 goal of treatment may be to increase systemic O delivery by increasing cardiac output with catecholamine2 infusions. Catecholamine-related increases in cardiac output during infusions of catecholamines have been repeatedly demonstrated in the adult. Catecholamines, nonetheless also stimulate the consumption of O2 through their effects on systemic metabolism, although in general, in the adult, the increase in O consumption is2 greatly outweighed by the increase in delivery (59). This may not be the case in the neonate in whom there are additional thermogenic effects of catecholamines through their actions on brown adipose tissue. Thus in a study of healthy neonatal lambs, infusions of dobutamine at high doses resulted in exaggerated increases in systemic O consumption which were of greater magnitude than the increase in delivery (2 60). Furthermore, in patients after the Norwood operation, dopamine induced a significant increase in systemic O consumption such that2 termination of the infusion improved the balance between O consumption and delivery (2 61). These data emphasize the importance of looking beyond cardiac output when examining the clinical effects of any agent which impacts on the cardiovascular system. The Physiology of the Developing Circulation The Central Circulation The central circulation is structured differently in the fetus to accommodate the different sites of oxygen uptake. Postnatally, O uptake occurs in the pulmonary vascular bed, which is perfused independently by the right2 ventricle, while the left ventricle separately supplies the regional systemic vascular beds. In the fetus, O uptake2 occurs in the placenta, which is perfused in parallel with the systemic vascular beds (62). To deliver relatively highly oxygenated blood to the metabolically active tissues (such as the heart and brain) and to deliver less oxygenated blood to the placenta for oxygen uptake, central shunts and preferential blood flow patterns exist. Shunts in the venous system (ductus venosus), the heart (foramen ovale), and the arterial system (ductus arteriosus) are remarkably efficient at achieving this goal (63). These shunts are abolished over a very short period of time after birth, and the mature postnatal central circulation is established within the first few days of life. The presence of the central shunts allows the fetal circulation to be remarkably efficient at distributing oxygen and substrate. The fetal right ventricle supplies most of its blood via the ductus arteriosus and descending aorta to the placenta for oxygen uptake, and the left ventricle supplies most of its blood via the ascending aorta to the heart and brain for oxygen delivery (Fig. For the central venous circulation to facilitate the efficient performance of these tasks, the least saturated venous blood should be directed to the right ventricle and the most saturated should be directed to the left. This blood is directed appropriately through the tricuspid valve to the right ventricle. The leftward and superior course of the eustachian valve directs >95% of the blood flowing caudally from the superior vena cava away from the foramen ovale and toward the tricuspid valve. In addition, the location of the coronary sinus caudad to the foramen ovale causes venous blood from the myocardium to flow through the tricuspid valve to the right ventricle. Blood returning from the lungs has an intermediate saturation, but by the nature of the normal drainage of the pulmonary veins to the left atrium, preferential flow to the right ventricle is not possible. However, pulmonary blood flow is a relatively small portion of combined venous return. It represents no more than 8% of combined ventricular output in the sheep fetus (64), and about twice that in the human, at most being 25% (65), so that it does not have a significant effect on oxygen delivery. Inferior vena caval return comes from the remaining two sources, the lower body and the placenta. Most lower body flow, except that from the liver, ascends the distal inferior vena cava (Fig. This stream of relatively desaturated blood enters the lateral margin of the right atrium and is directed primarily through the tricuspid valve. Under normal conditions in the fetal sheep, about 55% of the highly saturated umbilical venous return ascends via the ductus venosus to the inferior vena cava–right atrium junction (64), where it preferentially crosses the foramen ovale. Slightly less than half of the remaining umbilical venous return enters the left lobe of the liver, from which it reaches the left hepatic vein. The left hepatic vein joins the ductus venosus near the inferior vena cava, so that this highly saturated blood is also directed toward the foramen ovale. The limbus of the foramen ovale helps to direct this blood into the left atrium (66). The remainder of the umbilical venous blood, along with >95% of the poorly saturated portal venous blood, is directed to the right lobe of the liver. Ultrasound-based studies suggest that in the human the relative distribution of umbilical venous flow to the ductus venosus may be lower, although it is altered by fetal distress (67). From the right lobe, this much less saturated blood enters the right hepatic vein and tends to stream with the blood of the distal inferior vena cava to the tricuspid valve. The hepatic artery, which carries blood that is moderately well saturated, constitutes <10% of hepatic blood flow in the fetus, so it does not significantly contribute to oxygen supply. Thus, preferential streaming patterns among the different sources of venous return allow most of the poorly saturated blood from the upper body, myocardium, and lower body to reach the right ventricle, and the more highly saturated umbilical venous return to reach the left ventricle.