bubble_chart Overview

Single ventricle, also known as common ventricle or univentricular heart, is a relatively rare congenital malformation. Its incidence is approximately 1 in 6,500 live births, accounting for about 1.5% of congenital heart diseases. The ventricle receives blood from both the tricuspid and mitral valves or a common atrioventricular valve.

bubble_chart Etiology

From an embryological perspective, the formation of a single ventricle results from the failure of the atrioventricular canal to properly align with the developing ventricles, causing both atrioventricular valves to open into one ventricle. The commonly associated pulmonary stenosis may be due to the deviation of the infundibular septum caused by fistula disease.

bubble_chart Pathological Changes

The single ventricle itself can be further divided into many subtypes. Van Praagh et al. classified it into four types based on the morphology of the ventricular main body: Type A, morphologically a left ventricle accompanied by a primitive outflow tract portion including the right ventricular fistula disease infundibulum; Type B, morphologically a right ventricle without a left ventricular sinus (the remnant of the left ventricle may appear as a non-functional slit or pouch); Type C, the ventricle includes the main portions of both the left and right ventricles, with no ventricular septum or only its remnants; Type D, the ventricle lacks the characteristics of either a right or left ventricle (no right or left ventricular sinus). These four types can be further subdivided into Type I (normal), Type II (D-loop), or Type III (L-loop) based on their connection with the great vessels and the spatial arrangement of the great vessels. According to Van Praagh, Type A accounts for 78%, Type B for 5%, Type C for 7%, and Type D for 10%; as for the transposition of the great vessels, the cases of D-loop and L-loop are roughly similar, at 42% and 43% respectively, with normal arrangement of the great vessels accounting for 15%.

Autopsies can often classify the cases carefully according to the Van Praagh method, whereas clinicians, based on imaging data, often find it difficult to distinguish between Types B, C, and D in detail. Therefore, they simply group these three types without an outflow tract chamber into Type C, and those with an outflow tract chamber into Type A, then further classify them into Type I, II, or III based on the arrangement of the great vessels. Ritter et al. reported data from 145 clinical cases, showing the proportion of each type in the incidence of single ventricle and the percentage of cases complicated by pulmonary stenosis, as shown in Table 1.

Table 1: Distribution of 145 clinical cases of single ventricle by type and the percentage of cases complicated by pulmonary stenosis

The pathophysiology of a single ventricle depends on the presence and degree of pulmonary stenosis, subaortic stenosis, atrioventricular valve regurgitation, and the functional state of the ventricle. Patients with significant pulmonary stenosis exhibit cyanosis, which progresses to polycythemia over time. In cases without pulmonary stenosis, increased pulmonary blood flow leads to pulmonary congestion and symptoms of congestive heart failure, with late-stage [third stage] development of elevated pulmonary vascular resistance and pulmonary hypertension. Ventricular dysfunction and atrioventricular valve regurgitation may result from chronic ventricular volume overload or preexisting abnormalities of the atrioventricular valve. As atrioventricular valve regurgitation worsens and cardiac function deteriorates, manifestations of congestive heart failure progressively intensify. Subaortic stenosis often accompanies pulmonary stenosis or is particularly common in patients who have undergone pulmonary artery banding due to excessive pulmonary blood flow, primarily due to ventricular wall hypertrophy. These cases carry a particularly high mortality risk during corrective surgery. Due to abnormal positioning of the atrioventricular node and common conduction bundle, type A-III single ventricle has an exceptionally high incidence of spontaneous or surgically induced conduction block. According to McGoon et al., the preoperative incidence of atrioventricular block is 17%, with an additional 30% occurring after ventricular septation.

bubble_chart Clinical Manifestations

Most patients with a single ventricle exhibit obvious congenital heart disease symptoms early in life, such as cyanosis, tachycardia, or slow weight gain, drawing attention during the neonatal or early infant stages. For patients with increased pulmonary blood flow, early symptoms may often go unnoticed. Without treatment, the natural lifespan of single ventricle patients is relatively short. According to statistics from the Toronto Children's Hospital, among 182 cases, 117 (64%) died, with 50% dying within the first month of birth and 74% within the first six months. Moodie et al. analyzed 83 patients who did not undergo surgery, most of whom had survived infancy. From the time of diagnosis, 50% of Type A patients died within an average of 14 years. The prognosis for Type C patients was worse, with 50% dying within 4 years. The presence or absence of pulmonary {|###|} valve stenosis did not affect lifespan. The primary causes of death were congestive heart failure, arrhythmias, or sudden death of unknown cause.

Physical examination: Patients with reduced pulmonary blood flow may exhibit cyanosis and clubbing of fingers or toes. Those with abnormally increased pulmonary blood flow and chronic congestive heart failure show poor growth and emaciation. In cases of congestive heart failure or when there is right atrioventricular valve stenosis without an atrial septal defect, jugular veins may appear full or distended. If right atrioventricular valve regurgitation is severe, jugular veins and the liver may exhibit systolic pulsations.

On inspection and palpation, the cardiac impulse is often diffuse. Due to the relatively anterior position of the main {|###|} pulse in many patients, the closure of the main {|###|} valve can be felt along the left sternal border.

On auscultation, the first heart sound may be accentuated, and the second heart sound is often loud and single. Most patients have a prominent systolic murmur, originating from pulmonary {|###|} valve stenosis or subvalvular stenosis of the main {|###|} valve. Patients with increased pulmonary blood flow may also have a diastolic murmur at the apex due to relative stenosis of the left atrioventricular valve.

bubble_chart Auxiliary Examination

Electrocardiogram examination: It varies depending on the subtypes of single ventricle, but most patients exhibit ventricular hypertrophy.

Chest X-ray examination: Most patients show an enlarged cardiac shadow, while increased or decreased pulmonary blood flow depends on the presence or absence of pulmonary valve stenosis. Left atrial enlargement is seen in cases with increased pulmonary blood flow or atrioventricular valve insufficiency. Other findings vary according to the pathological anatomy of each subtype.

Cardiac catheterization and angiography: Before the advent of two-dimensional echocardiography and color Doppler diagnostic techniques, cardiac catheterization and angiography were necessary to confirm the diagnosis of single ventricle, its type, and associated anomalies. The objectives of the examination should include: ① the type of single ventricle; ② the presence and location of the outlet chamber; ③ the spatial relationship between the great arteries and the atrioventricular connection; ④ the presence and location of obstruction in pulmonary or systemic blood flow; ⑤ the number, position, functional status, deviation, and straddling of the atrioventricular valves; ⑥ pulmonary artery pressure and resistance; ⑦ ventricular function (ejection fraction and end-diastolic pressure); ⑧ the size, distribution, or distortion of the pulmonary arteries due to previous banding; ⑨ associated anomalies. Although venous blood from the systemic and pulmonary circulations mixes in the single ventricle, the oxygen saturation in the pulmonary and systemic arteries may not be identical due to differences in intracardiac blood flow. Therefore, to accurately calculate pulmonary and systemic vascular resistance, the oxygen saturation and pressure in both arteries must be measured separately.

Echocardiography: Two-dimensional echocardiography has largely replaced invasive cardiac catheterization for observing and analyzing various aspects of single ventricle, such as the basic intracardiac anatomy, the relationship of the great arteries, associated cardiac anomalies, the presence or absence of pulmonary valve stenosis, and the condition of the ventricular outlet. New Doppler techniques can also provide quantitative measurements of pulmonary stenosis, ventricular outflow obstruction, and atrioventricular valve insufficiency. Echocardiography is significantly superior to angiography in assessing the morphology, deviation, and straddling of the atrioventricular valves.

bubble_chart Treatment Measures

According to the specific pathological anatomy and pathophysiology of each subtype of single ventricle, the following surgical procedures are selected.

(1) Palliative surgery: To increase (systemic-pulmonary stirred pulse shunt) or decrease (pulmonary stirred pulse banding) pulmonary blood flow to improve symptoms. However, palliative surgery also has its drawbacks. For example, after a systemic-pulmonary stirred pulse shunt, the pulmonary stirred pulse often becomes distorted, making subsequent corrective surgery difficult; excessive increase in pulmonary blood flow can lead to heart failure due to increased ventricular volume load; the superior vena cava-pulmonary stirred pulse anastomosis (Glenn procedure) does not increase ventricular volume load, but sometimes advanced stage may develop ipsilateral pulmonary stirred pulse fistula; distal migration of the pulmonary stirred pulse band can cause pulmonary stirred pulse distortion, etc. Moodie et al. analyzed the effectiveness of palliative surgery for single ventricle and found that, regardless of whether the surgery was performed to increase or decrease pulmonary blood flow, 30% of Type A and 75% of Type C single ventricles died within 10 years of diagnosis. Therefore, palliative surgery is both useful and insufficient or unsatisfactory.

(2) Ventricular exclusion surgery (Fontan procedure): Directs pulmonary circulation from the atrium into the pulmonary stirred pulse (by closing the atrioventricular valve orifice and the root of the pulmonary stirred pulse), leaving the single ventricle dedicated to systemic circulation. As of 1983, the Mayo Clinic performed the Fontan procedure on 128 patients with single ventricle, with a surgical mortality rate of 25% (32 cases), which decreased to 14% (7 cases) in the last 50 cases. The risk of the Fontan procedure is particularly high when there is stenosis in the blood flow channel between the ventricle and the main stirred pulse.

(3) Ventricular septation: A large piece of artificial fibrous material is used to divide the ventricular cavity into two, each receiving blood from one atrioventricular valve and supplying the pulmonary stirred pulse and main stirred pulse, respectively. The surgery is complex and difficult, and despite continuous improvements in operative techniques, the early and advanced stage mortality rates remain unsatisfactory. Feldt from the Mayo Clinic reported 45 cases, with early and advanced stage mortality rates of 47% (21 cases) and 18% (8 cases), respectively. Among the 16 survivors, 12 were in good condition, while 4 had poor outcomes. In 11 cases with left-anterior positioned main stirred pulse subvalvular outflow tract, no preoperative congestive heart failure, no prior palliative surgery, and no significant preoperative cyanosis, the surgical survival rate reached 82%. Combined data from other reports also indicate that septation should be limited to cases with left-anterior positioned main stirred pulse subvalvular outflow chamber (Type A-III), normal atrioventricular valves, no obstructive lesions in the ventricular outflow tract, no prior palliative surgery, and no preoperative congestive heart failure or significant cyanosis.

bubble_chart Complications

Single ventricle combined with other congenital heart malformations is most commonly associated with pulmonary valve stenosis and atrial septal defects, occurring in 51% and 27% of patients, respectively. It may also be accompanied by coronary artery anomalies. The conduction system exhibits abnormal and variable positioning. In patients with an outlet chamber and those with discordant atrioventricular and ventriculoarterial relationships (L-loop), the atrioventricular node is abnormally anterior. In patients without an outlet chamber, the position of the atrioventricular node is unpredictable and may be posterior, lateral, or anterior. When the outlet chamber is left-anterior, the main conduction bundle encircles the anterior aspect of the subpulmonary outflow tract, close to the attachment of the pulmonary valve. If the outlet chamber is rightward and anterior, the conduction bundle is located inferoposterior to the pulmonary valve annulus. In cases without an outlet chamber, the conduction bundle is situated posterior to the ventricular mass.