Ultrasonographic Imaging of the Cervical Thoracic Duct in Children with Congenital or Acquired Heart Disease

Sonography for assessment of thoracic duct anatomy and physiology before and after meals

The interest in clinical anatomy of the thoracic duct (TD) has recently grown, owing to discoveries linking its morphology to pathologies such as heart failure or liver cirrhosis. In the light of this knowledge, a cost-efficient and reliable in-vivo imaging method of TD should be devised. Ultrasonography satisfies these criteria and hence is a promising tool for assessment of TD's anatomy and function. Thirty-one healthy volunteers attended the examination after 6 h of fasting and 2 h without drink. Ultrasound of the left supraclavicular fossa was performed in search of TD's orifice into the venous angle. In each case, the largest diameter, number of orifices, presence of valves, tributaries, and motility of the TD were examined. We performed examinations in three sessions: after fasting, after standardized meal and 1 h after the meal. The statistical significance has varied among the three sessions. The strongest connection was shown in the third examination. The TD was visualized in 31 cases, 35 orifices were found, most of which drained into the venous angle. Multiple orifices were seen in four cases and valves in 15 cases. Tributaries were present in 17 cases. Mean widest and orifice diameter measured 3.23 and 2.0 mm, respectively. Spontaneous peristaltic-like movements of the TD were observed in 25 cases. We demonstrated that ultrasound is useful for assessment of TD's anatomy, allowing to visualize and quantify its key features. Moreover, our study is presumably the first to capture and describe TD's motility in vivo.

Spontaneous contractions of the human thoracic duct-Important for securing lymphatic return during positive pressure ventilation?

The thoracic duct is responsible for the circulatory return of most lymphatic fluid. The return is a well-timed synergy between the pressure in the thoracic duct, venous pressure at the thoracic duct outlet, and intrathoracic pressures during respiration. However, little is known about the forces determining thoracic duct pressure and how these respond to mechanical ventilation. We aimed to assess human thoracic duct pressure and identify elements affecting it during positive pressure ventilation and a brief ventilatory pause. The study examined pressures of 35 patients with severe congenital heart defects undergoing lymphatic interventions. Thoracic duct pressure and central venous pressure were measured in 25 patients during mechanical ventilation and in ten patients during both ventilation and a short pause in ventilation. TD contractions, mechanical ventilation, and arterial pulsations influenced the thoracic duct pressure. The mean pressure of the thoracic duct was 16 ± 5 mmHg. The frequency of the contractions was 5 ± 1 min^-1 resulting in an average increase in pressure of 4 ± 4 mmHg. During mechanical ventilation, the thoracic duct pressure correlated closely to the central venous pressure. TD contractions were able to increase thoracic duct pressure by 25%. With thoracic duct pressure correlating closely to the central venous pressure, this intrinsic force may be an important factor in securing a successful return of lymphatic fluid. Future studies are needed to examine the return of lymphatic fluid and the function of the thoracic duct in the absence of both lymphatic complications and mechanical ventilation.

Lung lymphatic thrombosis and dysfunction caused by cigarette smoke exposure precedes emphysema in mice

The lymphatic vasculature is critical for lung function, but defects in lymphatic function in the pathogenesis of lung disease is understudied. In mice, lymphatic dysfunction alone is sufficient to cause lung injury that resembles human emphysema. Whether lymphatic function is disrupted in cigarette smoke (CS)-induced emphysema is unknown. In this study, we investigated the effect of CS on lung lymphatic function. Analysis of human lung tissue revealed significant lung lymphatic thrombosis in patients with emphysema compared to control smokers that increased with disease severity. In a mouse model, CS exposure led to lung lymphatic thrombosis, decreased lymphatic drainage, and impaired leukocyte trafficking that all preceded the development of emphysema. Proteomic analysis demonstrated an increased abundance of coagulation factors in the lymph draining from the lungs of CS-exposed mice compared to control mice. In addition, in vitro assays demonstrated a direct effect of CS on lymphatic endothelial cell integrity. These data show that CS exposure results in lung lymphatic dysfunction and a shift in thoracic lymph towards a prothrombic state. Furthermore, our data suggest that lymphatic dysfunction is due to effects of CS on the lymphatic vasculature that precede emphysema. These studies demonstrate a novel component of CS-induced lung injury that occurs early in the pathogenesis of emphysema.

The effect of respiration and body position on terminal thoracic duct diameter and the lymphovenous junction: An exploratory ultrasound study

The thoracic duct (TD) drains most of the body's lymph back to the venous system via its lymphovenous junction (LVJ), playing a pivotal role in fluid homeostasis, fat absorption and the systemic immune response. The respiratory cycle is thought to assist with lymph flow, but the precise mechanism underpinning terminal TD lymph flow into the central veins is not well understood. The aim of this study was to use ultrasonography (US) to explore the relationship between terminal TD lymph flow, the respiratory cycle, and gravity. The left supraclavicular fossa was scanned in healthy non-fasted volunteers using high-resolution (13-5 MHz) US to identify the terminal TD and the presence of a lymphovenous valve (LVV). The TD's internal diameter was measured in relation to respiration (inspiration vs. expiration) and body positioning (supine vs. Trendelenburg). The terminal TD was visualized in 20/33 (61%) healthy volunteers. An LVV was visualized in only 4/20 (20%) cases. The mean terminal TD diameter in the supine position was 1.7 mm (range 0.8-3.1 mm); this increased in full inspiration (mean 1.8 mm, range 0.9-3.2 mm, p < 0.05), and in the Trendelenburg position (mean 1.8 mm, range 1.2-3.1 mm, p < 0.05). The smallest mean terminal TD diameter occurred in full expiration (1.6 mm, range 0.7-3.1 mm, p < 0.05). Respiration and gravity impact the terminal TD diameter. Due to the challenges of visualizing the TD and LVJ, other techniques such as dynamic magnetic resonance imaging will be required to fully understand the factors governing TD lymph flow.

The Lymphovenous Junction of the Thoracic Duct: A Systematic Review of its Structural and Functional Anatomy

Background: The lymphovenous junction (LVJ) of the thoracic duct (TD) is the principle outlet of the lymphatic system. Interest in this junction is growing as its role in lymphatic outflow obstruction is being realized, and as minimally invasive procedures for accessing the terminal TD become more common. Despite the growing clinical significance of the LVJ, its precise form and function remain unclear. The aim of this article was to systematically review the literature surrounding the structure and function of the LVJ and its associated lymphovenous valve (LVV). Methods and Results: A systematic review of the structure and function of the LVJ and LVV was undertaken using the MEDLINE, Scopus, and Google Scholar databases. Human and animal studies up to November 2019, with no language or past date restriction, were included. Forty-six relevant articles were reviewed. The LVJ shows marked anatomical variation. A valve is frequently absent at the LVJ, but when present it displays numerous distinct morphologies. These include bicuspid semilunar, ostial, and flap-like structure. Other factors, such as functional platelet plugs, or the tangential/intramural course of the terminal TD across the vein wall, may work to prevent blood from entering the lymphatic system. Conclusions: The form and function of the LVJ remain unclear. Dedicated studies of this area in vivo are required to elucidate how this part of the body functions in both health and disease.

The anatomy and physiology of the terminal thoracic duct and ostial valve in health and disease: potential implications for intervention

The thoracic duct (TD) transports lymph drained from the body to the venous system in the neck via the lymphovenous junction. There has been increased interest in the TD lymph (including gut lymph) because of its putative role in the promotion of systemic inflammation and organ dysfunction during acute and critical illness. Minimally invasive TD cannulation has recently been described as a potential method to access TD lymph for investigation. However, marked anatomical variability exists in the terminal segment and the physiology regarding the ostial valve and terminal TD is poorly understood. A systematic review was conducted using three databases from 1909 until May 2017. Human and animal studies were included and data from surgical, radiological and cadaveric studies were retrieved. Sixty-three articles from the last 108 years were included in the analysis. The terminal TD exists as a single duct in its terminal course in 72% of cases and 13% have multiple terminations: double (8.5%), triple (1.8%) and quadruple (2.2%). The ostial valve functions to regulate flow in relation to the respiratory cycle. The patency of this valve found at the lymphovenous junction opening, is determined by venous wall tension. During inspiration, central venous pressure (CVP) falls and the valve cusps collapse to allow antegrade flow of lymph into the vein. During early expiration when CVP and venous wall tension rises, the ostial valve leaflets cover the opening of the lymphovenous junction preventing antegrade lymph flow. During chronic disease states associated with an elevated mean CVP (e.g. in heart failure or cirrhosis), there is a limitation of flow across the lymphovenous junction. Although lymph production is increased in both heart failure and cirrhosis, TD lymph outflow across the lymphovenous junction is unable to compensate for this increase. In conclusion the terminal TD shows marked anatomical variability and TD lymph flow is controlled at the ostial valve, which responds to changes in CVP. This information is relevant to techniques for cannulating the TD, with the aid of minimally invasive methods and high resolution ultrasonography, to enable longitudinal physiology and lymph composition studies in awake patients with both acute and chronic disease.

The Lymphatic System: The Achilles Heel of the Fontan-Kreutzer Circulation

In spite of excellent long term survival the Fontan Kreutzer procedure commonly presents late failure due to end-organ damage. Several advances have been described to refine single ventricle management and surgical techniques. However, very little research has been dedicated to the lymphatic circulation in the precarious Fontan hemodynamic state. The lymphatic circulation is clearly affected since there is increased lymph production, which requires to be drained at a similar or higher pressure than it is produced, commonly resulting in chronic lymphedema. Chronic lymphedema induces fibrosis and end-organ failure even in normal circulation. Diverting lymph drainage to the low-pressured systemic atrium in Fontan may represent a valid alternative for the treatment of devastating complications as protein-losing enteropathy and plastic bronchitis and may prevent or decrease the development of end-organ fibrosis or failure.

Lymphovenous hemostasis and the role of platelets in regulating lymphatic flow and lymphatic vessel maturation

Aside from the established role for platelets in regulating hemostasis and thrombosis, recent research has revealed a discrete role for platelets in the separation of the blood and lymphatic vascular systems. Platelets are activated by interaction with lymphatic endothelial cells at the lymphovenous junction, the site in the body where the lymphatic system drains into the blood vascular system, resulting in a platelet plug that, with the lymphovenous valve, prevents blood from entering the lymphatic circulation. This process, known as "lymphovenous hemostasis," is mediated by activation of platelet CLEC-2 receptors by the transmembrane ligand podoplanin expressed by lymphatic endothelial cells. Lymphovenous hemostasis is required for normal lymph flow, and mice deficient in lymphovenous hemostasis exhibit lymphedema and sometimes chylothorax phenotypes indicative of lymphatic insufficiency. Unexpectedly, the loss of lymph flow in these mice causes defects in maturation of collecting lymphatic vessels and lymphatic valve formation, uncovering an important role for fluid flow in driving endothelial cell signaling during development of collecting lymphatics. This article summarizes the current understanding of lymphovenous hemostasis and its effect on lymphatic vessel maturation and synthesizes the outstanding questions in the field, with relationship to human disease.