Anatomy of the Bronchial Circulation

Preservation of the blood supply of the donor and recipient airway is the most critical factor in obtaining a successful airway anastomosis in transplant patients. The primary blood supply to the common sites of anastomosis, the distal trachea and main bronchi, is derived from the bronchial circulation. A thorough understanding of the anatomy and physiology of this circulation is important in the management of the airway in lung transplantation. The bronchial circulation arises embryologically from the primitive pulmonary plexus, formed from the ventral branches of the dorsal aorta. The sixth branchial arch forms the pulmonary arteries, which invade the lung buds and fuse with the separately formed pulmonary microvasculature. The pulmonary and bronchial arteries therefore share a related embryologic derivation, which endures in the adult as a network of extensive anastomotic communications between the two systems, although the sites of origin of the respective circulations are by then separated into systemic and pulmonic circuits.

The bronchial arterial circulation may be divided into anterior branches, arising from the subclavian, internal mammary, or coronary arteries, and posterior branches, arising from the thoracic aorta or intercostal arteries. Anterior branches are uncommon and assume clinical significance only in instances in which the posterior branches are interrupted, such as following en bloc heart-lung transplantation. In these instances, anterior branches, particularly from the coronary circulation, provide significant systemic collateral blood flow to the airway for anastomotic healing and maintenance of structural integrity of the tracheobronchial tree.8

The branches of the posterior circulation are divided into left- and right-sided arteries. A left bronchial artery arises directly from the descending thoracic aorta in over 90% of cases. A right bronchial artery arises in relationship to a right intercostal artery as a right inter cost obronchial artery (RICBA) in 95% of cases. Additional right- and left-sided bronchial arteries may arise directly from the aorta or from ipsilateral intercostal arteries. The most common patterns of posterior bronchial circulation consist of one right and two left bronchial arteries (in approximately 25% of patients) or two right and two left bronchial arteries (in approximately 20% of patients). The RICBA arises from the descending thoracic aorta, crossing posterior to the esophagus and then passing between the azygous vein and the esophagus to the right main bronchus (Figure 44-1). It serves as a common trunk for the first or second right intercostal artery and the right bronchial artery. The RICBA supplies the right main bronchus and carina, and may supply the left bronchial tree as well, through subcarinal collaterals (Figure 44-2). This branch is useful for direct bronchial revascularization following en bloc double lung transplantation.9,10

The bronchial arteries enter the lungs through the hila and divide within the peribronchial connective tissue sheath surrounding the main bronchi. Typically, two or three branches of the bronchial artery wind around the wall of the bronchus, supplying the bronchial tree to the level of the terminal bronchioles. The diameter of the vessels decreases in size from 1.5 mm at the hilum to a 0.5 mm diameter at a broncho-pulmonary segment. The arteries anastomose freely, forming a peribronchial plexus, and also bear branches that penetrate the wall to the bronchial mucosa, forming a submucosal plexus (Figure 44-3). Additional branches are given off to the visceral pleura, the lymph nodes, and the vasa vasorum of the pulmonary arteries and veins.

Along the airways and beyond the terminal bronchioles, bronchopulmonary anastomoses are found. Two types of large anastomoses have been identified: short, narrow vessels (1 mm x 100 microns) and longer, wider coiled vessels (20 mm x 400 microns).11 The exact function of these vessels is not known,

Broncho Pulmonary Segment

figure 44-1 Anatomic location of the right intercostobronchial artery (RICBA). The RICBA is found in 95% of patients, originating from the right side of the descending thoracic aorta. It travels extrapleurally, posterior to the esophagus, and then passes between the esophagus and the azygous vein to the right main bronchus. The RICBA commonly bifurcates into a right bronchial artery and an intercostal artery to the first or second intercostal space. Reprinted with permission from Schreinemakers HHJ et al.9

although a role in pressure-dependent regulation of bronchopulmonary anastomotic flow via a Starling resistor mechanism has been postulated.12 These large anastomoses may be found within the pleura, along the bronchi, and between bronchial arteries and the pulmonary veins. In addition, at the level of the pulmonary lobules, the bronchial microvasculature merges with the pulmonary capillary network. These anastomoses become more numerous at the periphery of the lung.

Bronchial Circulation

figure 44-2 A line drawing from a selective arteriogram indicating the distribution of the right intercostobronchial artery. There is extensive arborization to the distal trachea, proximal right main bronchus, and carina. Significant subcarinal collaterals to the left main bronchus are also demonstrated. Reprinted with permission from Schreinemakers HHJ et al.9

Schreinemakers

artery vein figure 44-3 Schematic representation of the bronchial circulation in a bronchiole. The bronchial arterial circulation forms extensive peribronchial and submucosal plexi, which anastomose to bronchial venous plexi in these locations. Reprinted with permission from Deffenbach ME, Charan NB, Lakshminarayans S, Butler J. The bronchial circulation. Am Rev Respir Dis 1987;135:463-81.

artery vein figure 44-3 Schematic representation of the bronchial circulation in a bronchiole. The bronchial arterial circulation forms extensive peribronchial and submucosal plexi, which anastomose to bronchial venous plexi in these locations. Reprinted with permission from Deffenbach ME, Charan NB, Lakshminarayans S, Butler J. The bronchial circulation. Am Rev Respir Dis 1987;135:463-81.

The venous blood from the proximal tracheobronchial tree to the level of the lobar bronchi drains in an extrapulmonary route via bronchial veins to the azygous and hemiazygous veins. In addition, one or more deep bronchial veins are found in each lung. These deep veins drain intraparenchymal bronchial blood into the left atrium or inferior pulmonary vein near its left atrial confluence. In normal circumstances, however, the majority of intraparenchymal bronchial venous flow passes into the pulmonary circulation at the precapillary level via bronchial veins or the previously mentioned anastomotic pathways (Figure 44-4).

Functionally, flow in the bronchial circulation in the normal state is unidirectional, from bronchial arteries to both the bronchial veins and the pulmonary circulation via direct anastomoses. The bronchial veins also empty into the pulmonary circulation. Total bronchial blood flow and anastomotic flow in this state is directly proportional to the gradient between systemic arterial pressures and pulmonary arterial pressures.

The most common technique used for single or double lung transplantation consists of an isolated lung transplant without bronchial revascularization. In this instance, arterial pressure in the graft bronchial arteries remains close to zero after implantation of the graft, as the systemic connection to the bronchial arteries is not restored. Reversal of flow from the pulmonary circulation into the bronchial circulation via the bronchial venous network occurs, resulting in reperfusion of the peribronchial and submucosal plexi of the airway.13 The relative contributions of the pulmonary arterial and pulmonary venous anastomoses to this flow via the bronchial veins is not known, but is likely to be pressure dependent. In addition, the contribution of the two types of direct bronchopulmonary arterial anastomoses to the reversed bronchial blood flow is not known. Clinical experience suggests, however, that the amount of perfusion achieved by this

Drawing Varicose Bronchi

figure 44-4 Bronchial arterial and venous circulations. The bronchial arterial circulation to the extraparenchymal airways drains to systemic venous (azygous) vessels. The bronchial arterial circulation to the intraparenchymal airways may drain to deep veins, which anastomose to the left atrium or pulmonary veins or into the pulmonary circulation. The majority of flow is via the latter pathway. In addition, direct bronchial artery-pulmonary artery anastomotic vessels may be found. Reprinted with permission from Deffenbach ME, Charan NB, Lakshminarayans S, Butler J. The bronchial circulation. Am Rev Respir Dis 1987;135:463-81.

figure 44-4 Bronchial arterial and venous circulations. The bronchial arterial circulation to the extraparenchymal airways drains to systemic venous (azygous) vessels. The bronchial arterial circulation to the intraparenchymal airways may drain to deep veins, which anastomose to the left atrium or pulmonary veins or into the pulmonary circulation. The majority of flow is via the latter pathway. In addition, direct bronchial artery-pulmonary artery anastomotic vessels may be found. Reprinted with permission from Deffenbach ME, Charan NB, Lakshminarayans S, Butler J. The bronchial circulation. Am Rev Respir Dis 1987;135:463-81.

reversal of flow from the pulmonary to the bronchial circulation is sufficient to maintain airway viability and allow for anastomotic healing.

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment