The pulmonary trunk occupies the most anterior and leftward position among the great arterial roots. Its short intrapericardial course and bifurcation place it in close relationship with the ascending aorta, left atrial appendage, coronary arteries, and superior surface of the left atrium. For electrophysiologists, the essential feature is the variable extension of right ventricular myocardium into the pulmonary sinuses and the continuity of the root with the right ventricular outflow tract. [1-4]

1  Pulmonary root and ventriculoarterial junction

The pulmonary root connects the right ventricular outflow tract to the pulmonary trunk. Like the aortic root, it has three semilunar leaflets attached in a crown-shaped fashion. The basal points of attachment define an imaging plane, but there is no complete fibrous ring corresponding to a true anatomical annulus. The anatomical ventriculoarterial junction is the transition from infundibular myocardium to the fibroelastic wall of the trunk. [1,2]

The pulmonary sinuses are named according to their spatial position. Attitudinal description avoids transferring surgical or developmental terms that may obscure their actual relation to the aortic root. The hinge lines rise to the sinotubular junction, leaving interleaflet triangles between adjacent leaflets.

2  Relationship with the aortic root

The pulmonary root is anterior and leftward relative to the aortic root. The two roots are not arranged side by side in a single coronal plane. The posterior-facing pulmonary sinus is closely related to the adjacent aortic sinuses, while the muscular outlet septum and infundibular myocardium contribute to the separation between the outflow tracts. [1,2]

Because the roots spiral around one another, a catheter placed in a pulmonary sinus may lie near an aortic sinus or proximal coronary artery. This arrangement is particularly relevant when a ventricular arrhythmia can be mapped from more than one neighbouring chamber.

3  Pulmonary trunk and bifurcation

The pulmonary trunk passes superiorly and posteriorly from the right ventricular outlet before dividing into right and left pulmonary arteries. The bifurcation lies beneath the aortic arch and anterior to the left main bronchus. The arterial ligament connects the proximal left pulmonary artery to the undersurface of the arch.

The right pulmonary artery passes posterior to the ascending aorta and superior caval vein and anterior to the right main bronchus. The left pulmonary artery courses posteriorly and leftward, superior to the left superior pulmonary vein and anterior to the descending aorta. Cross-sectional imaging is particularly helpful because these relations cannot be appreciated reliably in a single fluoroscopic projection.

4  Myocardial extensions into the pulmonary root

Right ventricular myocardium commonly extends above the anatomical ventriculoarterial junction into the bases of one or more pulmonary sinuses. Large anatomical series have shown these extensions more frequently in the pulmonary than in the aortic root. Their height and circumferential distribution vary, and the myocardial crescents may be discontinuous. [3]

These extensions create a substrate for arrhythmias mapped above the pulmonary valvar hinge. The origin may remain connected to deeper infundibular myocardium, producing different activation patterns according to the recording site. The finding should be described as myocardial extension into the root, not as a generalized myocardial sleeve of the entire pulmonary trunk.

5  Relations with coronary arteries and adjacent structures

The left main coronary artery passes between the pulmonary trunk and left atrial appendage before bifurcating. The proximal anterior interventricular artery descends close to the anterior and leftward aspect of the right ventricular outflow tract, whereas the proximal right coronary artery is related to the opposite side of the outflow region. These relations impose limits on epicardial and supravalvar ablation.

The transverse pericardial sinus lies behind the arterial trunks. The left atrial appendage embraces the leftward aspect of the pulmonary trunk, while the roof of the left atrium and pulmonary veins lie more posteriorly. Pericardial reflections constrain catheter movement around the arterial bifurcation.

6  Electrophysiological relevance

Mapping within the pulmonary sinuses or proximal trunk may be required for arrhythmias arising from myocardial extensions above the valve or from neighbouring structures. Interpretation must integrate the right ventricular outflow tract, aortic sinuses, coronary arteries, and epicardial surface. Similar electrograms in adjacent locations do not prove a single anatomical chamber of origin.

Before energy delivery, the operator must establish the relation to the valvar leaflets and coronary arteries. This chapter defines the anatomy; mapping strategies and ablation parameters should remain within the dedicated ventricular arrhythmia section.