INTRODUCTION

Levo- or L-looped transposition of the great arteries (L-TGA) is a rare form of congenital heart disease characterized by atrioventricular and ventriculoarterial discordance. It is also commonly referred to as congenitally corrected TGA, double discordance, or ventricular inversion.

L-TGA usually does not present with cyanosis unless there are associated cardiac defects. Isolated L-TGA is "physiologically corrected" because systemic deoxygenated venous blood returns to the pulmonary circulation and oxygenated pulmonary venous blood returns to the systemic circulation. Patients with L-TGA are at increased risk for heart failure as adults due to progressive decline in systemic right ventricular function.

The pathophysiology, clinical features, diagnosis and management of L-TGA will be presented here. D-TGA is discussed separately. (See "Pathophysiology, clinical manifestations, and diagnosis of D-transposition of the great arteries" and "Management and outcome of D-transposition of the great arteries".)

EMBRYOLOGY AND ANATOMY

Cardiac loop formation — Looping of the straight heart tube during the third week of gestation is one of the key embryologic processes for correct anatomic alignment of the four chambers of the heart. Normally, the primitive heart tube “loops” to the right (dextro or [D] loop) resulting in the normal morphologic position of the right ventricle (RV) to the right of the left ventricle (LV). However, looping to the left (levo or [L] loop) leads to abnormal positioning of the ventricles and to abnormal connections among the atrial, ventricular, and arterial segments of the heart.

Anatomy of L-TGA — Levo-transposition of the great arteries (L-TGA) is due to the abnormal leftward looping of the primitive heart, which results in the morphologic LV being positioned to the right of the morphologic RV and to both atrioventricular and ventriculoarterial discordance (figure 1). In this lesion, deoxygenated systemic venous blood return flows from the correctly located right atrium to the discordant LV via the mitral valve, and into the lung through the pulmonary arteries. Oxygenated blood flows from the lungs through the pulmonary veins to the left atrium into the discordant RV via the tricuspid valve, and returns to the systemic circulation through the aorta [1]. The aorta is typically abnormally positioned anterior and to the left of the pulmonary artery.