Saddle thromboemboli occurs when pulmonary emboli occurs at  the level of the bifurcation of the pulmonary trunk and extends into the main right and left pulmonary arteries.

Such proximal thrombus are generally regarded as unstable, "in-transit" embolus, which can fragment spontaneously or secondary to treatment and obstruct multiple, distal pulmonary arteries. Often there are findings associated with right heart strain as a result of increased afterload on the right ventricle from a large thrombus burdun.  In the acute setting the RV cannot compensate for the afterload increase and there is development of right ventricular dilatation with pump failure (falling of the Starling Curve). 

The general understanding is that emergent interventional treament is required with catheter directed TPA. Patients with saddle embolism have also been clinically treated with various methods including IV heparin, IV TPA, or open thrombectomy. 

High mortality is generally associated with saddle pulmonary embolism and can be a cause of sudden cardiac arrest.