It has been estimated that hemodynamically unstable MPE constitutes 10% of all PE presentations, although this percentage may be higher given the aforementioned selection bias issues.22 In the UPET,35 the Urokinase-Streptokinase Embolism Trial (USPET),96 and the ICOPER,12 9% (14 of 160 patients), 7% (12 of 167 patients), and 4.2% (103 of 2,454 patients) of all patients, respectively, initially presented in shock. In the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED), 10% of all patients (38 of 383 patients) presented in circulatory collapse, as defined by the presence of shock or syncope.97–99 In the largest observational series ever performed, with entry criteria requiring acute right heart failure or pulmonary hypertension due to PE, the MAPPET13 reported that 59% of patients had hemodynamic instability on presentation (cardiac arrest, 18% [176 of 1,001 patients]; shock requiring vasopressor therapy support, 10% [102 of 1,001 patients]; and arterial hypotension of < 90 mm Hg not requiring vasopressor therapy, 31% [312 of 1,001 patients]). Syncope deserves special mention because, although it is not a presentation that is commonly recognized to be associated with PE, it has been reported in 13% of patients in large clinical trials35,96 and in a large case series.42 In the series by Thames et al,42 syncope was recurrent (35%) and was more prominent in women (82%) and patient presentations from outside the hospital (70%), but was distributed equally among patients with and without CPD. A comparison of PE patients with and without syncope revealed higher incidences of angiographic obstruction of ≥ 50% (82% vs 28%, respectively), RAP ≥ 8 mm Hg (88% vs 33%, respectively), cardiac index ≤ 2.5 L/min/m2 (70% vs 32%, respectively), arterial Po2 ≤ 60 mm Hg (83% vs 31%, respectively), new incomplete right bundlebranch block (RBBB) or S1Q3T3 pattern (60% vs 12%, respectively), and cardiac arrest (24% vs 1%, respectively) among patients with syncope. Cor pulmonale was found in 94% of patients, and hypotension, which was initially present in 76% of patients, resolved in 38% of patients but required continued vasoactive support in 62%.,42 Defining the true incidence of cardiac arrest is problematic, as this patient subgroup is infrequently reported. In the series by Miller et al100 of 68 patients without CPD and anatomically massive PE (ie, ≥ 50% obstruction), cardiac arrest occurred in 29% of patients and was more common in the group experiencing persistent shock. In the MAPPET,,13 which required pulmonary hypertension or right heart failure due to PE, the incidence of cardiac arrest was 18% (176 of 1,001 patients).