In an otherwise healthy male with previously normal pulmonary and cardiac function, how much of the pulmonary vascular bed must usually be occluded to produce an unstable cardiovascular state (shock)?

D. More than 50%.

DISCUSSION: If the patient had normal cardiovascular and respiratory function before the onset of pulmonary embolism, experimental and clinical studies have documented the fact that more than 50% of the pulmonary circulation must be occluded to produce cardiovascular collapse or shock. This can be considered similar to ligation of the pulmonary artery during the course of pneumonectomy, as occlusion by this procedure is also tolerated well, without development of hemodynamic instability. While bronchoconstriction may reduce pulmonary function in the normally perfused lung after embolism, this effect is generally short lived, as demonstrated by pulmonary ventilation scans.