Gas emboli may enter the arterial circulation in any of the following ways: From ruptured alveoli after lung barotrauma From within the arterial circulation itself in severe decompression sickness Via migration from the venous circulation (venous gas embolism) either via a right-to-left shunt (patent foramen ovale, atrial septal defect) or by overwhelming the filtering capacity of the lungs Even an otherwise asymptomatic venous gas embolism can cause serious manifestations (eg, stroke) in the presence of a right-to-left shunt.