The butterfly pattern of acute pulmonary edema

Among the factors causing uneven distribution of pulmonary edema, defective lymph drainage favors the accumulation of edematous fluid in areas of previous disease and scars, both pulmonary and pleural. The medulla and cortex of the lung are structurally different, and there are convincing reasons to assume nonuniformity of ventilation between them. Radiologic evidence shows that the cortex participates in the ventilatory play to a greater extent than the medulla; being readily distensible, it provides probably the greatest part of the ventilatory volume changes at rest. Lymphatic drainage, dependent on movements of the adjacent structures, is greatly promoted in the well ventilated parts, and the poorly moving central parts (medulla) seem to have a sluggish lymph flow. This would explain the frequently encountered preference of pulmonary edema for the central part of the lung or individual lobe. The involved central wet portion is occasionally set off abruptly from the dry cortex, because of the sharp morphologic and functional demarcation between medulla and cortex. The appearance of this demarcation is obviously dependent on factors of ventilation, perfusion, and others. It is, therefore, usually transient in cardiovascular edema.