Pink frothy sputum is most often caused by the infiltration of fluid from the capillaries into the interstitium or alveoli of the lungs. Clinically common pulmonary edema is cardiogenic pulmonary edema and nephrogenic pulmonary edema. Because acute left heart failure causes increased pulmonary venous pressure, increased plasma exudation from alveolar capillaries and even capillary rupture, forming acute pulmonary edema, this edema fluid is protein-containing, plasma or pink, and under the action of airflow, it will form foamy, just like there are bubbles if urine protein content increases. Meanwhile, the following causes can also lead to coughing up pink foamy sputum: 1. acute diffuse myocardial damage: such as myocarditis, heart attack. 2. Acute mechanical obstruction: e.g., diastasis, aortic stenosis. 3.Acute increased volume load: intravenous blood transfusion, excessive fluid infusion. 4.Acute ventricular diastolic restriction: pericardial effusion causing decreased cardiac output. The pathological reasons are for the sudden and severe weakening of cardiac contractility, the rapid decrease of cardiac blood output, or acute regurgitation of left ventricular valvularity, rapid increase of LVEDP, and poor pulmonary venous reflux. As the pulmonary venous pressure rises rapidly, the pulmonary capillary pressure rises, causing intravascular fluid to infiltrate into the interstitium and alveoli of the lungs to form acute pulmonary edema. The reason for the pink foamy appearance is the presence of red blood cells, which should be interspersed with some exudate, which is mucus-like. The pink foamy sputum is mainly due to the left lung stasis edema and therefore has red blood cells, hence the pink color.