Causes
Because of how deeply the heart and the lungs are intertwined, the causes of pulmonary edema are often related to an insult affecting the heart.
» To better understand these pathological entities, it helps to take a look at the relations between the heart, the lungs, and the blood vessels that connect them:
The human heart is composed of four chambers: Two small atria above, and two large ventricles below. The atria receive blood coming to the heart, and the ventricles pump blood away from the heart. The heart can thus be divided into two sides: Right and left. Hence, we have a right atrium, a right ventricle, a left atrium, and a left ventricle.
The right ventricle pumps blood to the lungs through the pulmonary artery. This blood travels through the pulmonary vascular tree and gets loaded with oxygen from the air sacs. The lungs, having performed their function of oxygenating the blood, pass this blood into the pulmonary veins, which are connected to the left atrium. The blood then ends up in the left ventricle, which pumps it to the rest of the body.
Regarding the mechanisms by which blood is pumped from the heart: An important structure that prevents the backflow of blood after pumping is the heart valve. Heart valves exist between atria and ventricles, and also between the heart and its great vessels: the aorta and the pulmonary artery. They close immediately after cardiac contraction, and re-open only to allow the blood flow to the correct direction.
With this in mind, how would a cardiac condition cause acute pulmonary edema? It often boils down to this: An abnormality of the heart impairs its ability to pump the blood it receives from the lungs. With each inefficient heartbeat, more and more blood accumulates in the left ventricle. The increased pressure is then transmitted to the left atrium above, then back into the pulmonary veins and pulmonary vasculature. The pulmonary vessels thus congested and with no method of relief begin to leak fluid into the air spaces. This makes it much more difficult for oxygen to enter the patient’s blood, thus impairing the main function of the lungs: Gas exchange.
Conditions that can cause this process include a heart attack (also called an acute myocardial infarction), which is often precipitated by decades of atherosclerosis, a condition which narrows the blood vessels of the body, often including those that supply the heart itself: The coronary arteries.
If the aforementioned heart valves fail to prevent the backflow of blood, blood will be regurgitated back into the left ventricle with each heartbeat. Examples of this include Mitral regurgitation and Aortic regurgitation. The mitral valve divides the left ventricle and the left atrium, and the aortic valve secures the largest blood vessel in the body: The aorta.
An abnormal cardiac rhythm may also compromise the orchestra of heart chamber contraction and relaxation, leading to a cacophony of uncoordinated heartbeats that end in acute pulmonary edema.
