A large pericardial effusion is the most common cause of pulsus paradoxus. Normally, you have some fluid in your pericardium, or sac around your heart. This gives your heart protection and cushioning. But sometimes, too much fluid builds up in your pericardium. Providers call this pericardial effusion. This fluid buildup presses on your heart because your pericardium can only stretch so much. When the fluid buildup squeezes the heart so much that blood can’t enter, it becomes cardiac tamponade. Other pulsus paradoxus causes include: Constrictive pericarditis. The sac around your heart (pericardium) stiffens and thickens. Pleural effusion. This is excess fluid around your lungs. Restrictive cardiomyopathy. Muscles in two of your heart’s four chambers get stiff. Asthma. Mucus blocks your narrowed airways. Tricuspid atresia. People with this condition are born without a tricuspid valve in their hearts. Pneumothorax (collapsed lung). Air gets outside your lung and presses against it. Thoracic outlet syndrome. These are compressed blood vessels in your upper chest. Chronic obstructive pulmonary disease (COPD). These are diseases that damage your lungs. Pectus excavatum. This is a congenital (since birth) issue in which your breastbone grows inward, creating a sunken chest wall. Other things that compress your heart, such as surgery or having a body mass index (BMI) greater than 30 (having obesity). In causes that don’t involve your pericardium, both blood pressure numbers (top and bottom) usually fall.