What is the medical term for the presence of air in the veins, which can lead to cardiac arrest?

The term you're looking for is "air embolism," which specifically refers to the presence of air within the circulatory system, particularly veins or arteries. This condition can occur due to various reasons, such as trauma, certain medical procedures, or decompression sickness. When air enters the venous system, it can travel to the heart and potentially lead to a blockage in the blood flow, especially if it reaches the right side of the heart and then the pulmonary arteries. This obstruction can result in serious complications, including cardiac arrest, as it disrupts the normal flow of oxygenated blood to the organs.

In contrast, airway obstruction pertains to a blockage in the upper respiratory tract and affects breathing rather than the circulatory system. Alveolar ventilation refers to the amount of fresh air that reaches the alveoli where gas exchange occurs, which is a different aspect of respiratory function. An allergic reaction involves the immune system and does not directly relate to air entering the bloodstream. Therefore, "air embolism" is the precise medical term for this dangerous condition that can impair circulation and result in cardiac complications.