A normal trachea has rings of cartilage that help keep it open. As the rings become weaker and start to collapse, the air tube becomes flatter. Inflammation in the esophagus can push on the trachea, also adding pressure to the cartilage rings. Veterinarians use a four-grade scale for tracheal collapse. With Grade 1 collapse, the cartilage rings still function but the tracheal lumen (the cells that support the trachea) have been reduced by about 25%. Grade 2 collapse indicates the lumen have decreased by 50% and Grade 3 indicates a 75% reduction in the lumen. With Grade 4, the cartilage has completely collapsed.