Fig. 4

The distance between the ring and the airway A: Grade III: “U”, the tissue between the trachea and the blood vessels forming the ring is clearly visible and far away B: Grade II: “U”, the tissue between the trachea and the blood vessels forming the ring is visible, and the distance is slightly close, so direct measurement is difficult C: Grade I: “U”, the tissue between the trachea and the blood vessel on the side of the ring is gone D: Grade I: PAS, “入”, the tissue between the trachea and the blood vessel on the side of the ring is gone, tracheal cross section deformation