A catheter is a tube inserted through the urethra into the bladder to drain urine. The main purpose of the catheter is to treat urinary retention, so the catheter is left in the urethra and bladder. Nowadays, the catheter is usually a double-lumen catheter with an air bladder. There is an air bladder at the head end of the catheter, and the head end of the catheter can be fixed in the bladder after the air bladder is inflated with water, so that the catheter can avoid slipping out of the bladder, so it can be left in place for a long time to achieve adequate drainage of urine. This is because for patients with acute urinary retention, it is usually recommended to leave the catheter in place for 5-7 days. This gives the bladder time to rest sufficiently, allowing the bladder muscles to relax and increasing the probability of successful self-extraction of urine after extubation.