Welcome to our lesson on the use of catheters in surgery. One of the side effects of a general anaesthetic is the inability for the patient to control their bladder. To get around this issue, a Foley catheter is usually placed prior to surgery which drains and empties the bladder throughout the operation. The catheter does not get removed until the patient is awake and alert enough to urinate normally again. A foley catheter has a balloon at the end of it which is inflated to secure it within the bladder and prevent it falling out. This is the most common type of catheter used during surgery, however other indwelling catheter types exist for other use cases. Urine drains straight from the bladder through the catheter tube and into a collected bag. Another benefit of having an indwelling catheter is the ability to measure urine output both during and after surgery – this is a good marker of hydration as well as renal function which can be worsened in some operations. Sterile technique is utilised in the insertion of a catheter, however there is always a risk of introducing a urinary tract infection. Theatres are a great place for medical students to sign off their catheters as they are often easier and less intimidating than on the ward. Remember to get patient consent to do this before they are anaesthetised.  ​ In summary, catheters are a useful tool to ensure safe urine flow and to accurately measure urine output. They are more necessary for longer procedures and must be inserted with sterile technique.