Final answer:
The correct modifier for a procedure that was started but discontinued due to complications such as respiratory distress is modifier -53, used in medical billing and coding.
Step-by-step explanation:
If a patient experiences respiratory distress during a medical procedure such as an arthroscopy of the knee and the procedure has to be terminated as a result, the correct modifier for the medical billing code would be -53. This modifier is used to indicate that a service or procedure was started but discontinued. In this scenario, because the arthroscope was inserted but the procedure had to be terminated due to respiratory complications, modifier -53 would be used when coding for insurance and billing purposes.