You may be referring to an injection given into the epidural space around the spinal cord. A local anesthetic combined with a narcotic is used for pain control. The doctor uses a needle inserted into the epidural space to put the drug in effect.
There may not be any space open for the needle to enter if there is a lot of inflammation and swelling in the area. Sometimes the needle is too large for the patient's anatomy preventing entry. The doctor can tell if the needle is in the right place. A special test called the whoosh test is used.
Air is injected through the needle into the space before the drug is used. The doctor listens to the spine with a stethoscope. If the needle is in the right place, the sound of air will be heard. The doctor can also use imaging X-rays to show the placement of the needle.