You hit a vein, aspirate, aspirate, aspirate.....what caused the cough is the BA being expeled through your lungs in the smaller veins/arteries.
To detect an oncoming coughing fit, as you inject, aspirate of course, if you begin to smell Fina....it's not in the air, it's in your lungs, take long slow, deep breaths and when the coughing fit occurs it will be much shorter lived. Learning this the hardway the first time is enough to be prepared.
bo