"Lasix", is a powerful diuretic that causes fluids locked up in the horse's body tissues to be released and expelled in the urine. This has the effect of lowering the horse's blood pressure, particularly in the aorta and pulmonary artery. The lower blood pressure in the capillaries mitigates the problem of bleeding, thus, the horse's performance returns to a normal level. In addition, Lasix has the side effect of increasing the Carbon Dioxide (CO2) in the blood.
So, it appears elevated CO2 is a sign the horse has been doped with the diuretic Lasix.
I had an OT TB that I bought for an eventer when they closed the DetroitRC barns.
He came with a lasix certificate because when he ran hard he would get nosebleeds. This apparently is not so uncommon so I’m not sure if Lasix is considered “doping” in the usual sense.
Lasix is legal in Ca.