Interesting articles (see links below) from John Pricci\'s email correspondence with Retired VET - Rod Cundy, D.V.M., Calgary, Alberta, Canada. The articles primarily focuses on Laxis, but provides a reasonable hypothesis on the TCO2 threshold level.
http://www.horseraceinsider.com/Saratoga-Diary/08302011-vet-racing-better-served-without-salix/
http://www.horseraceinsider.com/Saratoga-Diary/08312011-retired-vet-speaks-out-on-lasix-part-ii/
This is not really currently accepted physiology as causal for exercise-induced pulmonary hemorrhage.