The effect of posture and exercise on blood CO kinetics during the optimized carbon monoxide rebreathing procedure

An indispensable precondition for the determination of hemoglobin mass (Hbmass) and blood volume by CO rebreathing is complete mixing of CO in the blood. The aim of this study was to demonstrate the kinetics of CO in capillary and venous blood in different body positions and during moderate exercise. Six young subjects (4 male, 2 female) performed three 2-min CO rebreathing tests in seated (SEA) & supine (SUP) positions as well as during moderate exercise (EX) on a bicycle ergometer. Before, during, and until 15 min after CO rebreathing cubital venous and capillary blood samples were collected simultaneously and COHb% was determined. COHb% kinetics were significantly slower in SEA than in SUP or EX. Identical COHb% in capillary and venous blood were reached in SEA after 5.0 +/- 2.3 min, in SUP after 3.2 +/- 1.3 min and in EX after 1.9 +/- 1.2 min (EX vs. SEA p < .01, SUP vs. SEA p < .05). After 7th min, Hbmass did not differ between the resting positions (capillary: SEA 766 +/- 217 g, SUP 761 +/- 227 g; venous: SEA 759 +/- 224 g, SUP 744 +/- 207 g). Under exercise, however, a higher Hbmass (p < .05) was determined (capillary: 823 +/- 221 g, venous: 804 +/- 226 g). In blood, the CO mixing time in the supine position is significantly shorter than in the seated position. By the 6th minute complete mixing is achieved in either position giving similar Hbmass determinations. CO-rebreathing under exercise conditions, however, leads to similar to 7% higher Hbmass values.