The influence of hemoglobin concentration on exercise cardiac output.

Two experiments were performed to study the cardiac output (Q) vs. hemoglobin concentration (Hb) relationship during constant load submaximum eexercise. The first experiment examined the relationship between submaximum exercise Q and Hb in 28 females. A correlation of r = -0.83 (P less than 0.05) was observed between Q and Hb during xercise at 88 W (mean Q = 11.04 l . min-1) and 118 W (mean Q = 13.10 l . min-1) (mean steady rate VO2 = 1.40 l . min-1 and 1.79 l . min-1, respectively). By removing the influence of stroke volume (SV) from Q (part correlation analyses), the relationship between Q and Hb is compromised (r = -0.29, P greater than 0.05, 88 W and r = -0.33, P greater than 0.05, 118 W). The second experiment compared the VO2 max and submaximum exercise (118 W) Q responses in six males before and after blood donation. Experimentally reducing Hb 18.6% (P less than 0.05) caused a 6.2% decrease (P less than 0.05) in VO2 max. Additionally, submaximum exercise Q increased 12% (P less than 0.05) 6 days following hemodilution., The Q remained elevated 11 days (10% higher, P less than 0.05) and 16 days (9% higher, P less than 0.05) post-blood donation and progressively returned to pre-donation levels by 21 days post-withdrawal. The higher Q's during submaximum exercise were ascribed primarily to an 8% (P less than 0.05) higher SV in comparison to pre-donation levels. collectively, the data from the two experiments indicate that individual differences in submaximum exercise Q are due, in part, to individual differences in Hb concentration. Furthermore, it is suggested that the stimulus for the SV-induced Q vs. Hb association is related to maintaining maximal myocardial efficiency.