Exercise capacity deterioration and recovery after blood donation in women: a step forward

S ince the early studies by Karpovich and Millman it is well established that a reduction of total hemoglobin (Hb) decreases exercise capacity to a greater extent in longer than in shorter sporting events. However, some decades elapsed before Stangerup and coworkers have provided the first systematic assessment of the effect of a standard blood donation (450 mL) on the exercise capacity and its recovery in healthy women in this issue of TRANSFUSION. As expected, performance during a 3-km time trial deteriorated for 3 days after the blood donation, but no significant differences were observed after 14 days. However, the tendency line points toward 4 weeks for a 100% recovery of the baseline performance level. In fact, the VO2peak was clearly reduced 3 weeks after the donation and from the results reported it seems that a full recovery of VO2peak requires at least 4 weeks after the blood donation without iron supplements. VO2peak represents the higher flow of O2 than can be used in mitochondrial respiration during whole-body exercise and is a major determinant of performance in endurance events lasting from a few minutes to several hours. In healthy humans, O2 delivery is considered the main mechanism limiting VO2max, 5,6 together with the capacity to extract oxygen. Maximal O2 delivery is a function of arterial O2 content (CaO2) and peak cardiac output. The first negative effect of blood donation on performance is due to the reduction of blood volume, which progressively recovers during the next 24 to 72 hours, but the exact time needed to fully reestablish the baseline blood volume has not been clearly defined. A reduction of blood volume may result in a lower peak cardiac output because it is associated with lower central blood volume and filling pressures. Although blood volume was not measured, the results obtained by Stangerup and colleagues and previous studies provide indirect evidence supporting this effect. However, the main mechanism accounting for the reduction of exercise performance after a blood donation is the decrease in the O2-carrying capacity of blood. An isovolemic reduction of [Hb] decreases VO2peak and vice versa, but the relationship between the change in [Hb] and the changes in VO2peak is not linear, such that the reduction (or increase) in VO2peak is proportionally less than the change in [Hb] concentration (DVO2 5 0.22 1 0.7 D[Hb] 1 0.0054 D[Hb], 2 where D represents the percentage change from the starting value). This observation contrasts with the linear relationship reported between the reduction in CaO2 in acute hypoxia and the decrease in VO2peak. 10 The reduction in VO2peak is likely lower when the CaO2 has been diminished by isovolemic hemodilution than hypoxia, suggesting that for a similar reduction in CaO2, acute isovolemic hemodilution is better tolerated and has a lower impact on VO2peak than the same reduction elicited by acute hypoxia. Changes in the distribution of blood flow and O2 extraction could explain these differences, but also may be explained by the negative impact of hypoxia on pulmonary gas exchange. It has been reported that muscle O2-diffusing capacity is reduced with lower [Hb], presumably due to a greater intracapillary spacing of red blood cells (RBCs) or slower dissociation of O2 from Hb. 13,14 Nevertheless, O2 extraction at maximal exercise was not altered by isovolemic hemodilution with reductions of [Hb] by 20%. Previous studies were carried out in men and the [Hb] reduced from 8.9 6 2.4 to 7.1 6 1.2 mmol/L. It remains unknown whether a similar level of hemodilution in females, who have a lower basal [Hb] could reduce muscle O2-diffusing capacity. The women studied by Stangerup and colleagues remained higher than 7.1 mmol/L and therefore no impairment in O2 diffusion even during maximal exercise should be expected at the level of [Hb] reached after the blood donation. Moreover, it has been reported that the Fåhraeus effect, that is, the decrease in the hematocrit (Hct) of blood flowing into narrowing tubes below 300 mm in diameter, is less marked in the microvasculature of females. This observation means that the microvasculature in females contains more RBCs per unit volume of blood than in males at rest. However, muscle blood flow heterogeneity is reduced during exercise, and the Fåhraeus effect is unlikely to contribute to decreases in the microvascular Hct in contracting muscles. It is not clear at present whether a different regulation of microvascular Hct levels in women could attenuate the potentially negative effects of their lower [Hb] in muscle O2-diffusing capacity when exercising maximally. Although no study has directly compared the midterm responses to a blood donation in men and women, the same research group has applied a similar protocol to healthy men, although of slightly lower fitness levels doi:10.1111/trf.14224