Effects of 150- and 450-mL Acute Blood Losses on Maximal Oxygen Uptake and Exercise Capacity

Abstract
Purpose: This study aimed to determine how small acute blood loss (150 mL) affects maximal oxygen uptake (V˙O2max) and exercise capacity, and to identify the associated compensatory mechanisms.Methods: Thirteen male participants (V˙O2max: 63 ± 9 mL·kg^-1·min^-1) performed incremental exercise to exhaustion on a cycle ergometer under three conditions: normal blood volume (control; 6.0 ± 0.7 L), and immediately after acute reductions in blood volume of 150 mL (BVR150mL) and 450 mL (BVR450mL). Plasma volume (PV) and blood volume (BV) changes during exercise were assessed using hematocrit, hemoglobin concentration, and hemoglobin mass (carbon monoxide rebreathing).Results: The decrease in V˙O2max per milliliter of blood volume reduction was 2.5 times greater after BVR450mL compared to BVR150mL (-0.7 ± 0.3 vs. -0.3 ± 0.6 mL·min^-1·mL^-1, P = 0.029). V˙O2max remained unchanged after BVR150mL (-1% ± 2%, P = 0.124), but dropped by 7% ± 3% after BVR450mL (P < 0.001) compared to control. Peak power output decreased only after BVR450mL (P < 0.001). At maximal exercise, BV was restored after BVR150mL compared to control (-50 ± 185 mL, P = 0.375), attributed to PV restoration. However, BV was not fully restored after BVR450mL (-281 ± 184 mL, P < 0.001), despite PV restoration. Peak heart rate showed a tendency to increase (3 ± 5 bpm, P = 0.062), while O2 pulse (-2 ± 1 mL per beat, P < 0.001) and vastus lateralis tissue oxygenation index (-4% ± 8% points, P = 0.080) decreased after BVR450mL, indicating reduced stroke volume and increased leg O2 extraction.Conclusion: The negative impact on V˙O2max and maximal exercise capacity intensifies with the severity of acute blood loss. This is likely due to the ability to quickly restore PV to achieve BV-6 euvolemia after minor blood loss, but not after moderate blood loss.