Relation between resting spleen volume and apnea-induced increases in hemoglobin mass

Indigenous populations renowned for apneic diving have comparatively large spleen volumes. It has been proposed that a larger spleen translates to heightened apnea-induced splenic contraction and elevations in circulating hemoglobin mass (Hb_mass), which, in theory, improves O₂ carrying and/or CO₂/pH buffering capacities. However, the relation between resting spleen volume and apnea- induced increases in Hb_mass is unknown. Therefore, we tested the hypothesis that resting spleen volume is positively related to apnea-induced increases in total Hb_mass.

Fourteen healthy adults (six women; 29 ± 5 years) completed a two-minute carbon monoxide rebreathe procedure to measure pre-apneas Hb_mass and blood volume. Spleen length, width, and thickness were measured pre-and post-five maximal apneas via ultrasound. Spleen volume was calculated via the Pilström equation (test-retest CV:2 ± 2%). Hemoglobin concentration ([Hb]; g/dl) and hematocrit (%) were measured pre- and post-apneas via capillary blood samples. Post-apneas Hb_mass was estimated as post-apnea [Hb] x pre-apnea blood volume. Data are presented as mean ± SD.

Spleen volume decreased from pre- (247 ± 95 mL) to post- (200 ± 82 mL, p<0.01) apneas. [Hb] (14.6 ± 1.2 vs. 14.9 ± 1.2 g/dL, p<0.01), hematocrit (44 ± 3 vs. 45 ± 3%, p=0.04), and Hb_mass (1025 ± 322 vs. 1046 ± 339 g, p=0.03) increased from pre- to post-apneas. Pre-apneas spleen volume was unrelated to post-apneas increases in Hb_mass (r=-0.02, p=0.47). O₂ (+28 ± 31 mL, p<0.01) and CO₂ (+31 ± 35 mL, p<0.01) carrying capacities increased post-apneas.

Larger spleen volume is not associated with a greater rise in apneas-induced increases in Hb_mass in non-apnea-trained healthy adults.