Total haemoglobin mass and spleen contraction: a study on competitive apnea divers, non-diving athletes and untrained control subjects.

In diving mammals splenic contraction increases circulating red cell volume, whereas in humans increased haemoglobin concentrations have been reported. It is unknown, however, whether repetitive apnea diving also comprises an adaptive increase in total red cell volume as reported in endurance athletes. The first aim of the study therefore was to investigate the effect of repeated apnea dives on splenic size and putative red cell release in trained apnea divers (n = 10) and control subjects (SCUBA divers performing apneas without long-term apnea training, n = 7). Long-term effects of repetitive apnea diving may elevate the oxygen transport capacity by an adaptive increase in total haemoglobin mass as reported in endurance athletes. The second goal, therefore, was to compare the trained apnea divers' and the control divers' total haemoglobin mass (tHb-mass) with that of endurance-trained (n = 9) and untrained (n = 10) non-divers. Before and immediately after a series of five dives to a depth of 4 m in a heated pool, spleen volume was assessed with ultrasound tomography. tHb-mass and plasma volume were measured using the CO-rebreathing method. In the trained apnea divers, repeated apnea dives resulted in a 25% reduction of spleen size (P < 0.001), whereas no significant effect was observed in the control subjects. While tHb-mass did not differ between trained apnea divers, untrained SCUBA divers performing apneas and untrained non-divers, it was 30% lower than in endurance-trained non-divers. We conclude that prolonged apnea training causes marked apnea-induced splenic contraction. In contrast to athletes in endurance sports, the trained apnea divers did not present with increased total haemoglobin mass and, hence, no increase in blood oxygen stores.