Spleen reactivity during incremental ascent to altitude.

INTRODUCTION: The spleen contains a reservoir of red blood cells that are mobilized into circulation when under physiological stress. Despite the spleen having an established role in compensation to acute hypoxia, no previous work has assessed the role of the spleen during ascent to high altitude.

METHODS: Twelve participants completed 2 minutes of handgrip exercise at 30% of maximal voluntary contraction at 1045m, 3440m, and 4240m. In a subset of eight participants, an infusion of phenylephrine hydrochloride was administered at a dosage of 30 µg/l of predicted blood volume at each altitude. The spleen was imaged by ultrasound via a 2-5.5 MHz curvilinear probe. Spleen volume was calculated by the prolate ellipsoid formula. Finger capillary blood samples were taken to measure hematocrit. Spleen images and hematocrit were taken both before and at the end of both handgrip and phenylephrine infusion.

RESULTS: No changes in resting spleen volume were observed between altitudes. At low altitude, the spleen contracted in response to handgrip (272.8±102.3 vs 249.6±105.7 ml, p=0.009), leading to an increase in hematocrit (42.6±3.3% vs 44.3±3.3%, p=0.023) but did not contract nor increase hematocrit at the high-altitude locations. Infusion of phenylephrine led to spleen contraction at all altitudes, but only lead to an increase in hematocrit at low altitude.

CONCLUSION: These data reveal that the human spleen may not contribute to acclimatization to chronic hypoxia, contrary to its response to acute sympathoexcitation. These results are explained by alterations in spleen reactivity to increased sympathetic activation at altitude.