Spleen reactivity during incremental ascent to altitude

Abstract

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. Twelve participants completed 2 min of handgrip exercise at 30% of maximal voluntary contraction at 1,045, 3,440, and 4,240 m. 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- to 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. No changes in resting spleen volume were observed between altitudes. At low altitude, the spleen contracted in response to handgrip [272.8 ml (SD 102.3) vs. 249.6 ml (SD 105.7), P = 0.009], leading to an increase in hematocrit (42.6% (SD 3.3) vs. 44.3% (SD 3.3), P = 0.023] but did not contract or 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. 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. NEW & NOTEWORTHY The present study demonstrated that, despite the known role of the human spleen in increasing oxygen delivery to tissues during acute hypoxia scenarios, the spleen does not mobilize red blood cells during ascent to high altitude. Furthermore, the spleen's response to acute stressors at altitude depends on the nature of the stressor; the spleen's sensitivity to neurotransmitter is maintained, while its reflex response to stress is dampened.

Keywords: acclimatization; hypoxia; spleen; sympathetic activity.

Fig. 1.

Spleen volumes in response to 120 s of isometric handgrip exercise at 30% maximal voluntary contraction at increasing altitude (A) and phenylephrine hydrochloride infusion at increasing altitude (B). Values are means (SE). Black bars represent baseline (BL) spleen volume, and gray bars represent spleen volume at the end of the physiological stress period (HG, handgrip; PE, phenylephrine). *Significant decrease (P < 0.05) of spleen volume during stimulus period compared with baseline at the same altitude.

Fig. 2.

Change in spleen volume (%) and associated change in circulating hematocrit (%) in response to 120 s of isometric handgrip exercise at 30% of maximal voluntary contraction at increasing altitude (A) and infusion of phenylephrine hydrochloride at increasing altitude (B). Values are means (SE). *Significant decrease in spleen volume or increase in hematocrit from baseline to the end of the physiological stress period (P < 0.05).

Fig. 3.

Resting spleen volume (ml) of individual participants (n = 12) across 3 altitudes during incremental ascent to altitude. Individual dot/line combinations each represent a single participant.