Mid Sweden University

Spleen contraction resulting in an increase in circulating erythrocytes has been shown to occur during apnea. This effect, however, has not previously been studied during normobaric hypoxia whilst breathing. After 20 min of horizontal rest and normoxic breathing, five subjects underwent 20-min of normobaric hypoxic breathing (12.8% oxygen) followed by 10 min of normoxic breathing. Ultrasound measurements of spleen volume and samples for venous hemoglobin concentration (Hb) and hematocrit (Hct) were taken simultaneously at short intervals from 20 min before until 10 min after the hypoxic period. Heart rate, arterial oxygen saturation (SaO2) and respiration rate were recorded continuously. During hypoxia, a reduction in SaO2 by 34% (P < 0.01) was accompanied by an 18% reduction in spleen volume and a 2.1% increase in both Hb and Hct (P < 0.05). Heart rate increased 28% above baseline (P < 0.05). Within 3 min after hypoxia SaO2 had returned to pre-hypoxic levels, and spleen volume, Hb and Hct had all returned to pre-hypoxic levels within 10 min. Respiratory rate remained stable throughout the protocol. This study of short-term exposure to eupneic normobaric hypoxia suggests that hypoxia plays a key role in triggering spleen contraction and subsequent release of stored erythrocytes in humans. This response could be beneficial during early altitude acclimatization.

Introduction: Spleen contraction occurs in humans during exercise, apnea, and simulated altitude, resulting in ejection of stored red blood cells into circulation. The mechanisms responsible for initiating the contraction are not fully known: hypoxia is likely involved, but other, unknown factors may also contribute. To reveal the initiating factors, we studied its occurrence in two different situations involving similar reductions in arterial oxygen saturation (SaO2). We hypothesized that similar spleen responses would result if the level of hypoxia is the main factor involved. Methods: Five female and four male healthy volunteers performed two different trials on separate days: 1)20 min of normobaric hypoxic breathing (14.2% oxygen); and II) 2 min of apnea after a deep inspiration of air. Both trials started and ended with 10 min of sitting eupneic rest. Spleen diameter was intermittently measured via ultrasonic imaging in three dimensions to calculate volume. S aO2 and heart rate (HR) were recorded continuously with a pulse oximeter. Results: Exposures resulted in similar nadir SaO 2: 87% after normobaric hypoxia and 89% after apnea. During normobaric hypoxia, spleen volume was reduced by 16% and during apnea by 34%. HR increased by 7% during normobaric hypoxia, but fell by 25% during apnea. Discussion: Both normobaric hypoxia and apnea induced spleen contraction, but despite similar levels of SaO2 apnea evoked a significantly stronger response, possibly due to hypercapnia, faster desaturation, ortheapneic stimulus in itself. Spleen contraction may facilitate adaptation to altitude and to apneic diving by elevating blood gas storage capacity.

BACKGROUND:

Splenic contraction associated with apnea causes increased haemoglobin concentration and haematocrit (Hct), an effect that may promote prolonged breath-holding. Hypoxia has been shown to augment this effect, but hypercapnic influences have not been investigated previously.

METHODS:

Eight non-divers performed three series of apneas on separate days after inspiration of oxygen with different carbon dioxide (CO₂) levels. Each series consisted of three apneas 2 minutes apart: one with pre-breathing of 5% CO₂ in oxygen (O₂, 'Hypercapnia'); one with pre-breathing of 100% O₂ ('Normocapnia'); and one with hyperventilation of 100% O₂ ('Hypocapnia'). The apnea durations were repeated identically in all trials, determined from the maximum duration attained in the CO₂ trial. A fourth trial, breathing 5% CO₂ in O₂ for the same duration as these apneas was also performed ('Eupneic hypercapnia'). In three subjects, spleen size was measured using ultrasonic imaging.

RESULTS:

Haemoglobin increased by 4% after apneas in the 'Hypercapnia' trial (P = 0.002) and by 3% in the 'Normocapnia' trial (P = 0.011), while the 'Hypocapnia' and 'Eupneic hypercapnia' trials showed no changes. The 'easy' phase of apnea, i.e., the period without involuntary breathing movements, was longest in the 'Hypocapnia' trial and shortest in the 'Hypercapnia' trial. A decrease in spleen size was evident in the hypercapnic trial, whereas in the hypocapnia trial spleen size increased, while only minor changes occurred in the other trials. No differences were observed between trials in the cardiovascular diving response.

CONCLUSION:

There appears to be a dose-response effect of CO₂ on triggering splenic contraction during apnea in the absence of hypoxia.

Release of stored red blood cells resulting from spleen contraction improves human performance in various hypoxic situations. This study determined spleen volume resulting from two contraction-evoking stimuli: breath holding and exercise before and after altitude acclimatization during a Mount Everest ascent (8848m). Eight climbers performed the following protocol before and after the climb: 5min ambient air respiration at 1370m during rest, 20min oxygen respiration, 20min ambient air respiration at 1370m, three maximal-effort breath holds spaced by 2min, 10min ambient air respiration, 5min of cycling at 100 W, and finally 10min ambient air respiration. We measured spleen volume by ultrasound and capillary hemoglobin (HB) concentration after each exposure, and heart rate (HR) and arterial oxygen saturation (Sao2) continuously. Mean (SD) baseline spleen volume was unchanged at 213 (101) mL before and 206 (52) mL after the climb. Before the climb, spleen volume was reduced to 184 (83) mL after three breath holds, and after the climb three breath holds resulted in a spleen volume of 132 (26) mL (p=0.032). After exercise, the preclimb spleen volume was 186 (89) mL vs. 112 (389) mL) after the climb (p=0.003). Breath hold duration and cardiovascular responses were unchanged after the climb. We concluded that spleen contraction may be enhanced by altitude acclimatization, probably reflecting both the acclimatization to chronic hypoxic exposure and acute hypoxia during physical work. © Copyright 2014, Mary Ann Liebert, Inc. 2014.