Hemolysis during and after 21 days of head-down-tilt bed rest

Abstract

Hemoconcentration is observed in bed rest studies, descent from altitude, and exposure to microgravity. Hemoconcentration triggers erythrocyte losses to subsequently normalize erythrocyte concentration. The mechanisms of erythrocyte loss may involve enhanced hemolysis, but has never been measured directly in bed rest studies. Steady-state hemolysis was evaluated by measuring two heme degradation products, endogenous carbon monoxide concentration [CO] and urobilinogen in feces, in 10 healthy men, before, during, and after two campaigns of 21 days of 6° head-down-tilt (HDT) bed rest. The subjects were hemoconcentrated at 10 and 21 days of bed rest: mean concentrations of hemoglobin (15.0 ± 0.2 g/L and 14.6 ± 0.1 g/L, respectively) and erythrocytes (5.18 ± 0.06E6/μL and 5.02 ± 0.06E6/μL, respectively) were increased compared to baseline (all Ps < 0.05). In contrast, mean hemoglobin mass (743 ± 19 g) and number of erythrocytes (2.56 ± 0.07E13) were decreased at 21 days of bed rest (both Ps < 0.05). Indicators of hemolysis mean [CO] (1660 ± 49 ppb and 1624 ± 48 ppb, respectively) and fecal urobilinogen concentration (180 ± 23 mg/day and 199 ± 22 mg/day, respectively) were unchanged at 10 and 21 days of bed rest compared to baseline (both Ps > 0.05). A significant decrease in [CO] (-505 ppb) was measured at day 28 after bed rest. HDT bed rest caused hemoconcentration in parallel with lower hemoglobin mass. Circulating indicators of hemolysis remained unchanged throughout bed rest supporting that enhanced hemolysis did not contribute significantly to erythrocyte loss during the hemoconcentration of bed rest. At day 28 after bed rest, decreased hemolysis accompanied the recovery of erythrocytes, a novel finding.

Keywords: Carbon monoxide; head‐down‐tilt bed rest; hemolysis; urobilinogen.

Figure 1

CONSORT diagram of the study.

Figure 2

Hemoglobin, erythrocyte, and reticulocyte concentrations before, during, and after bed rest. Subjects remained hemoconcentrated during the 21 days of bed rest. A loss of erythrocytes is uncovered at reambulation day 1, with lower concentrations of hemoglobin and erythrocytes compared to baseline. The reticulocyte concentration was not significantly lower at R1 than at baseline. *P < 0.05 compared to baseline data collection (BDC). Reticulocyte concentrations were multiplied by 85 to be represented on the same Y2 axis as erythrocyte concentrations. Since the repeated measures ANOVA showed no effect of nutrition intervention (diet/control) or campaign (1 and 2) but a strong effect of duration of bed rest, data from all subjects are presented according to duration of bed rest. HDT: head down tilt; R: recovery.

Figure 3

Endogenous [CO] and urobilinogen before, during, and after bed rest. Two direct measures of hemolysis, endogenous [CO] and urobilinogen, showed similar patterns. CO and urobilinogen constitute 1:1 end‐products from the heme oxidase catalysis of heme. No significant increase in [CO] or urobilinogen was measured during the hemoconcentrated state of bed rest compared to baseline. There was a significant decline in hemolysis at R28/R28repeat. There was also a significant decline in endogenous [CO] at HDT5, measured only in campaign 2 with n = 9. *P < 0.05 compared to baseline data collection (BDC). Stools were collected over 3 consecutive days, and average daily amount of urobilinogen is represented. ppb: parts per billion; HDT: head down tilt; R: recovery.

Figure 4

tHb loss during bed rest. tHb declined between baseline data collection day 5 (BDC5) and the end of 21 days of head‐down‐tilt bed rest (HDT21). The contributors to this decline in tHb included venipuncture and calculated decrease in erythrocyte production. Hemolysis of the unexplained portion of the tHb loss would have contributed a 185 ppb increase in steady‐state endogenous [CO] during bed rest.