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Contribution of red blood cells to the compensation for hypocapnic alkalosis through plasmatic strong ion difference variations


Chloride shift is the movement of chloride between red blood cells (RBC) and plasma (and vice versa) caused by variations in pCO2. The aim of our study was to investigate changes in plasmatic strong ion difference (SID) during acute variations in pCO2 and their possible role in the compensation for hypocapnic alkalosis.


Patients admitted in this year to our ICU requiring extra-corporeal CO2 removal were enrolled. Couples of measurements of gases and electrolytes on blood entering (v) and leaving (a) the respiratory membrane were analyzed. SID was calculated as [Na+] + [K+] + 2[Ca2+] - [Cl-] - [Lac-]. Percentage variations in SID (SID%) were calculated as (SIDv - SIDa) × 100/SIDv. The same calculation was performed for pCO2 (pCO2%). Comparison between v and a values was performed by paired t test or the signed-rank test, as appropriate.


Analysis was conducted on 205 sample-couples of six enrolled patients. A significant difference (P < 0.001) between mean values of v-a samples was observed for pH (7.41 ± 0.05 vs. 7.51 ± 0.06), pCO2 (48 ± 6 vs. 35 ± 7 mmHg), [Na+] (136.3 ± 4.0 vs. 135.2 ± 4.0 mEq/l), [Cl-] (101.5 ± 5.3 vs. 102.8 ± 5.2 mEq/l) and therefore SID (39.5 ± 4.0 vs. 36.9 ± 4.1 mEq/l). pCO2% and SID% significantly correlated (r2 = 0.28, P < 0.001). Graphical representation by quartiles of pCO2% is shown in Figure 1.

Figure 1

* P < 0.05 versus first quartile. §P < 0.05 versus second. #P < 0.05 versus third. One-way ANOVA.


As a reduction in SID decreases pH, the observed movement of anions and cations probably limited the alkalinization caused by hypocapnia. In this model, the only source of electrolytes are blood cells (that is, no interstitium and no influence of the kidney is present); it is therefore conceivable to consider the observed phenomenon as the contribution of RBC for the compensation of acute hypocapnic alkalosis.

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Correspondence to T Langer.

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  • Public Health
  • Chloride
  • Blood Cell
  • Graphical Representation
  • Emergency Medicine