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

Introduction 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 diff erence (SID) during acute variations in pCO2 and their possible role in the compensation for hypocapnic alkalosis.Methods Patients admitted in this year to our ICU requiring extracorporeal 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) x 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. Results Analysis was conducted on 205 sample-couples of six enrolled patients. A signifi cant diff erence (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% signifi cantly correlated (r2 = 0.28, P <0.001). Graphical representation by quartiles of pCO2% is shown in Figure 1. Conclusions 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 infl uence 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