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