BACKGROUND: The dissociation constant of nonvolatile weak acids in plasma (KA), expressed as pKA, is essential for electroneutrality-based acid-base analysis. To date, its normal value in human plasma has been determined in only one study involving eight healthy volunteers. We hypothesized that pKA would differ in ICU patients, whose plasma protein composition is altered by disease and medication, and that changes in protein charge-rather than undetected strong acids-could account for the unexplained anions observed in sepsis. METHODS: Using CO2 tonometry, we determined pKA and total weak nonvolatile acids (ATOT) in plasma from 30 healthy volunteers and two ICU cohorts (27 postoperative and 30 septic patients). Additionally, we calculated the strong ion gap in plasma and protein-free serum filtrates from 10 healthy volunteers and 20 septic patients. RESULTS: In healthy volunteers, pKA was 7.55 ± 0.16 (KA = 2.8 × 10⁻⁸) and ATOT was 15.9 ± 3.0 mmol/L (0.222 ± 0.043 mmol/g of TP). In postoperative and septic patients, ATOT was significantly reduced (10.1 ± 5.4 and 11.9 ± 4.0 mmol/L, p < 0.001), but pKA and ATOT/TP remained unchanged, yielding an average pKA of 7.55 ± 0.35 (KA = 2.8 × 10⁻⁸) and ATOT/TP of 0.230 ± 0.097 mmol/g. We found elevated strong ion gap in both plasma and protein-free filtrates of septic patients, which confirms the presence of unmeasured low-molecular-weight anions. CONCLUSION: Our findings confirm stable pKA and ATOT/TP values in human plasma in both health and disease, supporting the Staempfli-Constable model for clinical acid-base diagnostics. Unexplained anions in sepsis are attributed to low molecular weight strong ions rather than alterations in plasma protein dissociation.

• Klíčová slova
• Acids, Acid–base equilibrium, Hydrogen-ion concentration, Models, Serum albumin, Theoretical,
• Publikační typ
• časopisecké články MeSH

It is commonly assumed that changes in plasma strong ion difference (SID) result in equal changes in whole blood base excess (BE). However, at varying pH, albumin ionic-binding and transerythrocyte shifts alter the SID of plasma without affecting that of whole blood (SIDwb), i.e., the BE. We hypothesize that, during acidosis, 1) an expected plasma SID (SIDexp) reflecting electrolytes redistribution can be predicted from albumin and hemoglobin's charges, and 2) only deviations in SID from SIDexp reflect changes in SIDwb, and therefore, BE. We equilibrated whole blood of 18 healthy subjects (albumin = 4.8 ± 0.2 g/dL, hemoglobin = 14.2 ± 0.9 g/dL), 18 septic patients with hypoalbuminemia and anemia (albumin = 3.1 ± 0.5 g/dL, hemoglobin = 10.4 ± 0.8 g/dL), and 10 healthy subjects after in vitro-induced isolated anemia (albumin = 5.0 ± 0.2 g/dL, hemoglobin = 7.0 ± 0.9 g/dL) with varying CO2 concentrations (2-20%). Plasma SID increased by 12.7 ± 2.1, 9.3 ± 1.7, and 7.8 ± 1.6 mEq/L, respectively (P < 0.01) and its agreement (bias[limits of agreement]) with SIDexp was strong: 0.5[-1.9; 2.8], 0.9[-0.9; 2.6], and 0.3[-1.4; 2.1] mEq/L, respectively. Separately, we added 7.5 or 15 mEq/L of lactic or hydrochloric acid to whole blood of 10 healthy subjects obtaining BE of -6.6 ± 1.7, -13.4 ± 2.2, -6.8 ± 1.8, and -13.6 ± 2.1 mEq/L, respectively. The agreement between ΔBE and ΔSID was weak (2.6[-1.1; 6.3] mEq/L), worsening with varying CO2 (2-20%): 6.3[-2.7; 15.2] mEq/L. Conversely, ΔSIDwb (the deviation of SID from SIDexp) agreed strongly with ΔBE at both constant and varying CO2: -0.1[-2.0; 1.7], and -0.5[-2.4; 1.5] mEq/L, respectively. We conclude that BE reflects only changes in plasma SID that are not expected from electrolytes redistribution, the latter being predictable from albumin and hemoglobin's charges.NEW & NOTEWORTHY This paper challenges the assumed equivalence between changes in plasma strong ion difference (SID) and whole blood base excess (BE) during in vitro acidosis. We highlight that redistribution of strong ions, in the form of albumin ionic-binding and transerythrocyte shifts, alters SID without affecting BE. We demonstrate that these expected SID alterations are predictable from albumin and hemoglobin's charges, or from the noncarbonic whole blood buffer value, allowing a better interpretation of SID and BE during in vitro acidosis.

• Klíčová slova
• albumin, hemoglobin, noncarbonic whole blood buffer value, plasma strong ion difference, whole blood base excess,
• MeSH
• acidobazická rovnováha MeSH
• acidóza * MeSH
• albuminy škodlivé účinky   MeSH
• anemie * MeSH
• elektrolyty MeSH
• hemoglobiny MeSH
• koncentrace vodíkových iontů MeSH
• lidé MeSH
• oxid uhličitý MeSH
• poruchy acidobazické rovnováhy * MeSH
• Check Tag
• lidé MeSH
• Publikační typ
• časopisecké články MeSH
• Názvy látek
• albuminy MeSH
• elektrolyty MeSH
• hemoglobiny MeSH
• oxid uhličitý MeSH

Non-carbonic buffer power (βNC) of blood is a pivotal concept in acid-base physiology as it is employed in several acid-base evaluation techniques, including the Davenport nomogram and the Van Slyke equation used for Base excess estimation in blood. So far, βNC has been assumed to be independent of metabolic acid-base status of blood, despite theoretical rationale for the contrary. In the current study, we used CO2 tonometry to assess βNC in blood samples from 10 healthy volunteers, simultaneously analyzing the electrolyte shifts across the red blood cell membrane as these shifts translate the action of intracellular non-carbonic buffers to plasma. The βNC of the blood was re-evaluated after experimental induction of metabolic acidosis obtained by adding a moderate or high amount of either hydrochloric or lactic acid to the samples. Moreover, the impact of βNC and pCO2 on the Base excess of blood was examined. In the control samples, βNC was 28.0 ± 2.5 mmol/L. In contrast to the traditional assumptions, our data showed that βNC rose by 0.36 mmol/L for each 1 mEq/l reduction in plasma strong ion difference (p < 0.0001) and was independent of the acid used. This could serve as a protective mechanism that increases the resilience of blood to the combination of metabolic and respiratory acidosis. Sodium and chloride were the only electrolytes whose plasma concentration changed relevantly during CO2 titration. Although no significant difference was found between the electrolyte shifts in the two types of acidosis, we observed a slightly higher rate of chloride change in hyperchloremic acidosis, while the variation of sodium was more pronounced in lactic acidosis. Lastly, we found that the rise of βNC in metabolic acidosis did not induce a clinically relevant bias in the calculation of Base excess of blood and confirmed that the Base excess of blood was little affected by a wide range of pCO2.

• Klíčová slova
• acid-base equilibrium, base excess, blood, blood-gas analysis, buffers, metabolic acidosis,
• Publikační typ
• časopisecké články MeSH