Purpose: To define the rule according to which crystalloid solutions characterized by different strong ion difference (SID) modify the acid-base variables of human plasma.
Methods: With a previously validated software, we computed the effects of diluting human plasma with crystalloid solutions ([SID] 0-60, 10 mEq/l stepwise). An equation was derived to compute the diluent [SID] required to maintain the baseline pH unchanged, at constant PCO₂ and at every dilution fraction. The results were experimentally tested using fresh frozen plasma, re-warmed at 37°C, equilibrated at PCO₂ 35 and 78 mmHg, at baseline and after the infusion of crystalloid solutions with 0, 12, 24, 36, 48 mEq/l [SID].
Results: The mathematical analysis showed that the diluent [SID] required to maintain unmodified the baseline pH equals the baseline bicarbonate concentration, [HCO₃⁻], assuming constant PCO₂ throughout the process. The experimental data confirmed the theoretical analysis. In fact, at the baseline [HCO₃⁻] of 18.3 ± 0.3 mmol/l (PCO₂ 35 mmHg) the pH was 7.332 ± 0.004 and remained 7.333 ± 0.003 when the diluting [SID] was 18.5 ± 0.0 mEq/l. At baseline [HCO₃⁻] of 19.5 ± 0.3 mmol/l (PCO₂ 78 mmHg) the pH was 7.010 ± 0.003 and remained 7.004 ± 0.003 when the diluting [SID] was 19.1 ± 0.1 mEq/l. At both PCO₂ values infusion with [SID] lower or greater than baseline [HCO₃⁻] led pH to decrease or increase, respectively.
Conclusions: The baseline [HCO₃⁻] dictates the pH response to crystalloid infusion. If a crystalloid [SID] equals baseline [HCO₃⁻], pH remains unchanged at constant PCO₂, whereas it increases or decreases if the [SID] is greater or lower, respectively.