The rule regulating pH changes during crystalloid infusion
To define the rule according to which crystalloid solutions characterized by different strong ion difference (SID) modify the acid–base variables of human plasma.
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].
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.
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.
KeywordsAcid–base equilibrium Volume resuscitation Stewart’s approach Balanced solution Crystalloids
We deeply thank A. De Mori S.p.A., Italy, that kindly provided the blood gas analyzer and tonometer used for the in vitro experiments.
- 9.Stewart PA (1981) How to understand acid-base. A quantitative acid-base primer for biology and medicine. Elsevier, New YorkGoogle Scholar
- 10.Stewart PA (2009) Stewart’s textbook of acid–base. Lulu.com, AmsterdamGoogle Scholar
- 15.Constable PD (2004) Stewart approach is not always a practical clinical tool (author reply). Anesth Analg 98:271–272Google Scholar