Crystalloid strong ion difference determines metabolic acid-base change during acute normovolaemic haemodilution

Abstract

Objective: To study the acid-base effects of crystalloid strong ion difference (SID) during haemodilution.

Design: Prospective in vivo study.

Setting: University laboratory.

Subjects: Anaesthetised, mechanically ventilated Sprague-Dawley rats.

Interventions: Rats were studied in seven groups of three. Each group underwent normovolaemic haemodilution with one of seven crystalloids, with SID values from 0 to 40 mEq/l. Six exchanges of 9 ml crystalloid for 3 ml blood were performed.

Measurements and main results: [Hb] fell from 142+/-17 to 44+/-10 g/l ( p<0.0001). Final plasma [lactate] was 1.1+/-0.6 mmol/l. Final standard base excess values ranged from -8 to +7 mmol/l, and were directly correlated with crystalloid SID (R(2)=0.91). By linear regression, the SID of a crystalloid balanced to maintain standard base excess=0 mmol/l was 23.7 mEq/l. Dilutions 2-5 produced similar results.

Conclusions: There is a linear relationship between crystalloid SID and post-dilutional metabolic acid-base status. The SID of a crystalloid balanced for normovolaemic haemodilution is 24 mEq/l. These principles are applicable in designing fluids for volume resuscitation, acute normovolaemic haemodilution and cardio-pulmonary bypass.