Fluid-induced
metabolic acidosis can be harmful and can complicate
cardiopulmonary bypass. In an attempt to prevent this disturbance, we designed a
bicarbonate-based
crystalloid circuit prime balanced on physico-chemical principles with a strong ion difference of 24 mEq/l and compared its
acid-base effects with those of
Plasma-Lyte 148, a multiple
electrolyte replacement
solution containing
acetate plus
gluconate totalling 50 mEq/l. Twenty patients with normal
acid-base status undergoing elective cardiac surgery were randomised 1:1 to a 2 litre prime of either
bicarbonate-balanced fluid or
Plasma-Lyte 148. With the trial fluid, metabolic
acid-base status was normal following bypass initiation (standard base excess 0.1 (1.3) mEq/l, mean, SD), whereas
Plasma-Lyte 148 produced a slight
metabolic acidosis (standard base excess -2.2 (2.1) mEq/l). Estimated group difference after baseline adjustment was 3.6 mEq/l (95% confidence interval 2.1 to 5.1 mEq/l, P=0.0001). By late bypass, mean standard base excess in both groups was normal (0.8 (2.2) mEq/l vs. -0.8 (1.3) mEq/l, P=0.5). Strong ion gap values were unaltered with the trial fluid, but with
Plasma-Lyte 148 increased significantly on bypass initiation (15.2 (2.5) mEq/l vs. 2.5 (1.5) mEq/l, P < 0.0001), remaining elevated in late bypass (8.4 (3.4) mEq/l vs. 5.8 (2.4) mEq/l, P < 0.05). We conclude that a
bicarbonate-based
crystalloid with a strong ion difference of 24 mEq/l is balanced for
cardiopulmonary bypass in patients with normal
acid-base status, whereas
Plasma-Lyte 148 triggers a surge of unmeasured
anions, persisting throughout bypass. These are likely to be
gluconate and/or
acetate. Whether surges of exogenous
anions during bypass can be harmful requires further study.