Dependence of the apparent bicarbonate space on initial plasma bicarbonate concentration and carbon dioxide tension in neonatal calves with diarrhea, acidemia, and metabolic acidosis

Background Marked strong ion (metabolic) acidosis in neonatal diarrheic calves usually is corrected by IV administration of NaHCO 3 . The distribution space for IV‐administered bicarbonate, called the apparent bicarbonate space (ABS), appears to depend on initial plasma bicarbonate concentration ( c HCO 3 ) and varies considerably in calves. Objective To determine whether ABS was associated with initial plasma c HCO 3 and other acid‐base variables. Animals Twenty‐five neonatal diarrheic calves with acidemia and metabolic acidosis. Methods Prospective observational study using a convenience sample. Calves received NaHCO 3 (10 mmol/kg) and glucose (1.4 mmol/kg) IV in a crystalloid solution at 25 mL/kg over 60 minutes. The ABS (L/kg) was calculated at 4 time points over 2 hours after the end of the infusion. The relationship between ABS and initial acid‐base variables was characterized using nonlinear, linear, and stepwise regression. Results The median value for ABS calculated from the initial plasma c HCO 3 increased from 0.53 L/kg (range, 0.40‐0.79) at the end of IV infusion to 0.96 L/kg (range, 0.54‐1.23) 120 minutes later. Data obtained at the end of infusion provided the best fit to initial plasma c HCO 3 and jugular venous blood Pco 2 , such that: ABS = 0.41 + 1.06/ c HCO 3 and ABS = 0.87‐0.0082 × Pco 2 . Conclusions and Clinical Importance The observed median value for ABS of 0.53 L/kg in our study was similar to the empirically used value of 0.6. However, ABS values varied widely and were increased in calves with severe metabolic acidosis. We therefore recommend calculating ABS using the initial plasma c HCO 3 or venous blood Pco 2 , if respective measurements are available.