Age-dependent response of the acid-base parameters (Henderson-Hasselbalch, Stewart) in healthy calves with experimentally induced metabolic acidosis

The intensity of the response to acid-base parameters in relation to the age after a defined acid load was studied in calves. 32 clinically healthy calves (age: 4-104 days) were infused with 5M NH4Cl solution (dose: 1.0 ml/kg) through a permanent intravenous catheter. Before (0 hrs) and after starting the infusion (2, 4, 6, 8 and 24 hrs) venous blood samples were collected for the determination of the various acid-base parameters. The intensity of the response of the acid-base parameters was estimated by using the "area under curve (AUC)" procedure. By 2-6hrs after the infusion of the NH4Cl solution, the Henderson-Hasselbalch parameters decreased significantly (down arrow pH, down arrow[HCO3-]) as did Stewart's variables (down arrow[Strong ion difference=SID3], down arrow[Acid total = A(tot), or A(-)]). A transient moderate hyperchloraemic acidosis with a slight hypoproteinaemic alkalosis was observed in all calves in association with a respiratory compensation (down arrow PCO2) The younger calves (1(st)-3(rd) week) showed a similar pattern of response to the same dose per kg(0.75) acid load with significantly greater acid-base parameters response (higher AUC values) than the older animals. The calculated pH was determined by using the three Stewart variables PvCO(2), serum-[SID3] and serum-[A(tot)). The mean difference was -0.03 to -0.09 compared with the measured pH (7.32-7.40). Conclusion:The Stewart model appears to be more successful in providing a comprehensive evaluation of acid-base status compared with the traditional Henderson-Hasselbalch model. The younger calves during the first week of life reacted more sensitively to an equal acidotic condition than the older animals.