Exchange of CO2 and bicarbonate between the circulation and cerebrospinal fluid in piglets

The dynamic exchange of CO2 and HCO-3 between the central circulation and peripheral sites such as the cerebrospinal fluid (CSF) is not completely understood. To examine this we administered a tracer dose of NaHCO3 labeled with the stable isotope 13C into the central circulation of nine 3- to 4-wk-old anesthetized, instrumented piglets. Serial samples of arterial and venous blood, breath, and CSF were obtained to determine the 13C/12C ratio by isotope-ratio mass spectrometry. Gas-exchange and hemodynamic parameters were obtained through standard techniques. The patterns of tracer washout in venous blood, arterial blood, and breath were nearly indistinguishable from each other, and the mean residence time was 113 +/- 24 min (average +/- SD). In contrast, the CSF 13C/12C ratio was not equivalent to that in venous blood, arterial blood, or breath until 20 min after tracer administration. Tracer washout data were used to determine the best-fit three-exponential-term equation and to calculate compartmental parameters of a three-compartment mammillary system in which CO2 and HCO3- residing in the central compartment (compartment 1) exchanges relatively rapidly with one peripheral compartment (compartment 2) and slowly with the other (compartment 3). The turnover time for the CSF was 6.8 +/- 2.5 min and the estimate for compartment 2 was 7.1 +/- 3.7 min. Accordingly, CSF appears to be one of the rapidly interchanging peripheral compartments.