Sodium-bicarbonate test : comparison between NaHCO3 and NaCl administration

Introduction. Sodium-bicarbonate (NaHCO3) test aims to assess the effective circulating volume (ECV), by analyzing urinary pH (pHU) after NaHCO3 administration. We aimed to elucidate its mechanisms as compared to sodium-chloride (NaCl) administration. We hypothesized that during normo-/increased-ECV the extra-Na+ reaching the kidney after NaHCO3 is accompanied by a weak anion (HCO3–), thereby leading to an increased urinary strong ion difference (SIDU) and pHU. Conversely, after NaCl, the extra-Na+ reaching the kidney will be associated to a strong anion (Cl–), thereby leaving SIDU and pHU unchanged. Methods. Twelve patients showing a normo-/increased-ECV during ICU stay (based upon a pre-test clinical evaluation) were enrolled and connected to the analyzer K.IN.G® (Kidney INstantaneous monitorinG), allowing quasi-continuous measurement of pHU and urinary electrolytes. In 6 patients, hemodynamic parameters were also assessed by PiCCO®. Afterwards, 0.3 mEq/kg of either NaHCO3 or NaCl were intravenously and randomly administrated, and pHU and urinary electrolytes were monitored for one hour. Results. After NaHCO3, pHU increased from 5.97±0.26 to 6.58±0.24 (p<0.001), while it remained unchanged after NaCl (not shown). At the maximal pHU variation after NaHCO3, SIDU increased in association to a Na+U variation greater than that of Cl–U (p=0.001, Fig.1). Conversely, NaCl administration led to a similar Na+U and Cl–U variation (6.1±3.7 and 6.3±4.0 mEq/l, respectively), leaving SIDU unchanged (Fig.1). CI and GEDI measured before each administration did not differ (3.8±0.5 vs. 3.9±0.4 l/min/m2; 773±104 vs. 868±158 ml/m2). Conclusions. These results indicates the importance of Na+U excretion in pHU variation during NaHCO3 test.