Letters Section Editor: Robert M. Golub,
MD, Senior Editor.
To the Editor: We found some of the results
in the study by Dr Merten and colleagues1 surprising.
The change in serum creatinine level after radiographic contrast (mean difference,
0.11 mg/dL; 95% confidence interval [CI], −1.10 to 0.30) was not significant
while the change in estimated glomerular filtration rate (GFR) (mean difference,
−8.6%; 95% CI, −17.0 to −0.2) was significant. Since in
using the Levey formula, serum creatinine is the major determinant in calculating
GFR, one might expect the change in creatinine and GFR to be of similar magnitude.
We wonder if changes other than in creatinine could explain the calculated
GFR change. The Levey formula uses age and sex, which would not change over
2 days, but also uses blood urea nitrogen (BUN) and serum albumin levels.
While these values are stable over the long-term, it is possible that the
infusion of sodium bicarbonate (NaHCO3) expanded extracellular
fluid volume to a greater amount and for a longer time than the control solution
of sodium chloride (NaCl). This might occur since the volume of distribution
of NaHCO3, in the absence of acidosis, is limited to extracellular
space while some chloride does enter cells. If this were the case, the BUN
and albumin values might show a change from the preinfusion values. We are
interested in knowing if the authors have these data. If it were the case
that NaHCO3led to greater and longer lasting extracellular fluid
volume expansion than NaCl, it might comprise a partial explanation for the
apparent protection against acute renal failure in this study. The relative
lack of a steady state of extracellular space would be a reason not to use
formulas such as that of Levey et al2 during
acute clinical changes.
Shavit L, Lifschitz M, Plaksin J, Slotki IN. Prevention of Contrast-Induced Nephropathy With Sodium Bicarbonate. JAMA. 2004;292(12):1428-1429. doi:10.1001/jama.292.12.1428-a