To the Editor:
We thank the writers for their astute comments about our calculations and the assumptions upon which they were based. Regarding our estimation of clearance, we need to correct an error in our original report: the procedure used in this patient was not continuous venovenous hemodialfiltration (CVVHD), but continuous venovenous hemofiltration (CVVH). There was no diafiltration, no dialysate, and therefore no dilution of the filtrate effluent; that is why it was appropriate to estimate the clearance using the effluent flow rate and measurement of effluent and serum acetaminophen levels, as described.
We agree that our estimated half-life was really an “apparent” half-life, which may reflect both elimination and continued absorption. We addressed this in our original discussion, and noted that continuing absorption would have made the half-life appear longer. Thus, our patient's true elimination half-life was probably shorter. As the writers pointed out, using this shorter half-life in the equation would yield a larger total body clearance value. This would bolster the argument that CVVH had a smaller impact on the total clearance of acetaminophen.
We did not mean to suggest that we endorse CVVH as a preferred procedure for extracorporeal removal of acetaminophen. It may help remove some of the ingested drug, and assist in restoration of a normal blood pH. However, if rapid removal of large amounts of acetaminophen is necessary, conventional high-flux, high-flow hemodialysis would be superior.