To the Editor: I was interested in your presentation of the formula for determining the efficiency of exchange in an exchange transfusion (Trossman, C. M., and Alzofon, F. E.: A.M.A.J. Dis. Child. 98:694 [Dec.] 1959). I assume these factors are determined on a mathematical basis and confirmed experimentally. These are factors which I have always estimated but never actually bothered to determine.
In general practice, I observe that there is another factor or error in efficiency of exchange which is much more important than that of dilution. Most exchange transfusions are performed by the single cannula method. In this method there is a "dead space," that volume required to fill the system from the syringe to the tip of the cannula. Most physicians are not aware that after the first withdrawals all subsequent withdrawals contain donor blood equal to the volume of the dead space (× cc.) and likewise ×