Atriopathic Arrhythmias — Sick Sinus Syndrome

autoradiographic approach to Xenoh/CT LCBF measurements" at a time when most CT scanners were slow 20 seconds scan time) and interscan delays were much longer O 30 sec) than those currently available on many machines. There is no doubt that this method is now outdated. Since 1979—80 we, along with other groups, have been devel­ oping and using more sophisticated multivariable methods of analysis such as the weighted monoor bi-compartmental least square fits. We believe that the true flow through a tissue volume should be used as a reference flow in error analyses and therefore, we do agree with Rottenberg, et al that what they define as E 2 is the error due to heterogeneity if the mono-compartmental model is used. Rottenberg'S data show that E 2 is significantly smaller than E, in tissue with a significant mixture (percent gray ^ 75%). Therefore, the statement we made in our recent review in STROKE is fully supported by their own data. The rest of the story has already been published and rebattled in the Journal of Cerebral Blood Flow and Metabolism and we see no reason to repeat it here. David Gur Sidney K. Wolfson, Jr. Howard Yonas Walter F. Good Leonard Shabason Department of Radiation Health University of Pittsburgh Pittsburgh, Pennsylvania 15261