Which circulates faster through the cerebral microcirculatory system, red cells or plasma?

To the Editor: In a recent paper describing interesting experiments to measure the plasma and red cell transit in a cerebral ischemic area of the cat brain (Stroke 12: 218-223, March-April, 1981), Little et al. reported that the transit time of plasma as represented by I albumin was shorter than that of red cells labeled with "Tc even in the control State. This is just the opposite of data reported by us in the cat brain using our photoelectric method. It also contrasts with the results of other groups including Larsen & Lassin in the brain, Pappenheimer & Kinter in the kidney, Rapaport et al. in the lung, Moore & Baker in the skeletal muscle, and Freis et al. through the forearm. All these authors found that the velocity of red cells exceeded that of plasma. These findings were supported by the larger plasma volume than red cell volume in the tissue." Such observations have led to the concept of a lower value of tissue hematocrit than large vessel hematocrit (Hct)v). The relationships in this situation can be explained mathematically as follows. The equation for the tissue hematocrit (Hct,) derived by Larsen & Lassen from the basic definition of the hematocrit value (Hct) as Hct = 100 x Vrc/(Vrc + Vp), where Vrc is the red cell volume and Vp the plasma volume, is Hct, = 100/[l + MCTp X (100 Hct|v)/MCTrc x Hctlv] where MCT is the mean circulation time which is by definition volume divided by flow, and is therefore equivalent to the mean transit time (t). If we consider the "relative" tissue hematocrit which means Hct, rela tive to Hct,v expressed in percentage, or rHct, = 100 x Hct,/Hctlv, and if we neglect the term Hctlv x (1 MCTp/MCTrc) which is much smaller than 100 x Mctp/MCTrc when MCTp/MCTrc is close to unity, the Larsen & Lassen equation reduces to rHct. 100 x trc/tp In other words, the ratio of the mean transit time for red cells and that for the plasma is approximately the tissue hematocrit relative to that of the large vessels. The data of Little et al. would imply therefore that the cerebral tissue hematocrit is higher than the large vessel hematocrit, which is incredi ble. A closer examination of their analytical method shows that the transit time was determined by them from measurements of the total peak time. We believe that the tissue concentration curve which they recorded was a cumulative distribution function, whose peak time rep resented only the arterial phase, and by no means the microcirculatory phase as they claimed. Even with any contribution from the arterial phase, their data are dubious due to the existence of the FahraeusLindqvist effect. We recommend that they recheck their data pertinent to the above criticism. Minoru Tomita, M.D. Fumio Gotoh, M.D. Department of Neurology School of Medicine Keio University 35 Shinanomachi Shinjuku-ku Tokyo 160 Japan