MICROVASCULAR
RESEARCH 9, 246 (1975)
LETTER
TO THE
EDITOR
Koo and Cheng have presented extremely interesting data in their recent paper (Microvas. Res. 8, 151-155, 1974). Their discussion fails, however, to give adequate attention to several important points. First of all, they conclude that autoregulation was produced by dilation of the cerebral blood vessels.Their text does not make it absolutely clear where they think dilation occurs, but strongly implies that it occurs in the pial vesselswhich they are directly observing. Figure 2, however, indicates that as pressurefall from 140to 50 mm of mercury, many vesselsfail to show a changein diameter yet volume flow remains constant. This suggests that in many instances, the dilation responsible for autoregulation is occurring either proximal to, or distal to, the 40-80 pm vesselsthat they are directly observing. Secondly, the authors usedan in uitro system to calibrate red cell velocity in terms of flow. Figure I implies that there is an identical linear relationship betweencenter line velocity and volume flow for all capillary tubes used in their study. If this is so, their data would appear to confirm the data of Baker and Wayland, who reported no change in velocity profile in capillary tubes of varying dimensions within the range used by Koo and Cheng. If there is indeed such an agreementbetweenthe two studies, it seemsparticularly strangethat the slope of the line obtained from Baker’s data is only 60 % of the slope obtained by Koo and Cheng. In any event, for the calibration shown in Fig. 1 to be applicable when in viuo velocity measurementsare used, one must assumethat the velocity profiles obtained in their nonpulsatile glass calibration system are the same as the velocity profiles obtained in vivo, where pulsatile flow is present even in the smallest arterioles. Unfortunately, no proof exists for this assumption, particularly at normal hematocrits. WILLIAM Virginia Commonwealth University Richmond, Virginia 23298
Copyright 0 1975 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
246
I. ROSENBLUM
RESEARCH 9, 246 (1975)
LETTER
TO THE
EDITOR
Koo and Cheng have presented extremely interesting data in their recent paper (Microvas. Res. 8, 151-155, 1974). Their discussion fails, however, to give adequate attention to several important points. First of all, they conclude that autoregulation was produced by dilation of the cerebral blood vessels.Their text does not make it absolutely clear where they think dilation occurs, but strongly implies that it occurs in the pial vesselswhich they are directly observing. Figure 2, however, indicates that as pressurefall from 140to 50 mm of mercury, many vesselsfail to show a changein diameter yet volume flow remains constant. This suggests that in many instances, the dilation responsible for autoregulation is occurring either proximal to, or distal to, the 40-80 pm vesselsthat they are directly observing. Secondly, the authors usedan in uitro system to calibrate red cell velocity in terms of flow. Figure I implies that there is an identical linear relationship betweencenter line velocity and volume flow for all capillary tubes used in their study. If this is so, their data would appear to confirm the data of Baker and Wayland, who reported no change in velocity profile in capillary tubes of varying dimensions within the range used by Koo and Cheng. If there is indeed such an agreementbetweenthe two studies, it seemsparticularly strangethat the slope of the line obtained from Baker’s data is only 60 % of the slope obtained by Koo and Cheng. In any event, for the calibration shown in Fig. 1 to be applicable when in viuo velocity measurementsare used, one must assumethat the velocity profiles obtained in their nonpulsatile glass calibration system are the same as the velocity profiles obtained in vivo, where pulsatile flow is present even in the smallest arterioles. Unfortunately, no proof exists for this assumption, particularly at normal hematocrits. WILLIAM Virginia Commonwealth University Richmond, Virginia 23298
Copyright 0 1975 by Academic Press. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
246
I. ROSENBLUM
