Filling of microcirculation in skeletal muscles during timed India ink perfusion

Lower leg muscles of anesthetized rabbits were perfused in situ with heparinized India ink at flows and pressures comparable to normal resting levels of 4 ml.min-1.100 g-1). Paired cross sections were counterstained with eosin to show ink-containing microvessels and reacted for alkaline phosphate to show all vessels. The fraction of microvessels filled with ink (Fi) increased progressively with perfusion time. At 3.5 s, mean Fi for the muscles studied fell between 0.12 and 0.19. At 60-90 s, the following levels were reached: medial gastrocnemius 0.74, lateral gastrocnemius 0.76, tibialis anterior 0.59, and soleus 0.80. The number of open capillaries and their distribution of flow velocities can be inferred from such data only if the perfusion rate is known or by recourse to a specified anatomic model. The time course of ink appearance shows best agreement with 60-80% of the vessels open and accessible to ink, with microvascular transit times ranging from less than 3 to greater than 30 s. If microvascular path lengths are assumed to be uniform, the range of velocities must be four times to one-fourth the mean, with 15-30% of the microvessels perfused at velocities equal to or greater than the mean. Alternatively, if microvascular velocity is assumed uniform, flow path lengths must vary from one-fourth to four times the average. Anatomic measurements of other suggest that less than one-half the variability in ink transit is attributable to differences in microvascular length. Thus both length and velocity must vary among alternate arteriovenous pathways.