Biomedical subjects

D H Damon

Publications and source records attributed to D H Damon.

23 records · Page 2Linked to original sources

PubMed1987

An examination of the measurement of flow heterogeneity in striated muscle.

This review leads us to a number of conclusions and suggestions for further study. First, we find wide differences in the meaning of flow heterogeneity, arising as a result of the different methods used. These differences will have to be reconciled to form a comprehensive view of the role of heterogeneity in determining vascular function. Second, in the future, the meaning of heterogeneity must be clearly defined and related to a particular microvascular component, and it is imperative that the differences in scale of heterogeneity be appreciated when comparing data from various laboratories. These heterogeneities have different implications for function, and failure to distinguish among them leads to confusion. Third, the degree to which perfusion heterogeneity is regulated in the microcirculation remains in doubt. Reports of variations in flow heterogeneity in response to physiological stimuli are for the most part based on highly questionable indirect methods. Fourth, the heterogeneity that can be demonstrated at the capillary level within striated muscle does not appear to be large relative to the capacity for the microcirculation to exchange most diffusible solutes. Thus, the inferences regarding heterogeneity, as evidenced by diffusible indicators, are likely to be the result of different preparations, damage to the preparations, or perhaps large-scale heterogeneities in the tissue. An alternate possibility would be that the heterogeneity occurs at the microvascular level but reflects some other aspect of microcirculatory function, such as length or hematocrit heterogeneities, but not flow heterogeneities. Fifth, flow heterogeneity within microvessels implies important consequences for capillary exchange and tissue oxygenation. Heterogeneities of velocity of a magnitude comparable to those observed by direct visualization of microcirculation can clearly produce reductions in oxygen supply to small tissue regions of a degree that may limit oxygen delivery, and thereby, tissue function. Sixth, flow heterogeneity may also influence capillary hematocrit and/or red cell spacing by producing cell separation at bifurcations and a resultant reduction in mean capillary tube hematocrit. There is as yet no agreement on why and how these hematocrits influence tissue oxygenation and function. Although several hypotheses are advanced to explain the distribution of blood flow and red cells within microcirculation, each lacks a critical experimental test at present.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1987

Are physiological changes in capillary tube hematocrit related to alterations in capillary perfusion heterogeneity?

Average capillary tube hematocrit is hypothesized to be reduced below the mean systemic value as a result of heterogeneous distribution of blood flow among microvessels. Furthermore, capillary tube hematocrit changes with vasomotor state and these have been proposed to be due to concomitant changes in perfusion heterogeneity. We reasoned that if alterations in average capillary tube hematocrit were related to changes in perfusion heterogeneity, then mean capillary tube hematocrit should be inversely related both to the heterogeneity of capillary blood flows and to the heterogeneity of capillary hematocrit. These inferences were tested by comparing estimates of average capillary tube hematocrit (Ht) and the heterogeneities of capillary blood flow and Ht in vasoconstricted and vasodilated hamster tibialis anterior muscles. Ht was estimated from the number of red blood cells per unit capillary length (n/l); heterogeneity of capillary blood flow and heterogeneity of Ht were estimated by calculating the coefficient of variation of capillary red blood cell velocities (vrbc) and n/l, respectively. Average Ht varied with vasomotor state. However, there was no correlation between average Ht and the heterogeneity of vrbc, our index of capillary blood flow: in fact, the heterogeneity of vrbc was constant under all experimental conditions. The heterogeneity of Ht varied with vasomotor state, but not in inverse proportion to average Ht as was expected. From these observations we conclude that alterations in average capillary tube hematocrit are not due to concomitant alterations in perfusion heterogeneity.

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PubMed1987

Capillary grouping in hamster tibials anterior muscles: flow patterns, and physiological significance.

We have used fluorescein-labeled albumin and epifluorescence videomicroscopy to visualize the organization and flow patterns in the capillaries of a postural muscle, the tibialis anterior, in the pentobarbital-anesthetized hamster. Video tape records were made of overlapping microscope fields comprising portions of the muscle up to two millimeters in length and as much as 180 micron in depth. Three-dimensional reconstructions made from these tapes incorporated observations on flow patterns and geometry of the microvessels. The microcirculation in the tibialis was found to be arranged in repeating modules or 'units', consisting of approximately 15 capillaries supplied by a common arteriole and drained by a common venule. The mean distance from arteriole to venule in a unit was 855 +/- 233 micron (X +/- SD) and the mean capillary length was 805 +/- 330 micron. Flow in the capillaries within a unit was almost entirely concurrent, and there was little interchange of blood flow between the capillaries of adjacent units. The flow in capillaries of adjacent units was both countercurrent and concurrent. For each capillary, a count of the adjacent capillaries with concurrent and countercurrent flow was made. The ratio of concurrent to countercurrent flow varied from 2.4 to 4.3, being greatest at the midpoint of the unit and least at the arterial and venous ends. Our observations suggest that the capillaries of striated muscle are arranged in modules which may function as the fundamental control and distribution elements in the microcirculation.

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PubMed1985

Evidence that capillary perfusion heterogeneity is not controlled in striated muscle.

We tested the hypothesis that the heterogeneity of capillary blood flow distribution in striated muscle is inversely proportional to tissue blood flow by examining the patterns of red blood cell flow in the capillaries of hamster tibialis anterior muscles. Capillary red blood cell velocities and capillary red blood cell fluxes were measured as indexes of blood flow and red blood cell distribution in vasoconstricted and vasodilated vascular beds of resting and working striated muscle. Standard statistical parameters describing dispersion of data (standard deviation and coefficient of variation) in addition to measured and normalized histograms were compared across treatments. With vasodilation the standard deviations of both variables increased linearly with the means, and measured distributions became broader. The coefficients of variation and normalized distributions of both variables did not differ across treatments. These observations do not support the idea that the heterogeneity of capillary perfusion is controlled. Rather they suggest that fractional flow dispersion among capillaries is constant and independent of muscle blood flow and/or O2 demand.

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PubMed1984

Distribution of capillary blood flow in the microcirculation of the hamster: an in vivo study using epifluorescent microscopy.

In vivo epifluorescent microscopy (EPI) was used to study capillary perfusion in superfused hamster cheek pouch, and cremaster and sartorius muscle preparations. In cheek pouches and cremaster muscles, in vivo epifluorescence microscopy (EPI) was compared to in vivo transillumination microscopy (TRANS) and the former was found to allow detection of a larger number of capillaries--34% more capillaries in cheek pouch and 21% more in cremasters were observed with EPI. The fraction of capillaries containing erythrocytes alone, plasma alone or plasma plus red cells was determined in all three tissues. Also, the fraction of capillaries unperfused was noted. Less than 2% of the capillaries contained plasma alone. The number of capillaries which contained stationary erythrocytes varied with vasomotor tone. In control cremaster and sartorius muscles 17 and 13% of observed capillaries were unperfused but contained erythrocytes. Ninety-eight percent of capillaries contained stationary erythrocytes in cremaster muscles vasoconstricted with 21% oxygen. From these observations we conclude (1) functional capillary counts obtained with TRANS may represent underestimates of the true number; (2) plasma shunts do not appear to be a significant factor in normal microcirculatory function in the hamster cheek pouch, or in cremaster and sartorius muscles; (3) since unperfused capillaries contain red cells, red cell counts in histological sections will overestimate functional capillarity. The magnitude of the overestimation will be a function of vascular tone and capillary hematocrit.

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