Sanjay Batra

Geometry of capillary networks in hypertrophied rat heart

Capillary geometry was examined in normal and hypertrophic myocardium. Hypertrophy was induced by aortic constriction in neonatal rats. Morphometric data were obtained from tissue sections exposed to a staining technique that distinguished the arteriolar and venular portions of capillaries by color. In sham-operated controls, the theoretical tissue region supplied by a single capillary decreased from the arteriolar to venular side (499 +/- 3 microns 2 and 456 +/- 5 microns 2, P less than 0.05; mean +/- SE) of capillaries. In hypertrophy, only arteriolar capillary tissue regions increased in size, thus enlarging the difference between arteriolar and venular ends (547 +/- 6 microns 2 and 464 +/- 5 microns 2, P less than 0.01). Intercapillary distances, measured at various levels along the capillary path length, decreased in a stepwise manner in both normal and hypertrophic hearts. In hypertrophic hearts, mean capillary path length was significantly longer than in controls, but the total length of the individual capillary nets was reduced. In both groups, arteriolar capillary segment length was longer (P less than 0.01) than venular capillary segment length. Given that PO2 values are lower on the venular side of capillaries, this spatially distinctive geometry in normal myocardium: smaller domains, shorter intercapillary distances and segment lengths, would provide favorable geometric conditions for oxygen diffusion. In hypertrophy, average intercapillary distance increased, and the distinction between arteriolar and venular portions of capillaries was further exacerbated.

Supporting tissue oxygenation during acute surgical bleeding using a perfluorochemical-based oxygen carrier.

Attempts to develop so-called “blood substitutes” have historically focused on three approaches: 1) acellular hemoglobin (Hb) solutions, 2) encapsulation of Hb, and 3) perfluorochemical emulsions. Work on Hb solutions first began over 100 years ago,1 by lysing red blood cells to extract the oxygen-carrying Hb molecules. For the past 60 years, efforts have concentrated on purifying the Hb from the contaminating stromal lipid,2 and on developing ways to crosslink or polymerize the Hb molecules3 to prevent them from splitting into dimers (i.e., half-molecules) which are rapidly filtered by the kidney. Based on the fact that free Hb has a very short intravascular persistence and is potentially toxic to the kidney,4 some investigators have used microencapsulation as a means to package the Hb inside sub-micron lipid-coated vesicles.5,6

Spatial distribution of coronary capillaries: A-V segment staggering.

The modelling of oxygen transport to tissue necessitates a concerted effort in linking structural and functional data. In our laboratory, we are interested in the geometrical distribution of coronary capillaries. Traditionally, capillary supply has been characterized only by measures of capillary density, from which it is possible to calculate average inter-capillary distance (ICD). The deficiency in such a calculation is the assumption of a uniform distribution of capillaries. The heterogeneity of inter-capillary spacing is clearly an important factor in myocardial oxygenation, over and above average ICD. Methods for assessing the heterogeneity of capillary spacing, and it’s effect on myocardial oxygenation have been recently analyzed (Rakusan and Turek, 1985 and Turek et. al., 1987). Another important parameter for modelling oxygen transport is the knowledge of the direction of blood flow in adjacent capillaries. Our recent application of coloured microspheres, for the analysis of myocardial flow pattern, revealed a predominance of concurrent flow in neighboring capillaries (Reeves and Rakusan, 1987). Nonetheless, a uniformity in flow direction does not ensure that the spatial position and P02 values of neighboring capillaries are synchronous. One may envision a situation where the transverse arteriole furnishes capillaries at staggered levels in the tissue.

Geometry of Capillary Networks in Volume Overloaded Rat Heart

Volume overload cardiac hypertrophy was induced in male Sprague-Dawley rats by experimental aortocaval fistula. This procedure resulted in considerable increases in left ventricular mass (70%) by 21-23 days. Our objective was to study the effect of volume overload on the geometry of coronary capillaries in the left ventricular midmyocardium. Tissue sections were stained according to a protocol that distinguished arteriolar (AC) and venular (VC) capillary regions by color. Morphometric data were then collected and compared between AC and VC regions. In sham-operated controls (CON; n = 8), the tissue area (capillary domain) supplied by a single capillary decreased from AC to VC regions (AC = 505 +/- 5 microns 2: VC = 452 +/- 7 microns 2; P less than 0.01; mean +/- SE). In volume overloaded hearts (VOL; n = 8), only VC domain areas were reduced from control values (P less than 0.01) and the differences between AC and VC regions were preserved (AC = 480 +/- 5 microns 2; VC = 395 +/- 6 microns 2; P less than 0.01). Minimal capillary length was significantly longer in volume overloaded hearts (VOL = 723 +/- 18; CON = 581 +/- 20 microns; P less than 0.01). In the control group, AC segment length was longer than VC segment length (AC = 93 +/- 2 microns: VC = 74 +/- 2 microns; P less than 0.01). In volume overload, AC segment length was also longer than VC segment length, but the divergence between AC and VC regions was increased (AC = 108 +/- 3 microns; VC = 71 +/- 2 microns; P less than 0.01). These changes in capillary geometry may be secondary to specific changes in the arrangement and dimension of myocytes in the left ventricular wall following volume overload hypertrophy.

Hemodilution and hyperoxia locally change distribution of regional pulmonary perfusion in dogs

In seven anesthetized dogs, the effects of acute normovolemic hemodilution (ANH) to a hematocrit of 20 and 8% and the effects of hyperoxic ventilation (100% oxygen) on distribution of regional pulmonary blood flow (rPBF; radioactive microspheres) were investigated. Normovolemia was monitored with blood volume measurements (indocyanine green dilution kinetics). Before ANH, fractal dimension (D) of rPBF in the whole lung was 1.19 +/- 0.09 (mean +/- SD). Spatial correlation (rho) of rPBF in the whole lung was 0.6 +/- 0.08. D is a resolution-independent measure for global rPBF distribution, and rho is the averaged flow relationship of directly neighboring lung samples. With regard to the entire lung, neither ANH nor hyperoxia changed D or rho. With regard to horizontal, isogravitational planes, ANH induced opposite changes of rPBF heterogeneity depending on the vertical location of the plane and the parameter used. In ventral planes, a change in relative dispersion (SD/mean) indicated decreased homogeneity. However, rho suggested more homogeneous perfusion. Hyperoxia restored baseline rPBF distribution. Our data suggest that ANH causes different alterations of heterogeneity of rPBF depending on location within the lung.In seven anesthetized dogs, the effects of acute normovolemic hemodilution (ANH) to a hematocrit of 20 and 8% and the effects of hyperoxic ventilation (100% oxygen) on distribution of regional pulmonary blood flow (rPBF; radioactive microspheres) were investigated. Normovolemia was monitored with blood volume measurements (indocyanine green dilution kinetics). Before ANH, fractal dimension ( D) of rPBF in the whole lung was 1.19 ± 0.09 (mean ± SD). Spatial correlation (ρ) of rPBF in the whole lung was 0.6 ± 0.08. D is a resolution-independent measure for global rPBF distribution, and ρ is the averaged flow relationship of directly neighboring lung samples. With regard to the entire lung, neither ANH nor hyperoxia changed D or ρ. With regard to horizontal, isogravitational planes, ANH induced opposite changes of rPBF heterogeneity depending on the vertical location of the plane and the parameter used. In ventral planes, a change in relative dispersion (SD/mean) indicated decreased homogeneity. However, ρ suggested more homogeneous perfusion. Hyperoxia restored baseline rPBF distribution. Our data suggest that ANH causes different alterations of heterogeneity of rPBF depending on location within the lung.

Morphometric Analysis of Capillary Nets in Rat Myocardium

The importance of relevant data with respect to capillary geometry cannot be under-estimated in the modelling of oxygen transport to tissue. C. R. Honig stated “knowledge of capillary length and it’s frequency distribution is essential in modelling O2 transport and indicator dilution data” (Honig et al., 1977). To this end, data have been collected for capillary lengths in skeletal muscle (eg. Honig et al., 1977; Skalak and Schmid-Schonbein, 1986; Potter et al., 1988). The situation in cardiac muscle, to date, is certainly more obscure. This is due to the exigency of developing appropriate facilities for viewing surface vessels of a beating heart in situ; or in the development of morphometric techniques that accurately demarcate capillaries from histological sections.

A New Approach for Quantitative Evaluation of Coronary Capillaries in Longitudinal Sections

A better knowledge of oxygen transport to tissue requires accurate, reliable quantification and modelling of a tissue’s vascular supply. Understanding the geometry of the capillary network is of particular importance, since capillaries are the regions primarily involved in the exchange of oxygen between red blood cells and working muscle.

The Effect of Realistic Geometry of Capillary Networks on Tissue PO2 in Hypertrophied Rat Heart

A multicylindrical Kroghian model was developed at our Departments. The distribution of the radii of the cylinders was logarithmic-normal and was defined by the mean or median value and the logarithmic standard deviation (i.e., SD of log-transformed variates, σlog10×, usually denoted by the abbreviation log SD) serving as the heterogeneity index. The mean value and log SD were obtained from capillary spacing on histological cross-sections, using the method of capillary domains (Hoofd et al., 1985). The model allowed calculation of PO2 histograms in a block of tissue and was specifically designed to depict the effect of the geometric heterogeneity in capillary spacing on tissue oxygenation in normal and hypertrophied heart (Rakusan et al., 1984; Turek et al., 1986). The model was later expanded by including also the facilitation of O2 by myoglobin, an additional resistance between blood and tissue (capillary barrier), and PO2-dependent O2 consumption. It was applied to skeletal (Turek et al., 1989) and cardiac muscle (Turek et al., 1991).

Use of a PFC-Based Oxygen Carrier to Lower the Transfusion Trigger in a Canine Model of Hemodilution and Surgical Blood Loss

Perfluorochemical (PFC) emulsions are inert compounds that demonstrate a high solubility for gases, and are being developed as temporary intravenous oxygen carriers for intraoperative use as an adjunct to surgical procedures involving autologous blood sparing techniques, i.e., predonation and acute normovolemic hemodilution (ANH).

Capillary Density, Distribution, and Length Parameters Related to Oxygen Supply in Myocardial Hypertrophy and Atrophy

Cardiac muscle is well known to be vulnerable to anoxia, even for a very short duration, due to its almost entirely oxidative metabolism. For proper cardiac function the balance between oxygen supply and demand must be met not only at the level of the whole organ but also at that of the individual working units, i.e. cardiac myocytes. Previous attempts to measure the adequacy of the oxygen supply-to-demand ratio were limited by an emphasis on measuring the basic parameters only at the level of the whole organ. This was due mainly to lack of data on the local distribution of oxygen sources and sinks, i.e. basic supply and demand elements. Recent developments in the quantitative analysis of cardiac tissue should permit such an investigation at the tissue level as well.