Marcos Intaglietta

Microvascular and tissue oxygen distribution

Understanding of oxygen delivery by the microcirculation has been dominated by the unitary component analysis of Krogh and Erlangen focussed on oxygen transport mediated by single capillaries, oxygenation of tissue as a whole being extrapolated from findings on oxygen exchange in these vessels. This analysis is under revision since capillaries are not sole sources of oxygen. It is increasingly apparent that arterioles are a significant equivalent source, while venules may serve as sinks for capillary and arteriolar oxygen. As a consequence detailed descriptions of the architecture of the microcirculation based on the tissue cylinder conceptualization does not yield new information given the non-exclusive role of capillaries as purveyors of oxygen to tissue. In the present study we investigate how tissue is oxygenated directly from the arteriolar supply on the basis of current results with newly developed optical techniques for the measurement of local intra- and extravascular pO2 by phosphorescence decay. This methodology shows that tissue regions between arterioles and venules have essentially uniform tissue pO2. The only experimentally detectable gradients in pO2 are those present in the immediate vicinity of arterioles. Findings on vascular longitudinal gradients are used to devise a model that links convective and diffusive processes, showing how blood viscosity, blood oxygen-carrying capacity and the slope of the oxygen dissociation curve are linked in determining intravascular and tissue pO2. The integrated approach provides a numerical basis for interpreting consequences of alterations in transport properties of blood applicable to the field of blood substitutes.

Microvascular measurements by video image shearing and splitting

Abstract A method is described for the measurement of dimensions from video displays by image splitting and shearing by electronic means only. Accuracy and repeatability are of the order of 0.5% of the video image width. The technique has the advantage that it can be adapted to provide multiple simultaneous measurements.

Capillary flow velocity measurements in vivo and in situ by television methods

Abstract A system for the measurement of red blood cell velocity in televised images of the microcirculation is described. The method measures the time delay between the photometric signals which originate from two “windows” positioned along the centerline of the televised capillary under study. The photometric windows are generated directly within the composite video signal, and are positioned by means of manual analog controls. Spatial and temporal resolution limitations set by the characteristics of broadcast television systems, in conjunction with the characteristic distances over which a configuration of red blood cells does not change substantially in its transit through the capillary, determine an upper limit for velocity measurements of 10 mm/sec. The system was utilized to measure RBC velocities in the capillaries of the rabbit omentum, cat tenuissimus muscle, and the pial microcirculation. These tissues present three limiting cases in terms of optical resolution, correlation length, and velocity patterns. In each case, adequate velocity measurements were routinely obtained.

A transparent access chamber for the rat dorsal skin fold.

Abstract The design and surgical implantation procedure of a new aluminum chamber with a transparent window for the rat dorsal skin fold are described. The chamber combines strength, low weight, and high optical transparency, and provides access to the subcutaneous tissue. The latter permits transplantation of different tissue, application of chemical substances, or micromanipulation. In the present work, a mammary adenocarcinoma was transplanted into the subcutaneous tissue within the chamber for observation of tumor growth and microcirculation.

Pressure measurements in the mammalian microvasculature

Abstract The servo nulling system for measuring pressures in the microvasculature system described by Wiederhielm was redesigned and utilized to measure pressures in the mammalian mesentric circulation, the hamster cheek pouch, and the rat cremaster muscle. A systems analysis of the components shows that direct-pressure measurements of suitable fidelity can be made with conventional transducers through microcannulas down to 10–15 μ diameter, and that for smaller diameters active systems are required. The system developed operates with a 25-Hz frequency response in the presence of routine electrical noise. Results obtained show that pressures in arterioles and venules in the mammalian mesenteric circulation are greater than the plasma colloidal osmotic pressure, while in corresponding vessels in muscle, venular pressures are lower than the colloidal osmotic pressure.

Low nitric oxide bioavailability contributes to the genesis of experimental cerebral malaria

The role of nitric oxide (NO) in the genesis of cerebral malaria is controversial. Most investigators propose that the unfortunate consequence of the high concentrations of NO produced to kill the parasite is the development of cerebral malaria. Here we have tested this high NO bioavailability hypothesis in the setting of experimental cerebral malaria (ECM), but find instead that low NO bioavailability contributes to the genesis of ECM. Specifically, mice deficient in vascular NO synthase showed parasitemia and mortality similar to that observed in control mice. Exogenous NO did not affect parasitemia but provided marked protection against ECM; in fact, mice treated with exogenous NO were clinically indistinguishable from uninfected mice at a stage when control infected mice were moribund. Administration of exogenous NO restored NO-mediated signaling in the brain, decreased proinflammatory biomarkers in the blood, and markedly reduced vascular leak and petechial hemorrhage into the brain. Low NO bioavailability in the vasculature during ECM was caused in part by an increase in NO-scavenging free hemoglobin in the blood, by hypoargininemia, and by low blood and erythrocyte nitrite concentrations. Exogenous NO inactivated NO-scavenging free hemoglobin in the plasma and restored nitrite to concentrations observed in uninfected mice. We therefore conclude that low rather than high NO bioavailability contributes to the genesis of ECM.

Microvascular perfusion upon exchange transfusion with stored red blood cells in normovolemic anemic conditions

BACKGROUND:  Transfusions are intended to augment oxygen‐carrying capacity. The ability of fresh and stored red blood cells (RBCs) to maintain microvascular perfusion and oxygen delivery to the tissue has not been directly measured.

Plasma viscosity regulates capillary perfusion during extreme hemodilution in hamster skinfold model

Effect of increasing blood viscosity during extreme hemodilution on capillary perfusion and tissue oxygenation was investigated in the awake hamster skinfold model. Two isovolemic hemodilution steps were performed with 6% Dextran 70 [molecular weight (MW) = 70,000] until systemic hematocrit (Hct) was reduced by 65%. A third step reduced Hct by 75% and was performed with the same solution [low viscosity (LV)] or a high-molecular-weight 6% Dextran 500 solution [MW = 500,000, high viscosity (HV)]. Final plasma viscosities were 1.4 and 2.2 cP (baseline of 1.2 cP). Hct was reduced to 11.2 ± 1.1% from 46.2 ± 1.5% for LV and to 11.9 ± 0.7% from 47.3 ± 2.1% for HV. HV produced a greater mean arterial blood pressure than LV. Functional capillary density (FCD) was substantially higher after HV (85 ± 12%) vs. LV (38 ± 30%) vs. baseline (100%).[Formula: see text] levels measured with Pd-porphyrin phosphorescence microscopy were not statistically changed from baseline until after the third hemodilution step. Wall shear rate (WSR) decreased in arterioles and venules after LV and only in arterioles after HV. Wall shear stress (WSR × plasma viscosity) was substantially higher after HV vs. LV. Increased mean arterial pressure and shear stress-dependent release of endothelium-derived relaxing factor are possible mechanisms that improved arteriolar and venular blood flow and FCD after HV vs. LV exchange protocols.

Micropressures and capillary filtration coefficients in single vessels of the cremaster muscle of the rat.

Abstract The pressure and filtration parameters of the microcirculation in the exteriorized cremaster muscle were studied in young rats anesthetized with pentobarbital or urethane. The filtration coefficient and capillary pressures were calculated by means of the Starling equation from fluid movements across the walls of selected capillaries during micro-occlusion, coupled with estimations of the osmotic pressure of the plasma colloids. The filtration coefficient “ K ” was determined as 0.0065 μ 3 /μ 2 /sec/cm H 2 O pressure and, the capillary velocity averaged 700 μ/sec. Pressure was calculated as 32 cm H 2 O on the arterial end of the capillary bed and 22 cm H 2 O in the efficuent venules. Pressures were measured directly in small arterioles and collecting venules with a micropipette using a null electronic servo system. The pressures determined by direct puncture and by calculation were essentially the same. The architecture of the bed in this muscle did not show the entensive postcapillary, venular drainage vessels described for the bat wing.

Mechanics of fluid movement across single capillaries in the rabbit

Abstract An analysis was made of the factors involved in the net movement of fluid through the walls of single capillaries. The method, as originally developed by Landis, utilized the movement of red blood cells in capillaries immediately after temporary occlusion with a glass microneedle as an indication of the relative predominance of either hydrostatic or colloid osmotic pressures. Comparison of transcapillary fluid movement before and after known osmotic transients with intravenous albumin made it possible to calculate the filtration constant and the net or effective driving pressure in selected capillaries. The filtration constant in the omental capillaries was lowest on the arterial side (2 to 8 × 10 −3 μ/sec) and highest on the venous side (16 to 25 × 10 −3 μ/sec). The effective hydrostatic pressure in the capillaries varied between 28−22 cm H 2 O, well above the colloid osmotic pressure of the plasma proteins. The findings reaffirm the need to reconsider the precise application of the linear, constitutive relationship governing transcapillary fluid movement as postulated by Starling.