Oxygen Partition Between Microvessels and Tissue: Signiﬁcance for the Design of Blood Substitutes

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Oxygen Partition Between Microvessels and Tissue: Signiﬁcance for the Design of Blood Substitutes

Amy G. Tsai

¹

, Barbara Friesenecker

²

, and Marcos Intaglietta

¹

Summary. Correction of blood losses with blood substitutes alter the pO

2

distribution in the microcirculation, with outcomes depending on the ﬁnal viscosity of the circulating blood and the vasoactivity induced to the restore normal distribution of pO

2

. Vasoactivity has an oxygen cost shown by the oxygen consumption of the arteriolar microcirculation. Vasodilators lower arteriolar oxygen consumption delivering more oxygen to the tissues, and vice versa. Increased oxygen delivery to the arterioles by right shifted oxygen dis- sociation causes autoregulatory vasoconstriction, a problem aggravated by low blood and plasma viscosity that lowers NO endothelial NO production.

Restoration of tissue function is achieved when no portion of the tissue falls below the threshold of anaerobic metabolism. This goal is attained by using high afﬁnity modiﬁed hemoglobins that act as a reservoir of oxygen only deployed when the circulating blood arrives at tissue regions where pO

2

is very low. Given these premises, restoration of tissue function after severe blood losses requires the re-establishment of oxygen delivery capacity and pO

2

distribution. This is attained by tailoring blood and plasma viscosity and oxygen dissociation properties to insure that no portion of the tissue lacks oxygen delivery, even though overall tissue pO

2

may be abnormally low.

Key words. Vessel wall oxygen consumption, Tissue oxygenation, Oxygen delivery

70

1Department of Bioengineering, 0412, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0412, USA

2Division of General and Surgical Intensive Care Medicine, Department of Anesthesia and Critical Care Medicine, The Leopold-Franzens-University of Innsbruck, Austria

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Introduction

The classical scheme for tissue oxygenation assumes that oxygen gathered by the lung capillaries from the atmosphere is distributed by the systemic cir- culation to the tissue capillaries, which yield their oxygen by being in close proximity with virtually every cell of the organism. In this process the lung capillaries receive oxygen that diffuses through a high oxygen concentration gradient, while tissue capillaries deliver oxygen by means of a low concen- tration gradient, a situation rendered possible because of the large surface area disparity between tissue and lung capillaries, which is about a factor of 10 ¥.

While the capillaries are in virtual equilibrium with tissue, there is also a large disparity between arterial and tissue pO

2

, which indicates that there is a signiﬁcant oxygen loss from blood as it transits from the lungs to the tissue.

This oxygen loss determines a longitudinal oxygen gradient mostly devel- oped in the microcirculation, leading to a speciﬁc distribution of intravascu- lar oxygen tension. There is substantial evidence that the circulation is adapted to this distribution and that the organism strives to maintain this speciﬁc pattern by the process of autoregulation [1,2].

Local autoregulation is based on arteriolar vasoactivity, a process that engages the smooth muscle of the arteriolar wall leading to vasodilation and vasoconstriction, and the control of blood ﬂow which regulates oxygen deliv- ery to counteract the changes in intravascular oxygen distribution.

The Oxygen Cost of Blood Flow Regulation

Arterioles yield a signiﬁcant amount of the oxygen that they carry directly to the tissue, instead of delivering this to the capillary system (leading to the presence of an intraluminal, longitudinal oxygen gradient), therefore in prin- ciple these microvessels should be a major source of oxygen for the tissue.

Mass balance analysis of the rate of exit of oxygen from arteriolar segments showed that it was much greater than that solely accounted by the process of diffusion, leading to the proposal that the diffusion constant of the arteriolar wall was of the order of 10¥ of that of normal tissue [3], a process for which there is no physical evidence.

An alternative explanation is that the vessels wall is a large oxygen sink due

to the high metabolic activity of the endothelium and possibly smooth muscle

in vivo and in situ. The presence of such an oxygen sink was demonstrated

by the measurements of Tsai et al. [4], who found large pO

2

gradients in the

region of the vessel wall, a ﬁnding that was recently conﬁrmed by Shibata

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et al. [5]. In this situation large oxygen gradients can also be due to a large resistance to oxygen exit, however this alternative is not compatible with the measured large rates of oxygen exit measured from arterioles.

Arteriolar wall oxygen consumption is not a static, ﬁxed rate, and it varies in accordance to microvessels tone or level of vasoconstriction. Under normal conditions 50 mm diameter arterioles of the unanesthetized hamster window chamber preparation consume about 25% of the oxygen convected by the blood stream, a rate that is evidenced by the these vessels exhibiting a vessel wall gradient, i.e., difference between in pO

2

across the vessel wall, of 18 .5 mmHg. The continuous infusion of PGE

1

(2.5 mg/kg min, i.v.) was found to increase ﬂow by a factor of 2.18x and lower the vessel wall oxygen gradi- ent to 15.5 mmHg. Geometrical considerations indicate that the oxygen gra- dient in 50 mm arterioles is 12 mmHg, the excess pO

2

difference being attributed to vessels wall oxygen consumption. The same analysis shows that the oxygen gradient of the vessels wall [6] is linearly related to its rate of oxygen consumption, thus the decrease due to PGE

1

halves vessel wall oxygen consumption. Not unexpectedly, the increased ﬂow and lowered vessels wall oxygen consumption causes tissue pO

2

to be 31.9 mmHg, vs. 24.5 mmHg found for normal conditions. The infusion of Vasopressin (0.001 IU/kg min i.v.) a vasoconstrictor, caused ﬂow to decrease to 0.60 of control, increased the vessel wall oxygen gradient to 31.2 mmHg, and lowered tissue pO

2

to 9.5 mmHg.

Similar ﬁndings have been reported by Ye et al. [7], who measured oxygen extraction from various tissues under different levels of tone.

The Signiﬁcance of Tissue pO

₂

Tissue pO

2

is controlled simultaneously by the rate at which oxygen is deliv-

ered by convection to the microcirculation and the rate that it is consumed

by the vessels walls, however the actual level of tissue pO

2

is only indicative

of tissue oxygen supply, a factor that is signiﬁcantly inﬂuenced by the shape

of the oxygen dissociation curve of hemoglobin, and therefore the intralumi-

nal distribution of oxygen in the microcirculation. In the normal hamster

window chamber model large arterioles and venules have respectively 56 and

33 mmHg blood oxygen partial pressure which corresponds to an arterio-

lar/venular difference in oxygen saturation of 34%, leading to a tissue pO

2

of

24 .5 ± 5.0 mmHg (mean ± SD) according to Intaglietta et al. [8]. By compari-

son, if tissue were regulated at the verge of the transition of tissue hypoxia,

or about 2 mmHg, the threshold for anaerobic metabolism, and all intravas-

cular pO

2

values were decreased by 22.5 mmHg the arteriolar/venular satura-

tion difference would by 45%, thus if blood ﬂow were the same, the tissue

would receive more oxygen.

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Normal tissue pO

2

is regulated at a level that is signiﬁcantly higher than that associated with anaerobic metabolism, which commences at pO

2

s lower than about 2 mmHg [9]. The disparity between level of regulation and limit of oxidative metabolism can be explained by considering a hypothetical situation in which the tissue is regulated at 7 mmHg pO

2

, under otherwise identical anatomical and microhemodynamic conditions. In this form of reg- ulation, if the pO

2

variability remains the same, 16% of the tissue would be beyond the anaerobic threshold, i.e., beyond one standard deviation or 1s. It is apparent that regulating tissue at 24.5 mmHg places 99.98% of the tissue within the aerobic metabolism, the anaerobic portion being beyond 4s.

Oxygen Distribution in the Design of Blood Substitutes

To the present, blood substitutes have been mostly formulated using hemo- globin as the oxygen carrier. Formulations based on human hemoglobin must in principle provide a material that is as plentiful an efﬁcacious as the natural blood, namely should be the result of a process that increase, or at least is equal to the amount of human blood utilized. In other words a practical product should yield an equivalent unit of blood utilizing at most a unit of natural blood. Formulations that use non-human hemoglobin or recombinant hemoglobin are not exempt from considerations of economy of oxygen carrier, since they are costly to produce.

An economic product can in principle be formulated if the material is vasoinactive and is utilized in minimal concentrations. However, overall hemoglobin concentrations (red blood hemoglobin plus product hemoglo- bin) that are signiﬁcantly lower than that of natural blood will correspond- ingly signiﬁcantly lower tissue pO

2

placing substantial portions of the tissue in conditions of anaerobic metabolism, as shown in our previous example.

The statistical nature of oxygen distribution, however, indicates that even at low tissue pO

2

conditions there are parts of the tissue whose pO

2

is higher than the tissue average, therefore a solution of this problem is to introduce an oxygen reservoir that is only deployed for tissue regions at very low pO

2

s.

This is readily accomplished by utilizing an oxygen carrier whose equivalent p50 is very low, i.e., a material that releases oxygen only in anoxic regions.

This analysis also shows that this approach is only possible if the material is

vasoinactive, otherwise the comparatively small amounts of oxygen trans-

ported by the low concentration formulation is utilized by the energy

requirements of vasoconstriction. Parenthetically and additional advantage

of oxygenating the tissue at low pO

2

is that vessel wall oxygen consumption

is proportional to blood pO

2

[6], thus less oxygen would be consumed by the

arteriolar walls.

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These concepts have been validated by the experiments of Sakai et al., 1999 [10], who tested tissue oxygenation by hemoglobin vesicles formulated at varying p50s in a hemodilution protocol in hamster and determined that for this type of oxygen carrier optimal performance was attained at p50 = 16 mmHg vs. p50 = 36 mmHg for hamster blood. A material incorporating these features, namely vasoinactivity, low p50, i.e., 5 mmHg, and formulated at 4% hemoglobin concentration (MalPEG-hemoglobin, Hemospan) is presently produced by Sangart Inc, San Diego, has shown to be highly efﬁca- cious in experimental studies [11] and is presently in clinical trials.

Acknowledgments. This work was supported by Bioengineering Research Partnership grant R24-HL 64395 and the grants R01-HL 62318 and R01-HL 62354 .

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