Is red cell flow heterogeneity a critical variable in the regulation and limitation of oxygen transport to tissue?

Functional implications of microvascular heterogeneity for oxygen uptake and utilization

In the vascular system, an extensive network structure provides convective and diffusive transport of oxygen to tissue. In the microcirculation, parameters describing network structure, blood flow, and oxygen transport are highly heterogeneous. This heterogeneity can strongly affect oxygen supply and organ function, including reduced oxygen uptake in the lung and decreased oxygen delivery to tissue. The causes of heterogeneity can be classified as extrinsic or intrinsic. Extrinsic heterogeneity refers to variations in oxygen demand in the systemic circulation or oxygen supply in the lungs. Intrinsic heterogeneity refers to structural heterogeneity due to stochastic growth of blood vessels and variability in flow pathways due to geometric constraints, and resulting variations in blood flow and hematocrit. Mechanisms have evolved to compensate for heterogeneity and thereby improve oxygen uptake in the lung and delivery to tissue. These mechanisms, which involve long-term structural adaptation and short-term flow regulation, depend on upstream responses conducted along vessel walls, and work to redistribute flow and maintain blood and tissue oxygenation. Mathematically, the variance of a functional quantity such as oxygen delivery that depends on two or more heterogeneous variables can be reduced if one of the underlying variables is controlled by an appropriate compensatory mechanism. Ineffective regulatory mechanisms can result in poor oxygen delivery even in the presence of adequate overall tissue perfusion. Restoration of endothelial function, and specifically conducted responses, should be considered when addressing tissue hypoxemia and organ failure in clinical settings.

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

The physiologic perspective in fluid management in vascular anesthesiology

Vascular surgery patients frequently suffer from atherosclerosis and peripheral arterial occlusive disease generating endothelial dysfunction. Furthermore, ischemia and reperfusion during surgery damage endothelial cells and, especially, the endothelial glycocalix. The damage of the glycocalix promotes an increase in permeability. Not only crystalloids, which freely diffuse between the intravascular and the interstitial compartment, but also colloidal fluids cross from the intravascular space in the interstitial space with the consequence of edema formation. Possible tissue edema may result in an impairment of tissue oxygenation, leading to wound healing disturbances and initiation of inflammatory responses up to tissue apoptosis. Particularly in vascular anesthesia, this possibly means that colloids only should be administered in acute volume resuscitation immediately after unclamping a big vessel for immediate volume restoration. Which colloidal fluid should be administered is still under intense discussion. From a theoretical physiologic point of view, iso-osmolar albumin is the best choice regarding volume effect, antioxidative properties, and protection against destruction of the glycocalix. Nonetheless, albumin experimentally has not lived up to its promise in the clinical setting. Thus, further well-conducted large randomized clinical trials are necessary to ascertain the optimal fluid therapy in vascular surgery patients.

Dependence of oxygen delivery on blood flow in rat brain: a 7 tesla nuclear magnetic resonance study

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used at a magnetic field strength of 7 T to measure CBF and CMRO2 in the sensorimotor cortex of mature rats at different levels of cortical activity. In rats maintained on morphine anesthesia, transitions to lower activity and higher activity states were produced by administration of pentobarbital and nicotine, respectively. Under basal conditions of morphine sulfate anesthesia, CBF was 0.75 +/- 0.09 mL x g(-1) x min(-1) and CMRO2 was 3.15 +/- 0.18 micromol x g(-1) x min(-1). Administration of sodium pentobarbital reduced CBF and CMRO2 by 66% +/- 16% and 61% +/- 6%, respectively (i.e., "deactivation"). In contrast, administration of nicotine hydrogen tartrate increased CBF and CMRO2 by 41% +/- 5% and 30% +/- 3%, respectively (i.e., "activation"). The resting values of CBF and CMRO2 for alpha-chloralose anesthetized rats were 0.40 +/- 0.09 mL x g(-1) x min(-1) and 1.51 +/- 0.06 micromol x g(-1) x min(-1), respectively. Upon forepaw stimulation, CBF and CMRO2 were focally increased by 34% +/- 10% and 26% +/- 12%, respectively, above the resting nonanesthetized values (i.e., "activation"). Incremental changes in CBF and CMRO2, when expressed as a percentage change for "deactivation" and "activation" from the respective control conditions, were linear (R2 = 0.997) over the entire range examined with the global and local perturbations. This tight correlation for cerebral oxygen delivery in vivo is supported by a recent model where the consequence of a changing effective diffusivity of the capillary bed for oxygen, D, has been hypothetically shown to be linked to alterations in CMRO2 and CBF. This assumed functional characteristic of the capillary bed can be theoretically assessed by the ratio of fractional changes in D with respect to changes in CBF, signified by omega. A value 0.81 +/- 0.23 was calculated for omega with the in vivo data presented here, which in turn corresponds to a supposition that the effective oxygen diffusivity of the capillary bed is not constant but presumably varies to meet local requirements in oxygen demand in a similar manner with both "deactivation" and "activation."

Cardiovascular responses to dynamic exercise with acute anemia in humans

We hypothesized that reducing arterial O2 content (CaO2) by lowering the hemoglobin concentration ([Hb]) would result in a higher blood flow, as observed with a low PO2, and maintenance of O2 delivery. Seven young healthy men were studied twice, at rest and during two-legged submaximal and peak dynamic knee extensor exercise in a control condition (mean control [Hb] 144 g/l) and after 1-1.5 liters of whole blood had been withdrawn and replaced with albumin [mean drop in [Hb] 29 g/l (range 19-38 g/l); low [Hb]]. Limb blood flow (LBF) was higher (P < 0.01) with low [Hb] during submaximal exercise (i.e., at 30 W, LBF was 2.5 +/- 0.1 and 3.0 +/- 0.1 l/min for control [Hb] and low [Hb], respectively; P < 0.01), resulting in a maintained O2 delivery and O2 uptake for a given workload. However, at peak exercise, LBF was unaltered (6.5 +/- 0.4 and 6.6 +/- 0.6 l/min for control [Hb] and low [Hb], respectively), which resulted in an 18% reduction in O2 delivery (P < 0.01). This occurred despite peak cardiac output in neither condition reaching >75% of maximal cardiac output (approximately 26 l/min). It is concluded that a low CaO2 induces an elevation in submaximal muscle blood flow and that O2 delivery to contracting muscles is tightly regulated.