Microvascular Oxygen Delivery Research Articles

Muscle microvascular oxygen delivery limitations during the contraction phase of intermittent maximal effort contractions.

The end-test torque (ETT) during intermittent maximal effort contractions reflects the highest contraction intensity at which a muscle metabolic steady-state can be attained. This study determined if ETT is the highest intensity at which the contraction phase of intermittent exercise does not limit the matching of microvascular oxygen delivery to muscle oxygen demand. Microvascular oxygenation characteristics of the biceps brachii muscle were measured in sixteen young, healthy individuals (8M/8F, 22 ± 3years, 80.9 ± 20.3kg) by near-infrared spectroscopy during maximal effort elbow flexion under control conditions (CON) and with complete circulatory occlusion (OCC). Increases in total-[heme] were blunted during OCC compared to CON (225 ± 87 vs. 264 ± 88μM, p < 0.001) but OCC did not elicit a compensatory increase in deoxygenated-[heme] at any timepoint (108 ± 62 vs. 101 ± 61μM, p > 0.05). Deoxygenated-[heme] was significantly elevated during contraction, relative to relaxation, above ETT (107 ± 60 vs. 98.8 ± 60.5μM, p < 0.001), but not at ETT (91.7 ± 54.1 vs. 98.4 ± 62.2μM, p = 0.174). Total-[heme] was significantly reduced during contraction, relative to relaxation, at all contraction intensities during CON (p < 0.05) and OCC (p < 0.05). These data suggest that ETT may reflect the highest contraction intensity at which contraction-induced increases in intramuscular pressures do not limit muscle perfusion to a degree that requires further increases in fractional oxygen extraction (i.e., deoxygenated-[heme]) despite limited microvascular diffusive conductance (i.e., total-[heme]).

Hemodilution on microvascular oxygen delivery potential of the blood during coronary bypass surgery.

The hematocrit-to-whole blood viscosity ratio (Hct/WBV) reflects the blood O2 delivery potential (O2-DP). WBV is variable to the dynamic vascular shear rate (SR), 1-5/s at microcirculation and 300/s at larger vessels. To estimate the impact of hemodilution on the blood O2-DP to the myocardium, we analyzed the hemodilution-induced change of Hct/WBV at SR 5/s (Hct/WBV5) during off-pump coronary bypass (OPCAB) surgery. During OPCAB surgery (n = 21), 10% acute normovolemic hemodilution (HD 10%) was applied. Arterial blood samples were taken: one before and two after HD 10%. One of which after HD 10% underwent an additional 33% in vitro hemodilution (reaching 40% hemodilution in total, HD 40%). WBV of all blood samples was determined using a scan-capillary tube viscometer (Hemovister™). The changes of Hct/WBV5 were analyzed as a primary measure of the study and compared with those of Hct/WBV at SR 300/s (Hct/WBV300). Median[IQR] of Hct/WBV5 [3.5 (2.8-4.2)%/cPoise] was significantly increased by HD 10 and HD 40% [3.6 (3.2-4.6)%/cPoise and 4.2 (3.3-5.2)%/cPoise, respectively, all P < 0.001], but the degrees of changes after HD 10 and HD 40% were not different. Median[IQR] of Hct/WBV300 [10.3(8.6‒10.8)%/cPoise] was not changed by HD 10% [10.3(9.1-11.1)%/cPoise], but it was significantly decreased by HD 40% [8.4(7.4‒9.2)%/cPoise, P < 0.001]. The increased Hct/WBV5 suggests that 10-40% hemodilution improves the blood O2-DP to the myocardium during OPCAB surgery. The SR-specific discrepancy in Hct/WBV changes advocates using microvascular WBV and Hct/WBV to evaluate the blood O2-DP changes to the myocardium. Further study is warranted to assess the actual changes in myocardial O2 delivery.

The acute effects of passive heating on endothelial function, muscle microvascular oxygen delivery, and expression of serum HSP90α.

Passive heating has been a therapeutic tool used to elevate core temperature and induce increases in cardiac output, blood flow, and shear stress. We aimed to determine the effects of a single bout of passive heating on endothelial function and serum heat shock protein 90α (HSP90α) levels in young, healthy subjects. 8 healthy subjects were recruited to participate in one bout of whole-body passive heating via immersion in a 40°C hot tub to maintain a 1°C increase in rectal temperature for 60min. Twenty-four hours after heating, shear-rate corrected endothelium-dependent dilation increased (pre: 0.004±0.002%SRAUC; post: 0.006±0.003%SRAUC; p=0.034) but serum [HSP90α] was not changed (pre: 36.7±10.3ng/mL; post: 40.6±15.9ng/mL; p=0.39). Neither resting muscle O2 utilization (pre: 0.17±0.11mL O2 min-1 (100g)-1; post: 0.14±0.09mL O2 min-1 (100g)-1); p=0.28) nor mean arterial pressure (pre: 74±11mmHg; post: 73±11mmHg; p=0.79) were influenced by the heating intervention. Finally, time to peak after cuff release was significantly delayed for % O2 sat (TTPpre=39±8.9s and TTPpost=43.5±8.2s; p=0.007) and deoxy-[heme] (TTPpre=41.3±18.1s and TTPpost=51.4±16.3s; p=0.018), with no effect on oxy-[heme] (p=0.19) and total-[heme] (p=0.41). One bout of passive heating improved endothelium-dependent dilation 24h later in young, healthy subjects. This data suggests that passive heat treatments may provide a simple intervention for improving vascular health.

S-Nitrosylated hemoglobin predicts organ yield in neurologically-deceased human donors

Current human donor care protocols following death by neurologic criteria (DNC) can stabilize macro-hemodynamic parameters but have minimal ability to preserve systemic blood flow and microvascular oxygen delivery. S-nitrosylated hemoglobin (SNO-Hb) within red blood cells (RBCs) is the main regulator of tissue oxygenation (StO2). Based on various pre-clinical studies, we hypothesized that brain death (BD) would decrease post-mortem SNO-Hb levels to negatively-impact StO2 and reduce organ yields. We tracked SNO-Hb and tissue oxygen in 61 DNC donors. After BD, SNO-Hb levels were determined to be significantly decreased compared to healthy humans (p = 0·003) and remained reduced for the duration of the monitoring period. There was a positive correlation between SNO-Hb and StO2 (p < 0.001). Furthermore, SNO-Hb levels correlated with and were prognostic for the number of organs transplanted (p < 0.001). These clinical findings provide additional support for the concept that BD induces a systemic impairment of S-nitrosylation that negatively impacts StO2 and reduces organ yield from DNC human donors. Exogenous S-nitrosylating agents are in various stages of clinical development. The results presented here suggest including one or more of these agents in donor support regimens could increase the number and quality of organs available for transplant.

Spatial matching of microvascular oxygen delivery to demand in skeletal muscle: Has the missing link been found?

Spatial matching of microvascular oxygen delivery to demand in skeletal muscle: Has the missing link been found?

A two-compartment model of oxygen transport in skeletal muscle using continuously distributed capillaries

For future application to studying regulation of microvascular oxygen delivery, a model is developed for O2 transport within an idealized volume of tissue, that is perfused by a continuous distribution of capillaries. Considering oxygen diffusion, convection, and consumption, an O2-dependent transfer term between the capillaries and tissue is used to extend previous single-compartment approaches to include separate tissue and capillary compartments. The coupled tissue–capillary PDE system is considered for unidirectional capillary flow in z, as a simplified model of O2 transport in skeletal muscle, and steady-state 2D solutions are obtained using boundary conditions in x that are consistent with two experimental situations of interest. To validate the continuous capillary model, comparisons are made of an exact nonlinear solution (for no flux at x=0) to results of an established discrete capillary model (solved via finite differences) for varying capillary density, O2 consumption rate, and red blood cell velocity. In addition, comparisons of an approximate linearized solution (for fixed PO2 at x=0) are made to the corresponding discrete capillary solution. Results of the continuous capillary model are presented for varying inlet O2 saturation, showing the utility of the new model for studying physiological problems. Numerical solution of the new model for problems with time dependence and complex geometry is expected to be substantially more efficient than for the corresponding discrete capillary problems.

Limb blood flow and muscle oxygenation responses to rhythmic exercise below and above critical force

This study aimed to determine brachial artery blood flow (Q̇BA) and microvascular oxygen delivery responses to rhythmic exercise above and below critical force (CF; the isometric analog of critical power). We hypothesized that Q̇BA, deoxygenated‐heme (deoxy‐[heme]; an estimate of microvascular fractional oxygen extraction), and total‐heme concentrations (total‐[heme]; an estimate of changes in microvascular hematocrit) would demonstrate physiological maximums above CF despite increases in exercise intensity. 13 healthy subjects performed 1) a 5‐min rhythmic isometric‐handgrip maximal‐effort test (MET) to determine CF and 2) constant target‐force tests above (severe‐intensity; S1 and S2) and below (heavy‐intensity; H1 and H2) CF. CF was 189.3 ± 16.7 N (29.7 ± 1.6 %MVC). At end‐exercise, Q̇BA was greater during S1 (418 ± 147 mL/min) and S2 (403 ± 137 mL/min) than during H1 (287 ± 97 mL/min) and H2 (340 ± 116 mL/min; p &lt; 0.05) but was not different between S1 and S2. Further, Q̇BA for S1 and S2 were not significantly different from the Q̇BA estimated for CF (392 ± 37 mL/min). At end‐exercise, deoxy‐[heme] was not different between S1 (150 ± 50 μM) and S2 (155 ± 57 μM), but was greater during S1 and S2 than during H1 (101 ± 24 μM) and H2 (111 ± 21 μM; p &lt; 0.05). Total‐[heme] was not different between S1 (404 ± 58 μM) and S2 (397 ± 73 μM), but was greater during S1 and S2 than H1 (352 ± 58 μM; p &lt; 0.01) but not H2 (371 ± 57 μM). These data suggest limb blood flow limitations and maximal levels of muscle microvascular oxygen delivery and extraction during exercise above, but not below, CF.End‐exercise brachial artery blood flow (Q̇BA) values. Q̇BA at end‐exercise below (● H1 and ○ H2) and above (▼ S1 and Δ S2) critical force (CF), and Q̇BA predicted at CF by linear regression (♦). * Significantly greater than Q̇BA at end‐exercise during H1 (p &lt; 0.05). ** Significantly greater than Q̇BA at end‐exercise during H2 (p &lt; 0.05). Note no differences among end‐exercise Q̇BA values at (estimated) and above CF, suggesting a maximal response in the severe‐intensity domain.Figure 1Frequency‐domain near‐infrared spectroscopy (FD‐NIRS) measurements of muscle oxygenation during exercise below (● H1 and ○ H2) and above (▼ S1 and Δ S2) critical force (CF). Dashed lines indicate average endexercise values of respective FD‐NIRS signals during the maximal‐effort test (MET). * Significantly different from end‐exercise during H1 and H2 (p &lt; 0.05). ** Significantly greater than total‐[heme] at end‐exercise during H2 (p &lt; 0.05).Figure 2

The Effect of Acute Passive Heating on Microvascular Oxygen Delivery, Exercise Tolerance and Recovery

The purpose of this study was to determine the effect of 1 bout of passive heating on oxygen delivery during knee extension exercise and neuromuscular recovery. We tested the hypotheses that: 1) 1 bout of passive heating would result in a decrease in the diffusive oxygen delivery (total‐[heme]) and an increase in the perfusive oxygen delivery (deoxy‐[heme]) in the exercising muscle, 2) peripheral fatigue would recover at a faster rate post heating and 3) passive heating would increase time to exhaustion.MethodsTwo men (183.9 ± 5.6 cm; 74.2 ± 10.8 kg) and two women (160.5 ± 0.7 cm; 53.8 ± 0.3 kg) performed intermittent isometric knee extension tests to exhaustion at 40% MVC pre and post 1 bout of passive heating. Absolute concentrations of deoxy‐[heme] (perfusive oxygen delivery) and total‐[heme] (diffusive oxygen delivery) of the vastus lateralis muscle were measured continuously via frequency‐domain multi‐distance near‐infrared spectroscopy (OxiplexTS, ISS) during exercise. Neuromuscular measurements of central (as measured by maximal voluntary contraction force, MVC; and voluntary activation, VA) and peripheral fatigue (as measured by potentiated twitch force, Qtw) were made every 30 s pre exercise and immediately after task failure for a total of 6 sets each. The passive heating protocol consisted of immersion up to the shoulder in a 40°C hot tu b until rectal temperature reached 38.5°C or increased by 1°C for 60 minutes.ResultsTime to exhaustion for pre and post passive heating was 553 ± 296 sec and 808 ± 491 sec, respectively (p=0.227), but 3 of the 4 subjects showed an increase in time to exhaustion post heating. From baseline to the last 60 seconds of exercise there was no significant difference in the increase in deoxy‐[heme] for pre (47.5 ± 30.8 μM) and post passive heating (38.5 ± 21.5 μM; p=0.526). Likewise, the increase in total‐[heme] during exercise was not significantly different following passive heating (p=0.117) (pre and post passive heating Δ total‐[heme] was 20.8 ± 19.8 μM and 8.3 ± 13.4 μM, respectively). There was no difference in baseline MVC pre to post heating (p=0.841), but MVC was significantly decreased following exercise for both pre and post passive heating (p&lt;0.001). % change in VA from pre to post exercise was not different between pre and post heating (p=0.892). Qtw was not significantly different post exercise between pre to post heating. However, Qtw began to recover faster post compared to pre heating following exercise to exhaustion (30 sec vs. 90 sec, respectively).ConclusionThe trend of increased time to exhaustion after 1 bout of passive heating cannot be explained by increased perfusive or diffusive oxygen delivery. Passive heating led to a faster onset of recovery of peripheral fatigue (Qtw) following exercise to exhaustion, but the indices of central fatigue were not different pre vs. post heating.

Limb blood flow and muscle oxygenation responses during handgrip exercise above vs. below critical force

This study compared the brachial artery blood flow (Q̇BA) and microvascular oxygen delivery responses during handgrip exercise above vs. below critical force (CF; the isometric analog of critical power). Q̇BA and microvascular oxygen delivery are important determinants of oxygen utilization and metabolite accumulation during exercise, both of which increase progressively during exercise above CF. However the Q̇BA and microvascular oxygen delivery responses above vs. below CF remain unknown. We hypothesized that Q̇BA, deoxygenated-heme (deoxy-[heme]; an estimate of microvascular fractional oxygen extraction), and total-heme concentrations (total-[heme]; an estimate of changes in microvascular hematocrit) would demonstrate physiological maximums above CF despite increases in exercise intensity. Seven men and six women performed 1) a 5-min rhythmic isometric-handgrip maximal-effort test (MET) to determine CF and 2) two constant target-force tests above (severe-intensity; S1 and S2) and two constant target-force tests below (heavy-intensity; H1 and H2) CF. CF was 189.3 ± 16.7 N (29.7 ± 1.6%MVC). At end-exercise, Q̇BA was greater for tests above CF (S1: 418 ± 147 mL/min; S2: 403 ± 137 mL/min) compared to tests below CF (H1: 287 ± 97 mL/min; H2: 340 ± 116 mL/min; all p < 0.05) but was not different between S1 and S2. Further, end-test Q̇BA during both tests above CF was not different from Q̇BA estimated at CF (392 ± 37 mL/min). At end-exercise, deoxy-[heme] was not different between tests above CF (S1: 150 ± 50 μM; S2: 155 ± 57 μM), but was greater during tests above CF compared to tests below CF (H1: 101 ± 24 μM; H2: 111 ± 21 μM; all p < 0.05). At end-exercise, total-[heme] was not different between tests above CF (S1: 404 ± 58 μM; S2: 397 ± 73 μM), but was greater during tests above CF compared to H1 (352 ± 58 μM; p < 0.01) but not H2 (371 ± 57 μM). These data suggest limb blood flow limitations exist and maximal levels of muscle microvascular oxygen delivery and extraction occur during exercise above, but not below, CF.

Effects of Passive Heating on Perfusive and Diffusive Microvascular Oxygen Delivery

Previous studies have demonstrated that passive heating has led to increases in endothelial function and vasodilation of the brachial artery. The increase in vasodilation is thought to originate from increased nitric oxide bioavailability, thus increasing blood flow into the limb. However, the different aspects of the downstream microvascular oxygen delivery (i.e. perfusive and diffusive) to the exercising muscle have yet to be described. PURPOSE: The purpose of this study was to determine the effect of seven days of passive heating on oxygen delivery during handgrip exercise. We tested the hypothesis that, 7 days of passive heating would result in a decrease in the diffusive oxygen delivery (total-[heme]) and an increase in the perfusive oxygen delivery (deoxy-[heme]) in the exercising muscle. METHODS: Three participants (2 women, 23.0 ± 1.0 yrs, 70.9 ± 15.7 kg, 171 ± 10.1 cm) participated in this study. Peak power was determined by an incremental two-hand handgrip exercise test. Subjects performed 10 minutes of dynamic handgrip exercise at 40% peak power pre and post 7 days of passive heating. Absolute concentrations of deoxy-[heme] and total-[heme] of the flexor digitorum superficialis muscle were measured continuously via frequency-domain multi-distance near-infrared spectroscopy (OxiplexTS, ISS). The passive heating protocol consisted of immersion up to the shoulder in a 40°C hot tub until rectal temperature reached 38.5°C or increased by 1°C for 60 minutes. Data reported as mean ± SE. RESULTS: From baseline to the last 30 seconds of exercise there was no significant difference in the ∆ deoxy-[heme] (perfusive oxygen delivery) for pre (52.3 ± 2.2 μM) and post passive heating (47.6 ± 16.4 μM; p=0.822). However, the ∆ total-[heme] (diffusive oxygen delivery) was significantly lower following passive heating (p<0.001). Pre and post passive heating ∆ total-[heme] was 75.1 ± 13.8 μM and 30.7 ± 13.3 μM, respectively. CONCLUSION: The significant decrease in∆ total-[heme] after passive heating suggests that the diffusion of oxygen into the exercising muscle was reduced. This finding, along with no change in the perfusive oxygen delivery as represented by the ∆ deoxy-[heme], suggests that the oxygen uptake of the exercising muscle was decreased.

Repeated sprint training in hypoxia – an innovative method

The year 2018 marked the 50th anniversary of the Mexico Olympic games, which represents the starting point of scientific research on hypoxic training. Since the original “Live High – Train High”, many altitude/hypoxic training methods have been developed. The aim of the present review is to present the most recent method called “Repeated Sprint training in Hypoxia” (RSH). RSH is of unprecedented interest in the altitude training area with 25 studies published in the 5-year period following the pioneer article in 2013, and with only two studies that did not report any beneficial effects. Potential mechanisms include transcriptional factors involved in oxygen-signaling and oxygen-carrying capacity and mitochondrial metabolism enzymes, improved behavior of fasttwitch fibers notably via compensatory vasodilatation, improved vascular relaxation and greater microvascular oxygen delivery as well as faster rate of phosphocreatine resynthesis. In general, RSH leads to superior repeated-sprint ability (i.e., faster mean sprint times or higher power outputs associated with a better resistance to fatigue during a repeated-sprint test) in normoxic conditions. RSH where hypoxia is induced by voluntary hypoventilation at low lung volume (named VHL) may also improve repeated-sprint performance more than in normoxia. Practically, RSH benefits have been demonstrated for a large range of team- (rugby, football, LaCrosse, Australian Football, field hockey), endurance (cycling, track and field, cross-country ski), racket (tennis) or combat (Jiu-Jitsu) sports.

Novel Insight Into the Determinants of Skeletal Muscle Oxygen Consumption by Dual‐Wavelength Diffuse Correlation Spectroscopy

Near‐infrared diffuse correlation spectroscopy (DCS) is a promising new optical imaging technique to noninvasively quantify microvascular perfusion. Combining DCS with conventional near‐infrared spectroscopy (NIRS) provides novel insight into the determinants of skeletal muscle oxygen consumption, by measuring microvascular oxygen delivery (by DCS) and oxygen utilization (by NIRS, i.e. tissue saturation). To test this hypothesis, we compared DCS‐derived measures of skeletal muscle oxygen delivery and utilization against Doppler ultrasound and direct measures of oxygen saturation from the venous effluent of the exercising muscle. Eight healthy male subjects (age 27.2 ± 4.2; BMI 26.9 ± 4.3) performed a graded handgrip exercise test. Subjects were instructed to perform rhythmic hand grip exercise on a two‐second duty cycle, at 10, 13, 16, and 19 kg. All subjects were instrumented with the DCS probe placed over the belly of the flexor digitorum profundus, a retrograde venous catheter (deep vein) inserted into the antecubital vein, and Doppler ultrasound over the brachial artery. Heart rate was monitored by ECG and blood pressure by continuous finger‐pressure of the non‐gripping hand. The major findings were two‐fold: First, we observed a strong intra‐individual relationship between DCS‐derived muscle perfusion and Doppler‐derived brachial artery blood flow (r2 = 0.95). Second, we observed an unexpected dissociation between skeletal muscle saturation, as measured by DCS, which declined progressively with each incremental workload (BL 71.9 ± 0.29%, 10kg 68.4 ± 0.91%, 13 kg 64.2 ± 1.04%, 16kg 61.8 ± 0.54%, 19kg 59.4 ± 1.26%), while directly measured venous oxygen saturation declined with the onset of exercise but remained unchanged thereafter (BL, 65.3 ± 7.20%; 10kg, 39.8 ± 8.42%; 13 kg, 39.5 ± 7.83%, 16kg, 42 ± 4.14%, 19kg, 41 ± 3.38%). Taken together, these results support DCS as a valuable tool, capable of providing novel insight into the determinants of skeletal muscle oxygen consumption at the microvascular level. The fact that tissue saturation was dissociated from venous saturation suggests that DCS may provide insight beyond conventional measurements.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Managing the power grid: how myoglobin can regulate PO2 and energy distribution in skeletal muscle.

Managing the power grid: how myoglobin can regulate PO2 and energy distribution in skeletal muscle.

Compromised microvascular oxygen delivery increases brain tissue vulnerability with age

Despite the possible role of impaired cerebral tissue oxygenation in age-related cognition decline, much is still unknown about the changes in brain tissue pO2 with age. Using a detailed investigation of the age-related changes in cerebral tissue oxygenation in the barrel cortex of healthy, awake aged mice, we demonstrate decreased arteriolar and tissue pO2 with age. These changes are exacerbated after middle-age. We further uncovered evidence of the presence of hypoxic micro-pockets in the cortex of awake old mice. Our data suggests that from young to middle-age, a well-regulated capillary oxygen supply maintains the oxygen availability in cerebral tissue, despite decreased tissue pO2 next to arterioles. After middle-age, due to decreased hematocrit, reduced capillary density and higher capillary transit time heterogeneity, the capillary network fails to compensate for larger decreases in arterial pO2. The substantial decrease in brain tissue pO2, and the presence of hypoxic micro-pockets after middle-age are of significant importance, as these factors may be related to cognitive decline in elderly people.

Perfusive and Diffusive Microvascular Oxygen Delivery During Simulated Hypovolemia and Dynamic Forearm Exercise

Perfusive and Diffusive Microvascular Oxygen Delivery During Simulated Hypovolemia and Dynamic Forearm Exercise

The noninvasive simultaneous measurement of tissue oxygenation and microvascular hemodynamics during incremental handgrip exercise.

Limb blood flow increases linearly with exercise intensity; however, invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We tested the hypotheses that 1) brachial artery blood flow (Q̇BA) would increase with increasing exercise intensity until task failure, 2) blood flow index of the flexor digitorum superficialis (BFIFDS) measured noninvasively via diffuse correlation spectroscopy would plateau at a submaximal work rate, and 3) muscle oxygenation characteristics (total-[heme], deoxy-[heme], and percentage saturation) measured noninvasively with near-infrared spectroscopy would demonstrate a plateau at a similar work rate as BFIFDS. Sixteen subjects (23.3 ± 3.9 yr, 170.8 ± 1.9 cm, 72.8 ± 3.4 kg) participated in this study. Peak power (Ppeak) was determined for each subject (1.8 ± 0.4 W) via an incremental handgrip exercise test. Q̇BA, BFIFDS, total-[heme], deoxy-[heme], and percentage saturation were measured during each stage of the exercise test. On a subsequent testing day, muscle activation measurements of the FDS (RMSFDS) were collected during each stage of an identical incremental handgrip exercise test via electromyography from a subset of subjects ( n = 7). Q̇BA increased with exercise intensity until the final work rate transition ( P < 0.05). No increases in BFIFDS or muscle oxygenation characteristics were observed at exercise intensities greater than 51.5 ± 22.9% of Ppeak. No submaximal plateau in RMSFDS was observed. Whereas muscle activation of the FDS increased until task failure, noninvasively measured indices of perfusive and diffusive muscle microvascular oxygen delivery demonstrated submaximal plateaus. NEW & NOTEWORTHY Invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We demonstrate that indices of perfusive and diffusive microvascular oxygen transport to skeletal muscle, measured completely noninvasively, plateau at submaximal work rates during incremental exercise, even though limb blood flow and muscle recruitment continued to increase.

Redesign of EAF Peg Hb As an Oxygen Transfer Catalyst for Improved Tissue Oxygenation of Hypoxic Areas: A Therapeutic for Anemia and Sickle Cell Disease

BackgroundSurface decoration of Hb with PEG chains has been advanced as an approach to attenuate the vasoconstrictive activity of acellular Hb in circulation. PEGylated Hbs with higher oxygen affinities relative to unmodified Hb has an advantage when designing blood substitutes in achieving targeted transport of O2 from lungs to tissues. Extension Arm Facilitated (EAF) PEGylation of Hb with six copies of PEG 5K (EAF P5K6 Hb) was advanced as an optimum level of PEGylation that does not induce weakening of interdimeric interactions in the quaternary structure of Hb. EAF P5K6 Hb at 4 gm % is an O2 carrying colloidal plasma expander. However, a prototype of EAF P5K6 Hb, developed by Sangart as MP4, did not increase microvascular oxygen delivery or improve tissue oxygenation in models of extreme hemodilution,whereas the EAF P5K6 Hb and P5K2 did better than the non-oxygen carrying semisynthetic colloidal plasma expander and EAF P5K6 albumin.Also, P5K2 Hb does a better tissue oxygenation even though its oxygen affinity is comparable to that of EAF P5K6 Hb1.As PEG chains increases from 2 to 10 copies, there is a lesser effect in the oxygen affinity of PEG Hbs in the presence of allosteric effectors like IHP and L-35. This suggests that higher degree of PEGylation preferentially stabilize the oxy conformation of PEG Hb, decreasing the efficacy of O2 release in the circulation. We attributed this decreased efficiency to the larger PEG shell which might lower the diffusion of O2out to the plasma. We reasoned that we could reduce the size of the PEG shell using six copies of PEG 3K and that we could use this new PEG Hb (EAF P3K6 Hb) at 6 gm% to get nearly the same viscosity as 4 gm % EAF P5K6 Hb. We compared tissue oxygenation by EAF P3K6 Hb with EAF P5K6 Hb and P5K2 Hb in an experimentally induced anemia model and tested its therapeutic efficacy in attenuating the experimentally induced acute crisis in a mild murine model of SCD, NY1DD.MethodsEAF P3K6 Hb was prepared with PEG3K and characterized as EAF P5K6 Hb2. Molecular models and the shapes of PEG Hbs have been developed using InsightII program (Accelrys). Tissue oxygenation studies were carried out in the extreme hemodilution hamster model as described previously.1 Acute vaso-occlusion was studied using intravital microscopy in the venules of cremaster muscles of NY1DD SCD mice subjected to hypoxia-reoxygenation (18 hrs of hypoxia with 8% O2 and then normoxia).3The efficacy was measured after treatment with 10% top load of 6 gm % solution of EAF P3K6 Hb, given at the transitioning stage from hypoxia to normoxiaResultsPEG 3K chains reduced the molecular radius and volume while increased the packing density of PEG shell of EAF P3K6 Hb as compared to EAF P5K6 Hb (Table 1). The packing of the PEG chains induced a globular shape to the EAF P3K6 Hb vs ellipsoidal shape to EAF P5K6 Hb (Fig 1). Oxygen affinity of EAF P3K6 was comparable to that of EAF P5K6 Hb. In the extreme hemodilution hamster model of anemia, the efficacy of EAF P3K6 Hb to increase tissue oxygenation was comparable to that of P5K2 Hb (Fig 2). From the intravital micrographs of cremaster muscles (Fig 3), P3K6 Hb markedly attenuated the induced vaso-occlusive crisis in NY1DD mice.Discussion and ConclusionsWe have previously shown that EAF P3K6 Hb is effective in normalizing the chronic state of vaso-occlusion in BERK sickle mice. We demonstrate here that EAF P3K6 Hb protects NY1DD sickle mice from developing acute vaso-occlusion. These studies suggest that a smaller compact PEG shell around Hb increases the ability of the PEG Hb to oxygenate the tissues. NY1DD mice are not very anemic (Hb > 10 gm/dl); a 10% top load of 6 gm% high oxygen affinity Hb represents no significant increase in Hb level (≈0.06 gm %). We suggest that the ability of such a miniscule amount of PEG Hb in the plasma to protect the animal from hypoxia is due to its enhanced ability to deliver O2 under anemic and hypoxic conditions. We suggest that EAF P3K6 Hb facilitates improved extraction of O2 from RBC to plasma and to the subsequent diffusion to tissues, i.e. small amount of PEG Hb in plasma acts as a catalyst for O2 transfer while the oxyHb in RBC serves as the local reservoir for O2. This improved O2 extractionmay be effective in anemia in general, and have a unique application in sickle cell disease.

Simultaneous sampling of tissue oxygenation and oxygen consumption in skeletal muscle

Under physiologic conditions, microvascular oxygen delivery appears to be well matched to oxygen consumption in respiring tissues. We present a technique to measure interstitial oxygen tension (PISFO2) and oxygen consumption (VO2) under steady-state conditions, as well as during the transitions from rest to activity and back. Phosphorescence Quenching Microscopy (PQM) was employed with pneumatic compression cycling to achieve 1 to 10Hz sampling rates of interstitial PO2 and simultaneous recurrent sampling of VO2 (3/min) in the exteriorized rat spinotrapezius muscle. The compression pressure was optimized to 120–130mmHg without adverse effect on the tissue preparation. A cycle of 5s compression followed by 15s recovery yielded a resting VO2 of 0.98±0.03ml O2/100cm3min while preserving microvascular oxygen delivery. The measurement system was then used to assess VO2 dependence on PISFO2 at rest and further tested under conditions of isometric muscle contraction to demonstrate a robust ability to monitor the on-kinetics of tissue respiration and the compensatory changes in PISFO2 during contraction and recovery. The temporal and spatial resolution of this approach is well suited to studies seeking to characterize microvascular oxygen supply and demand in thin tissues.

Abstract 14822: Impact of Proportional Assist Ventilation on Skeletal Muscle Reoxygenation and Central Hemodynamic During Constant Work Rate Exercise in Chronic Heart Failure

Introduction: Progressive chronic heart failure slows the recovery of microvascular oxygen delivery and utilization, which produce deleterious implications to exercise capacity. Respiratory muscle unloading can enhance the responses to exercise thereby allowing a closer matching between skeletal muscle oxygen delivery and utilization in patients with chronic heart failure (CHF). Hypothesis: We reasoned that noninvasive ventilation administered by proportional assisted ventilation (PAV) could accelerate skeletal muscle reoxygenation after high intensity exercise in CHF humans. To test this hypothesis, we conducted a study with 12 patients with stable CHF who were randomized to receive PAV or sham ventilation during high-intensity constant work exercise and compared the effects of these interventions on oxygen pulmonary (O2p), cardiac output and [[Unable to Display Character: &amp;#8710;]][deoxi-Hb+Mb] off kinetics. Methods: Twelve patients with CHF (NYHA class II and III and left ventricle ejection fraction= 26±9%) underwent two high-intensity, constant-work rate (80% peak) cycle ergometer tests receiving PAV or sham ventilation. Off-exercise kinetics of the primary component of oxygen uptake, an index of fractional oxygen extraction by near infrared spectroscopy (~[[Unable to Display Character: &amp;#8710;]][deoxy-Hb+Mb]) in the vastus lateralis) and cardiac output (QT) by impedance cardiography were assessed. Results: PAV significantly accelerated the recovery of O2p when compared with sham τ = 56±22 vs. 77±42s, respectively, p&lt;0.05). Interestingly, PAV was associated with faster fractional O2 extraction (~[[Unable to Display Character: &amp;#8710;]][deoxy-Hb+Mb] by near-infrared spectroscopy) (τ= 31±19 vs. 42±22s, respectively, p&lt;0.05) . In addition, kinetics of QT were significantly faster with PAV than sham (τ = 39±22 vs. 78±46s, respectively, p&lt;0.05). Conclusions: These data indicate that PAV has beneficial effects on recovery of muscle metabolism and central hemodynamics after high-intensity exercise in CHF patients. Financial Support: FAPESP 2009-01842-0

Effect of oxygenated polyethylene glycol decorated hemoglobin on microvascular diameter and functional capillary density in the transgenic mouse model of sickle cell anemia

Malemide polyethylene glycol-conjugated Hb (MP4OX, Sangart Inc.), a high-affinity low-concentration acellular hemoglobin (P50 = 5 mmHg, 4.3 g/dl) solution, has been shown to optimize microvascular perfusion and target oxygen delivery to anoxic tissue. Microvascular perfusion during an acute hypoxic challenge in a transgenic anemic sickle cell disease mouse model was studied with MP4OX and saline. Arterioles were dilated in both groups. Functional capillary density (FCD) was maintained at a higher level with MP4OX. In conclusion, MP4OX treatment reduced the hypoxia-mediated decline in FCD, an effect in part due to higher arterial pressure resulting in increased microvascular perfusion pressures.