Microvascular Oxygen Research Articles

Bilateral Nephrectomy Impairs Cerebral Oxygen Delivery After Acute Hemodilution Anemia in Rats

HypothesisWe hypothesize that renal sensing of hypoxia is essential for activation of cardiovascular responses needed to maintain cerebral oxygen delivery during acute anemia and that bilateral nephrectomy will negatively affect brain oxygen delivery during acute hemodilutional anemia.MethodsAll experiments were approved by the Animal Care and Use Committee at St. Michael’s Hospital and conducted in accordance with ARRIVE‐2 guidelines. Sham or bilateral nephrectomy (n=5) were performed on isoflurane anesthetized Sprague‐Dawley rats prior to performing acute hemodilution (50% estimated blood volume) 1:1 with 6% hydroxyethyl starch. Heart rate (HR), mean arterial pressure (MAP) and rectal temperature were monitored continuously. Arterial blood gases and cooximetry samples were measured before and after hemodilution. Continuous measurements of brain microvascular oxygen tension (PbrO2) were measured via a phosphorescence quenching method using an intravascular oxygen probe, Oxyphor PdG4, and an OxyLED phosphorometer. Data were tested for normality and analysis performed by two‐way ANOVA with p<0.05 taken to be significant.ResultsBilateral nephrectomy resulted in a reduction in baseline HR (p<0.004). Hemodilution resulted in a comparable reduction in hemoglobin concentration in both groups (Sham: 5.1±0.3 g/dL; Nephrectomy: 4.2±0.4 g/dL) with no differences in arterial PaO2 or PaCO2. After hemodilution, HR increased significantly in both groups (p<0.05). MAP tended to be lower in the bilateral nephrectomy group and decreased transiently in both groups following hemodilution (p<0.01). Brain microvascular pO2 (PbrO2)was not significantly different between nephrectomy and sham at baseline. Post‐hemodilution the PbrO2 was significantly lower in nephrectomised rats (11.5±5.1 mmHg) compared to sham operated rats (25.4±11.3 mmHg) (p=0.031).ConclusionsWe observed that bilateral nephrectomy resulted in changes in baseline physiology, including a lower HR. Hemodilution resulted in a comparable reduction in hemoglobin concentration, without difference in PaO2 or PaCO2. MAP decreased more profoundly immediately after hemodilution in nephrectomised animals, and recovered with time. Nephrectomy resulted in a significant drop in brain microvascular pO2 (PbrO2) immediately after hemodilution. This was not observed in sham controls. These data suggest that the kidney serves an important sensing role during acute anemia which supports cardiovascular responses and preserves the adequacy of brain tissue oxygen delivery. Hypoxia sensing by the kidney, and other integrative systemic mechanisms, may be central to these findings.

Local Mucosal CO2 but Not O2 Insufflation Improves Gastric and Oral Microcirculatory Oxygenation in a Canine Model of Mild Hemorrhagic Shock.

IntroductionAcute hemorrhage results in perfusion deficit and regional hypoxia. Since failure of intestinal integrity seem to be the linking element between hemorrhage, delayed multi organ failure, and mortality, it is crucial to maintain intestinal microcirculation in acute hemorrhage. During critical bleeding physicians increase FiO2 to raise total blood oxygen content. Likewise, a systemic hypercapnia was reported to maintain microvascular oxygenation (μHbO2). Both, O2 and CO2, may have adverse effects when applied systemically that might be prevented by local application. Therefore, we investigated the effects of local hyperoxia and hypercapnia on the gastric and oral microcirculation.MethodsSix female foxhounds were anaesthetized, randomized into eight groups and tested in a cross-over design. The dogs received a local CO2-, O2-, or N2-administration to their oral and gastric mucosa. Hemorrhagic shock was induced through a withdrawal of 20% of estimated blood volume followed by retransfusion 60 min later. In control groups no shock was induced. Reflectance spectrophotometry and laser Doppler were performed at the gastric and oral surface. Oral microcirculation was visualized by incident dark field imaging. Systemic hemodynamic parameters were recorded continuously. Statistics were performed using a two-way-ANOVA for repeated measurements and post hoc analysis was conducted by Bonferroni testing (p < 0.05).ResultsThe gastric μHbO2 decreased from 76 ± 3% to 38 ± 4% during hemorrhage in normocapnic animals. Local hypercapnia ameliorated the decrease of μHbO2 from 78 ± 4% to 51 ± 8%. Similarly, the oral μHbO2 decreased from 81 ± 1% to 36 ± 4% under hemorrhagic conditions and was diminished by local hypercapnia (54 ± 4%). The oral microvascular flow quality but not the total microvascular blood flow was significantly improved by local hypercapnia. Local O2-application failed to change microvascular oxygenation, perfusion or flow quality. Neither CO2 nor O2 changed microcirculatory parameters and macrocirculatory hemodynamics under physiological conditions.DiscussionLocal hypercapnia improved microvascular oxygenation and was associated with a continuous blood flow in hypercapnic individuals undergoing hemorrhagic shock. Local O2 application did not change microvascular oxygenation, perfusion and blood flow profiles in hemorrhage. Local gas application and change of microcirculation has no side effects on macrocirculatory parameters.

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.

Muscle oxygenation and time to task failure of submaximal holding and pulling isometric muscle actions and influence of intermittent voluntary muscle twitches

BackgroundIsometric muscle actions can be performed either by initiating the action, e.g., pulling on an immovable resistance (PIMA), or by reacting to an external load, e.g., holding a weight (HIMA). In the present study, it was mainly examined if these modalities could be differentiated by oxygenation variables as well as by time to task failure (TTF). Furthermore, it was analyzed if variables are changed by intermittent voluntary muscle twitches during weight holding (Twitch). It was assumed that twitches during a weight holding task change the character of the isometric muscle action from reacting (≙ HIMA) to acting (≙ PIMA).MethodsTwelve subjects (two drop outs) randomly performed two tasks (HIMA vs. PIMA or HIMA vs. Twitch, n = 5 each) with the elbow flexors at 60% of maximal torque maintained until muscle failure with each arm. Local capillary venous oxygen saturation (SvO2) and relative hemoglobin amount (rHb) were measured by light spectrometry.ResultsWithin subjects, no significant differences were found between tasks regarding the behavior of SvO2 and rHb, the slope and extent of deoxygenation (max. SvO2 decrease), SvO2 level at global rHb minimum, and time to SvO2 steady states. The TTF was significantly longer during Twitch and PIMA (incl. Twitch) compared to HIMA (p = 0.043 and 0.047, respectively). There was no substantial correlation between TTF and maximal deoxygenation independently of the task (r = − 0.13).ConclusionsHIMA and PIMA seem to have a similar microvascular oxygen and blood supply. The supply might be sufficient, which is expressed by homeostatic steady states of SvO2 in all trials and increases in rHb in most of the trials. Intermittent voluntary muscle twitches might not serve as a further support but extend the TTF. A changed neuromuscular control is discussed as possible explanation.

Effects of low load exercise with and without blood-flow restriction on microvascular oxygenation, muscle excitability and perceived pain

ABSTRACT This paper aimed to examine the acute effect of low-load (LL) exercise with blood-flow restriction (LL-BFR) on microvascular oxygenation and muscle excitability of the vastus medialis (VM) and vastus lateralis (VL) muscles during a single bout of unilateral knee extension exercise performed to task failure. Seventeen healthy recreationally resistance-trained males were enrolled in a within-group randomized cross-over study design. Participants performed one set of unilateral knee extensions at 20% of one-repetition maximum (1RM) to task failure, using a LL-BFR or LL free-flow (LL-FF) protocol in a randomized order on separate days. Changes in microvascular oxygenation and muscle excitability in VL and VM were assessed using near-infrared spectroscopy (NIRS) and surface electromyography (sEMG), respectively. Pain measures were collected using the visual analog scale (VAS) before and following set completion. Within- and between- protocol comparisons were performed at multiple time points of set completion for each muscle. During LL-BFR, participants performed 43% fewer repetitions and reported feeling more pain compared to LL-FF (p<0.05). Normalized to time to task failure, LL-BFR and LL-FF generally demonstrated similar progression in microvascular oxygenation and muscle excitability during exercise to task failure. The present results demonstrate that LL-BFR accelerates time to task failure, compared with LL-FF, resulting in a lower dose of mechanical work to elicit similar levels of oxygenation, blood-pooling, and muscle excitability. LL-BFR may be preferable to LL-FF in clinical settings where high workloads are contraindicated, although increased pain experienced during BFR may limit its application. Highlights Compared to free flow (FF), neuromuscular fatigue mechanisms are accelerated during blood flow restricted (BFR) training. This can be observed as changes in microvascular oxygenation and muscle excitability occurring at a ∼43% faster mean rate during BFR compared to FF. BFR exercise seems to elicit the same level of neuromuscular fatigue as FF training within a shorter timeframe. This reduces total joint load and may be especially helpful in cases where high training volumes may be contraindicated (e.g. recovering from a sports injury or orthopedic surgery).

Vascular feature as a modulator of the aging brain.

The cerebral functional reorganization and declined cognitive function of aging might associate with altered vascular features. Here, we explored the altered cerebral hierarchical functional network of 2 conditions (task-free and naturalistic stimuli) in older adults and its relationship with vascular features (systemic microvascular and perfusion features, measured by magnetic resonance imaging) and behavior. Using cerebral gradient analysis, we found that compressive gradient of resting-state mainly located on the primary sensory-motor system and transmodal regions in aging, and further compress in these regions under the continuous naturalistic stimuli. Combining cerebral functional gradient, vascular features, and cognitive performance, the more compressive gradient in the resting-state, the worse vascular state, the lower cognitive function in older adults. Further modulation analysis demonstrated that both vascular features can regulate the relationship between gradient scores in the insula and behavior. Interestingly, systemic microvascular oxygenation also can modulate the relationship between cerebral gradient and cerebral perfusion. Furthermore, the less alteration of the compressive gradient with naturalistic stimuli came with lower cognitive function. Our findings demonstrated that the altered cerebral hierarchical functional structure in aging was linked with changed vascular features and behavior, offering a new framework for studying the physiological mechanism of functional connectivity in aging.

Role of nitric oxide in convective and diffusive skeletal muscle microvascular oxygen kinetics

Progress in understanding physiological mechanisms often consists of discrete discoveries made across different models and species. Accordingly, understanding the mechanistic bases for how altering nitric oxide (NO) bioavailability impacts exercise tolerance (or not) depends on integrating information from cellular energetics and contractile regulation through microvascular/vascular control of O2 transport and pulmonary gas exchange. This review adopts state-of-the-art concepts including the intramyocyte power grid, the Wagner conflation of perfusive and diffusive O2 conductances, and the Critical Power/Critical Speed model of exercise tolerance to address how altered NO bioavailability may, or may not, affect physical performance. This question is germane from the elite athlete to the recreational exerciser and particularly the burgeoning heart failure (and other clinical) populations for whom elevating O2 transport and/or exercise capacity translates directly to improved life quality and reduced morbidity and mortality. The dearth of studies in females is also highlighted, and areas of uncertainty and questions for future research are identified.

Test-Retest Reliability of the Microvascular Oxygenation Recovery Response Subsequent to Submaximal Cycling Exercise

Introduction: The purpose was to quantify the within-session and between-session reliability of halftime (HT) and monoexponential curve fitting (EXP) analyses, when assessing the microvascular tissue oxygenation (StO2) recovery response via near-infrared spectroscopy (NIRS). Methods: Seventeen subjects completed a submaximal cycling test and 6-minute cool-down on three occasions. The protocol was completed twice during session 1 and once during session 2. StO2 were collected via NIRS from a randomized vastus lateralis. StO2 response from the last minute of exercise and the entire cool-down was analyzed using HT and EXP. Within-session and between-session reliability were examined by mixed, absolute agreement intraclass correlation coefficients (ICC) and standard error of the measurement (SEM). Results: HT resulted in higher within-session reliability compared to EXP for exercising StO2 (ICCHT=0.920, ICCEXP=0.865, SEMHT=4.9 ∆BSL, SEMEXP=6.2 ∆BSL) and StO2 recovery time (ICCHT=0.772, ICCEXP=0.720, SEMHT=7 sec, SEMEXP=9 sec). Similar between-session reliability for exercising StO2 was observed (ICCHT=0.895, ICCEXP=0.879, SEMHT=5.2 ∆BSL, SEMEXP=5.4 ∆BSL), however HT elicited higher between-session reliability for StO2 recovery time (ICCHT=0.583, ICCEXP=-0.211, SEMHT=7 sec, SEMEXP=15 sec). Conclusions: Due to the better within-session (exercising StO2, StO2 recovery time) and between-session (StO2 recovery time) reliability, practitioners are encouraged to use HT when assessing exercising StO2 and StO2 recovery time.

NIRS Studies Show Reduced Interhemispheric Functional Connectivity in Individuals with Multiple Sclerosis That Exhibit Cortical Hypoxia.

Many with multiple sclerosis (MS) have low cortical microvascular oxygen levels (hypoxia), which have been previously proposed to exacerbate inflammation in MS. We do not know if hypoxia impacts or relates to brain function. We hypothesise that within the MS population, those who have hypoxia may show reduced brain functional connectivity (FC). We recruited 20 MS participants and grouped them into normoxic and hypoxic groups (n=10 in each group) using frequency-domain near-infrared spectroscopy (fdNIRS). Functional coherence of the haemodynamic signal, quantified with functional near-infrared spectroscopy (fNIRS) was used as a marker of brain function and was carried out during resting-state, finger-tapping, and while completing two neurocognitive tasks. Reduced FC was detected in the hypoxic MS group. fNIRS measures of haemodynamic coherence in MS could be a biomarker of functional impairment and/or disease progression.

Ocular surface microcirculation is better preserved with pulsatile versus continuous flow during cardiopulmonary bypass-An experimental pilot.

Non-pulsatile cardiopulmonary bypass (CPB) may induce microvascular dysregulation. In piglets, we compared ocular surface microcirculation during pulsatile versus continuous flow (CF) bypass. Ocular surface microcirculation in small tissue volumes (~0.1 mm3 ) at limbus (high metabolic rate) and bulbar conjunctiva (low metabolic rate) was examined in a porcine model using computer assisted video microscopy and diffuse reflectance spectroscopy, before and after 3 and 6h of pulsatile (n=5 piglets) or CF (n=3 piglets) CPB. Functional capillary density, capillary flow velocity and microvascular oxygen saturation were quantified. At limbus, velocities improved with pulsatility (p < 0.01) and deteriorated with CF (p < 0.01). In bulbar conjunctiva, velocities were severely reduced with CF (p < 0.01), accompanied by an increase in capillary density (p < 0.01). Microvascular oxygen saturation decreased in both groups. Ocular surface capillary densities and flow patterns are better preserved with pulsatile versus CF during 6h of CPB in sleeping piglets.

Skin trauma rapidly induces thermoregulatory plexus hyperemia, while an increased nutritive papillary capillary function can be detected after 24h.

Clinical assessments and laser Doppler perfusion measurements (LDPM) of skin microcirculation have limited value, as they fail to capture events regulated by local metabolic needs at a papillary capillary level. This study aimed to examine the ability of computer-assisted video microscopy (CAVM) and diffuse reflectance spectroscopy (DRS) to assess skin nutritive perfusion-compared to LDPM. Healthy volunteers (n=10) were examined after (≈1 and ≈24h) an incision (5×1mm) on the forearm, at 0.1mm (only with CAVM), 2-3mm, and 30mm from the trauma. No changes were detected by CAVM after ≈1h. After ≈24h, 0-1mm from the trauma, both CAVM parameters were increased: functional capillary density (capillary crossings/mm, 11.8±1.4 vs. 7.3±1.2, p<.01) and capillary flow velocities (CFV, %capillaries with brisk flow, 10±6.8 vs. 1±1, p<.01). At a distance of 2-3mm, only CFV was increased (6.2±6.1 vs. 1±1, p<.05). DRS and LDPM measurements increased 2-3mm from the trauma line in relation to baseline after both ≈1 and ≈24h, that is, with DRS (%microvascular oxygen saturation): 45.8±7.4% (baseline), 70.0±12.5% (≈1h), and 73.1±10.4% (≈24h), p<.01 and with LDPM (a.u.): 7.2±2.5 (baseline), 28.3±18.7 (≈1h), and 45.9±16.3 (≈24h), p<.01. ≈24h after skin trauma, an increased function of the nutritive papillary capillaries can be detected by CAVM.

P-100. Retinal microvascular oxygen saturation in women 3 years after hypertensive disorders of pregnancy

Male breast cancer has a low incidence, and its metastases are considered even rarer and a sign of seriousness for the patient. Breast metastases are difficult to diagnose even in female patients, with few cases of gastric metastasis described in the current literature.Male patient, 54 years old, obese, diagnosed with breast cancer for 8 years, underwent 5 years of cancer follow-up. After this period, he began to experience epigastric pain and weight loss, among other symptoms. Endoscopy was performed, showing rounded elevated lesions with central depression. After anatomopathological and biochemical tests, a diagnosis of differentiated carcinoma as a metastasis of breast origin was concluded.MBC is rare when compared to the incidence in females. Its risk factors differ between genders, and due to its rarity, there are few studies in the male population. This fact makes the evaluation of metastases more difficult. Gastric metastasis has a very low incidence even in women, approximately 8.9% of all breast cancer metastasis's locations. Other pathologies can mimic GM, always associating anatomopathological and biochemical tests. Oncological follow-up must always be carried out, and the possibility of screening must be evaluated.Despite the rare incidence and little information in the literature, the association between MBC and GM should be considered. Currently, there is no guidance for conducting systematic screenings and treatment is usually palliative due to the severity of the disease.

Increased Hepatic Microvascular Density, Oxygenation, and VEGF in the Hypertrophic Lobe following Portal Vein Embolization in Rabbits

Introduction: The microvascular events following portal vein embolization (PVE) are poorly understood despite the pivotal role of the microcirculation in liver regeneration and tumor progression. We aimed to assess the changes in hepatic microvascular perfusion and neo-angiogenesis after experimental PVE. Methods: PVE of the cranial liver lobes was performed in 12 New Zealand White rabbits divided into 2 groups of permanent (P-PVE) and reversible PVE (R-PVE), respectively. Hepatobiliary scintigraphy and CT were used to evaluate hepatic function and volume. Hepatic microcirculation was assessed using a handheld vital microscope (Cytocam) to measure microvascular density (total vessel density; TVD) before PVE, right after PVE, and 20 min after PVE, as well as at 14 days (D14 post-PVE) and 35 days (D35 post-PVE). Additionally, on D35, microvascular PO₂ and liver parenchymal VEGF were assessed. Results: Eleven rabbits were included after PVE (R-PVE, n = 5; P-PVE, n = 6). TVD in the nonembo­lized (hypertrophic) lobes was higher than in the embolized (atrophic) lobes of the P-PVE group at D35 post-PVE (36.7 ± 7.2 vs. 23.4 ± 4.9 mm/mm²; p < 0.05). In the R-PVE group, TVD in the nonembolized lobes was not increased at D35. Function and volume were increased in the nonembolized lobes of the P-PVE group compared to the embolized lobes, but not in the R-PVE group. Likewise, the mmicrovascular PO₂ and VEGF staining rate were higher in the nonembolized lobes of the P-PVE group at D35 post-PVE. Discussion/Conclusion: Successful volumetric and functional hypertrophy of the nonembolized lobe was accompanied by microvascular alterations featuring increased neo-angiogenesis, microvascular density, and microvascular oxygen pressure following P-PVE.

Localized Oxygen Exchange Platform for Intravital Video Microscopy Investigations of Microvascular Oxygen Regulation.

Intravital microscopy has proven to be a powerful tool for studying microvascular physiology. In this study, we propose a gas exchange system compatible with intravital microscopy that can be used to impose gas perturbations to small localized regions in skeletal muscles or other tissues that can be imaged using conventional inverted microscopes. We demonstrated the effectiveness of this system by locally manipulating oxygen concentrations in rat extensor digitorum longus muscle and measuring the resulting vascular responses. A computational model of oxygen transport was used to partially validate the localization of oxygen changes in the tissue, and oxygen saturation of red blood cells flowing through capillaries were measured as a surrogate for local tissue oxygenation. Overall, we have demonstrated that this approach can be used to study dynamic and spatial responses to local oxygen challenges to the microenvironment of skeletal muscle.

Impact of sodium glucose linked cotransporter-2 inhibition on renal microvascular oxygen tension in a rodent model of diabetes mellitus.

BackgroundThe mechanisms whereby inhibitors of sodium‐glucose linked cotransporter‐2 (SGLT2) exert their nephroprotective effects in patients with diabetes are incompletely understood but have been hypothesized to include improved tissue oxygen tension within the renal cortex. The impact of SGLT2 inhibition is likely complex and region specific within the kidney. We hypothesize that SGLT2 inhibitors have differential effects on renal tissue oxygen delivery and consumption in specific regions of the diabetic kidney, including the superficial cortex, containing SGLT2‐rich components of proximal tubules, versus the deeper cortex and outer medulla, containing predominantly SGLT1 receptors.MethodsWe measured glomerular filtration rate (GFR), microvascular kidney oxygen tension (PkO2), erythropoietin (EPO) mRNA, and reticulocyte count in diabetic rats (streptozotocin) treated with the SGLT2 inhibitor, dapagliflozin. Utilizing phosphorescence quenching by oxygen and an intravascular oxygen sensitive probe (Oxyphor PdG4); we explored the effects of SGLT2 inhibition on PkO2 in a region‐specific manner, in vivo, in diabetic and non‐diabetic rats. Superficial renal cortical or deeper cortical and outer medullary PkO2 were measured utilizing excitations with blue and red light wavelengths, respectively.ResultsIn diabetic rats treated with dapagliflozin, measurement within the superficial cortex (blue light) demonstrated no change in PkO2. By contrast, measurements in the deeper cortex and outer medulla (red light) demonstrated a significant reduction in PkO2 in dapagliflozin treated diabetic rats (p = 0.014). Consistent with these findings, GFR was decreased, hypoxia‐responsive EPO mRNA levels were elevated and reticulocyte counts were increased with SGLT2 inhibition in diabetic rats (p < 0.05 for all).ConclusionsThese findings indicate that microvascular kidney oxygen tension is maintained in the superficial cortex but reduced in deeper cortical and outer medullary tissue, possibly due to the regional impact of SGLT‐2 inhibition on tissue metabolism. This reduction in deeper PkO2 had biological impact as demonstrated by increased renal EPO mRNA levels and circulating reticulocyte count.

Oxygen Uptake and Near Infrared Spectroscopy Responses during Heavy Intensity Pseudorandom Binary Sequence Exercise

Activities of daily living are characterized by continuous changes in exercise intensity and duration resulting in many abrupt changes in metabolic demand. The ability to readily meet these demands is described by the rate of adaptation in oxygen uptake measured at either the mouth (VO2p) or at the site of active muscle. Many studies have used a single step transition to assess this rate of adaptation. However, a continually changing exercise, such as a pseudorandom binary sequence (PRBS) protocol, provides more transition phases to analyze. We examined the VO2p and microvascular oxygenation responses using near infrared spectroscopy (NIRS) during PRBS exercise. Five healthy males (30 ± 6 yrs, 83.4 ± 20.9 kg, VO2pk, 38.9 ± 5.9 mL/kg/min; mean ± SD) completed a ramp exercise to fatigue for the determination of the lactate threshold (LT) and peak VO2p (VO2pk). Each bout of PRBS exercise transitioned from a baseline of 20 W to a WR corresponding to either 90% of LT (MOD, 100 ± 7 W)) or to a WR at 40% of the difference between LT and VO2pk (HVY, 173 ± 13 W). The PRBS exercise consisted of 30 units of 15 s that alternated between BSL and the higher WR in a pseudorandom pattern; the pattern was repeated 3 times. VO2p was measured breath-by-breath using a metabolic cart and interpolated to 1 s intervals. For each subject and WR, the 3 repetitions for each trial were ensembled averaged yielding a single 450 s response. Changes in hemoglobin oxygenation (deoxy-[Hb+Mb]) and total hemoglobin concentration ([THC]) were measured from the vastus lateralis (VL) using frequency-domain NIRS. Data were collected at 1 Hz and ensembled averaged to yield a single 450 s response. VO2p and deoxy-[Hb+Mb] responses were transformed into the frequency domain and analyzed using standard approaches and the mean-normalized gain (MNG). The harmonic amplitudes (Amph) for VO2p/WR were lower (p<0.05) during HVY at 0.0067 Hz (Mod, 56.9 ± 13.0 %; HVY, 49.3 ± 6.8 %), 0.011 Hz (Mod, 37.6 ± 17.0 %, HVY: 25.8 ± 5.7 %) and 0.013 Hz (Mod, 33.2 ± 12.8 %; HVY, 15.8 ± 10.2 %). The Amph for deoxy-[Hb+Mb]/WR were lower (p<0.05) for VO­2p/WR in HVY compared to MOD (Mod, 39.5 ± 6.0 %; HVY, 30.2 ± 3.9 %). MNG was also lower (p<0.05) for deoxy-[Hb+Mb]/WR in HVY compared to MOD (Mod, 77.4 ± 26.6 %; HVY, 47.3 ± 10.0 %). The VO2p and deoxy-[Hb+Mb] responses from a PRBS trial represent the responses to individual sinusoidal inputs at each frequency tested. The normalized Amph and MNG measured in the current study described the temporal dynamics of the variables of interest (i.e., VO2p and deoxy-[Hb+Mb]). The results of the current study showed a significant decrease in both normalized Amph and MNG for VO2p and deoxy-[Hb+Mb] suggesting that exercise performed above the LT results in a slowing of both the VO2p and microvascular deoxy-[Hb+Mb] responses. These findings are consistent with previous studies demonstrating a slowing of VO2p above the LT. Compared to studies using a single step protocol, responses to PRBS protocols include many transition periods to identify potential control mechanisms that are more discernable during non-steady state exercise.

Dietary Nitrate Influence on Oxygen Uptake Response to Pseudorandom Binary Sequence Cycling Protocols

Dietary nitrate supplementation in the form of beetroot juice has been the subject of numerous studies in recent literature with inconclusive results. Beetroot juice (BRJ) has been consistently tested using constant-load or maximal-exertion exercise, but the influence of nitrate on pulmonary oxygen uptake (VO2p) and microvascular oxygenation to continually-changing exercise demands has received less attention. Five healthy males (30 ± 6 yrs, 83.4 ± 20.9 kg, VO2pk, 38.9 ± 5.9 mL/kg/min; mean ± SD) completed a ramp exercise to fatigue for the determination of the lactate threshold (LT) and peak VO2p (VO2pk). Each bout of PRBS exercise transitioned from a baseline of 20 W to a WR corresponding to 90% of LT (MOD, 100 ± 7 W). The PRBS exercise consisted of 30 units of 15 s that alternated between BSL and 90% LT in a pseudorandom pattern; the pattern was repeated 3 times. VO2p was measured breath-by-breath using a metabolic cart and interpolated to 1 s intervals. For each subject and WR, the 3 repetitions for each trial were ensemble averaged yielding a single 450 s response. Changes in hemoglobin oxygenation (deoxy-[Hb+Mb]) and total hemoglobin concentration ([THC]) were measured from the vastus lateralis (VL) using frequency-domain NIRS. Data were collected at 1 Hz and ensemble averaged to yield a single 450 s response. VO2p gain was calculated in the time domain by dividing the ΔVO2p by the ΔWR. VO2p and deoxy-[Hb+Mb] responses were transformed into the frequency domain and analyzed using standard approaches and the mean-normalized gain (MNG). Subjects completed the first trial under control conditions (CON), and then were put on a dosing regimen of 400mg of dietary nitrate (Beet It! Beetroot Juice) 2x/day for 3 days before returning to the lab for the second trial (BRJ). The gain (VO2/WR) was decreased in the BRJ condition versus CON (CON: 5.58 ± 0.65 mL/min/W, BRJ: 4.66 ± 1.03 mL/min/W, p = 0.049). The VO2 harmonics were not different between conditions. The deoxy-[Hb+Mb] harmonics were lower at all frequencies in the BRJ condition versus CON (p < 0.05). MNG is a measure that isolates the temporal characteristics of the investigated variable, using a normalization procedure. MNG for VO2 was not significantly different between conditions. The deoxy-[Hb+Mb] MNG was lower in BRJ (CON: 77.4 ± 26.6 %, BRJ: 42.3 ± 11.8 %, p = 0.011). The results of the current study are consistent with previous studies showing that BRJ supplementation decreases the oxygen cost of exercise for a given work rate. The similarity in the harmonics and the MNG for VO2 suggest that there was no impact of BRJ on the speed of VO2 adaptation, but the significant decrease in the MNG for the deoxy-[Hb+Mb] suggests that the rate of adaptation of deoxy-[Hb+Mb] was slower in the BRJ condition. The PRBS protocol has many on- and off-transitions that may identify influences of dietary nitrate that are not discernable during the rapid adaptation of deoxy-[Hb+Mb] to a single-step transition. Additionally, the results of the current study indicate that further investigation is warranted into the influences of dietary nitrate on the microvascular response to exercise.

Quantifying Intestinal Capillary Oxygenation Using Two‐photon Phosphorescence Lifetime Microscopy

Impaired oxygen delivery reduces the diversity of the gut microbiota1,2. Quantifying the microvascular oxygenation in the small intestines may help us better understand progression of the intestinal diseases such as inflammatory bowel disease (IBD), their relation with the changes in the gut microbiota, as well as implications to the microbiota–gut–brain axis. To start addressing these questions, we applied two-photon phosphorescence lifetime microscopy (2PLM) to quantify partial pressure of oxygen (pO2) and Red Blood Cell (RBC) flux in the capillary plexus through entire cecum wall in healthy, 2-month-old, female C57BL6 mice. Mice were anesthetized with isoflurane mixed with air (3% during surgery and 2% during imaging). The animals were breathing spontaneously through a loose mask. A small incision was made into the abdomen and the cecum was gently pulled out and positioned into a custom-made chamber with a cover glass on top. The exposed intestine was kept moist with a warmed 1x phosphate buffered saline (PBS) solution. Both core body temperature and temperature over the cecum were maintained at 37 °C. Experiments were terminated when the peristaltic movement prevented microscopic measurements. We measured pO2 and RBC flux during normoxia (n=4 mice, mean pO2 = 60 ± 6 mmHg, mean capillary RBC flux = 23 ± 8 RBCs/s) and hyperoxia (100% O2 inhalation, n=3, mean pO2= 120 ± 15 mmHg, mean capillary RBC flux = 31 ± 17 RBCs/s), as a function of depth in the cecum wall from 0 to 120 µm. Our results show expected significant increase in pO2 in response to hyperoxia, small capillary mean RBC flux values, and approximately uniform capillary pO2 across the thickness of the cecum wall during normoxia. Future steps include investigation of the capillary oxygenation and RBC flux in pathological conditions and in relation with the diversity of the gut microbiome.

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.