Endurance exercise training changes the limitation on muscle V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ in normoxia from the capacity to utilize O2 to the capacity to transport O2.

Maximal oxygen (O2 ) uptake ( V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ ) is an important parameter with utility in health and disease. However, the relative importance of O2 transport and utilization capacities in limiting muscle V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ before and after endurance exercise training is not well understood. Therefore, the present study aimed to identify the mechanisms determining muscle V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ pre- and post-endurance exercise training in initially sedentary participants. In five initially sedentary young males, radial arterial and femoral venous P O 2 ${P}_{{{\mathrm{O}}}_{\mathrm{2}}}$ (blood samples), leg blood flow (thermodilution), and myoglobin (Mb) desaturation (1 H nuclear magnetic resonance spectroscopy) were measured during maximal single-leg knee-extensor exercise (KE) breathing either 12%, 21% or 100% O2 both pre and post 8 weeks of KE training (1 h, 3 times per week). Mb desaturation was converted to intracellular P O 2 ${P}_{{{\mathrm{O}}}_{\mathrm{2}}}$ using an O2  half-saturation pressure of 3.2 mmHg. Pre-training muscle V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ was not significantly different across inspired O2 conditions (12%: 0.47 ± 0.10; 21%: 0.52 ± 0.13; 100%: 0.54 ± 0.01 L min-1 , all q > 0.174), despite significantly greater muscle mean capillary-intracellular P O 2 ${P}_{{{\mathrm{O}}}_{\mathrm{2}}}$ gradients in normoxia (34 ± 3 mmHg) and hyperoxia (40 ± 7 mmHg) than hypoxia (29 ± 5 mmHg, both q < 0.024). Post-training muscle V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ was significantly different across all inspired O2 conditions (12%: 0.59 ± 0.11; 21%: 0.68 ± 0.11; 100%: 0.76 ± 0.09 mmHg, all q < 0.035), as were the muscle mean capillary-intracellular P O 2 ${P}_{{{\mathrm{O}}}_{\mathrm{2}}}$ gradients (12%: 32 ± 2; 21%: 37 ± 2; 100%: 45 ± 7 mmHg, all q < 0.029). In these initially sedentary participants, endurance exercise training changed the basis of limitation on muscle V ̇ O 2 max ${\dot{V}}_{{{\mathrm{O}}}_{\mathrm{2}}{\mathrm{max}}}$ in normoxia from the mitochondrial capacity to utilize O2 to the capacity to transport O2 to the mitochondria. KEY POINTS: Maximal O2 uptake is an important parameter with utility in health and disease. The relative importance of O2 transport and utilization capacities in limiting muscle maximal O2 uptake before and after endurance exercise training is not well understood. We combined the direct measurement of active muscle maximal O2 uptake with the measurement of muscle intracellular P O 2 ${P}_{{{\mathrm{O}}}_{\mathrm{2}}}$ before and after 8 weeks of endurance exercise training. We show that increasing O2 availability did not increase muscle maximal O2 uptake before training, whereas increasing O2 availability did increase muscle maximal O2 uptake after training. The results suggest that, in these initially sedentary participants, endurance exercise training changed the basis of limitation on muscle maximal O2 uptake in normoxia from the mitochondrial capacity to utilize O2 to the capacity to transport O2 to the mitochondria.

Intrapulmonary shunt and alveolar dead space in a cohort of patients with acute COVID-19 pneumonitis and early recovery.

BACKGROUND: Pathological evidence suggests that coronavirus disease 2019 (COVID-19) pulmonary infection involves both alveolar damage (causing shunt) and diffuse microvascular thrombus formation (causing alveolar dead space). We propose that measuring respiratory gas exchange enables detection and quantification of these abnormalities. We aimed to measure shunt and alveolar dead space in moderate COVID-19 during acute illness and recovery. METHODS: We studied 30 patients (22 males; mean±sd age 49.9±13.5 years) 3-15 days from symptom onset and again during recovery, 55±10 days later (n=17). Arterial blood (breathing ambient air) was collected while exhaled oxygen and carbon dioxide concentrations were measured, yielding alveolar-arterial differences for each gas (P A-aO2 and P a-ACO2 , respectively) from which shunt and alveolar dead space were computed. RESULTS: For acute COVID-19 patients, group mean (range) for P A-aO2 was 41.4 (-3.5-69.3) mmHg and for P a-ACO2 was 6.0 (-2.3-13.4) mmHg. Both shunt (% cardiac output) at 10.4% (0-22.0%) and alveolar dead space (% tidal volume) at 14.9% (0-32.3%) were elevated (normal: <5% and <10%, respectively), but not correlated (p=0.27). At recovery, shunt was 2.4% (0-6.1%) and alveolar dead space was 8.5% (0-22.4%) (both p<0.05 versus acute). Shunt was marginally elevated for two patients; however, five patients (30%) had elevated alveolar dead space. CONCLUSIONS: We speculate impaired pulmonary gas exchange in early COVID-19 pneumonitis arises from two concurrent, independent and variable processes (alveolar filling and pulmonary vascular obstruction). For most patients these resolve within weeks; however, high alveolar dead space in ∼30% of recovered patients suggests persistent pulmonary vascular pathology.

Determinants of maximal oxygen consumption.

This article lays out the determinants of maximal O2 consumption (VO2max) achieved during high intensity endurance exercise. It is not a traditional topical review but rather an educational essay that intertwines chance observations made during an unrelated research project with a subsequent program of stepwise thought, analysis and experimentation to reveal how O2 is delivered to and used by the mitochondria. The centerpiece is the recognition that O2 is delivered by an inter-dependent system of transport components functioning as a "bucket brigade", made up of the lungs, heart, blood and circulation, and the muscles themselves, each of which affects O2 transport by similar amounts as they change. There is thus no single "limiting factor" to VO2max. Moreover, each component is shown to quantitatively affect the performance of the others. Mitochondrial respiration is integrated into the O2 transport system analysis to reveal its separate contribution to VO2max, and to show that mitochondrial PO2 at VO2max must be extremely low. Clinical application of the O2 transport systems analysis is described to separate central cardiopulmonary from peripheral tissue contributions to exercise limitation, illustrated by a study of patients with COPD. Finally, a short discussion of why muscles operating maximally must endure an almost anoxic state is offered. The hope is that in sum, both the increased understanding of O2 transport and the scientific approach to achieving that understanding described in the review can serve as a model for solving other complex problems going forward.

Blood Gas Transport: Carriage of Oxygen and Carbon Dioxide in Blood.

The ways in which oxygen (O2) and carbon dioxide (CO2) are carried in the blood are well known and well understood, with a plethora of textbooks, both general and lung specific, all presenting the topic in a very similar manner. This first of two companion chapters similarly summarizes this information. First, carriage of gases by physical solution is described, followed by discussion of O2, carbon monoxide, and CO2 transport in that order. However, what available texts have not emphasized is why knowing how gases are carried in blood matters, and the second, companion, chapter specifically addresses that critical aspect of gas exchange physiology. In fact, each of the chapters in this volume describes physiological behavior that depends more or less directly on the dissociation curves of O2 and CO2.

Blood Gas Transport: Implications for O2 and CO2 Exchange in Lungs and Tissues.

The well-known ways in which O2 and CO2 (and other gases) are carried in the blood were presented in the preceding chapter. However, what the many available texts about O2 and CO2 transport do not emphasize is why knowing how gases are carried in blood matters, and this second, companion, article specifically addresses that critical aspect of gas exchange physiology. During gas exchange, both at the lungs and in the peripheral tissues, it is the shapes and the slopes of the O2 and CO2 binding curves that explain almost all of the behaviors of each gas and the quantitative differences observed between them. This conclusion is derived from first principle considerations of the gas exchange processes. Dissociation curve shape and slope differences explain most of the differences between O2 and CO2 in both diffusive exchange in the lungs and tissues and convective exchange/transport in, and between, the lungs and tissues. In fact, each of the chapters in this volume describes physiological behavior that depends more or less directly on the dissociation curves of O2 and CO2.

Exercise training in COPD: muscle O2 transport plasticity.

Both convective oxygen (O2) transport to, and diffusive transport within, skeletal muscle are markedly diminished in patients with COPD. However, it is unknown how these determinants of peak muscle O2 uptake (V'mO2peak) respond to exercise training in patients with COPD. Therefore, the purpose of this study was to assess the plasticity of skeletal muscle O2 transport determinants of V'mO2peak in patients with COPD.Adaptations to 8 weeks of single-leg knee-extensor exercise training were measured in eight patients with severe COPD (mean±sem forced expiratory volume in 1 s (FEV1) 0.9±0.1 L) and eight healthy, well-matched controls. Femoral arterial and venous blood samples, and thermodilution-assessed leg blood flow were used to determine muscle O2 transport and utilisation at maximal exercise pre- and post-training.Training increased V'mO2peak in both COPD (by ∼26% from 271±29 to 342±35 mL·min-1) and controls (by ∼32% from 418±37 to 553±41 mL·min-1), restoring V'mO2peak in COPD to only ∼80% of pre-training control V'mO2peak Muscle diffusive O2 transport increased similarly in both COPD (by ∼38% from 6.6±0.9 to 9.1±0.9 mL·min-1·mmHg-1) and controls (by ∼36% from 10.4±0.7 to 14.1±0.8 mL·min-1·mmHg-1), with the patients reaching ∼90% of pre-training control values. In contrast, muscle convective O2 transport increased significantly only in controls (by ∼26% from 688±57 to 865±69 mL·min-1), leaving patients with COPD (438±45 versus 491±51 mL·min-1) at ∼70% of pre-training control values.While muscle diffusive O2 transport in COPD was largely restored by exercise training, V'mO2peak remained constrained by limited plasticity in muscle convective O2 transport.

Determinants of the diminished exercise capacity in patients with chronic obstructive pulmonary disease: looking beyond the lungs.

KEY POINTS: Peak oxygen uptake, a primary determinant of prognosis, mortality and quality of life, is diminished in patients with chronic obstructive pulmonary disease (COPD), with mounting evidence supporting an important role for peripheral dysfunction, particularly within skeletal muscle. In patients with severe COPD and activity-matched controls, muscle oxygen transport and utilization were assessed at peak effort during single-leg knee-extensor exercise (KE), where ventilation is assumed to be submaximal. This strategy removes ventilation as the major constraint to exercise capacity in COPD, allowing maximal muscle function to be attained and evaluated. During maximal KE, both convective arterial oxygen delivery to the skeletal muscle microvasculature and subsequent diffusive oxygen delivery to the mitochondria were diminished in patients with COPD compared to control subjects. These findings emphasize the importance of factors, beyond the lungs, that influence exercise capacity in this patient population and may, ultimately, influence the prognosis, mortality and quality of life for patients with COPD. ABSTRACT: Peak oxygen uptake ( V̇O2peak ), a primary determinant of prognosis, mortality and quality of life, is diminished in patients with chronic obstructive pulmonary disease (COPD). Mounting evidence supports an important role of the periphery, particularly skeletal muscle, in the diminished V̇O2peak with COPD. However, the peripheral determinants of V̇O2peak have not been comprehensively assessed in this cohort. Thus, the hypothesis was tested that both muscle convective and diffusive oxygen (O2 ) transport, and therefore skeletal muscle peak O2 uptake ( V̇MO2peak ), are diminished in patients with COPD compared to matched healthy controls, even when ventilatory limitations (i.e. attainment of maximal ventilation) are minimized by using small muscle mass exercise. Muscle O2 transport and utilization were assessed at peak exercise from femoral arterial and venous blood samples and leg blood flow (by thermodilution) in eight patients with severe COPD (forced expiratory volume in 1s (FEV1 ) ± SEM = 0.9 ± 0.1 l, 30% of predicted) and eight controls during single-leg knee-extensor exercise. Both muscle convective O2 delivery (0.44 ± 0.06 vs. 0.69 ± 0.07 l min-1 , P < 0.05) and muscle diffusive O2 conductance (6.6 ± 0.8 vs. 10.4 ± 0.9 ml min-1  mmHg-1 , P < 0.05) were ∼1/3 lower in patients with COPD than controls, resulting in an attenuated V̇MO2peak in the patients (0.27 ± 0.04 vs. 0.42 ± 0.05 l min-1 , P < 0.05). When cardiopulmonary limitations to exercise are minimized, the convective and diffusive determinants of V̇MO2peak , at the level of the skeletal muscle, are greatly attenuated in patients with COPD. These findings emphasize the importance of factors, beyond the lungs, that may ultimately influence this population's prognosis, mortality and quality of life.

Greater exercise tolerance in COPD during acute interval, compared to equivalent constant-load, cycle exercise: physiological mechanisms.

KEY POINTS: Exercise intolerance is common in chronic obstructive pulmonary disease (COPD) patients. In patients with COPD, we compared an interval exercise (IE) protocol (alternating 30 s at 100% peak work rate (WRpeak ) with 30 s at 50% WRpeak ) with moderate-intensity constant-load exercise (CLE) at 75% WRpeak , which yielded the same work rate. Exercise endurance time and total work output were almost twice as high for IE than CLE. At exercise isotime (when work completed was the same between IE and CLE), IE was associated with less dynamic hyperinflation, lower blood lactate concentration, and greater respiratory and locomotor muscle oxygenation, but there were no differences in ventilation or cardiac output. However, at the limit of tolerance for each modality, dynamic hyperinflation was not different between IE and CLE, while blood lactate remained lower and muscle oxygenation higher with IE. Taken together, these findings suggest that dynamic hyperinflation and not muscle-based factors dictate the limits of tolerance in these COPD patients. ABSTRACT: The relative importance of ventilatory, circulatory and peripheral muscle factors in determining tolerance to exercise in patients with chronic obstructive pulmonary disease (COPD) is not known. In 12 COPD patients (forced expiratory volume in one second: 58 ± 17%pred.) we measured ventilation, cardiac output, dynamic hyperinflation, local muscle oxygenation, blood lactate and time to exhaustion during (a) interval exercise (IE) consisting of 30 s at 100% peak work rate alternating with 30 s at 50%, and (b) constant-load exercise (CLE) at 75% peak work rate, designed to produce the same average work rate. Exercise time was substantially longer during IE than CLE (19.5 ± 4.8 versus 11.4 ± 2.1 min, p = 0.0001). Total work output was therefore greater during IE than CLE (81.3 ± 27.7 versus 48.9 ± 23.8 kJ, p = 0.0001). Dynamic hyperinflation (assessed by changes from baseline in inspiratory capacity, ΔIC) was less during IE than CLE at CLE exhaustion time (isotime, p = 0.009), but was similar at exhaustion (ΔICCLE : -0.38 ± 0.10 versus ΔICIE : -0.33 ± 0.12 l, p = 0.102). In contrast, at isotime, minute ventilation, cardiac output and systemic oxygen delivery did not differ between protocols (P > 0.05). At exhaustion in both protocols, the vastus lateralis and intercostal muscle oxygen saturation were higher in IE than CLE (p = 0.014 and p = 0.0002, respectively) and blood lactate concentrations were lower (4.9 ± 2.4 mmol l-1 versus 6.4 ± 2.2 mmol l-1 , p = 0.039). These results suggest that (1) exercise tolerance with COPD is limited by dynamic hyperinflation; and (2) cyclically lower (50%) effort intervals in IE help to preserve muscle oxygenation and reduce metabolic acidosis compared with CLE at the same average work rate; but these factors do not appear to determine time to exhaustion.

Respiratory and locomotor muscle blood flow during exercise in health and chronic obstructive pulmonary disease.

NEW FINDINGS: What is the topic of this review? The work presented here focuses mostly on testing the theory of blood flow redistribution from the locomotor to the respiratory muscles during heavy exercise in healthy participants and in patients with COPD. What advances does it highlight? Studies presented and the direct experimental approach to measure muscle blood flow by indocyanine green dye detected by near infrared spectroscopy, show that exercise interferes with respiratory muscle blood flow especially in COPD, but even in healthy. ABSTRACT: We have developed an indicator-dilution method to measure muscle blood flow at rest and during exercise using the light absorbing tracer indocyanine green dye (ICG) injected as an intravenous bolus, with surface optodes placed over muscles of interest to record the ICG signal by near-infrared spectroscopy. Here we review findings for both quadriceps and intercostal muscle blood flow (measured simultaneously) in trained cyclists and in patients with chronic obstructive pulmonary disease (COPD). During resting hyperpnoea in both athletes and patients, intercostal muscle blood flow increased with ventilation, correlating closely and linearly with the work of breathing, with no change in quadriceps flow. During graded exercise in athletes, intercostal flow at first increased, but then began to fall approaching peak effort. Unexpectedly, in COPD, intercostal muscle blood flow during exercise fell progressively from resting values, contrasting sharply with the response to resting hyperpnoea. During exercise at peak intensity, we found no quadriceps blood flow reduction in favour of the respiratory muscles in either athletes or patients. In COPD at peak exercise, when patients breathed 21% oxygen in helium or 100% oxygen, there was no redistribution of blood flow observed between legs and respiratory muscles in either direction. Evidence of decrease in leg blood flow and increase in respiratory muscle flow was found only when imposing expiratory flow limitation (EFL) during exercise in healthy individuals. However, because EFL caused substantial physiological derangement, lowering arterial oxygen saturation and raising end-tidal PCO2 and heart rate, these results cannot be projected onto normal exercise.

Synergistic effect of vascular endothelial growth factor gene inactivation in endothelial cells and skeletal myofibres on muscle enzyme activity, capillary supply and endurance exercise in mice.

NEW FINDINGS: What is the central question of this study? Does vascular endothelial growth factor (VEGF) expressed by both endothelial cells and skeletal myofibres maintain the number of skeletal muscle capillaries and regulate endurance exercise? What is the main finding and its importance? VEGF expressed by both endothelial cells and skeletal myofibres is not essential for maintaining capillary number but does contribute to exercise performance. ABSTRACT: Many chronic diseases lead to exercise intolerance, with loss of skeletal muscle capillaries. While many muscle cell types (myofibres, satellite cells, endothelial cells, macrophages and fibroblasts) express vascular endothelial growth factor (VEGF), most muscle VEGF is stored in myofibre vesicles which can release VEGF to signal VEGF receptor-expressing cells. VEGF gene ablation in myofibres or endothelial cells alone does not cause capillary regression. We hypothesized that simultaneously deleting the endothelial cell (EC) and skeletal myofibre (Skm) VEGF gene would cause capillary regression and impair exercise performance. This was tested in adult mice by simultaneous conditional deletion of the VEGF gene (Skm/EC-VEGF-/- mice) through the use of VEGFLoxP, HSA-Cre-ERT2 and PDGFb-iCre-ERT2 transgenes. These double-deletion mice were compared to three control groups - WT, EC VEGF gene deletion alone and myofibre VEGF gene deletion alone. Three weeks after initiating gene deletion, Skm/EC-VEGF-/- mice, but not SkmVEGF-/- or EC-VEGF-/- mice, reached exhaustion 40 min sooner than WT mice in treadmill tests (P = 0.002). WT, SkmVEGF-/- and EC-VEGF-/- , but not Skm/EC-VEGF-/- , mice gained weight over the 3 weeks. Capillary density, fibre area and capillary: fibre ratio in soleus, plantaris, gastrocnemius and cardiac papillary muscle were similar across the groups. Phosphofructokinase and pyruvate dehydrogenase activities increased only in Skm/EC-VEGF-/- mice. These data suggest that deletion of the VEGF gene simultaneously in endothelial cells and myofibres, while reducing treadmill endurance and despite compensatory augmentation of glycolysis, is not required for muscle capillary maintenance. Reduced endurance remains unexplained, but may possibly be related to a role for VEGF in controlling perfusion of contracting muscle.

Precapillary pulmonary gas exchange is similar for oxygen and inert gases.

KEY POINTS: Precapillary gas exchange for oxygen has been documented in both humans and animals. It has been suggested that, if precapillary gas exchange occurs to a greater extent for inert gases than for oxygen, shunt and its effects on arterial oxygenation may be underestimated by the multiple inert gas elimination technique (MIGET). We evaluated fractional precapillary gas exchange in canines for O2 and two inert gases, sulphur hexafluoride and ethane, by measuring these gases in the proximal pulmonary artery, distal pulmonary artery (1 cm proximal to the wedge position) and systemic artery. Some 12-19% of pulmonary gas exchange occurred within small (1.7 mm in diameter or larger) pulmonary arteries and this was quantitatively similar for oxygen, sulphur hexafluoride and ethane. Under these experimental conditions, this suggests only minor effects of precapillary gas exchange on the magnitude of calculated shunt and the associated effect on pulmonary gas exchange estimated by MIGET. ABSTRACT: Some pulmonary gas exchange is known to occur proximal to the pulmonary capillary, although the magnitude of this gas exchange is uncertain, and it is unclear whether oxygen and inert gases are similarly affected. This has implications for measuring shunt and associated gas exchange consequences. By measuring respiratory and inert gas levels in the proximal pulmonary artery (P), a distal pulmonary artery 1 cm proximal to the wedge position (using a 5-F catheter) (D) and a systemic artery (A), we evaluated precapillary gas exchange in 27 paired samples from seven anaesthetized, ventilated canines. Fractional precapillary gas exchange (F) was quantified for each gas as F = (P - D)/(P - A). The lowest solubility inert gases, sulphur hexafluoride (SF6 ) and ethane were used because, with higher solubility gases, the P-A difference is sufficiently small that experimental error prevents accurate assessment of F. Distal samples (n = 12) with oxygen (O2 ) saturation values that were (within experimental error) equal to or above systemic arterial values, suggestive of retrograde capillary blood aspiration, were discarded, leaving 15 for analysis. D was significantly lower than P for SF6 (D/P = 88.6 ± 18.1%; P = 0.03) and ethane (D/P = 90.6 ± 16.0%; P = 0.04), indicating partial excretion of inert gas across small pulmonary arteries. Distal pulmonary arterial O2 saturation was significantly higher than proximal (74.1 ± 6.8% vs. 69.0 ± 4.9%; P = 0.03). Fractional precapillary gas exchange was similar for SF6 , ethane and O2 (0.12 ± 0.19, 0.12 ± 0.20 and 0.19 ± 0.26, respectively; P = 0.54). Under these experimental conditions, 12-19% of pulmonary gas exchange occurs within the small pulmonary arteries and the extent is similar between oxygen and inert gases.

Intra-pulmonary arteriovenous anastomoses and pulmonary gas exchange: evaluation by microspheres, contrast echocardiography and inert gas elimination.

KEY POINTS: Imaging techniques such as contrast echocardiography suggest that anatomical intra-pulmonary arteriovenous anastomoses (IPAVAs) are present at rest and are recruited to a greater extent in conditions such as exercise. IPAVAs have the potential to act as a shunt, although gas exchange methods have not demonstrated significant shunt in the normal lung. To evaluate this discrepancy, we compared anatomical shunt with 25-µm microspheres to contrast echocardiography, and gas exchange shunt measured by the multiple inert gas elimination technique (MIGET). Intra-pulmonary shunt measured by 25-µm microspheres was not significantly different from gas exchange shunt determined by MIGET, suggesting that MIGET does not underestimate the gas exchange consequences of anatomical shunt. A positive agitated saline contrast echocardiography score was associated with anatomical shunt measured by microspheres. Agitated saline contrast echocardiography had high sensitivity but low specificity to detect a ≥1% anatomical shunt, frequently detecting small shunts inconsequential for gas exchange. ABSTRACT: The echocardiographic visualization of transpulmonary agitated saline microbubbles suggests that anatomical intra-pulmonary arteriovenous anastomoses are recruited during exercise, in hypoxia, and when cardiac output is increased pharmacologically. However, the multiple inert gas elimination technique (MIGET) shows insignificant right-to-left gas exchange shunt in normal humans and canines. To evaluate this discrepancy, we measured anatomical shunt with 25-µm microspheres and compared the results to contrast echocardiography and MIGET-determined gas exchange shunt in nine anaesthetized, ventilated canines. Data were acquired under the following conditions: (1) at baseline, (2) 2 µg kg-1  min-1 i.v. dopamine, (3) 10 µg kg-1  min-1 i.v. dobutamine, and (4) following creation of an intra-atrial shunt (in four animals). Right to left anatomical shunt was quantified by the number of 25-µm microspheres recovered in systemic arterial blood. Ventilation-perfusion mismatch and gas exchange shunt were quantified by MIGET and cardiac output by direct Fick. Left ventricular contrast scores were assessed by agitated saline bubble counts, and separately by appearance of 25-µm microspheres. Across all conditions, anatomical shunt measured by 25-µm microspheres was not different from gas exchange shunt measured by MIGET (microspheres: 2.3 ± 7.4%; MIGET: 2.6 ± 6.1%, P = 0.64). Saline contrast bubble score was associated with microsphere shunt (ρ = 0.60, P < 0.001). Agitated saline contrast score had high sensitivity (100%) to detect a ≥1% shunt, but low specificity (22-48%). Gas exchange shunt by MIGET does not underestimate anatomical shunt measured using 25-µm microspheres. Contrast echocardiography is extremely sensitive, but not specific, often detecting small anatomical shunts which are inconsequential for gas exchange.

Exercise Intolerance in Heart Failure With Preserved Ejection Fraction: Diagnosing and Ranking Its Causes Using Personalized O2 Pathway Analysis.

BACKGROUND: Heart failure with preserved ejection fraction (HFpEF) is a common syndrome with a pressing shortage of therapies. Exercise intolerance is a cardinal symptom of HFpEF, yet its pathophysiology remains uncertain. METHODS: We investigated the mechanism of exercise intolerance in 134 patients referred for cardiopulmonary exercise testing: 79 with HFpEF and 55 controls. We performed cardiopulmonary exercise testing with invasive monitoring to measure hemodynamics, blood gases, and gas exchange during exercise. We used these measurements to quantify 6 steps of oxygen transport and utilization (the O2 pathway) in each patient with HFpEF, identifying the defective steps that impair each one's exercise capacity (peak Vo2). We then quantified the functional significance of each O2 pathway defect by calculating the improvement in exercise capacity a patient could expect from correcting the defect. RESULTS: Peak Vo2 was reduced by 34±2% (mean±SEM, P<0.001) in HFpEF compared with controls of similar age, sex, and body mass index. The vast majority (97%) of patients with HFpEF harbored defects at multiple steps of the O2 pathway, the identity and magnitude of which varied widely. Two of these steps, cardiac output and skeletal muscle O2 diffusion, were impaired relative to controls by an average of 27±3% and 36±2%, respectively (P<0.001 for both). Due to interactions between a given patient's defects, the predicted benefit of correcting any single one was often minor; on average, correcting a patient's cardiac output led to a 7±0.5% predicted improvement in exercise intolerance, whereas correcting a patient's muscle diffusion capacity led to a 27±1% improvement. At the individual level, the impact of any given O2 pathway defect on a patient's exercise capacity was strongly influenced by comorbid defects. CONCLUSIONS: Systematic analysis of the O2 pathway in HFpEF showed that exercise capacity was undermined by multiple defects, including reductions in cardiac output and skeletal muscle diffusion capacity. An important source of disease heterogeneity stemmed from variation in each patient's personal profile of defects. Personalized O2 pathway analysis could identify patients most likely to benefit from treating a specific defect; however, the system properties of O2 transport favor treating multiple defects at once, as with exercise training.

Acute and chronic exercise in patients with heart failure with reduced ejection fraction: evidence of structural and functional plasticity and intact angiogenic signalling in skeletal muscle.

KEY POINTS: The vascular endothelial growth factor (VEGF) responses to acute submaximal exercise and training effects in patients with heart failure with reduced ejection fraction (HFrEF) were investigated. Six patients and six healthy matched controls performed knee-extensor exercise (KE) at 50% of maximum work rate before and after (only patients) KE training. Muscle biopsies were taken to assess skeletal muscle structure and the angiogenic response. Before training, during this submaximal KE exercise, patients with HFrEF exhibited higher leg vascular resistance and greater noradrenaline spillover. Skeletal muscle structure and VEGF response were generally not different between groups. Following training, resistance was no longer elevated and noradrenaline spillover was curtailed in the patients. Although, in the trained state, VEGF did not respond to acute exercise, capillarity was augmented. Muscle fibre cross-sectional area and percentage area of type I fibres increased and mitochondrial volume density exceeded that of controls. Structural/functional plasticity and appropriate angiogenic signalling were observed in skeletal muscle of patients with HFrEF. ABSTRACT: This study examined the response to acute submaximal exercise and the effect of training in patients with heart failure with reduced ejection fraction (HFrEF). The acute angiogenic response to submaximal exercise in HFrEF after small muscle mass training is debated. The direct Fick method, with vascular pressures, was performed across the leg during knee-extensor exercise (KE) at 50% of maximum work rate (WRmax ) in patients (n = 6) and controls (n = 6) and then after KE training in patients. Muscle biopsies facilitated the assessment of skeletal muscle structure and vascular endothelial growth factor (VEGF) mRNA levels. Prior to training, HFrEF exhibited significantly higher leg vascular resistance (LVR) (≈15%) and significantly greater noradrenaline spillover (≈385%). Apart from mitochondrial volume density, which was significantly lower (≈22%) in HFrEF, initial skeletal muscle structure, including capillarity, was not different between groups. Resting VEGF mRNA levels, and the increase with exercise, was not different between patients and controls. Following training, LVR was no longer elevated and noradrenaline spillover was curtailed. Skeletal muscle capillarity increased with training, as assessed by capillary-to-fibre ratio (≈13%) and number of capillaries around a fibre (NCAF ) (≈19%). VEGF mRNA was now not significantly increased by acute exercise. Muscle fibre cross-sectional area and percentage area of type I fibres both increased significantly with training (≈18% and ≈21%, respectively), while the percentage area of type II fibres fell significantly (≈11%), and mitochondrial volume density now exceeded that of controls. These data reveal structural and functional plasticity and appropriate angiogenic signalling in skeletal muscle of HFrEF patients.

Skeletal myofiber vascular endothelial growth factor is required for the exercise training-induced increase in dentate gyrus neuronal precursor cells.

KEY POINTS: Peripheral vascular endothelial growth factor (VEGF) is necessary for exercise to stimulate hippocampal neurogenesis. Here we report that skeletal myofiber VEGF directly or indirectly regulates exercise-signalled proliferation of neuronal precursor cells. Our results found skeletal myofiber VEGF to be necessary for maintaining blood flow through hippocampal regions independent of exercise training state. This study demonstrates that skeletal myofiber VEGF is required for the hippocampal VEGF response to acute exercise. These results help to establish the mechanisms by which exercise, through skeletal myofiber VEGF, affects the hippocampus. ABSTRACT: Exercise signals neurogenesis in the dentate gyrus of the hippocampus. This phenomenon requires vascular endothelial growth factor (VEGF) originating from outside the blood-brain barrier, but no cellular source has been identified. Thus, we hypothesized that VEGF produced by skeletal myofibers plays a role in regulating hippocampal neuronal precursor cell proliferation following exercise training. This was tested in adult conditional skeletal myofiber-specific VEGF gene-ablated mice (VEGFHSA-/- ) by providing VEGFHSA-/- and non-ablated (VEGFf/f ) littermates with running wheels for 14 days. Following this training period, hippocampal cerebral blood flow (CBF) was measured by functional magnetic resonance imaging (fMRI), and neuronal precursor cells (BrdU+/Nestin+) were detected by immunofluorescence. The VEGFf/f trained group showed improvements in both speed and endurance capacity in acute treadmill running tests (P < 0.05). The VEGFHSA-/- group did not. The number of proliferating neuronal precursor cells was increased with training in VEGFf/f (P < 0.05) but not in VEGFHSA-/- mice. Endothelial cell (CD31+) number did not change in this region with exercise training or skeletal myofiber VEGF gene deletion. However, resting blood flow through the hippocampal region was lower in VEGFHSA-/- mice, both untrained and trained, than untrained VEGFf/f mice (P < 0.05). An acute hypoxic challenge decreased CBF (P < 0.05) in untrained VEGFf/f , untrained VEGFHSA-/- and trained VEGFHSA-/- mice, but not trained VEGFf/f mice. VEGFf/f , but not VEGFHSA-/- , mice were able to acutely run on a treadmill at an intensity sufficient to increase hippocampus VEGF levels. These data suggest that VEGF expressed by skeletal myofibers may directly or indirectly regulate both hippocampal blood flow and neurogenesis.

Selective Life-Long Skeletal Myofiber-Targeted VEGF Gene Ablation Impairs Exercise Capacity in Adult Mice.

Exercise is dependent on adequate oxygen supply for mitochondrial respiration in both cardiac and locomotor muscle. To determine whether skeletal myofiber VEGF is critical for regulating exercise capacity, independent of VEGF function in the heart, ablation of the VEGF gene was targeted to skeletal myofibers (skmVEGF-/-) during embryogenesis (∼ E9.5), leaving intact VEGF expression by all other cells in muscle. In adult mice, VEGF levels were decreased in the soleus (by 65%), plantaris (94%), gastrocnemius (74%), EDL (99%) and diaphragm (64%) (P < 0.0001, each muscle). VEGF levels were unchanged in the heart. Treadmill speed (WT 86 ± 4 cm/sec, skmVEGF-/- 70 ± 5 cm/sec, P = 0.006) and endurance (WT 78 ± 24 min, skmVEGF-/- 18 ± 4 min, P = 0.0004) were severely limited in skmVEGF-/- mice in contrast to minor effect of conditional skmVEGF gene deletion in the adult. Body weight was also reduced (WT 22.8 ± 1.6 g, skmVEGF-/-, 21.1 ± 1.5, P = 0.02), but the muscle mass/body weight ratio was unchanged. The capillary/fiber ratio was lower in skmVEGF-/- plantaris (WT 1.51 ± 0.12, skmVEGF-/- 1.16 ± 0.20, P = 0.01), gastrocnemius (WT 1.61 ± 0.08, skmVEGF-/- 1.39 ± 0.08, P = 0.01), EDL (WT 1.36 ± 0.07, skmVEGF-/- 1.14 ± 0.13, P = 0.03) and diaphragm (WT 1.39 ± 0.18, skmVEGF-/- 0.79 ± 0.16, P = 0.0001) but, not in soleus. Cardiac function (heart rate, maximal pressure, maximal dP/dt, minimal dP/dt,) in response to dobutamine was not impaired in anesthetized skmVEGF-/- mice. Isolated soleus and EDL fatigue times were 16% and 20% (P < 0.02) longer, respectively, in skmVEGF-/- mice than the WT group. These data suggest that skeletal myofiber VEGF expressed during development is necessary to establish capillary networks that allow maximal exercise capacity.