Near-infrared spectroscopy using indocyanine green dye for minimally invasive measurement of respiratory and leg muscle blood flow in patients with COPD

Respiratory and locomotor muscle blood flow measurements using near-infrared spectroscopy and indocyanine green dye in health and disease

Measuring respiratory and locomotor muscle blood flow during exercise is pivotal for understanding the factors limiting exercise tolerance in health and disease. Traditional methods to measure muscle blood flow present limitations for exercise testing. This article reviews a method utilising near-infrared spectroscopy (NIRS) in combination with the light-absorbing tracer indocyanine green dye (ICG) to simultaneously assess respiratory and locomotor muscle blood flow during exercise in health and disease. NIRS provides high spatiotemporal resolution and can detect chromophore concentrations. Intravenously administered ICG binds to albumin and undergoes rapid metabolism, making it suitable for repeated measurements. NIRS-ICG allows calculation of local muscle blood flow based on the rate of ICG accumulation in the muscle over time. Studies presented in this review provide evidence of the technical and clinical validity of the NIRS-ICG method in quantifying respiratory and locomotor muscle blood flow. Over the past decade, use of this method during exercise has provided insights into respiratory and locomotor muscle blood flow competition theory and the effect of ergogenic aids and pharmacological agents on local muscle blood flow distribution in COPD. Originally, arterial blood sampling was required via a photodensitometer, though the method has subsequently been adapted to provide a local muscle blood flow index using venous cannulation. In summary, the significance of the NIRS-ICG method is that it provides a minimally invasive tool to simultaneously assess respiratory and locomotor muscle blood flow at rest and during exercise in health and disease to better appreciate the impact of ergogenic aids or pharmacological treatments.

The exercise pressor reflex - a pressure-raising mechanism with a limited role in regulating leg perfusion during locomotion in young healthy men

We investigated the role of the exercise pressor reflex (EPR) in regulating the haemodynamic response to locomotor exercise. Eight healthy participants (23 ± 3 years,V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ : 49 ± 6 ml/kg/min) performed constant-load cycling exercise (∼36/43/52/98%V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ ; 4 min each) without (CTRL) and with (FENT) lumbar intrathecal fentanyl attenuating group III/IV locomotor muscle afferent feedback and, thus, the EPR. To avoid different respiratory muscle metaboreflex and arterial chemoreflex activation during FENT, subjects mimicked the ventilatory response recorded during CTRL. Arterial and leg perfusion pressure (femoral arterial and venous catheters), femoral blood flow (Doppler-ultrasound), microvascular quadriceps blood flow index (indocyanine green), cardiac output (inert gas breathing), and systemic and leg vascular conductance were quantified during exercise. There were no cardiovascular and ventilatory differences between conditions at rest. Pulmonary ventilation, arterial blood gases and oxyhaemoglobin saturation were not different during exercise. Furthermore, cardiac output (-2% to -12%), arterial pressure (-7% to -15%) and leg perfusion pressure (-8% to -22%) were lower, and systemic (up to 16%) and leg (up to 27%) vascular conductance were higher during FENT compared to CTRL. Leg blood flow, microvascular quadriceps blood flow index, and leg O2 -transport and utilization were not different between conditions (P > 0.5). These findings reflect a critical role of the EPR in the autonomic control of the heart, vasculature and, ultimately, arterial pressure during locomotor exercise. However, the lack of a net effect of the EPR on leg blood flow challenges the idea of this cardiovascular reflex as a key determinant of leg O2 -transport during locomotor exercise in healthy, young individuals. KEY POINTS: The role of the exercise pressor reflex (EPR) in regulating leg O2 -transport during human locomotion remains uncertain. We investigated the influence of the EPR on the cardiovascular response to cycling exercise. Lumbar intrathecal fentanyl was used to block group III/IV leg muscle afferents and debilitate the EPR at intensities ranging from 30% to 100%V ̇ O 2 max ${\dot V_{{{\mathrm{O}}_{\mathrm{2}}}{\mathrm{max}}}}$ . To avoid different respiratory muscle metaboreflex and arterial chemoreflex activation during exercise with blocked leg muscle afferents, subjects mimicked the ventilatory response recorded during control exercise. Afferent blockade increased leg and systemic vascular conductance, but reduced cardiac output and arterial-pressure, with no net effect on leg blood flow. The EPR influenced the cardiovascular response to cycling exercise by contributing to the autonomic control of the heart and vasculature, but did not affect leg blood flow. These findings challenge the idea of the EPR as a key determinant of leg O2 -transport during locomotor exercise in healthy, young individuals.

Do Cerebral Cortex Perfusion, Oxygen Delivery, and Oxygen Saturation Responses Measured by Near-Infrared Spectroscopy Differ Between Patients Who Fail or Succeed in a Spontaneous Breathing Trial? A Prospective Observational Study

BACKGROUND

Alterations in perfusion to the brain during the transition from mechanical ventilation (MV) to a spontaneous breathing trial (SBT) remain poorly understood. The aim of the study was to determine whether changes in cerebral cortex perfusion, oxygen delivery (DO₂), and oxygen saturation (%StiO₂) during the transition from MV to an SBT differ between patients who succeed or fail an SBT.

METHODS

This was a single-center prospective observational study conducted in a 16-bed medical intensive care unit of the University Hospital Leuven, Belgium. Measurements were performed in 24 patients receiving MV immediately before and at the end of a 30-min SBT. Blood flow index (BFI), DO₂, and %StiO₂ in the prefrontal cortex, scalene, rectus abdominis, and thenar muscle were simultaneously assessed by near-infrared spectroscopy using the tracer indocyanine green dye. Cardiac output, arterial blood gases, and systemic oxygenation were also recorded.

RESULTS

During the SBT, prefrontal cortex BFI and DO₂ responses did not differ between SBT-failure and SBT-success groups (p > 0.05). However, prefrontal cortex %StiO₂ decreased in six of eight patients (75%) in the SBT-failure group (median [interquartile range 25-75%]: MV = 57.2% [49.1-61.7] vs. SBT = 51.0% [41.5-62.5]) compared to 3 of 16 patients (19%) in the SBT-success group (median [interquartile range 25-75%]: MV = 65.0% [58.6-68.5] vs. SBT = 65.1% [59.5-71.1]), resulting in a significant differential %StiO₂ response between groups (p = 0.031). Similarly, a significant differential response in thenar muscle %StiO₂ (p = 0.018) was observed between groups. A receiver operating characteristic analysis identified a decrease in prefrontal cortex %StiO₂ > 1.6% during the SBT as an optimal cutoff, with a sensitivity of 94% and a specificity of 75% to predict SBT failure and an area under the curve of 0.79 (95% CI: 0.55-1.00). Cardiac output, systemic oxygenation, scalene, and rectus abdominis BFI, DO₂, and %StiO₂ responses did not differ between groups (p > 0.05); however, during the SBT, a significant positive association in prefrontal cortex BFI and partial pressure of arterial carbon dioxide was observed only in the SBT-success group (SBT success: Spearman's ρ = 0.728, p = 0.002 vs. SBT failure: ρ = 0.048, p = 0.934).

CONCLUSIONS

This study demonstrated a reduced differential response in prefrontal cortex %StiO₂ in the SBT-failure group compared with the SBT-success group possibly due to the insufficient increase in prefrontal cortex perfusion in SBT-failure patients. A > 1.6% drop in prefrontal cortex %StiO₂ during SBT was sensitive in predicting SBT failure. Further research is needed to validate these findings in a larger population and to evaluate whether cerebral cortex %StiO₂ measurements by near-infrared spectroscopy can assist in the decision-making process on liberation from MV.

Inspiratory and leg muscle blood flows during inspiratory muscle metaboreflex activation in heart failure with preserved ejection fraction

The purpose of this study was to determine the cardiovascular consequences elicited by activation of the inspiratory muscle metaboreflex in patients with heart failure with preserved ejection fraction (HFpEF) and controls. Patients with HFpEF (n = 15; 69 ± 10 yr; 33 ± 4 kg/m2) and controls (n = 14; 70 ± 8 yr; 28 ± 4 kg/m2) performed an inspiratory loading trial at 60% maximal inspiratory pressure (PIMAX) until task failure. Mean arterial pressure (MAP) was measured continuously. Near-infrared spectroscopy and bolus injections of indocyanine green dye were used to determine the percent change in blood flow index (%ΔBFI) from baseline to the final minute of inspiratory loading in the vastus lateralis and sternocleidomastoid muscles. Vascular resistance index (VRI) was calculated. Time to task failure was shorter in HFpEF than in controls (339 ± 197 s vs. 626 ± 403 s; P = 0.02). Compared with controls, patients with HFpEF had a greater increase from baseline in MAP (16 ± 7 vs. 10 ± 6 mmHg) and vastus lateralis VRI (76 ± 45 vs. 32 ± 19%) as well as a greater decrease in vastus lateralis %ΔBFI (-32 ± 14 vs. -17 ± 9%) (all, P < 0.05). Sternocleidomastoid %ΔBFI normalized to absolute inspiratory pressure was higher in HFpEF compared with controls (8.0 ± 5.0 vs. 4.0 ± 1.9% per cmH2O·s; P = 0.03). These data indicate that patients with HFpEF exhibit exaggerated cardiovascular responses with inspiratory muscle metaboreflex activation compared with controls.NEW & NOTEWORTHY Respiratory muscle dysfunction is thought to contribute to exercise intolerance in heart failure with preserved ejection fraction (HFpEF); however, the underlying mechanisms are unknown. In the present study, patients with HFpEF had greater increases in leg muscle vascular resistance index and greater decreases in leg muscle blood flow index compared with controls during inspiratory resistive breathing (to activate the metaboreflex). Furthermore, respiratory muscle blood flow index responses normalized to pressure generation during inspiratory resistive breathing were exaggerated in HFpEF compared with controls.

Mapping the global research landscape and hotspot of exercise therapy and chronic obstructive pulmonary disease: A bibliometric study based on the web of science database from 2011 to 2020

Background: The application of exercise therapy (ET) in chronic obstructive pulmonary disease (COPD) is generating increasing clinical efficacy and social-economic value. In this study, research trends, evolutionary processes and hot topics in this field are detailed, as well as predictions of future development directions.

Methods: Search for literature in the field of COPD and ET and analyze data to generate knowledge graphs using VOSiewer and CiteSpace software. The time frame for the search was from 2011 to January 2021. Then we extracted full-text key information (such as title, journal category, publication date, author, country and institution, abstract, and keyword) and obtained the co-citation analysis. Use hierarchal clustering analysis software developed by VOSviewer to map common citations, and use Citespace software to plot trend networks.

Results: The United States topped the list with 27.91% of the number of articles posted, followed by the UK at 25.44%. Imperial College London was the highest number of article publications in institutions, followed by Maastricht University and the University of Toronto. The Royal Brompton Harefield NHS Foundation Trust was one of many research institutions and currently holds the highest average citations per item (ACI) value, followed by Imperial College London and the University of Leuven. Judging from the number of publications related to ET and COPD, it is mainly published in cell biology, respiratory pulmonary diseases, and rehabilitation experiments study medicine. The European Respiration Journal is the most widely published in this field, followed by the International Journal of Chronic Obstructive Pulmonary Disease and Respiratory Medicine.

Conclusion: COPD combined with ET is widely used in clinical practice and is on the rise. A distinctive feature of the field is multidisciplinary integration. Rehabilitation research for COPD involves multidisciplinary collaboration, tissue engineering, and molecular biology mechanism studies to help patients remodel healthy breathing. Multidisciplinary rehabilitation measures provide a solid foundation for advancing clinical efficacy in the field of COPD.

Intercostal muscle oxygenation and expiratory loaded breathing at rest: Respiratory pattern effect

In patients with airway obstruction, an increase in breathing frequency at rest is commonly associated with a dynamic hyperinflation (DH). In such a situation, intercostal muscle oxygenation may be disturbed. This hypothesis was examined in a context of simulated airway obstruction in healthy subjects. After a control period of 5 min, twelve participants (20 ± 2 years) breathed at rest through a 20-cmH₂O expiratory threshold load, either by increasing or reducing their respiratory rate (_ETLF⁺ or _ETLF). Tissue saturation index (TSI) and concentration changes in oxyhaemoglobin (oxy[Hb+Mb]) were measured as well as cardiorespiratory variables. Inspiratory capacity was decreased in _ETLF⁺ (p < 0.001) and correlated with dyspnea. An increase in oxy[Hb+Mb] occurred in _ETLF⁺ that was higher than in _ETLF (p < 0.01). TSI was not different between conditions. In healthy subjects at rest, an increase in respiratory rate during a simulated obstruction with an expiratory threshold load resulted in paradoxical response with DH emergence while intercostal muscle oxygenation was preserved.

Effect of portable noninvasive ventilation on thoracoabdominal volumes in recovery from intermittent exercise in patients with COPD

We previously showed that use of portable noninvasive ventilation (pNIV) during recovery periods within intermittent exercise improved breathlessness and exercise tolerance in patients with COPD compared with pursed-lip breathing (PLB). However, in a minority of patients recovery from dynamic hyperinflation (DH) was better with PLB, based on inspiratory capacity. We further explored this using Optoelectronic Plethysmography to assess total and compartmental thoracoabdominal volumes. Fourteen patients with COPD (means ± SD) (FEV₁: 55% ± 22% predicted) underwent, in a balanced order sequence, two intermittent exercise protocols on the cycle ergometer consisting of five repeated 2-min exercise bouts at 80% peak capacity, separated by 2-min recovery periods, with application of pNIV or PLB in the 5 min of recovery. Our findings identified seven patients showing recovery in DH with pNIV (DH responders) whereas seven showed similar or better recovery in DH with PLB. When pNIV was applied, DH responders compared with DH nonresponders exhibited greater tidal volume (by 0.8 ± 0.3 L, P = 0.015), inspiratory flow rate (by 0.6 ± 0.5 L/s, P = 0.049), prolonged expiratory time (by 0.6 ± 0.5 s, P = 0.006), and duty cycle (by 0.7 ± 0.6 s, P = 0.007). DH responders showed a reduction in end-expiratory thoracoabdominal DH (by 265 ± 633 mL) predominantly driven by reduction in the abdominal compartment (by 210 ± 494 mL); this effectively offset end-inspiratory rib-cage DH. Compared with DH nonresponders, DH responders had significantly greater body mass index (BMI) by 8.4 ± 3.2 kg/m², P = 0.022 and tended toward less severe resting hyperinflation by 0.3 ± 0.3 L. Patients with COPD who mitigate end-expiratory rib-cage DH by expiratory abdominal muscle recruitment benefit from pNIV application.NEW & NOTEWORTHY Compared with the pursed-lip breathing technique, acute application of portable noninvasive ventilation during recovery from intermittent exercise improved end-expiratory thoracoabdominal dynamic hyperinflation (DH) in 50% of patients with COPD (DH responders). DH responders, compared with DH nonresponders, exhibited a reduction in end-expiratory thoracoabdominal DH predominantly driven by the abdominal compartment that effectively offset end-expiratory rib cage DH. The essential difference between DH responders and DH nonresponders was, therefore, in the behavior of the abdomen.

Intercostal and vastus lateralis microcirculatory response to a sympathoexcitatory manoeuvre in patients with chronic obstructive pulmonary disease

Patients with COPD present with systemic vascular malfunctioning and their microcirculation is possibly more fragile to overcome an increase in the sympathetic vasoconstrictor outflow during sympathoexcitatory situations. To test the skeletal muscle microvascular responsiveness to sympathoexcitation, we asked patients with COPD and age- and sex-matched controls to immerse a hand in iced water [Cold Pressor Test (CPT)]. Near-infrared spectroscopy detection of the indocyanine green dye in the intercostal and vastus lateralis microcirculation provided a blood flow index (BFI). BFI divided by mean blood pressure (MBP) provided an index of microvascular conductance (BFI/MBP). The CPT decreased BFI and BFI/MBP in the intercostal (P = 0.01 and < 0.01, respectively) and vastus lateralis (P = 0.08 and 0.03, respectively) only in the COPD group, and the per cent BFI and BFI/MBP decrease was similar between muscles (P = 0.78 and 0.85, respectively). Thus, our findings support that sympathoexcitation similarly impairs intercostal and vastus lateralis microvascular regulation in patients with COPD.

Muscle and cerebral oxygenation during cycling in chronic obstructive pulmonary disease: A scoping review

To synthesize evidence for prefrontal cortex (PFC), quadriceps, and respiratory muscle oxygenation using near-infrared spectroscopy (NIRS) during cycling in individuals with chronic obstructive pulmonary disease (COPD). A scoping review was performed searching databases (inception-August 2020): Ovid MEDLINE, EMBASE, Cochrane Systematic Reviews, Cochrane Central Register of Controlled Clinical Trials, CINAHL, SPORTDiscus and Pedro. The search focused on COPD, cycling, and NIRS outcomes. 29 studies (541 COPD participants) were included. Compared to healthy individuals (8 studies), COPD patients at lower cycling workloads had more rapid increases in vastus lateralis (VL) deoxygenated hemoglobin (HHb); lower increases in VL total hemoglobin (tHb) and blood flow; and lower muscle tissue saturation (StO₂). Heliox and bronchodilators were associated with smaller and slower increases in VL HHb. Heliox increased VL and intercostal blood flow compared to room air and supplemental oxygen in COPD patients (1 study). PFC oxygenated hemoglobin (O₂Hb) increased in COPD individuals during cycling in 5 of 8 studies. Individuals with COPD and heart failure demonstrated worse VL and PFC NIRS outcomes compared to patients with only COPD-higher or more rapid increase in VL HHb and no change or decrease in PFC O₂Hb. Individuals with COPD present with a mismatch between muscle oxygen delivery and utilization, characterized by more rapid increase in VL HHb, lower muscle O₂Hb and lower muscle StO₂. PFC O₂Hb increases or tends to increase in individuals with COPD during exercise, but this relationship warrants further investigation. NIRS can be used to identify key deoxygenation thresholds during exercise to inform PFC and muscle oxygenation.

Differences in Respiratory Muscle Responses to Hyperpnea or Loaded Breathing in COPD

INTRODUCTION

We aimed to compare acute mechanical and metabolic responses of the diaphragm and rib cage inspiratory muscles during two different types of respiratory loading in patients with chronic obstructive pulmonary disease.

METHODS

In 16 patients (age, 65 ± 13 yr; 56% male; forced expiratory volume in the first second, 60 ± 6%pred; maximum inspiratory pressure, 82 ± 5%pred), assessments of respiratory muscle EMG, esophageal pressure (Pes) and gastric pressures, breathing pattern, and noninvasive assessments of systemic (V˙O2, cardiac output, oxygen delivery and extraction) and respiratory muscle hemodynamic and oxygenation responses (blood flow index, oxygen delivery index, deoxyhemoglobin concentration, and tissues oxygen saturation [StiO2]), were performed during hyperpnea and loaded breathing.

RESULTS

During hyperpnea, breathing frequency, minute ventilation, esophageal and diaphragm pressure-time product per minute, cardiac output, and V˙O2 were higher than during loaded breathing (P < 0.05). Average inspiratory Pes and transdiaphragmatic pressure per breath, scalene (SCA), sternocleidomastoid, and intercostal muscle activation were higher during loading breathing compared with hyperpnea (P < 0.05). Higher transdiaphragmatic pressure during loaded breathing compared with hyperpnea was mostly due to higher inspiratory Pes (P < 0.05). Diaphragm activation, inspiratory and expiratory gastric pressures, and rectus abdominis muscle activation did not differ between the two conditions (P > 0.05). SCA-blood flow index and oxygen delivery index were lower, and SCA-deoxyhemoglobin concentration was higher during loaded breathing compared with hyperpnea. Furthermore, SCA and intercostal muscle StiO2 were lower during loaded breathing compared with hyperpnea (P < 0.05).

CONCLUSION

Greater inspiratory muscle effort during loaded breathing evoked larger rib cage and neck muscle activation compared with hyperpnea. In addition, lower SCA and intercostal muscle StiO2 during loaded breathing compared with hyperpnea indicates a mismatch between inspiratory muscle oxygen delivery and utilization induced by the former condition.

High-intensity exercise impairs extradiaphragmatic respiratory muscle perfusion in patients with COPD

The study investigated whether high-intensity exercise impairs inspiratory and expiratory muscle perfusion in patients with chronic obstructive pulmonary disease (COPD). We compared respiratory local muscle perfusion between constant-load cycling[sustained at 80% peak work rate (WRpeak)] and voluntary normocapnic hyperpnea reproducing similar work of breathing (WoB) in 18 patients [forced expiratory volume in the first second (FEV₁): 58 ± 24% predicted]. Local muscle blood flow index (BFI), using indocyanine green dye, and fractional oxygen saturation (%StiO₂) were simultaneously assessed by near-infrared spectroscopy (NIRS) over the intercostal, scalene, rectus abdominis, and vastus lateralis muscles. Cardiac output (impedance cardiography), WoB (esophageal/gastric balloon catheter), and diaphragmatic and extradiaphragmatic respiratory muscle electromyographic activity (EMG) were also assessed throughout cycling and hyperpnea. Minute ventilation, breathing pattern, WoB, and respiratory muscle EMG were comparable between cycling and hyperpnea. During cycling, cardiac output and vastus lateralis BFI were significantly greater compared with hyperpnea [by +4.2 (2.6-5.9) L/min and +4.9 (2.2-7.8) nmol/s, respectively] (P < 0.01). Muscle BFI and %StiO₂ were, respectively, lower during cycling compared with hyperpnea in scalene [by -3.8 (-6.4 to -1.2) nmol/s and -6.6 (-8.2 to -5.1)%], intercostal [by -1.4 (-2.4 to -0.4) nmol/s and -6.0 (-8.6 to -3.3)%], and abdominal muscles [by -1.9 (-2.9 to -0.8) nmol/s and -6.3 (-9.1 to -3.4)%] (P < 0.001). The difference in respiratory (scalene and intercostal) muscle BFI between cycling and hyperpnea was associated with greater dyspnea (Borg CR10) scores (r = -0.54 and r = -0.49, respectively, P < 0.05). These results suggest that in patients with COPD, 1) locomotor muscle work during high-intensity exercise impairs extradiaphragmatic respiratory muscle perfusion and 2) insufficient adjustment in extradiaphragmatic respiratory muscle perfusion during high-intensity exercise may partly explain the increased sensations of dyspnea.NEW & NOTEWORTHY We simultaneously assessed the blood flow index (BFI) in three respiratory muscles during hyperpnea and high-intensity constant-load cycling sustained at comparable levels of work of breathing and respiratory neural drive in patients with COPD. We demonstrated that high-intensity exercise impairs respiratory muscle perfusion, as intercostal, scalene, and abdominal BFI increased during hyperpnea but not during cycling. Insufficient adjustment in respiratory muscle perfusion during exercise was associated with greater dyspnea sensations in patients with COPD.

Effect of high-intensity exercise on cerebral, respiratory and peripheral muscle oxygenation of HF and COPD-HF patients

OBJECTIVE

To investigate cerebral oxygenation (Cox) responses as well as respiratory (Res) and active peripheral muscle (Pm) O₂ delivery during high-intensity cycling exercise and contrast responses between patients with coexistent chronic obstructive pulmonary disease (COPD)-heart failure (HF) and HF alone.

METHODS

Cross-sectional study involving 11 COPD-HF and 11 HF patients. On two different days, patients performed maximal incremental cardiopulmonary exercise testing (CPET) and constant load exercise on a cycle ergometer until the limit of tolerance (Tlim). The high-intensity exercise session was 80% of the peak CPET work rate. Relative blood concentrations of oxyhemoglobin ([O₂Hb]), deoxyhemoglobin ([HHb]) of Res, Pm (right vastus lateralis) and Cox (pre-frontal) were measured using near infrared spectroscopy.

RESULTS

We observed a greater decrease in [O₂Hb] at a lower Tlim in COPD-HF when compared to HF (P < 0.05). [HHb] of Res was higher (P < 0.05) and Tlim was lower in COPD-HF vs. HF. Pm and Cox were lower and Tlim was higher in (P < 0.05) HF vs. COPD-HF. In HF, there was a lower ∆[O₂Hb] and higher ∆ [HHb] of Pm when contrasted to Cox observed during exercise, as well as a lower ∆ [O₂Hb] and higher ∆ [HHb] of Res when contrasted with Cox (P < 0.05). However, COPD-HF patients presented with a higher ∆ [HHb] of Res and Pm when contrasted with Cox (P < 0.05).

CONCLUSION

The coexistence of COPD in patients with HF produces negative effects on Cox, greater deoxygenation of the respiratory and peripheral muscles and higher exertional dyspnea, which may help to explain an even lower exercise tolerance in this multimorbidity phenotype.

Near-Infrared Spectroscopy in Bio-Applications

Near-infrared (NIR) spectroscopy occupies a specific spot across the field of bioscience and related disciplines. Its characteristics and application potential differs from infrared (IR) or Raman spectroscopy. This vibrational spectroscopy technique elucidates molecular information from the examined sample by measuring absorption bands resulting from overtones and combination excitations. Recent decades brought significant progress in the instrumentation (e.g., miniaturized spectrometers) and spectral analysis methods (e.g., spectral image processing and analysis, quantum chemical calculation of NIR spectra), which made notable impact on its applicability. This review aims to present NIR spectroscopy as a matured technique, yet with great potential for further advances in several directions throughout broadly understood bio-applications. Its practical value is critically assessed and compared with competing techniques. Attention is given to link the bio-application potential of NIR spectroscopy with its fundamental characteristics and principal features of NIR spectra.

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 (WR_peak ) with 30 s at 50% WR_peak ) with moderate-intensity constant-load exercise (CLE) at 75% WR_peak , 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 (ΔIC_CLE : -0.38 ± 0.10 versus ΔIC_IE : -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 P C O 2 and heart rate, these results cannot be projected onto normal exercise.

Cerebral cortex and respiratory muscles perfusion during spontaneous breathing attempts in ventilated patients and its relation to weaning outcomes: a protocol for a prospective observational study

INTRODUCTION

In addition to the well-documented factors that contribute to weaning failure, increased energy demands of the respiratory muscles during spontaneous breathing trials (SBTs) might not be met by sufficient increases in energy supplies. This discrepancy may deprive blood and oxygen of other tissues. In this context, restrictions in perfusion of splanchnic organs and non-working muscles during SBT have been associated with weaning failure. However, alterations in perfusion of the brain during the weaning process are less well understood.

OBJECTIVE AND HYPOTHESIS

To investigate whether cerebral cortex perfusion evolves differentially during the transition from mechanical ventilation (MV) to spontaneous breathing between patients failing or succeeding the SBT. We hypothesise that patients failing the SBT will exhibit reduced cerebral cortex perfusion during the transition from MV to spontaneous breathing as compared with patients succeeding the SBT.

METHODS AND ANALYSIS

This single-centre, prospective, observational study will be conducted in a medical Intensive Care unit of University Hospital Leuven, Belgium in ready to wean patients. Blood flow index in the cerebral cortex (prefrontal area), inspiratory (scalene) and expiratory muscle (upper rectus abdominis) and a non-working muscle (thenar eminence) will be simultaneously assessed by near-infrared spectroscopy (NIRS) using the tracer indocyanine green dye. Measurements will be performed on the same day during MV and during SBT. NIRS-derived tissue oxygenation index and cardiac output (by pulse contour analyses) will be recorded continuously. Twenty patients failing an SBT are estimated to be sufficient for detecting a significant difference in the change of cerebral cortex perfusion from MV to SBT (primary outcome) between SBT failure and success patients.

ETHICS AND DISSEMINATION

Ethics approval was obtained from the local ethical committee (Ethische Commissie Onderzoek UZ/KU Leuven protocol ID: S60516). Results from this study will be presented at scientific meetings and congresses and published in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

NCT03240263; Pre-results.

More Impaired Dynamic Ventilatory Muscle Oxygenation in Congestive Heart Failure than in Chronic Obstructive Pulmonary Disease

Patients with chronic obstructive pulmonary disease (COPD) and congestive heart failure (CHF) often have dyspnea. Despite differences in primary organ derangement and similarities in secondary skeletal muscle changes, both patient groups have prominent functional impairment. With similar daily exercise performance in patients with CHF and COPD, we hypothesized that patients with CHF would have worse ventilatory muscle oxygenation than patients with COPD. This study aimed to compare differences in tissue oxygenation and blood capacity between ventilatory muscles and leg muscles and between the two patient groups. Demographic data, lung function, and maximal cardiopulmonary exercise tests were performed in 134 subjects without acute illnesses. Muscle oxygenation and blood capacity were measured using frequency-domain near-infrared spectroscopy (fd-NIRS). We enrolled normal subjects and patients with COPD and CHF. The two patient groups were matched by oxygen-cost diagram scores, New York Heart Association functional classification scores, and modified Medical Research Council scores. COPD was defined as forced expired volume in one second and forced expired vital capacity ratio ≤0.7. CHF was defined as stable heart failure with an ejection fraction ≤49%. The healthy subjects were defined as those with no obvious history of chronic disease. Age, body mass index, cigarette consumption, lung function, and exercise capacity were different across the three groups. Muscle oxygenation and blood capacity were adjusted accordingly. Leg muscles had higher deoxygenation (HHb) and oxygenation (HbO₂) and lower oxygen saturation (S_mO₂) than ventilatory muscles in all participants. The S_mO₂ of leg muscles was lower than that of ventilatory muscles because S_mO₂ was calculated as HbO₂/(HHb+HbO₂), and the HHb of leg muscles was relatively higher than the HbO₂ of leg muscles. The healthy subjects had higher S_mO₂, the patients with COPD had higher HHb, and the patients with CHF had lower HbO₂ in both muscle groups throughout the tests. The patients with CHF had lower S_mO₂ of ventilatory muscles than the patients with COPD at peak exercise (p < 0.01). We conclud that fd-NIRS can be used to discriminate tissue oxygenation of different musculatures and disease entities. More studies on interventions on ventilatory muscle oxygenation in patients with CHF and COPD are warranted.