Inspiratory Threshold Loading Research Articles

Comparative Study of Different Respiratory Muscle Training Methods: Effects on Cardiopulmonary Indices and Athletic Performance in Elite Short-Track Speedskaters.

Respiratory muscle training (RMT) improves endurance performance, balance, and ability to repeat high-intensity exercise bouts, providing a rationale to be applied in short-track speedskating. To establish a preferable RMT method for short-track speedskating, the influence of inspiratory pressure threshold loading (IPTL) and voluntary isocapnic hyperpnoea (VIH) on cardiopulmonary indices and athletic performance was investigated. Sixteen elite short-track speedskaters completed 6 weeks of RMT based on IPTL or VIH. Wingate Anaerobic Tests (WAnTs), cardiopulmonary exercise tests (CPETs), spirometry assessments, and on-ice time trials were performed before and after RMT intervention. Repeated measures ANOVA was used to assess the differences between each method's influence. No statistically significant (p > 0.05) differences between RMT methods were found in performance during the WAnT, CPET, or specific on-ice time trials. Spirometry measures were similar between both methods. Significant effects were found for the interaction between maximum breathing frequency during CPET (BFmax) and method (p = 0.009), as well as for the interaction between BFMax, method, and sex (p = 0.040). BFmax decreased for IPTL and increased for VIH. The interaction between method and sex revealed that BFmax increased only in males performing VIH. Our findings suggest that IPTL and VIH lead to analogous effects in the study participants, highlighting a negligible practical disparity in the impact of different RMT methods in elite short-track speedskaters.

Sex-based differences in the blood pressure responses to muscle metaboreflex activation are consistent between limb and respiratory muscle.

The purpose of this study was to compare sex-based differences in the mean arterial blood pressure (MAP) response to limb and inspiratory metaboreflex activation, during relative and absolute workloads. Healthy males (M; n=9) and females (F; n=8) completed pulmonary function testing, forearm volume and circumference measurements, and bouts of limb and inspiratory muscle exercise. The exercises performed included bouts of rhythmic handgrip exercise (RHG) and inspiratory pressure threshold loading (PTL) to task failure, performed in a randomized order, and separated by 30 minutes of rest. Participants performed both RHG and PTL at predetermined relative (R) and absolute (A) workloads while cardiopulmonary measurements were recorded continuously. A time-dependent rise in MAP was observed in all participants, regardless of sex, muscle, or workload (p<0.001). MAP was greater in males than females during all exercise bouts regardless of muscle group or workload (p<0.001). The change in MAP from baseline was also greater in males (R-RHG: ∆31±12 mmHg; R-PTL: ∆31±9; A-RHG: ∆35±6; A-PTL: ∆30±7) than females (R-RHG: ∆21±7 mmHg; R-PTL: ∆13±7; A-RHG: ∆21±7; A-PTL: ∆14±3) (p<0.001). Results from this study show that when the forearm and diaphragm perform the same relative or absolute work, the blood pressure response is statistically similar, and both responses are greater in males than females. The findings from the present study suggest that the sex-based difference in the response to metaboreflex activation is similar between the limb and respiratory musculature.

Medical hypnosis mitigates laboratory dyspnoea in healthy humans: a randomised, controlled experimental trial.

Dyspnoea persisting despite treatments of underlying causes requires symptomatic approaches. Medical hypnosis could provide relief without the untoward effects of pharmacological approaches. We addressed this question through experimentally induced dyspnoea in healthy humans (inspiratory threshold loading (excessive inspiratory effort) and carbon dioxide stimulation (air hunger)). 20 volunteers (10 women, aged 21-40 years) were studied on four separate days. The order of the visits was randomised in two steps: firstly, the "inspiratory threshold loading first" versus "carbon dioxide first" group (n=10 in each group); secondly, the "medical hypnosis first" versus "visual distraction first" subgroup (n=5 in each subgroup). Each visit comprised three 5-min periods (reference, intervention, washout) during which participants used visual analogue scales (VAS) to rate the sensory and affective dimensions of dyspnoea, and after which they completed the Multidimensional Dyspnea Profile. Medical hypnosis reduced both dimensions of dyspnoea significantly more than visual distraction (inspiratory threshold loading: sensory reduction after 5 min 34% of full VAS versus 8% (p=0.0042), affective reduction 17.6% versus 2.4% (p=0.044); carbon dioxide: sensory reduction after 5 min 36.9% versus 3% (p=0.0015), affective reduction 29.1% versus 8.7% (p=0.0023)). The Multidimensional Dyspnea Profile showed more marked sensory effects during inspiratory threshold loading and more marked affective effects during carbon dioxide stimulation. Medical hypnosis was more effective than visual distraction at attenuating the sensory and affective dimensions of experimentally induced dyspnoea. This provides a strong rationale for clinical studies of hypnosis in persistent dyspnoea patients.

Continuous measurements of respiratory muscle blood flow and oxygen consumption using noninvasive frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy.

Prior studies of muscle blood flow and muscle-specific oxygen consumption have required invasive injection of dye and magnetic resonance imaging, respectively. Such measures have limited utility for continuous monitoring of the respiratory muscles. Frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy (FD-NIRS & DCS) can provide continuous surrogate measures of blood flow index (BFi) and metabolic rate of oxygen consumption (MRO2). This study aimed to validate sternocleidomastoid FD-NIRS & DCS outcomes against electromyography (EMG) and mouth pressure (Pm) during incremental inspiratory threshold loading (ITL). Six female and six male healthy adults (means ± SD; 30 ± 7 yr, maximum inspiratory pressure 118 ± 61 cmH2O) performed incremental ITL starting at low loads (8 ± 2 cmH2O) followed by 50-g increments every 2 min until task failure. FD-NIRS & DCS continuously measured sternocleidomastoid oxygenated and deoxygenated hemoglobin + myoglobin (oxy/deoxy[Hb + Mb]), tissue saturation of oxygen (StO2), BFi, and MRO2. Ventilatory parameters including inspiratory Pm were also evaluated. Pm increased during incremental ITL (P < 0.05), reaching -47[-74 to -34] cmH2O (median [IQR: 25%-75%]) at task failure. Ventilatory parameters were constant throughout ITL (all P > 0.05). Sternocleidomastoid BFi and MRO2 increased from the start of the ITL (both P < 0.05). Deoxy[Hb + Mb] increased close to task failure, concomitantly with a constant increase in MRO2, and decreased StO2. Sternocleidomastoid deoxy[Hb + Mb], BFi, StO2, and MRO2 obtained during ITL via FD-NIRS & DCS correlated with sternocleidomastoid EMG (all P < 0.05). In healthy adults, FD-NIRS & DCS can provide continuous surrogate measures of respiratory BFi and MRO2. Increasing sternocleidomastoid oxygen consumption near task failure was associated with increased oxygen extraction and reduced tissue saturation.NEW & NOTEWORTHY This study introduces a novel approach, frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy (FD-NIRS & DCS), for noninvasive continuous monitoring of respiratory muscle blood flow and metabolic rate of oxygen consumption. Unlike prior methods involving invasive dye injection and magnetic resonance imaging, FD-NIRS & DCS offers the advantage of continuous measurement without the need for invasive procedures. It holds promise for advancing muscle physiology understanding and opens avenues for real-time monitoring of respiratory muscles.

Cognitive interference of respiratory versus limb muscle dual tasking in healthy adults.

Inspiratory threshold loading (ITL) and associated dyspnoea have been shown to interfere with cognition during cognitive-motor dual tasking. However, ITL has not been compared with another rhythmic muscle activity, such as lower limb pedalling. While ITL has been shown to interfere with cognition, the mechanism of the prefrontal cortex (PFC) during ITL or other rhythmical muscle dual tasking, has not been elucidated. Given the cognitive interference that arises during ITL, we hypothesise that ITL cognitive-motor dual tasking will result in greater cognitive decrements and increased PFC activity compared with the pedalling cognitive-motor dual task. 30 healthy participants (16 females; median age 23 (interquartile range 23-24) years) were recruited. They performed five 3-min tasks in a single visit in a random order: single tasks were ITL, pedalling and Stroop task and dual tasks were ITL-Stroop and pedalling-Stroop. Participant's PFC activity was assessed bilaterally using functional near-infrared spectroscopy throughout each task. Single- and dual-task cognitive performance was evaluated by measuring Stroop task reaction time and accuracy. Dyspnoea and rating of perceived exertion were evaluated at the end of each task. ITL-Stroop resulted in greater impairments in reaction time (p<0.001), accuracy (p<0.01) and increased medial/dorsolateral PFC activity (p≤0.006) than pedalling-Stroop. ITL-Stroop elicited greater Borg dyspnoea and rating of perceived exertion than pedalling-Stroop (p<0.001), despite pedalling-Stroop having a greater heart rate response (p<0.001). The heightened cognitive decrements, perceptual response and PFC activity suggest that inspiratory muscle loading and its accompanied dyspnoea results in greater cognitive interference than rhythmic pedalling.

Quantifying Diaphragm Blood Flow With Contrast-Enhanced Ultrasound in Humans

Despite the known interplay between blood flow and function, to our knowledge, there is currently no minimally invasive method to monitor diaphragm hemodynamics. We used contrast-enhanced ultrasound to quantify relative diaphragm blood flow (Q˙DIA) in humans and assessed the technique's efficacy and reliability during graded inspiratory pressure threshold loading. We hypothesized that: (1) Q˙DIA would linearly increase with pressure generation, and (2) that there would be good test-retest reliability and interanalyzer reproducibility. Can we validate what is, to our knowledge, the first minimally invasive method to measure relative diaphragm blood flow in humans? Quantitative contrast-enhanced ultrasound of the costal diaphragm was performed in healthy participants (10 male participants, 6 female participants; mean age 28 ± 5 years; BMI 22.8 ± 2.0kg/m) during unloaded breathing and three stages of loaded breathing on two separate days. Gastric and esophageal balloon catheters measured transdiaphragmatic pressure. Ultrasonography was performed during a constant-rate IV infusion of lipid-stabilized microbubbles following each stage. Ultrasound images were acquired after a destruction-replenishment sequence and diaphragm specific time-intensity data were used to determine Q˙DIA by two individuals. Transdiaphragmatic pressure for unloaded and each loading stage were 15.2 ± 0.8, 26.1 ± 0.8, 34.6 ± 0.8, and 40.0 ± 0.8 percentage of the maximum, respectively. Q˙DIA increased with each stage of loading (3.1 ± 3.1, 6.9 ± 3.6, 11.0 ± 4.9, and 13.5 ± 5.4 acoustic units/s; P< .0001). The linear relationship between diaphragmatic flow and pressure was reproducible from day to day. Q˙DIA had good to excellent test-retest reliability (0.86 [0.77, 0.92]; P< .0001) and excellent interanalyzer reproducibility (0.93 [0.90, 0.95]; P< .0001) with minimal bias. Relative Q˙DIA measurements had valid physiological underpinnings, were reliable day-to-day, and were reproducible analyzer-to-analyzer. This study indicated that contrast-enhanced ultrasound is a viable, minimally invasive method for assessing costal Q˙DIA in humans and may provide a tool to monitor diaphragm hemodynamics in clinical settings.

Diaphragm thickness and mobility elicited by two different modalities of inspiratory muscle loading in heart failure participants: A randomized crossover study.

To analyze diaphragmatic thickness, at end-inspiration and end-expiration, diaphragmatic thickening index and mobility via US under two different modalities of inspiratory muscle loading, in two different modalities of inspiratory muscle loading and different load intensities at full-vital capacity maneuvers and the relationship between diaphragmatic thickness with pulmonary function tests in participants with HF. This randomized crossover trial, enrolled with 17 HF subjects, evaluated diaphragm thickness (Tdi, mm), fractional thickness (TFdi, %), and mobility (mm) US during low and high intensities (30% and 60% of maximal inspiratory pressure-MIP) with two modalities of inspiratory muscle loading mechanical threshold loading (MTL) and tapered flow-resistive loading (TFRL). Both MTL and TFRL produced a increase in Tdi, but only with high intensity loading compared to baseline-2.21 (0.26) vs. 2.68 (0.33) and 2.73 (0.44) mm; p = .01. TFdi was greater than baseline under all conditions, except during low intensity of TFRL. Diaphragm mobility was greater than baseline under all conditions, and high intensity of TFRL elicited greater mobility compared to all other conditions. Additionally, baseline Tdi was moderately correlated with pulmonary function tests. MTL and TFRL modalities elicit similar increases in diaphragm thickness at loads, but only during high intensity loading it was greater than baseline. Diaphragm mobility was significantly greater than baseline under both loads and devices, and at high intensity compared to low intensity, although TFRL produced greater mobility compared to modalities of inspiratory muscle loading. There is an association between diaphragm thickness and pulmonary function tests.

Inspiratory Pressure Threshold Loading: Effects on Inspiratory Pattern, Perception and Prefrontal Cortical Activity

Cognitive awareness of breathing can lead to behavioral ventilatory compensation, respiratory interoception and activation of brain regions associated with decision processes, including the dorsal prefrontal cortex (dPFC). The dPFC is a critical cortical region mediating executive functions and dPFC activity is modulated during increased breathing load. This study investigated load compensation and perception response to sustained breathing against inspiratory pressure threshold (PT) loads. We hypothesized that increasing magnitudes of PT loads require increasing time to valve opening resulting in; greater inspiratory motor drive, increased perception of breathing effort (INT), increased perception of unpleasantness (UPL) and increased dPFC oxygen consumption (OXY, a measure of increased neural activity). Functional near-infrared spectroscopy (fNIRS) was used to measure OXY changes in the dPFC. Healthy subjects were exposed to 5 cmH2O (LoPT) and 20 cmH2O (HiPT) inspiratory PT loads for 1 minute load-on and 1 minute load-off trials. A total of 8 trials for each load were presented, 4 INT trials and 4 UPL trials. Perception of inspiratory INT and UPL was rated with a 0-10 modified Borg scale. There was a significant PT magnitude dependent increase in valve opening duration (0.18±0.15 LoPT vs 0.57±0.17 HiPT) and pressure (-4.4±0.75 LoPT vs -21.3±2.2 HiPT, p&lt;0.001). The pressure-time-product (PtP), a measure of inspiratory motor drive, significantly (p&lt;0.001) increased with PT magnitude. A significant PT magnitude dependent increase in INT (2.8±1.3 vs 7.9±1.2, p&lt;0.001) and UPL (1.9±1.4 vs 7.8±1.4, p&lt;0.001) ratings occurred but perception ratings did not significantly change throughout the 1-minute trial. The average 1-minute trial OXY LoPT was significantly (p&lt;0.02) less than HiPT. The right dPFC OXY did not significantly change between trial Breath 1 and Breath 10. The left dPFC OXY significantly (p&lt;0.01) increased from trial Breath 1 to Breath 10. These results suggest increased PT load magnitude required greater inspiratory drive to breathe. Breathing against a HiPT load was perceived as highly effortful and unpleasant. Sustained breathing against PT loads progressively increased left dPFC neural activity. Supported by the University of Florida, College of Veterinary Medicine. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Respiratory muscle training induces additional stress and training load in well-trained triathletes-randomized controlled trial.

Background: Respiratory muscle training (RMT) has been investigated in the context of improved athletic performance and pulmonary function. However, psychophysiological costs of RMT remain understudied. Voluntary isocapnic hyperpnoea (VIH) and inspiratory pressure threshold loading (IPTL) are widely applied RMT methods. The main purposes of this study were to assess whether RMT induces additional load on well-trained triathletes and determine differences in RMT-induced load between sexes and applied methods. Materials and Methods: 16 well-trained triathletes (n = 16, 56% males) underwent 6weeks of VIH or IPTL program with progressive overload. Blood markers, subjective measures, cardiac indices, near-infrared spectroscopy indices, inspiratory muscle fatigue, and RMT-induced training load were monitored pre-, in and post-sessions. We used multiple ANOVA to investigate effects of sex, training method, and time on measured parameters. Results: There were significant interactions for acid-base balance (p = 0.04 for sex, p < 0.001 for method), partial carbon dioxide pressure (p = 0.03 for sex, p < 0.001 for method), bicarbonate (p = 0.01 for method), lactate (p < 0.001 for method), RMT-induced training load (p = 0.001 for method for single session, p = 0.03 for method per week), average heart rate (p = 0.03 for sex), maximum heart rate (p = 0.02 for sex), intercostales muscle oxygenation (p = 0.007 for testing week), and intercostales muscle oxygenation recovery (p = 0.003 for testing week and p = 0.007 for method). Conclusion: We found that RMT induced additional load in well-trained triathletes. Elicited changes in monitored variables depend on sex and training method. VIH significantly increased subjective training load measures. IPTL was associated with disbalance in blood gasometry, increase in lactate, and reports of headaches and dizziness. Both methods should be applied with consideration in high-performance settings.

The metabolic cost of inspiratory muscle training in mechanically ventilated patients in critical care

BackgroundDiaphragmatic dysfunction is well documented in patients receiving mechanical ventilation. Inspiratory muscle training (IMT) has been used to facilitate weaning by strengthening the inspiratory muscles, yet the optimal approach remains uncertain. Whilst some data on the metabolic response to whole body exercise in critical care exist, the metabolic response to IMT in critical care is yet to be investigated. This study aimed to quantify the metabolic response to IMT in critical care and its relationship to physiological variables.MethodsWe conducted a prospective observational study on mechanically ventilated patients ventilated for ≥ 72 h and able to participate in IMT in a medical, surgical, and cardiothoracic intensive care unit. 76 measurements were taken on 26 patients performing IMT using an inspiratory threshold loading device at 4 cmH2O, and at 30, 50 and 80% of their negative inspiratory force (NIF). Oxygen consumption (VO2) was measured continuously using indirect calorimetry.ResultsFirst session mean (SD) VO2 was 276 (86) ml/min at baseline, significantly increasing to 321 (93) ml/min, 333 (92) ml/min, 351(101) ml/min and 388 (98) ml/min after IMT at 4 cmH2O and 30, 50 and 80% NIF, respectively (p = 0.003). Post hoc comparisons revealed significant differences in VO2 between baseline and 50% NIF and baseline and 80% NIF (p = 0.048 and p = 0.001, respectively). VO2 increased by 9.3 ml/min for every 1 cmH2O increase in inspiratory load from IMT. Every increase in P/F ratio of 1 decreased the intercept VO2 by 0.41 ml/min (CI − 0.58 to − 0.24 p < 0.001). NIF had a significant effect on the intercept and slope, with every 1 cmH2O increase in NIF increasing intercept VO2 by 3.28 ml/min (CI 1.98–4.59 p < 0.001) and decreasing the dose–response slope by 0.15 ml/min/cmH2O (CI − 0.24 to − 0.05 p = 0.002).ConclusionsIMT causes a significant load-dependent increase in VO2. P/F ratio and NIF impact baseline VO2. The dose–response relationship of the applied respiratory load during IMT is modulated by respiratory strength. These data may offer a novel approach to prescription of IMT.Take home messageThe optimal approach to IMT in ICU is uncertain; we measured VO2 at different applied respiratory loads to assess whether VO2 increased proportionally with load and found VO2 increased by 9.3 ml/min for every 1 cmH2O increase in inspiratory load from IMT. Baseline NIF has a significant effect on the intercept and slope, participants with a higher baseline NIF have a higher resting VO2 but a less pronounced increase in VO2 as the inspiratory load increases; this may offer a novel approach to IMT prescription.Trial registration ClinicalTrials.gov, registration number: NCT05101850. Registered on 28 September 2021, https://clinicaltrials.gov/ct2/show/NCT05101850

Dyspnea induced by inspiratory loading limits dual-tasking in healthy young adults.

Dyspnea is a common and multidimensional experience of healthy adults and those with respiratory disorders. Due to its neural processing, it may limit or interfere with cognition, which may be examined with a dual-task paradigm. The aim of this study was to compare single-task performance of Stroop Colour and Word Test (SCWT) or inspiratory threshold loading (ITL) to their combined dual-task performance. Secondly, whether mood was related to dyspnea or cognitive performance was also evaluated. A virtual pre-post design examined single (SCWT and ITL) and dual-task (SCWT+ITL) performance. For ITL, a Threshold Trainer™ was used to elicit a "somewhat severe" rating of dyspnea. The SCWT required participants to indicate whether a colour-word was congruent or incongruent with its semantic meaning. The Depression, Anxiety and Stress Scale-21 (DASS-21) was completed to assess mood. Breathing frequency, Borg dyspnea rating, and breathing endurance time were ascertained. Thirty young healthy adults (15F, 15M; median age = 24, IQR [23-26] years) completed the study. SCWT+ITL had lower SCWT accuracy compared to SCWT alone (98.6%, [97.1-100.0] vs 99.5%, [98.6-100.0]; p = 0.009). Endurance time was not different between ITL and SCWT+ITL (14.5 minutes, [6.9-15.0]) vs 13.7 minutes, [6.1-15.0]; p = 0.59). DASS-21 scores positively correlated with dyspnea scores during ITL (rho = 0.583, p<0.001) and SCWT+ITL (rho = 0.592, p<0.001). ITL significantly reduced dual-task performance in healthy young adults. Lower mood was associated with greater perceived dyspnea during single and dual-task ITL. Considering the prevalence of dyspnea in respiratory disorders, the findings of this dual task paradigm warrant further exploration to inform dyspnea management during daily activities.

Biomarkers to measure respiratory muscle damage following inspiratory pressure threshold loading in healthy young men.

Elevated respiratory muscle work is encountered during strenuous exercise, acute and chronic respiratory disorders, and during inspiratory pressure threshold loading (ITL). ITL can induce respiratory muscle damage, evidenced by increases in fast and slow skeletal troponin-I (sTnI). However, other blood markers of muscle damage have not been measured. We investigated respiratory muscle damage following ITL using a skeletal muscle damage biomarkers panel. Seven healthy men (33 ± 2 yr) undertook 60 min of ITL at a resistance equivalent to ∼0% (Sham ITL) and 70% of their maximal inspiratory pressure 2 wk apart. Serum was collected before and at 1, 24, and 48 h after each ITL session. Creatine kinase muscle-type (CKM), myoglobin, fatty acid-binding protein-3 (FABP3), myosin light chain-3, and fast and slow sTnI were measured. Two-way ANOVA revealed time × load interaction effects (P < 0.05) for CKM, slow and fast sTnI. All of these were higher for 70% compared with Sham ITL. CKM was higher at 1 and 24 h, fast sTnI at 1 h, whereas slow sTnI was higher at 48 h. There were main effects of time (P < 0.01) for FABP3 and myoglobin, but no time × load interaction effects. Hence, CKM and fast sTnI could be used to assess respiratory muscle damage immediately (1 h), whereas CKM and slow sTnI could be used to assess respiratory muscle damage 24 and 48 h following conditions that elevate inspiratory muscle work. The specificity of these markers for different time points needs further exploration in other protocols that cause elevated inspiratory muscle work.NEW & NOTEWORTHY We investigated inspiratory pressure threshold loading-induced respiratory muscle damage using a skeletal muscle damage biomarkers panel. Our investigation showed that creatine kinase muscle-type, and fast skeletal troponin I could be used to assess respiratory muscle damage immediately (1 h), whereas creatine kinase muscle-type, and slow skeletal troponin I could be used to assess respiratory muscle damage 24 and 48 h following conditions that cause elevated inspiratory muscle work.

Effectiveness of Respiratory Muscles Training by Voluntary Isocapnic Hyperpnea Versus Inspiratory Threshold Loading on Intercostales and Vastus Lateralis Muscles Deoxygenation Induced by Exercise in Physically Active Adults

Respiratory muscle training (RMT) improves physical performance, although it is still debated whether this effect depends on the type of training. The purpose of this study was to compare the effects of two different types of RMT, i.e., voluntary isocapnic hyperpnea (VIH) and inspiratory threshold loading (ITL), on the deoxygenation of intercostal (ΔSmO2-m. intercostales) and vastus lateralis (ΔSmO2-m. vastus lateralis) muscles during exercise. Twenty-four participants performed eight weeks of RMT by: (i) VIH (3 days·week-1 for 12 min at 60% maximal voluntary ventilation) or (ii) ITL (5 sets·week-1 of 30 breaths·minute-1 at 60% maximal inspiratory pressure). Cardiopulmonary exercise testing (CPET) included ΔSmO2 (the change from baseline to end of test) of intercostal and vastus lateralis muscles. After RMT, both groups showed decreased ΔSmO2-m. intercostales (VIH = 12.8 ± 14.6%, p = 0.04 (effect size, ES = 0.59), and ITL = 8.4 ± 9.8%, p = 0.04 (ES = 0.48)), without a coincident change of ∆SmO2-m. vastus lateralis. ITL training induced higher V˙O2-peak absolute values than VIH (mean Δ post-pre, ITL = 229 ± 254 mL·min-1 [95% CI 67-391] vs. VIH, 39 ± 153 mL·min-1 [95% CI -58-136.0], p = 0.01). In conclusion, both RMT improved the balance between supply and oxygen consumption levels of m. intercostales during CPET, with ITL also inducing an increase of aerobic capacity.

Potentiated respiratory metaboreflex in older males and females: afferent or efferent mechanisms?

Potentiated respiratory metaboreflex in older males and females: afferent or efferent mechanisms?

Effects of sex and ageing on the human respiratory muscle metaboreflex.

Intense inspiratory muscle work evokes a sympathetically mediated pressor reflex, termed the respiratory muscle metaboreflex, in which young females demonstrate an attenuated response relative to males. However, the effects of ageing and female sex hormones on the respiratory muscle metaboreflex are unclear. We tested the hypothesis that the pressor response to inspiratory work would be similar between older males and females, and higher relative to their younger counterparts. Healthy, normotensive young (26±3years) males (YM; n=10) and females (YF; n=10), as well as older (64±5years) males (OM; n=10) and females (OF; n=10), performed inspiratory pressure threshold loading (PTL) to task failure. Older adults had a greater mean arterial pressure (MAP) response to PTL than young (P<0.001). YF had a lower MAP compared to YM (+10±6 vs. +19±15mmHg, P=0.026); however, there was no difference observed between OF and OM (+26±11 vs. +27±11mmHg, P=0.162). Older adults had a lower heart rate response to PTL than young (P=0.002). There was no effect of sex between young females and males (+19±9 and +27±11bpm, P=0.186) or older females and males (+17±7 and +20±7bpm, P=0.753). We conclude the respiratory muscle metaboreflex response is heightened in older adults, and the sex effect between older males and post-menopause females is absent, suggesting an effect of circulating sex hormones. KEY POINTS: The arterial blood pressure response to the respiratory muscle metaboreflex is greater in older males and females. Compared to sex-matched young individuals, there is no sex differences in the blood pressure response between older males and post-menopause females. Our results suggest the differences between males and females in the cardiovascular response to high levels of inspiratory muscle work is abolished with reduced circulating female sex hormones.

Voluntary activation of the diaphragm after inspiratory pressure threshold loading.

After a bout of isolated inspiratory work, such as inspiratory pressure threshold loading (IPTL), the human diaphragm can exhibit a reversible loss in contractile function, as evidenced by a decrease in transdiaphragmatic twitch pressure (PDI,TW ). Whether or not diaphragm fatigability after IPTL is affected by neural mechanisms, measured through voluntary activation of the diaphragm (D-VA) in addition to contractile mechanisms, is unknown. It is also unknown if changes in D-VA are similar between sexes given observed differences in diaphragm fatigability between males and females. We sought to determine whether D-VA decreases after IPTL and whether this was different between sexes. Healthy females (n=11) and males (n=10) completed an IPTL task with an inspired duty cycle of 0.7 and targeting an intensity of 60% maximal transdiaphragmatic pressure until task failure. PDI,TW and D-VA were measured using cervical magnetic stimulation of the phrenic nerves in combination with maximal inspiratory pressure maneuvers. At task failure, PDI,TW decreased to a lesser degree in females vs. males (87 ± 15 vs. 73 ± 12% baseline, respectively, p=0.016). D-VA decreased after IPTL but was not different between females and males (91 ± 8 vs. 88 ± 10% baseline, respectively, p=0.432). When all participants were pooled together, the decrease in PDI,TW correlated with both the total cumulative diaphragm pressure generation (R2 =0.43; p=0.021) and the time to task failure (TTF, R2 = 0.40; p = 0.30) whereas the decrease in D-VA correlated only with TTF (R2 =0.24; p=0.041). Our results suggest that neural mechanisms can contribute to diaphragm fatigability, and this contribution is similar between females and males following IPTL.

Altering magnetic field strength impacts the assessment of diaphragmatic function using cervical magnetic stimulation

Quantifying diaphragm neuromuscular function using cervical magnetic stimulation (CMS) typically uses only a single stimulator (1-Stim) which may be inadequate to maximally stimulate the phrenic nerves. We questioned if using two stimulators (2-Stim) together alters diaphragm neuromuscular function at baseline and following inspiratory pressure threshold loading. Six (n = 3 female) healthy young participants were instrumented with esophageal and gastric balloon tipped catheters and electrodes over the 7–8th intercostal space. With either 1-Stim or 2-Stim an incremental protocol, where the stimulator intensity was progressively increased was completed prior to a series of potentiated twitches. The inspiratory threshold loading test consisted of loaded breathing to failure. Compared to 1-Stim, 2-Stim resulted in significantly greater unpotentiated Pditw and M-waves during the incremental protocol (both p < 0.01). Similarly, 2-Stim resulted in greater potentiated Pditw (31 ± 8 vs. 41 ± 9 cmH2O; p = 0.02) and M-waves (6.4 ± 2.9 vs. 8.6 ± 2.4 V; p = 0.02). Our findings suggest that CMS using 1-Stim is unlikely to generate a sufficient magnetic field to maximally stimulate the phrenic nerves and may underestimate diaphragm function.

Non-invasive assessment of fatigue and recovery of inspiratory rib cage muscles during endurance test in healthy individuals.

Fatigue is defined as loss of capacity to develop muscle force and/or velocity that is reversible at rest. We assessed non-invasively the fatigue and recovery of inspiratory rib cage muscles during two respiratory endurance tests in healthy individuals. The sniff nasal inspiratory pressure (SNIP) was assessed before and after two respiratory endurance tests: normocapnic hyperpnea (NH) and inspiratory pressure threshold loading (IPTL). Contractile (maximum rate of pressure development and time to peak pressure) and relaxation parameters (maximum relaxation rate [MRR], time constant of pressure decay [τ], and half relaxation time) obtained from sniff curves and shortening velocity and mechanical power estimated using optoelectronic plethysmography were analyzed during SNIP maneuvers. Respiratory muscle activity (electromyography) and tissue oxygenation (near-infrared spectroscopy-NIRS) were obtained during endurance tests and SNIP maneuvers. Fatigue development of inspiratory rib cage muscles was assessed according to the slope of decay of median frequency. Peak pressure during SNIP decreased after both protocols (p <0.05). MRR, shortening velocity, and mechanical power decreased (p <0.05), whereas τ increased after IPTL (p <0.05). The median frequency of inspiratory rib cage muscles (i.e., sum of sternocleidomastoid, scalene, and parasternal) decreased linearly during IPTL and exponentially during NH, mainly due to the sternocleidomastoid. Fatigue development behaved differently between protocols and relaxation properties (MRR and τ), shortening velocity, and mechanical power changed only in the IPTL.

Inspiratory threshold loading negatively impacts attentional performance.

RationaleThere are growing concerns over the occurrence of adverse physiologic events (PEs) occurring in pilots during operation of United States Air Force and Navy high-performance aircraft. We hypothesize that a heightened inspiratory work of breathing experienced by jet pilots by virtue of the on-board life support system may constitute a “distraction stimulus” consequent to an increased sensation of respiratory muscle effort. As such, the purpose of this study was to determine whether increasing inspiratory muscle effort adversely impacts on attentional performance.MethodsTwelve, healthy participants (age: 29 ± 6 years) were recruited for this study. Participants completed six repetitions of a modified Masked Conjunctive Continuous Performance Task (MCCPT) protocol while breathing against four different inspiratory threshold loads to assess median reaction times (RTs). A computer-controlled threshold loading device was used to set the inspiratory threshold loads. Repeated measures analysis of variances (ANOVAs) were performed to examine: (i) the efficacy of the threshold loading device to impose significantly higher loading at each loading condition; (ii) the effects of loading condition on respiratory muscle effort sensation; and (iii) the influence of hypercapnia on MCCPT scores during inspiratory threshold loading. Generalized additive mixed effects models (GAMMs) were used to examine the potential non-linear effects of respiratory muscular effort sensation, device loading, and hypercapnia, on MCCPT scores during inspiratory threshold loading.ResultsInspiratory threshold loading significantly augmented (P < 0.05) inspiratory effort sensation and the inspiratory pressure-time product (PTP). Our analyses also revealed that median hit RT was positively associated with inspiratory effort sensation during inspiratory loading trials.ConclusionThe findings of this work suggest that it was not increasing inspiratory muscle effort (i.e., PTP) per se, but rather participant’s subjective perception of inspiratory “load” that impacts negatively on attentional performance; i.e., as the degree of inspiratory effort sensation increased, sotoo did median hit RT. As such, it is reasonable to suggest that minimizing inspiratory effort sensation (independent of the mechanical output of the inspiratory muscles) during high-performance flight operations may prove useful in reducing pilot RTs during complex behavioral tasks.

Respiratory muscle fatigability in patients with spinal muscular atrophy.

Respiratory failure is a major cause of morbidity and mortality in patients with Spinal Muscular Atrophy (SMA). Lack of endurance, or "fatigability," is an important symptom of SMA. In addition to respiratory muscle weakness, respiratory function in SMA may be affected by Respiratory Muscle Fatigability (RMF). The purpose of this study was to explore RMF in patients with SMA. We assessed a Respiratory Endurance Test (RET) in 19 children (median age [years]: 11) and 36 adults (median age [years]: 34) with SMA types 2 and 3. Participants were instructed to breath against an inspiratory threshold load at either 20%, 35%, 45%, 55%, or 70% of their individual maximal inspiratory mouth pressure (PImax). RMF was defined as the inability to complete 60 consecutive breaths. Respiratory fatigability response was determined by change in maximal inspiratory mouth pressure (ΔPImax) and perceived fatigue (∆perceived fatigue). The probability of RMF during the RET increased by 59%-69% over 60 breaths with every 10% increase in inspiratory threshold load (%PImax). Fatigability response was characterized by a large variability in ΔPImax (-21% to +16%) and a small increase in perceived fatigue (p = 0.041, range 0 to +3). Patients with SMA demonstrate a dose-dependent increase in RMF without severe increase in exercise-induced muscle weakness or perceived fatigue. Inspiratory muscle loading in patients with SMA seems feasible and its potential to stabilize or improve respiratory function in patients with SMA needs to be determined in further research.