Inspiratory Loading Research Articles

FNIRS analysis of rostral prefrontal cortex activity and perception of inspiratory loads

It is well-established that the brainstem is responsible for the automatic control of breathing, however, cortical areas control perception and conscious breathing. This study investigated activity in the prefrontal cortex (PFC) during breathing difficulty using functional near-infrared spectroscopy (fNIRS). It was hypothesized that extrinsic inspiratory loads will elicit regional changes in PFC activity and increased perception ratings, as a function of load magnitude and type. Participants were exposed to varying magnitudes of resistive (R) and pressure threshold (PT) inspiratory loads to increase breathing effort. Perception ratings of breathing effort and load magnitude were positively correlated (p < 0.05). PT loads were rated more effortful than R loads (p < 0.05). Differences in perceived effort were a function of inspiratory pressure-time-product (PTP) and inspiratory work of breathing (WoB). PFC activity increased with the largest PT load (p < 0.01), suggesting that the PFC is involved in processing respiratory stimuli. The results support the hypothesis that the PFC is an element of the neural network mediating effortful breathing perception.

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

Thoracoabdominal asynchrony in a virtual preterm infant: computational modeling and analysis.

Thoracoabdominal asynchrony (TAA), the asynchronous volume changes between the rib cage and abdomen during breathing, is associated with respiratory distress, progressive lung volume loss, and chronic lung disease in the newborn infant. Preterm infants are prone to TAA risk factors such as weak intercostal muscles, surfactant deficiency, and a flaccid chest wall. The causes of TAA in this fragile population are not fully understood and, to date, the assessment of TAA has not included a mechanistic modeling framework to explore the role these risk factors play in breathing dynamics and how TAA can be resolved. We present a dynamic compartmental model of pulmonary mechanics that simulates TAA in the preterm infant under various adverse clinical conditions, including high chest wall compliance, applied inspiratory resistive loads, bronchopulmonary dysplasia, anesthesia-induced intercostal muscle deactivation, weakened costal diaphragm, impaired lung compliance, and upper airway obstruction. Sensitivity analyses performed to screen and rank model parameter influence on model TAA and respiratory volume outputs show that risk factors are additive so that maximal TAA occurs in a virtual preterm infant with multiple adverse conditions, and addressing risk factors individually causes incremental changes in TAA. An abruptly obstructed upper airway caused immediate nearly paradoxical breathing and tidal volume reduction despite greater effort. In most simulations, increased TAA occurred together with decreased tidal volume. Simulated indices of TAA are consistent with published experimental studies and clinically observed pathophysiology, motivating further investigation into the use of computational modeling for assessing and managing TAA.NEW & NOTEWORTHY A novel model of thoracoabdominal asynchrony incorporates literature-derived mechanics and simulates the impact of risk factors on a virtual preterm infant. Sensitivity analyses were performed to determine the influence of model parameters on TAA and respiratory volume. Predicted phase angles are consistent with prior experimental and clinical results, and influential parameters are associated with clinical scenarios that significantly alter phase angle, motivating further investigation into the use of computational modeling for assessing and managing thoracoabdominal asynchrony.

Differential effects of repeated inspiratory and limb muscle loading on effort perception in patients with obstructive sleep apnea and healthy males.

Obstructive sleep apnea (OSA) is characterized by collapse of the upper airways during sleep. The contribution of alterations in effort perception is not understood. This study investigated the response of inspiratory and quadriceps muscles to repetitive loading on effort perception in OSA patients, pre and post continuous positive airway pressure (CPAP) treatment, and in healthy individuals. Twenty-one OSA patients and 40 healthy participants completed protocols for repetitive inspiratory and leg muscle loading combined with intermittent rating of perceived exertion (RPE 14-somewhat hard/hard) to assess effort sensitivity. Electromyography, inspiratory pressure and isometric force were measured. OSA patients reported higher fatiguability of respiratory and leg muscles than controls. OSA patients revealed lower effort sensitivity in the leg muscles compared with controls, while repetitive loading led to a decline in force production. In the respiratory system, OSA patients revealed similar effort sensitivity at baseline compared with controls, but a large reduction in effort sensitivity after loading. Baseline effort sensitivity was correlated with apnea-hypopnea index (AHI). After CPAP treatment, OSA patients revealed a decreased baseline effort sensitivity with a missing loading response. Effort sensitivity was differentially affected in the respiratory and leg systems with outcomes of CPAP treatment suggesting a full reversibility. Outcomes suggest that reversible adaptive response of effort perception in the respiratory system might contribute to the severity of OSA.

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.

Effects and Usefulness of Inspiratory Muscle Training Load in Patients with Advanced Lung Cancer with Dyspnea.

Patients with advanced lung cancer tend to experience dyspnea. Pulmonary rehabilitation has been reported as a method for relieving dyspnea. However, exercise therapy imposes a high burden on patients, and it is difficult to sustain in many cases. Inspiratory muscle training (IMT) imposes a relatively low burden on patients with advanced lung cancer; however, its benefits have not been demonstrated. We retrospectively analyzed 71 patients who were hospitalized for medical treatment. The participants were divided into an exercise therapy group and an IMT load + exercise therapy group. Changes in maximal inspiratory pressure (MIP) and dyspnea were examined using a two-way repeated measures analysis of variance. MIP variations significantly increase in the IMT load group, with significant differences between baseline and week 1, between week 1 and week 2, and between baseline and week 2. The analysis also showed that the variations in dyspnea decreased in the IMT load + exercise therapy group with significant differences between baseline and week 1 and between baseline and week 2. The results show that IMT is useful and has a high persistence rate in patients with advanced lung cancer who present dyspnea and cannot perform high-intensity exercise therapy.

The Relationship Between Esophageal Pressure and Diaphragm Thickening Fraction in Spontaneously Breathing Sedated Children: A Feasibility Study.

Diaphragm ultrasound is a novel alternative to esophageal pressure measurements in the evaluation of diaphragm function and activity, but data about its reliability in a pediatric setting are lacking. We aimed to compare the esophageal pressure swing (∆Pes, gold standard) with the diaphragmatic thickening fraction (DTF) as a measure of inspiratory effort in sedated children. Additionally, we studied the effect of positive end-expiratory pressure (PEEP) on the end-expiratory thickness of the diaphragm (DTee). Prospective open-label non-randomized interventional physiological cohort study. Operating room in tertiary academic hospital. Children 28 days to 13 years old scheduled for elective surgery with general anesthesia, spontaneously breathing through a laryngeal mask airway, were eligible for inclusion. Exclusion criteria were disorders or previous surgery of the diaphragm, anticipated difficult airway or acute cardiopulmonary disease. All measurements were performed prior to surgery. Patients were subjected to different levels of respiratory load, PEEP and anesthetic depth in a total of seven respiratory conditions. The esophageal pressure and diaphragm thickening fraction were simultaneously recorded for five breaths at each respiratory condition. The relation between ∆Pes and DTF was studied in a mixed model. We analyzed 407 breaths in 13 patients. Both DTF (p = 0.03) and ∆Pes (p = 0.002) could detect respiratory activity, and ∆Pes and DTF were associated across respiratory conditions (p < 0.001; R2 = 31%). With increasing inspiratory load, ∆Pes increased significantly, while DTF did not (p = 0.08). Additionally, DTee did not differ significantly between 10, 5, and 0 cm H2O PEEP (p = 0.08). In spontaneously breathing sedated children and across different respiratory conditions, DTF could differentiate minimal or no inspiratory effort from substantial inspiratory effort and was associated with ∆Pes. Increased efforts resulted in higher ∆Pes but not larger DTF.

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.

Effects of Inspiratory Muscle Warm-Up on Physical Exercise: A Systematic Review

This study aimed to systematically review the literature to examine the effects of inspiratory-muscle warm-up (IMW) on the inspiratory, metabolic, respiratory and performance parameters of a main exercise performed by athletes and healthy and active individuals. Methods: This systematic review included randomized studies in English based on the criteria of the PICOS model. The exclusion criteria adopted were studies that applied inspiratory exercise to: i. promote long-term adaptations through inspiratory training (chronic responses); ii. obtain acute responses to inspiratory load (overload) during and in breaks from physical effort and in an inspiratory-exercise session (acute training effect); iii. evaluate the effects of IMW on participants with cardiorespiratory and/or metabolic disease. Data Sources: PubMed, Embase, MedLine, Scopus, SPORTDiscus and Google Scholar (until 17 January 2023). Results: Thirty-one studies were selected. The performance and respiratory parameters were the most investigated (77% and 74%, respectively). Positive effects of IMW were reported by 88% of the studies that investigated inspiratory parameters and 45% of those that evaluated performance parameters. Conclusions: The analyzed protocols mainly had positive effects on the inspiratory and performance parameters of the physical exercises. These positive effects of IMW are possibly associated with the contractile and biochemical properties of inspiratory muscles.

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.

The use of time-of-flight camera to assess respiratory rates and thoracoabdominal depths in patients with chronic respiratory disease.

Over the last 5 years, the analysis of respiratory patterns presents a growing usage in clinical and research purposes, but there is still currently a lack of easy-to-use and affordable devices to perform such kind of evaluation. The aim of this study is to validate a new specifically developed method, based on Kinect sensor, to assess respiratory patterns against spirometry under various conditions. One hundred and one participants took parts in one of the three validations studies. Twenty-five chronic respiratory disease patients (14 with chronic obstructive pulmonary disease (COPD) [65 ± 10 years old, FEV1 = 37 (15% predicted value), VC = 62 (20% predicted value)], and 11 with lung fibrosis (LF) [64 ± 14 years old, FEV1 = 55 (19% predicted value), VC = 62 (20% predicted value)]) and 76 healthy controls (HC) were recruited. The correlations between the signal of the Kinect (depth and respiratory rate) and the spirometer (tidal volume and respiratory rate) were computed in part 1. We then included 66 HC to test the ability of the system to detect modifications of respiratory patterns induced by various conditions known to modify respiratory pattern (cognitive load, inspiratory load and combination) in parts 2 and 3. There is a strong correlation between the depth recorded by the Kinect and the tidal volume recorded by the spirometer: r = 0.973 for COPD patients, r = 0.989 for LF patients and r = 0.984 for HC. The Kinect is able to detect changes in breathing patterns induced by different respiratory disturbance conditions, gender and oral task. Measurements performed with the Kinect sensors are highly correlated with the spirometer in HC and patients with COPD and LF. Kinect is also able to assess respiratory patterns under various loads and disturbances. This method is affordable, easy to use, fully automated and could be used in the current clinical context. Respiratory patterns are important to assess in daily clinics. However, there is currently no affordable and easy-to-use tool to evaluate these parameters in clinics. We validated a new system to assess respiratory patterns using the Kinect sensor in patients with chronic respiratory diseases.

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.

Pediatric Simulation of Intrinsic PEEP and Patient-Ventilator Trigger Asynchrony During Mechanical Ventilation.

Intrinsic PEEP during mechanical ventilation occurs when there is insufficient time for expiration to functional residual capacity before the next inspiration, resulting in air trapping. Increased expiratory resistance (RE), too rapid of a patient or ventilator breathing rate, or a longer inspiratory to expiratory time ratio (TI/TE) can all be causes of intrinsic PEEP. Intrinsic PEEP can result in increased work of breathing and patient-ventilator asynchrony (PVA) during patient-triggered breaths. We hypothesized that the difference between intrinsic PEEP and ventilator PEEP acts as an inspiratory load resulting in trigger asynchrony that needs to be overcome by increased respiratory muscle pressure (Pmus). Using a Servo lung model (ASL 5000) and LTV 1200 ventilator in pressure control mode, we developed a passive model demonstrating how elevated RE increases intrinsic PEEP above ventilator PEEP. We also developed an active model investigating the effects of RE and intrinsic PEEP on trigger asynchrony (expressed as percentage of patient-initiated breaths that failed to trigger). We then studied if trigger asynchrony could be reduced by increased Pmus. Intrinsic PEEP increased significantly with increasing RE (r = 0.97, P = .006). Multivariate logistic regression analysis showed that both RE and negative Pmus levels affect trigger asynchrony (P < .001). A passive lung model describes the development of increasing intrinsic PEEP with increasing RE at a given ventilator breathing rate. An active lung model shows how this can lead to trigger asynchrony since the Pmus needed to trigger a breath is greater with increased RE, as the inspiratory muscles must overcome intrinsic PEEP. This model will lend itself to the study of intrinsic PEEP engendered by a higher ventilator breathing rate, as well as higher TI/TE, and will be useful in ventilator simulation scenarios of PVA. The model also suggests that increasing ventilator PEEP to match intrinsic PEEP can improve trigger asynchrony through a reduction in RE.

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.

The detection and sensory perception of inspiratory resistive loads in people with chronic tetraplegia.

This study investigated sensations of breathing following tetraplegia. Fifteen people with chronic tetraplegia and fifteen healthy able-bodied controls matched for age, sex, height, and weight participated. Sensations of breathing were quantified by determining the threshold for detecting an added resistance during inspiration. In a separate task, the perceived magnitudes of six suprathreshold resistive loads were determined with a modified Borg scale. The detection threshold of 0.34 cmH2O/L/s [standard deviation (SD) 0.14] in the tetraplegia group was higher than the 0.23 cmH2O/L/s (SD 0.10) threshold for able-bodied controls (P = 0.004). Both participant groups perceived larger loads to be more effortful, with the Borg effort rating increasing linearly with the peak inspiratory pressure generated at each load. The relationship between Borg effort rating and peak inspiratory pressure was steeper in participants with tetraplegia than in able-bodied controls (P = 0.001), but there was no difference when pressure was divided by maximal inspiratory pressure (P = 0.95). Despite a higher detection threshold, the findings suggest that the perceived magnitude of a suprathreshold inspiratory load is not impaired in chronic tetraplegia and that load magnitude perception is related to the maximal, and not absolute, inspiratory muscle force.NEW & NOTEWORTHY Sensations of breathing are thought to be impaired following chronic tetraplegia. The detection threshold for an added resistive load during inspiration was higher in people with tetraplegia than in healthy able-bodied participants. However, for inspiratory loads above the detection threshold, the perceived magnitude of a resistive load as a function of the peak inspiratory pressure was greater in tetraplegia. Load magnitude perception was comparable between participant groups when peak pressure was divided by maximal inspiratory pressure.

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.