Effect of resistive loads on pattern of respiratory muscle recruitment during exercise

High Flow Nasal Cannula compared to Continuous Positive Airway Pressure: a bench and physiological study

High-flow nasal cannula (HFNC) is extensively used for acute respiratory failure. However, questions remain regarding its physiological effects. We explored 1) whether HFNC produced similar effects to continuous positive airway pressure (CPAP); 2) possible explanations of respiratory rate changes; 3) the effects of mouth opening. Two studies were conducted: a bench study using a manikin's head with lungs connected to a breathing simulator while delivering HFNC flow rates from 0 to 60L/min; a physiological cross-over study in 10 healthy volunteers receiving HFNC (20 to 60L/min) with the mouth open or closed and CPAP 4cmH2O delivered through face-mask. Nasopharyngeal and esophageal pressures were measured; tidal volume and flow were estimated using calibrated electrical impedance tomography. In the bench study, nasopharyngeal pressure at end-expiration reached 4cmH2O with HFNC at 60L/min, while tidal volume decreased with increasing flow. In volunteers with HFNC at 60L/min, nasopharyngeal pressure reached 6.8cmH2O with mouth closed and 0.8cmH2O with mouth open; p<0.001. When increasing HFNC flow, respiratory rate decreased by lengthening expiratory time, tidal volume did not change, and effort decreased (pressure-time product of the respiratory muscles); at 40L/min, effort was equivalent between CPAP and HFNC40L/min and became lower at 60L/min (p=0.045). During HFNC with mouth closed, and not during CPAP, resistance to breathing was increased, mostly during expiration. In conclusion, mouth closure during HFNC induces a positive nasopharyngeal pressure proportional to flow rate and an increase in expiratory resistance that might explain the prolonged expiration and reduction in respiratory rate and effort, and contribute to physiological benefits.

The effect of the protective face mask on cardiorespiratory response during aerobic exercise

Protective face mask (PFM) has been widely used for safety purposes and, after the advent of the COVID-19 pandemic, its use is growing steadily, not only among healthcare personnel but also the general population. While PFM is important to preserve the wearer from contaminating agents present in the airflow, they are well known to increase the subjective perception of breathing difficulty. Although some studies demonstrated that PFM use worsens exercise tolerance, there are several studies stating that there is no such limitation with the use of PFM. Moreover, no serious adverse effects during physical exercise have been found in the literature. Physical exercise represents a significant challenge to the human body through a series of integrated changes in function that involve most of its physiologic systems. In this respect, cardiovascular and respiratory systems provide the capacity to sustain physical tasks over extended periods. Within this scenario, both convective oxygen (O₂ ) transport (product of arterial O₂ content x blood flow) to the working locomotor muscles and O₂ diffusive transport from muscle capillaries to mitochondria are of paramount importance to endurance performance. Interestingly, the effects of PFM on cardiorespiratory response during aerobic exercise depends on the type of mask and exercise (i.e., walking, running, or cycling), including decrease in ventilatory demands, arterial oxygen levels, maximal oxygen consumption, and endurance performance. The purpose of this review was to elucidate the effect of protective face mask-wearing on 1) cardiorespiratory responses during aerobic exercise and 2) endurance performance.

Sonographic assessment of changes in diaphragmatic kinetics induced by inspiratory resistive loading

BACKGROUND AND OBJECTIVE

Diaphragmatic breathing patterns under resistive loading remain poorly documented. To our knowledge, this is the first study assessing diaphragmatic motion under conditions of inspiratory resistive loading with the use of sonography.

METHODS

We assessed diaphragmatic motion during inspiratory resistive loading in 40 healthy volunteers using M-mode sonography. In phase I of the study, sonography was performed during normal quiet breathing without respiratory loading. In phase II, sonography was performed after application of a nose clip and connection of the subjects to a pneumotachograph through a mouth piece. In phase III, the participants were assessed while subjected to inspiratory resistive loading of 50 cm H(2)O/L/s.

RESULTS

Compared with baseline, the application of a mouth piece and nose clip induced a significant increase in diaphragmatic excursion (from 1.7 to 2.3 cm, P < 0.001) and a decrease in respiratory rate (from 13.4 to 12.2, P < 0.01). Inspiratory resistive loading induced a further decrease in respiratory rate (from 12.2 to 8.0, P < 0.01) and a decrease in diaphragmatic velocity contraction (from 1.2 to 0.8 cm/s, P < 0.01), and also an increase in tidal volume (from 795 to 904 mL, P < 0.01); diaphragmatic excursion, however, did not change significantly.

CONCLUSIONS

Inspiratory resistive loading induced significant changes in diaphragmatic contraction pattern, which mainly consisted of decreased velocity of diaphragmatic displacement with no change in diaphragmatic excursion. Tidal volume, increased significantly; the increase in tidal volume, along with the unchanged diaphragmatic excursion, provides sonographic evidence of increased recruitment of extradiaphragmatic muscles under inspiratory resistive loading.

Effects of inspiratory loading on the chaotic dynamics of ventilatory flow in humans

Human ventilation at rest exhibits complexity and chaos. The aim of this study was to determine whether suprapontine interferences with the automatic breathing control could contribute to ventilatory chaos. We conducted a post hoc analysis of a previous study performed in awake volunteers exhibiting cortical pre-motor potentials during inspiratory loading. In eight subjects, flow was recorded at rest, while breathing against inspiratory threshold loads (median 21.5 cm H(2)O) and resistive loads (50 cm H(2)Ol(-1)s(-1)) loads, and while inhaling 7% CO(2)-93% O(2). Chaos was identified through noise titration (noise limit, NL) and the sensitivity to initial conditions was assessed through the largest Lyapunov exponent (LLE). Breath-by-breath variability was evaluated using the coefficient of variation of several ventilatory variables. Chaos was consistently present in ventilatory flow recordings, but mechanical loading did not alter NL, LLE, or variability. In contrast, CO(2) altered chaos and reduced variability. In conclusion, inspiratory loading - and any resultant respiratory-related cortical activity - were not associated with changes in ventilatory chaos in this study, arguing against suprapontine contributions to ventilatory complexity.

Electroencephalographic evidence for pre-motor cortex activation during inspiratory loading in humans

Faced with mechanical inspiratory loading, awake animals and anaesthetized humans develop alveolar hypoventilation, whereas awake humans do defend ventilation. This points to a suprapontine compensatory mechanism instead of or in addition to the 'traditional' brainstem respiratory regulation. This study assesses the role of the cortical pre-motor representation of inspiratory muscles in this behaviour. Ten healthy subjects (age 19-34 years, three men) were studied during quiet breathing, CO2-stimulated breathing, inspiratory resistive loading, inspiratory threshold loading, and during self-paced voluntary sniffs. Pre-triggered ensemble averaging of Cz EEG epochs starting 2.5 s before the onset of inspiration was used to look for pre-motor activity. Pre-motor potentials were present during voluntary sniffs in all subjects (average latency (+/-s.d.): 1325 +/- 521 ms), but also during inspiratory threshold loading (1427 +/- 537 ms) and during inspiratory resistive loading (1109 +/- 465 ms). Pre-motor potentials were systematically followed by motor potentials during inspiratory loading. Pre-motor potentials were lacking during quiet breathing (except in one case) and during CO2-stimulated breathing (except in two cases). The same pattern was observed during repeated experiments at an interval of several weeks in a subset of three subjects. The behavioural component of inspiratory loading compensation in awake humans could thus depend on higher cortical motor areas. Demonstrating a similar role of the cerebral cortex in the compensation of disease-related inspiratory loads (e.g. asthma attacks) would have important pathophysiological implications: it could for example contribute to explain why sleep is both altered and deleterious in such situations.

Inspiratory flow rates during hard work when breathing through different respirator inhalation and exhalation resistances

There has been a long-standing debate regarding the adequacy of airflow rates used in respirator certification testing and whether these test flow rates underestimate actual values. This study investigated breath by breath inspiratory peak flow rate, minute ventilation, and instantaneous flow rates of eight young, healthy volunteers walking on a treadmill at 80-85% of maximal aerobic capacity until exhaustion while wearing an air-purifying respirator with one of eight combinations of inhalation and exhalation resistance. An analysis of variance was performed to identify differences among the eight conditions. Scheffe's post hoc analysis indicated which means differed. The group of conditions with the highest average value for each parameter was identified and considered to represent a worst-case scenario. Data was reported for these conditions. A Gaussian distribution was fit to the data and the 99.9% probability levels determined. The 99.9% probability level for the peak and instantaneous flow rates were 374 L/min and 336 L/min, respectively. The minute ventilation distribution was not Gaussian. Less than 1% of the recorded minute ventilations exceeded 135 L/min. Instantaneous flow rates exceeded the National Institute for Occupational Safety and Health's respirator test standards of 64, 85, and 100 L/min constant flow 91%, 87%, and 82% of the time, respectively. The recorded minute ventilations exceeded the 40 L/min minute ventilation test standard (for tests with a sinusoidal flow pattern) 100% of the time. This study showed that young, healthy respirator wearers generated peak flow rates, minute ventilations, and instantaneous flow rates that consistently exceeded current test standards. Their flow rates should be higher than those of a respirator wearer performing occupational work and could be considered upper limits. Testing respirators and respirator cartridges using a sinusoidal breathing pattern with a minute ventilation of 135 L/min (peak flow rate approximately 424 L/min) would encompass 99% of the recorded minute ventilations and 99.9% of the predicted peak and instantaneous flow rates from this study and would more accurately reflect human respiration during strenuous exercise.

Inspiratory resistances facilitate the diaphragm response to transcranial stimulation in humans

Background

Breathing in humans is dually controlled for metabolic (brainstem commands) and behavioral purposes (suprapontine commands) with reciprocal modulation through spinal integration. Whereas the ventilatory response to chemical stimuli arises from the brainstem, the compensation of mechanical loads in awake humans is thought to involve suprapontine mechanisms. The aim of this study was to test this hypothesis by examining the effects of inspiratory resistive loading on the response of the diaphragm to transcranial magnetic stimulation.

Results

Six healthy volunteers breathed room air without load (R0) and then against inspiratory resistances (5 and 20 cmH₂O/L/s, R5 and R20). Ventilatory variables were recorded. Transcranial magnetic stimulation (TMS) was performed during early inspiration (I) or late expiration (E), giving rise to motor evoked potentials (MEPs) in the diaphragm (Di) and abductor pollicis brevis (APB). Breathing frequency significantly decreased during R20 without any other change. Resistive breathing had no effect on the amplitude of Di MEPs, but shortened their latency (R20: -0.903 ms, p = 0.03) when TMS was superimposed on inspiration. There was no change in APB MEPs.

Conclusion

Inspiratory resistive breathing facilitates the diaphragm response to TMS while it does not increase the automatic drive to breathe. We interpret these findings as a neurophysiological substratum of the suprapontine nature of inspiratory load compensation in awake humans.

An additive interaction between different qualities of dyspnea produced in normal human subjects

We evaluated the sensation of dyspnea induced by hypercapnia alone and a combination of hypercapnia and flow-resistive loading by the use of visual analogue scale (VAS) and the use of 13 listed descriptors in 23 healthy subjects. Hypercapnia alone caused a modest degree of dyspnea characterized by both air hunger and work/effort sensations. An addition of inspiratory flow-resistive loading (IRL) caused an increase in inspiratory difficulty and some attenuation of 'work/effort.' The addition of expiratory flow-resistive loading (ERL) caused an increase in expiratory difficulty and attenuation of 'air hunger.' The addition of both IRL and ERL caused a marked increase in dyspnea, the amount of which was close to the sum of the increases obtained individually by IRL and by ERL, while the quality of dyspnea was characterized predominantly by work/effort. These results suggest that despite the difference in quality of sensations, the intensity of dyspnea would sum linearly when the two kinds of loads are presented at the same time.

Performance when breathing through different respirator inhalation and exhalation resistances during hard work

Respirator inspiratory and expiratory breathing resistances impact ventilation and performance when studied independently. However, it is less clear as to how various combinations of inhalation and exhalation resistance affect user performance. The present study investigated the performance of 11 individuals during constant load, demanding work to exhaustion while wearing respirators with eight different combinations of inhalation and exhalation resistance. Exercise performance time, performance rating, minute volume, and peak inspiratory and expiratory airflow were recorded at the end of each test trial, and independent correlations with inhalation resistance and exhalation resistance were assessed. The combined impacts of respirator inhalation and exhalation resistances were quantified as the total external work of breathing (WOB(tot)) and correlations between the test variables and WOB(tot) were also examined. Significantly linear decreases in performance were found with increased inhalation resistances independent of exhalation resistance (R(2) = 0.99; p < 0.001) and with increased WOB(tot) (R(2) = 0.92; p < 0.001). Performance also decreased with increased exhalation resistance but no significant relationships were found. Minute volume decreased linearly with increased inhalation resistance independent of exhalation resistance (R(2) = 0.99; p < 0.001), but the linear decrease observed between minute volume and WOB(tot) was weak (R(2) = 0.36; p < 0.05). These findings suggest that WOB(tot) serves as a reliable estimate of the combined impacts of respirator inhalation and exhalation resistances on user performance during hard work, but that inhalation resistance alone serves as a better predictor of ventilation during respirator wear.

Effect of inspiratory threshold loading on ventilatory kinetics during constant-load exercise

Humoral factors play an important role in the control of exercise hyperpnea. The role of neuromechanical ventilatory factors, however, is still being investigated. We tested the hypothesis that the afferents of the thoracopulmonary system, and consequently of the neuromechanical ventilatory loop, have an influence on the kinetics of oxygen consumption (VO2), carbon dioxide output (VCO2), and ventilation (VE) during moderate intensity exercise. We did this by comparing the ventilatory time constants (tau) of exercise with and without an inspiratory load. Fourteen healthy, trained men (age 22.6 +/- 3.2 yr) performed a continuous incremental cycle exercise test to determine maximal oxygen uptake (VO2max = 55.2 +/- 5.8 ml x min(-1) x kg(-1)). On another day, after unloaded warm-up they performed randomized constant-load tests at 40% of their VO2max for 8 min, one with and the other without an inspiratory threshold load of 15 cmH2O. Ventilatory variables were obtained breath by breath. Phase 2 ventilatory kinetics (VO2, VCO2, and VE) could be described in all cases by a monoexponential function. The bootstrap method revealed small coefficients of variation for the model parameters, indicating an accurate determination for all parameters. Paired Student's t-tests showed that the addition of the inspiratory resistance significantly increased the tau during phase 2 of VO2 (43.1 +/- 8.6 vs. 60.9 +/- 14.1 s; P < 0.001), VCO2 (60.3 +/- 17.6 vs. 84.5 +/- 18.1 s; P < 0.001) and VE (59.4 +/- 16.1 vs. 85.9 +/- 17.1 s; P < 0.001). The average rise in tau was 41.3% for VO2, 40.1% for VCO2, and 44.6% for VE. The tau changes indicated that neuromechanical ventilatory factors play a role in the ventilatory response to moderate exercise.

Accuracy of noninvasive estimates of respiratory muscle effort during spontaneous breathing in restrictive diseases

Mean inspiratory pressure (Pi), estimated from the occlusion pressure at the mouth and the inspiratory time, is useful as a noninvasive estimate of respiratory muscle effort during spontaneous breathing in normal subjects and patients with chronic obstructive pulmonary disease. The aim of this study was to compare the Pi with respect to mean esophageal pressure (Pes) in patients with restrictive disorders. Eleven healthy volunteers, 12 patients with chest wall disease, 14 patients with usual interstitial pneumonia, and 17 patients with neuromuscular diseases were studied. Pi, Pes, and mean transdiaphragmatic pressure were simultaneously measured. Tension-time indexes of diaphragm (TTdi) and inspiratory muscles (TTmu) were also determined. In neuromuscular patients, significant correlations were found between Pi and Pes, Pi and transdiaphragmatic pressure, and TTmu and TTdi. A moderate agreement between Pi and Pes and between TTmu and TTdi was found. No significant correlation between these parameters was found in the other patient groups. These findings suggest that Pi is a good surrogate for the invasive measurement of respiratory muscle effort during spontaneous breathing in neuromuscular patients.

Associative conditioning of the exercise ventilatory response in humans

Repeated hypercapnic exercise augmented the ventilatory response to subsequent trials of exercise alone in running goats and in humans performing arm exercise, suggesting a form of associative conditioning or 'long-term modulation' had taken place. These studies did not include 'control' single stimulus conditioning paradigms. This study demonstrated that ten repeated trials of familiar leg bicycling exercise with dead-space induced hypercapnia also elicited similar significant increases in inspired ventilation (+ 22%; P < 0.009) and tidal volume (VT; + 255 +/- 73 ml(BTPS); mean +/- S.E.M.; P = 0.004) within the first 20 sec of subsequent exercise only trials. Long-term modulation of the early ventilatory response to cycling was not fully replicated by ten trials of 'control' paradigms involving either repeated exercise alone or resting dead space alone. This study thus demonstrated that long term modulation of the early ventilatory response exercise was due to an explicit effect of associative conditioning and not simply sensitisation to repeated trials of a single stimulus.

Effect of expiratory resistive loading on the noninvasive tension-time index in COPD

Expiratory resistive loading (ERL) is used by chronic obstructive pulmonary disease (COPD) patients to improve respiratory function. We, therefore, used a noninvasive tension-time index of the inspiratory muscles (TT(mus) = I/PI(max) x TI/TT, where I is mean inspiratory pressure estimated from the mouth occlusion pressure, PI(max) is maximal inspiratory pressure, TI is inspiratory time, and TT is total respiratory cycle time) to better define the effect of ERL on COPD patients. To accomplish this, we measured airway pressures, mouth occlusion pressure, respiratory cycle flow rates, and functional residual capacity (FRC) in 14 COPD patients and 10 normal subjects with and without the application of ERL. TT(mus) was then calculated and found to drop in both COPD and normal subjects (P<0.05). The decline in TT(mus) in both groups resulted solely from a prolongation of expiratory time with ERL (P<0.001 for COPD, P<0.05 for normal subjects). In contrast to the COPD patients, normal subjects had an elevation in I and FRC, thus minimizing the decline in TT(mus). In conclusion, ERL reduces the potential for inspiratory muscle fatigue in COPD by reducing TI/TT without affecting FRC and I.

Non-invasive quantification of diaphragm kinetics using m-mode sonography

PURPOSE

The standard conditions of spirometry (i.e., wearing a noseclip and breathing through a mouthpiece and a pneumotachograph) are likely to alter the ventilatory pattern. We used "time motion" mode (M-mode) sonography to assess the changes in diaphragm kinetics induced by spirometry during quiet breathing.

METHODS

An M-mode sonographic study of the right diaphragm was performed before and during standard spirometry in eight patients without respiratory disease (age 34 to 68 yr).

RESULTS

During spirometry, the diaphragm inspiratory amplitude (DIA) increased from 1.34 +/- 0.18 cm to 1.80 +/- 0.18 cm (P = 0.007), whereas the diaphragmatic inspiratory (T1 diaph) increased from 1.27 +/- 0.15 to 1.53 +/- 0.23 sec, (P = 0.015, without change in diaphragmatic total time interval (Ttot diaph). Therefore, the diaphragm duty cycle (T1 diaph/Ttot diaph) increased from 38% +/- 1% to 44% +/- 4% (P = 0.023). The diaphragm inspiratory (DIV) and expiratory (DEV) motion velocity (P = 0.007).

CONCLUSION

M-mode sonography enabled us to demonstrate that the wearing of a nose clip and breathing through a mouthpiece and a pneumotachograph induce measurable changes in diaphragm kinetics.

Breathing when chest wall muscle are tonically contracted for isometric, non-respiratory tasks

We have previously shown that regional chest wall impedance increases when the chest wall muscles are tonically contracted to perform isometric, non-respiratory tasks. To test how this affects breathing, we measured respiratory frequency, tidal volume, end-tidal PCO2, electromyographic activity (EMG) at four points on the chest wall surface, and regional displacements across six planes of the chest wall during maintenance of three different postures that necessitated strong tonic respiratory muscle contraction. These postures included a static push-up, a bilateral leg-lift and a partial sit-up. The subjects (n = 8) were able to maintain the postures for 1.5-2.5 min, and strong tonic EMG activity was observed in each posture at all points measured. The rate and depth of breathing and pattern of regional chest wall displacements were variable within the group of subjects and among the three postures. However, minute ventilation increased and end-tidal PCO2 decreased in each subject during each posture (P < 0.05). In six of the eight subjects, transdiaphragmatic pressure (Pdi) was measured during 1 min of the same exercises. The ratio of the breathing fluctuation in Pdi to tidal volume was at least twice as high compared with rest, except for two subjects during the leg-lifts. We conclude that strong tonic contraction of the chest wall muscles impedes, but does not limit, breathing, and that there is no single breathing strategy used during such conditions.

Lack of importance of respiratory muscle load in ventilatory regulation during heavy exercise in humans

1. Seven active subjects (24 +/- 1 years; maximal oxygen uptake (VO2,max), 3.77 +/- 0.2 l min-1; mean +/- S.E.M.) performed constant work rate heavy exercise (CWHE, approximately 80% of maximal incremental work rate) to exhaustion on 2 days, one with (unload) and one without (control) respiratory muscle unloading. 2. With unloading, a special device applied flow-proportional mouth pressure assist (positive with inspiratory (I), negative with expiratory (E) flows) throughout each breath. No pressure assist occurred during control CWHE. To confirm unloading, respiratory muscle pressures (Pmus) were derived (n = 5) from measured pleural pressure and chest wall elastic and resistive pressures. 3. Other than minor differences in early exercise, the temporal course of minute ventilation (VE) was similar in both tests as exercise progressed. The fall in estimated mean alveolar CO2 (PA,CO2) throughout CWHE was identical in both tests. There were no significant differences (ANOVA) in VE, tidal volume, frequency, oxygen consumption rate (VO2), heart rate or PA,CO2, between unload and control CWHE, at matched times (at 50% of control duration and at the end of exercise). Unloading reduced Pmus significantly throughout CWHE; at 50% control duration, peak Pmus,I and Pmus,E fell by 24 and 41%, respectively, with unloading, as did mean Pmus,I and Pmus,E (21 and 44%). 4. The lack of any significant changes in VE, PA,CO2 or breathing pattern, despite a marked reduction in respiratory muscle load throughout CWHE, indicates that the load on the respiratory muscles has only a minor role in the regulation of ventilation during heavy exercise. 5. The absence of improvement in CWHE duration (control, 11.4 +/- 1.2 min; unload, 12.6 +/- 2.1 min, n.s.) with unloading implies that respiratory muscle function does not limit endurance exercise performance during cycling in healthy humans.

Dyspnea in a patient years after severe poliomyelitis. The role of cardiopulmonary exercise testing

Dyspnea after polio can occur for a variety of reasons, including neuromuscular disease and upper airway abnormalities resulting from prolonged intubation, including tracheal stenosis, tracheomalacia, and vocal cord paralysis. Routine studies such as spirometry and maximum voluntary ventilation (MVV) measurements can give similar results in these conditions. We present a 50-year-old woman who as a child developed poliomyelitis that required tracheostomy and negative pressure ventilation. Thirty-nine years later, she developed breathlessness with normal spirometry but decreased MVV. The flow volume loop showed flattening of the inspiratory and expiratory limbs, consistent with a fixed upper airway obstruction or neuromuscular weakness. Exercise testing with measurement of exercise flow volume loops and respiratory pressures was performed. The patient was ventilatory limited with increasing end-expiratory lung volume through exercise. Flow volume loops confirmed flow limitation. Respiratory pressures did not change after maximal exercise. Further evaluation confirmed left vocal cord paralysis and tracheomalacia. This patient demonstrates that the causes of dyspnea after poliomyelitis can be multifactorial, and that routine evaluation may fail to elucidate the limiting factor. In this case, exercise testing provided valuable insight into the limiting factor for this patient and provided useful data for counseling and for further management.