Physiological Factors Associated with Unsatisfied Inspiration at Peak Exercise in Healthy Adults.

INTRODUCTION: Contrary to common belief, a growing body of evidence suggests that unsatisfied inspiration (UI), an inherently uncomfortable quality of dyspnea, is experienced by ostensibly healthy adults during high-intensity exercise. Based on our understanding of the mechanisms of UI among people with chronic respiratory conditions, this analysis tested the hypothesis that the experience of UI at peak exercise in young, healthy adults reflects the combination of high ventilatory demand and critical inspiratory constraints. METHODS: In a retrospective analysis design, data included 321 healthy individuals (129 females) aged 25 ± 5 yr. Data were collected during one visit to the laboratory, which included anthropometrics, spirometry, and an incremental cardiopulmonary cycling test to exhaustion. Metabolic and cardiorespiratory variables were measured at peak exercise, and qualitative descriptors of dyspnea at peak exercise were assessed using a list of 15 descriptor phrases. RESULTS: Thirty-four percent of participants ( n = 109) reported sensations of UI at peak exercise. Compared with the non-UI group, the UI group achieved a significantly higher peak work rate (243 ± 77 vs 235 ± 69 W, P = 0.016, d = 0.10), rate of O 2 consumption (3.32 ± 1.02 vs 3.27 ± 0.96 L·min -1 , P = 0.018, d = 0.05), minute ventilation (120 ± 38 vs 116 ± 35 L·min -1 , P = 0.047, d = 0.11), and breathing frequency (50 ± 9 vs 47 ± 9 breaths per minute, P = 0.014, d = 0.33), while having a lower exercise-induced change (peak-baseline) in inspiratory capacity (0.07 ± 0.41 vs 0.20 ± 0.49 L, P = 0.023, d = 0.29). The inspiratory reserve volume to minute ventilation ratio at peak exercise was also lower in the UI versus non-UI group. Dyspnea intensity and unpleasantness ratings were significantly higher in the UI versus non-UI group at peak exercise (both P < 0.001). CONCLUSIONS: Healthy individuals reporting UI at peak exercise have relatively greater inspiratory constraints compared with those who do not select UI.

Inspiratory response and side-effects to rapid bilateral magnetic phrenic nerve stimulation using differently shaped coils: implications for stimulation-assisted mechanical ventilation.

BACKGROUND: Rapid magnetic stimulation (RMS) of the phrenic nerves may serve to attenuate diaphragm atrophy during mechanical ventilation. With different coil shapes and stimulation location, inspiratory responses and side-effects may differ. This study aimed to compare the inspiratory and sensory responses of three different RMS-coils either used bilaterally on the neck or on the chest, and to determine if ventilation over 10 min can be achieved without muscle fatigue and coils overheating. METHODS: Healthy participants underwent bilateral anterior 1-s RMS on the neck (RMSBAMPS) (N = 14) with three different pairs of magnetic coils (parabolic, D-shape, butterfly) at 15, 20, 25 and 30 Hz stimulator-frequency and 20% stimulator-output with + 10% increments. The D-shape coil with individual optimal stimulation settings was then used to ventilate participants (N = 11) for up to 10 min. Anterior RMS on the chest (RMSaMS) (N = 8) was conducted on an optional visit. Airflow was assessed via pneumotach and transdiaphragmatic pressure via oesophageal and gastric balloon catheters. Perception of air hunger, pain, discomfort and paresthesia were measured with a numerical scale. RESULTS: Inspiration was induced via RMSBAMPS in 86% of participants with all coils and via RMSaMS in only one participant with the parabolic coil. All coils produced similar inspiratory and sensory responses during RMSBAMPS with the butterfly coil needing higher stimulator-output, which resulted in significantly larger discomfort ratings at maximal inspiratory responses. Ten of 11 participants achieved 10 min of ventilation without decreases in minute ventilation (15.7 ± 4.6 L/min). CONCLUSIONS: RMSBAMPS was more effective than RMSaMS, and could temporarily ventilate humans seemingly without development of muscular fatigue. Trial registration This study was registered on clinicaltrials.gov (NCT04176744).