Influences Of Extradiaphragmatic Muscle Recruitment On The Inspiratory Muscle Metaboreflex

The inspiratory muscle metaboreflex is activated during fatiguing diaphragmatic contractions when metabolites activate type III and IV afferents, increasing sympathetic nerve activity, mean arterial pressure (MAP), and heart rate (HR). The extent to which extradiaphragmatic inspiratory muscle effort augments the cardiovascular response to inspiratory loading is unknown. PURPOSE: To determine if the inspiratory muscle metaboreflex is augmented by increasing extradiaphragmatic inspiratory muscle effort during targeted diaphragmatic loading. METHODS: Sixteen healthy participants (8 female), age 32 ± 8 years (mean ± SD), completed two, 5-minute inspiratory threshold loading tasks in random order. In each task, participants targeted the same transdiaphragmatic pressure (PDI) of 65 cmH2O, breathing rate of 15 breaths/min, and duty cycle of 0.7. In one task, absolute mouth pressure (PM) was low, 30 cmH2O, while in another task, absolute mouth pressure was high, 50 cmH2O, requiring increased extradiaphragmatic inspiratory muscle effort. Metaboreflex sensitivity was determined as the change in MAP and HR from baseline during the last two minutes of loading. Data were compared across tasks and loads using a mixed effects linear model, reported as means and 95% confidence intervals. RESULTS: The absolute cumulative diaphragmatic work (8484 [7064, 9903] vs. 8920 [7501, 10340] cmH2O s) was not different between tasks (P = 0.31) or sexes (P = 0.62). The peak inspiratory PDI was not different between tasks (64 [57,70] vs. 67 [60,73] cmH2O, P = 0.38) but peak inspiratory PM was greater when extradiaphragmatic inspiratory muscle effort was increased (+20 [19,23] cmH2O, P < 0.0001). The change in MAP (+12 [6,17] vs. +10 [5,15] mmHg, P = 0.465) and HR (+13 [8, 18] vs +16 [11, 21] beats/min, P = 0.095) did not differ between tasks when extradiaphragmatic inspiratory muscle effort was increased, and was not different between sexes (P > 0.56). CONCLUSION: Extradiaphragmatic inspiratory muscle effort did not augment the inspiratory muscle metaboreflex when diaphragmatic effort was constant, suggesting that type III or IV afferents originating from the diaphragm are key determinants of the inspiratory muscle metaboreflex. Supported by: NSERC, BC Lung Foundation, HBI RHISE and UCalgary Transdisciplinary Connector Grants.