P245 Acute thoracoabdominal and central haemodynamic responses to inspiratory muscle loading in healthy young adults

Introduction Inspiratory muscle training (IMT) has been shown to improve inspiratory muscle strength and exercise tolerance in healthy and diseased populations, however the acute physiological effects of short bouts of tapered flow resistive loading (TFRL) remain unclear. We investigated the acute responses of TFRL at low, moderate, and high IMT intensities and aimed to determine an optimal training load.

Methods Twelve healthy adults (26±3 years) performed 3 loaded trials (at 30, 50 and 70% maximal inspiratory pressure; PImax) applied in a balanced ordered sequence and lasting 3 minutes each. Thoracoabdominal volumes (captured by Optoelectronic Plethysmography), cardiac output (recorded by Cardio-impedance), gas exchange, and dyspnoea scores were assessed throughout.

Results Inspiratory loading induced significant increases in thoracoabdominal tidal volumes compared to QB (0.69±0.06 L): by 2.71±0.30 L at 30% PImax (p=0.003); 3.01±0.27 L at 50% PImax (p=0.002); and 3.02±0.27 L at 70% PImax (p=0.002). Increased end-inspiratory rib cage volume and decreased end-expiratory abdominal volume contributed to the expansion of thoracoabdominal tidal volumes. A significant difference in thoracoabdominal tidal volumes was observed between 30 and 50% PImax (p=0.033) and between 30 and 70% PImax (p=0.049). Cardiac output was significantly increased from rest (6.11±0.28 L/min) to 7.74±0.31 L/min at 30% PImax (p=0.004), 8.38±0.66 L/min at 50% PImax (p=0.003), and 8.36±0.57 L/min at 70% PImax (p=0.003). With increasing inspiratory intensity, BORG ratings for dyspnoea progressively increased from 2.36±0.20 at 30% PImax, to 3.45±0.21 at 50% PImax (p=0.003), and to 4.91±0.25 at 70% PImax (p=0.003). A significant difference in dyspnoea ratings was also observed between 50 and 70% PImax (p=0.002). End-tidal carbon dioxide pressure (PETCO2) progressively decreased from QB during 30% PImax (26.23±0.59 mmHg; p=0.005), 50% PImax (25.87±1.02 mmHg; p=0.005) and 70% PImax (24.30±0.82 mmHg; p=0.005). Significant differences in PETCO2 were found between 30% and 70% PImax (p=0.017) and 50% and 70% PImax (p=0.037).

Discussion Thoracoabdominal tidal volumes and cardiac output responses were nearly identical between 50% and 70% PImax, however adverse physiological responses, such as hyperventilation (decreased PETCO2) and dyspnoea scores were significantly greater at 70% PImax. This study suggests that 50% PImax is the optimal intensity for IMT via TFRL in healthy subjects.