The influence of mouth pressure on the cardiovascular response during inspiratory muscle loading at a constant transdiaphragmatic pressure

Purpose To investigate whether the cardiovascular response during inspiratory pressure-threshold loading (ITL) is augmented by higher mouth pressure (Pm) when transdiaphragmatic pressure is maintained, and whether greater negative intrathoracic pressure increases intra-breath fluctuations in mean arterial pressure (MAP). Methods Sixteen healthy participants (6 females) completed two, 5-min ITL tasks while targeting a transdiaphragmatic pressure of 65 cmH2O, breathing rate of 15 breaths/min, and duty cycle of 0.7. In one task, absolute Pm was low (20–30 cmH2O; low Pm task), while in the other task it was higher (40–50 cmH2O; high Pm task). Results Both tasks resulted in an increase in heart rate (HR; low Pm: + 13 [95% Confidence Interval: 7,19] beats/min; high Pm: + 15 [9,21] beats/min; P < 0.0001] and MAP (low Pm: + 12 [5,19] mmHg; high Pm: + 11 [4,18] mmHg; P < 0.0001). For the high Pm task, the HR response was greater (+ 3 [2,5] beats/min; P < 0.0001), the MAP response was lower (−2 [−4,0] mmHg; P = 0.0209), and there was a greater inspiratory-expiratory MAP difference (P = 0.0009), compared to the low Pm task. Conclusion Increasing Pm during ITL elicited a greater HR response and a blunted MAP response. The smaller changes in MAP during inspiration compared to expiration during the high compared to low Pm task may suggest an influence of negative intrathoracic pressure on cardiac loading. Future studies are required to determine how cardiac output changes with increases in Pm.