Relationship between inspiratory mouth pressure and respiratory muscle activity in normal subjects.

The relationship between inspiratory mouth pressure and respiratory muscle activation was investigated in 20 normal non-smoking subjects (13 males). Surface electromyography (EMG) was recorded from the seventh intercostal space bilaterally during different levels of inspiratory effort monitored by a respiratory mouth pressure gauge. The mouth pressure and EMG relationship was non-linear between 20 and 100% of inspiratory effort. Quadratic regression analysis was performed between percent inspiration and percent EMG on each side of the chest. The regression equation and coefficient of determination (r2) for the right side were y = 9.73 (e0.024x-1), r2 = 0.91, and for left side were y = 10.79 (e0.023x-1), r2 = 0.88. An independent t-test did not reveal significant difference (P > 0.05) between the slopes of the regression lines from the two sides of the chest. Therefore, results of the two sides were pooled and the predictive regression equation for the combined results was y = 10.05 (e0.0236x-1). Reasons for such a non-linear relationship may include the loading mechanism of respiratory muscles, inefficiency of intercostal muscles at high levels of inspiration, their recruitment pattern and the histochemical characteristics of respiratory muscles. Since absolute EMG values cannot be used to assess changes in activation on different days, further work is required to establish the between-day repeatability of the technique described which is potentially useful for assessing respiratory muscle function pre- and post-operatively in chest surgery patients.