Effect of Inspiratory Time and Lung Compliance on Flow Bias Generated During Manual Hyperinflation: A Bench Study

Manual hyperinflation can be used to assist mucus clearance in intubated patients. The technique's effectiveness to move mucus is underpinned by its ability to generate flow bias in the direction of expiration, and this must exceed specific thresholds. It is unclear whether the inspiratory times commonly used by physiotherapists generate sufficient expiratory flow bias based on previously published thresholds and whether factors such as lung compliance affect this.

In a series of laboratory experiments, we applied manual hyperinflation to a bench model to examine the role of 3 target inspiratory times and 2 lung compliance settings on 3 measures of expiratory flow bias.

Longer inspiratory times and lower lung compliances were associated with improvements in all measures of expiratory flow bias. In normal compliance lungs, achievement of the expiratory flow bias thresholds were (1) never achieved with an inspiratory time of 1 s, (2) rarely achieved with an inspiratory time of 2 s, and (3) commonly achieved with an inspiratory time of 3 s. In lower compliance lungs, achievement of the expiratory flow bias thresholds was (1) rarely achieved with an inspiratory time of 1 s, (2) sometimes achieved with an inspiratory time of 2 s, and (3) nearly always achieved with an inspiratory time of 3 s. Peak inspiratory pressures exceeded 40 cm H2O in normal compliance lungs with inspiratory times of 1 s and in lower compliance lungs with inspiratory times of 1 and 2 s.

Inspiratory times of at least 3 s with normal compliance lungs and at least 2 s with lower compliance lungs appear necessary to achieve expiratory flow bias thresholds during manual hyperinflation. Inspiratory times shorter than this may lead to excessive peak inspiratory pressures. Verification of these findings in relation to the movement of mucus should be examined in further bench or animal models and/or human clinical trials.