The assessment of inspiratory muscle fatigue in healthy individuals: a systematic review.

Inspiratory muscle fatigue (IMF) may contribute to the development of exercise limitation and respiratory failure. Identifying fatigue of the inspiratory muscles requires a rigorous and integrative methodological approach. However, there is no consensus about an optimal protocol to induce and assess the fatigability of the inspiratory muscles. A systematic review was performed to identify, evaluate, and summarize the literature related to the assessment of induced IMF in healthy individuals. The aim was to identify factors that are related consistently to IMF, as well as to suggest possible assessment methods. MEDLINE and EMBASE were searched for relevant articles until February 2012. Only studies with a quantitative description of assessment and outcome were included. The search yielded 460 citations and a total of 77 studies were included. Inspiratory muscle fatigue was produced acutely by inspiratory resistive loading (IRL), whole body exercise (WBE), hyperpnea, or WBE combined with IRL, and under normocapnic, hypoxic or hypercapnic conditions. To detect IMF, most studies (64%) used phrenic nerve stimulation, 44% used a maximal voluntary inspiratory maneuver and the remainder used electromyography. The heterogeneity of the published reports precluded a quantitative analysis. Inspiratory resistive loadings at intensities of 60-80% of maximum, and cycling at 85% of maximum were found to produce IMF most consistently. Hypoxic or hypercapnic conditions, and WBE combined with IRL, exacerbated IMF. The specific outcome measures employed to detect IMF, the magnitude of their change, as well as their functional significance, are ultimately dependent upon the research question being addressed.