Determination of normal values for an isocapnic hyperventilation endurance test

Aim: Evaluation of respiratory muscle function relies mainly on maximal inspiratory/expiratory pressures (MIP/MEP). However, their sensitivity to detect respiratory muscle weakness is low. Alternatively, it has been proposed to assess diaphragmatic performance through a progressive isocapnic hyperventilation test. However, normal values for this test are lacking precluding its use in a clinical setting. Subjects and methods: We measured spirometry, MIP, MEP and maximal pressure during a snif-test according to recommendations. Isocapnic hyperventilation was performed using a Spirotiger® device in healthy volunteers. The subjects were asked to breathe in a rebreathing bag at a fixed tidal volume and predetermined levels of ventilation were reached by changing breathing frequency (f b ). The test began at 30% of predicted maximal voluntary ventilation (MVV), and f b was then increased every 3 min to reach the target ventilation, up to MVV. The test was stopped when the subjects could no longer hyperventilate and sustain the target f b. Endurance time was expressed as Tlim in minutes and as the percentage of MVV sustained (%MVV). Results: We studied 58 subjects (23 males), aged 44±17 yrs [range 25-77]. Mean Tlim was 21±7 min [6-30] and reached on average 88±24% MVV. Tlim%MVV was significantly related to MIP (R=0.42, p=0.003) and MEP (R=0.34, p=0.02) but not to SNIP (p=0.975). Conclusions: These preliminary results showed that the evaluation of respiratory muscle endurance is feasible using this hyperventilation test using an incremental protocol up to task failure. Normative values according to age should be derived from a larger group of subjects, which will allow to detect diaphragmatic weakness in patients.