Measurement of inspiratory muscle performance with incremental threshold loading.

We found that breathing strategies affect measurement of sustainable inspiratory pressure (SIP). After allowing time for learning, the maximum sustainable inspiratory pressure (SIPmax) was 46% greater than SIP. We therefore developed a test of ventilatory muscle performance that used progressive 2-minute increments in threshold inspiratory resistance. Subjects started with a low load and continued to breathe until they could no longer inspire. With increasing load there was a fall in minute ventilation and time of inspiration, and an increase in oxygen consumption and PETCO2. Power was greatest when loads were 55 to 75% of maximum static inspiratory pressure (MIP). The inspiratory mouth pressure corresponding to the greatest load achieved (PmPeak) was the same in trained and naive subjects. PmPeak/MIP was reproducible and was not influenced by fixing subjects' breathing frequency. We concluded that tests of ventilatory muscle performance should allow subjects to develop breathing strategies to handle high inspiratory loads. Two-minute incremental loading is a simple assessment of ventilatory muscle performance and the test may have clinical application where reproducibility is necessary.