Repeated Maximum Inspiratory Pressures

Methodology. McKenzie and Gandevia (6, 62, 63) have developed a technique that uses 18 repeated Pi,max maneuvers. The test begins with measurement of Pi,max and practice efforts using visual feedback of airway opening pressure. Three different breathing patterns have been described (6, 62). The most practical appears to be a series of 18 Pi,max contractions lasting 10 seconds each, with 5 seconds of rest between contractions (duty cycle = 0.67) (62). A similar approach has been used to measure expiratory and limb muscle endurance (6). The only equipment required is a manometer for measuring airway opening pressure. This technique has been generally used to measure inspiratory muscle endurance.

Normal values. in normal young subjects (n = 12), with a duty cycle of 0.67, the average inspiratory mouth pressure attained in the last contraction is 87 ± 3% of Pi,max (mean ± SD) (6). The PTimo at this point is approximately 0.58. Using a similar protocol, but with a slower frequency, pressures dropped to approximately 77% (6). interestingly, when the duty cycle is reduced to 0.5, no drop in pressure generation is observed across the 18 contractions in normal subjects (6). The PTimo is then 0.25, which may be just below the threshold for fatigue for the rib cage muscles (2, 43).

Advantages. The technique provides a measurement that is entirely independent of lung and chest wall mechanics, as well as mechanical work of breathing, making it potentially useful for understanding endurance properties of the respiratory muscles without interference from chest wall or lung mechanics. it appears to be sensitive to the influence of lung disease (6), is simple to perform, and lends itself to the potential for a pulmonary function testing environment.

Disadvantages. Potential disadvantages include the fact that the endurance characteristics may reflect the anaerobic capacity of the muscles to sustain force, rather than aerobic endurance, because it is likely that blood flow is largely occluded to the muscles during the prolonged contractions. it also does not appear that in 18 contractions a sustainable level of pressure is fully attained (62). Furthermore, patients with severe lung disease may find it difficult to perform such extended maximum inspiratory maneuvers without discomfort or dyspnea. This technique has yet to be independently tested in patients.

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