Ventilatory Pattern and Inspiratory Work in Normal Subjects Breathing through an Inspiratory Muscle Trainer

Abstract

We measured the pattern and work of breathing in eight normal subjects while they breathed through a commercial inspiratory muscle trainer. The purpose of the study was to measure the immediate effects of inspiratory trainers on ventilatory muscle use that might result in the desired training effect. Methods: We connected the trainer to a metabolic computer and randomized the order of changes in inspiratory aperture diameter from 7 mm to 2 mm. During quiet steady-state breathing we measured respiratory rate, tidal volume, minute ventilation, inspiratory and expiratory mouth pressure, O₂ consumption, CO₂ production, and end-tidal PCO₂. On a separate occasion we also measured end-tidal thoracic gas volume by body plethysmography at the lowest and highest resistance. Results: From the lowest to highest resistance, respiratory rate decreased 68%, from 14.2 ±0.7 to 4.5 ± 0.4 breaths/min; tidal volume more than doubled, from 880 ± 70 to 2070 ± 160 ml; and minute ventilation decreased from 12.5 ± 0.9 to 8.7 ±0.6 L/min. End-tidal PCO₂ did not change. Inspiratory pressure was -35 ± 3 cm H₂O, and Vo₂ rose by 26 ml/min at the 2-mm diameter. Thoracic gas volume was not affected. Inspiratory time increased to approximately 80% of the total ventilatory cycle. We conclude that a fixed-orifice inspiratory trainer causes large changes in ventilatory pattern and increased ventilatory work during quiet breathing. The pattern of breathing involves reduction in inspiratory flow at high negative pressure, consistent with prolonged isotonic contraction of the ventilatory muscles.