Diaphragm Ultrasound Evaluation of the Effects of High Intensity or Low Intensity Inspiratory Muscle Training in Patients With Prolonged Mechanical Ventilation

Prolonged mechanical ventilation (PMV) is associated with substantial morbidity, mortality, and healthcare resource utilization. In Taiwan, approximately 170,000 patients receive mechanical ventilation annually, of whom an estimated 13%-16% remain ventilator-dependent for more than 21 days and 3.7%-5.6% require ventilatory support for more than 63 days. Diaphragm dysfunction is an important physiological contributor to difficult ventilator liberation. Prolonged mechanical unloading, systemic inflammation, critical illness, and disuse may result in diaphragmatic atrophy, structural injury, and impaired contractility.

Inspiratory muscle training (IMT) is used during ventilator rehabilitation to improve inspiratory muscle strength and endurance. IMT may be delivered using resistive-loading or threshold-loading devices. Threshold-loading devices provide a relatively constant, flow-independent inspiratory resistance and permit training intensity to be prescribed according to maximal inspiratory pressure (Pimax). Similar to peripheral skeletal muscle training, respiratory muscle adaptation is influenced by the magnitude and progression of the applied workload. However, the appropriate training intensity for patients with PMV and diaphragm dysfunction remains uncertain. Higher-intensity training may provide a stronger physiological stimulus but may be less well tolerated, whereas lower-intensity training may be easier to perform but may produce a smaller training response.

The effects of IMT have traditionally been evaluated using global respiratory measurements such as Pimax, tidal volume, respiratory rate, minute ventilation, and the rapid shallow breathing index. Although these measurements provide clinically relevant information, most are effort-dependent and do not directly assess diaphragm-specific mechanical recovery. Point-of-care diaphragm ultrasound provides a noninvasive bedside method for serial evaluation of diaphragmatic function. Diaphragm excursion reflects craniocaudal diaphragmatic displacement during inspiration, while diaphragm thickening fraction reflects inspiratory muscle contraction within the zone of apposition.

This prospective, single-center, randomized controlled trial compared high-intensity and low-intensity inspiratory muscle training (IMT) in adults with prolonged mechanical ventilation and inspiratory muscle weakness who had been transferred from an intensive care unit to a specialized respiratory care center. Eligible participants were randomly assigned to receive an individualized IMT workload based on baseline maximal inspiratory pressure (Pimax). The high-intensity group trained at 50% of baseline Pimax, and the low-intensity group trained at 20% of baseline Pimax.

IMT was administered using a Dofin® adjustable-resistance threshold inspiratory muscle trainer (GaleMed Corporation, Taiwan). During each training session, the device was connected directly to the endotracheal or tracheostomy tube by a respiratory therapist after temporary disconnection from the mechanical ventilator. Each cycle included 6-10 inspiratory efforts, and five cycles were performed daily. Rest periods of 1-3 minutes were provided between cycles to reduce acute respiratory muscle fatigue. Pimax was reassessed twice weekly, and the training resistance was adjusted according to the updated measurement.