Effect of Training on Inspiratory Load Compensation in Weaned and Unweaned Mechanically Ventilated ICU Patients

Abstract

BACKGROUND While inspiratory muscle weakness is common in prolonged mechanical ventilation, inspiratory muscle strength training (IMST) can facilitate strengthening and ventilator weaning. However, the inspiratory load compensation (ILC) responses to threshold loads are not well characterized in patients. We retrospectively compared ILC responses according to the clinical outcomes of IMST (ie, maximum inspiratory pressure [PImax], weaning outcome), in difficult-to-wean ICU patients. METHODS Sixteen tracheostomized subjects (10 weaned, 6 unweaned) from a previous clinical trial underwent IMST 5 days/week, at the highest tolerated load, in conjunction with daily, progressive spontaneous breathing trials. PImax and ILC with a 10 cm H2O load were compared in the subjects before and after IMST. Changes in ILC performance were further characterized (5, 10, 15 cm H2O loads) in the trained subjects who weaned. RESULTS Demographics, respiratory mechanics, and initial PImax (52 ± 26 cm H2O vs 42 ± 13 cm H2O) did not significantly differ between the groups. Upon enrollment, PImax significantly correlated with flow ILC responses with the 10 cm H2O load (r = 0.64, P = .008). After IMST, PImax significantly increased in the entire sample (P = .03). Both before and after IMST, subjects who weaned generated greater flow and volume ILC than subjects who failed to wean. Additionally, ILC flow, tidal volume, and duty cycle increased upon ventilator weaning, at loads of 5, 10, and 15 cm H2O. CONCLUSIONS Flow ILC at a threshold load of 10 cm H2O in ventilated, tracheostomized subjects positively correlated with PImax. Although PImax improved in both groups, the flow and volume ILC responses of the weaned subjects were more robust, both before and after IMST. The results suggest that ILC response is different in weaned and unweaned subjects, reflecting dynamic inspiratory muscular efforts that could be influential in weaning.

What Are the Implications of Blunted Load Compensation Responses in Prolonged-Weaning Patients?

“…26 Smith and co-workers 18 found that prolonged-weaning patients who continued to fail SBTs had blunted ILC responses, similar to those who failed weaning for the first time. 27 Blunted ILC response may be due to depressed neuromuscular output and/or neuroventilatory inefficiency.…”

“…After training the average PIF and V T increased significantly: PIF range 16 -18%, V T range 14 -42%. However, there was a large overlap between before and after IMST (see figure 6 in Smith et al 18 ).…”

“…In this issue of the Journal, Smith and co-workers, 18 of the same group, analyzed 16 patients who underwent IMST from the same data set: those with inspiratory load compensation (ILC) responses to a pressure threshold load of 10 cm H 2 O. Ten patients weaned successfully, and 6 patients failed weaning.…”

“…6 Intervention to strengthen the inspiratory muscles seems a logical step to facilitate weaning from mechanical ventilation. Several methods of inspiratory muscle training have been employed; these include isocapnic hyperpnea to improve respiratory muscle endurance, 14 inspiratory resistive-flow training, 15,16 inspiratory threshold-pressure training, 17,18 and adjustments of ventilator pressure sensitivity. 19 While the first method focuses on endurance, the latter 3 are to improve inspiratory muscle strength.…”

“…The method employing the inspiratory threshold-pressure training device, which maintains a given constant pressure, independent of flow, 20 and is commercially available, has therefore been commonly employed. 17,18 Most studies on inspiratory muscle strength training (IMST) for facilitating weaning from mechanical ventilation are case series without controls, 21 making IMST efficacy difficult to interpret. However, IMST appears safe.…”