Inspiratory muscle weakness is a consequence of prolonged mechanical ventilation and may contribute to the residual physical disability which has been observed in intensive care survivors. Inspiratory muscle weakness is associated with duration of mechanical ventilation, with those ventilated for 7 days or longer most at risk of developing both strength and endurance deficits.
Specific resistance training of the inspiratory muscles (inspiratory muscle training) improves inspiratory muscle strength and endurance in normal people, athletes and patients with a wide variety of underlying pathologies. Mechanisms of improvement include proliferation of both Type 1 and Type 2 inspiratory muscle fibres, enhanced metabolism resulting in reduced lactate production, attenuation of a fatigue-induced metaboreflex, adaptations to neural pathways and modulation of the perception of dyspnoea. Furthermore, inspiratory muscle training enhances exercise tolerance and reduces dyspnoea in both patients and athletes, while improving quality of life for patients with chronic lung or heart disease.
There is a lack of evidence for inspiratory muscle training in intensive care patients, despite the potential benefits of training in this group. This project explores the feasibility, safety and efficacy of inspiratory muscle training in intensive care patients who have been mechanically ventilated for 7 days or longer, as this subset of patients is most likely to demonstrate inspiratory muscle weakness and therefore benefit from specific training. This project includes patient-centred outcome measures, as most studies of inspiratory muscle training to date have focused solely on impairments, rather than the patient experiences of quality of life, physical function or dyspnoea.
Study 1 establishes the safety and feasibility of high-intensity interval-based inspiratory muscle training in selected ventilator-dependent patients. Across 195 inspiratory muscle training sessions, there were no adverse events recorded during or immediately following the treatment. No significant changes were observed in heart rate, blood pressure, respiratory rate or oxygen saturation. Furthermore, mean training pressures increased by a mean difference of 18.6 cm H2O across the 10 patients studied. Thus Study 1 confirms that inspiratory muscle training is safe in selected ventilator-dependent patients.
At time of project design, there was a lack of established outcome measures to assess global physical function in intensive care patients. Study 2 explores the clinimetric properties of the Acute Care Index of Function (ACIF) in a heterogeneous group of intensive care patients. Study 2 demonstrates that the ACIF has excellent inter-rater reliability (ICC 0.94 for total ACIF scores), and correlates well with the ICU Mobility Scale (r=0.84). Moreover, an ACIF score of less than 0.40 at intensive care discharge predicts discharge from hospital to a destination other than home (sensitivity 0.78). Thus Study 2 confirms the reliability and validity of ACIF as a tool to measure physical function in Studies 3, 4 and 5.
Study 3 is an observational study that describes the residual impairments of inspiratory muscle strength and endurance in a cohort of 43 patients recently weaned from mechanical ventilation. In this group, 37% demonstrated impaired inspiratory muscle endurance (fatigue resistance index < 0.80), while mean strength scores were only 38% of predicted values (mean 38.6, SD 19.7). This study also captured deficits in physical function, with mean ACIF score of 0.40/1.00, and raised perception of exertion both at rest (1.95/10) and during exercise (3.40/10). Thus even in an intensive care unit where minimal sedation, early rehabilitation and spontaneous modes of ventilation are the norm, patients recently weaned from mechanical ventilation have major residual impairments and functional deficits.
Study 4 is a randomised trial of high-intensity inspiratory muscle training in patients recently weaned from mechanical ventilation. Using concealed allocation, blinded outcome assessors and intention-to-treat analysis, Study 4 measures the effects of 2 weeks of inspiratory muscle training (in addition to usual care) compared to usual care. Patients in the experimental group demonstrated greater improvements in inspiratory muscle strength (mean difference 11%), but not endurance, while quality of life improved more in the experimental group than the control (mean difference 12%). Improvements in physical function and dyspnoea were equivalent. Thus 2 weeks of inspiratory muscle training improves inspiratory muscle strength and quality of life in the post-weaning period.
Finally, Study 5 is a protocol for a randomised trial of inspiratory muscle training performed by patients while mechanically-ventilated. Study 5 measures the impact of inspiratory muscle training on duration of mechanical ventilation, as well as residual inspiratory muscle strength and endurance, quality of life, physical function and perceived exertion.
The clinical implications of this project are that inspiratory muscle training can be used in the post-weaning period to ameliorate respiratory muscle weakness, which may enhance quality of life for patients mechanically ventilated for seven days or longer. Furthermore, the ACIF can be utilised in intensive care patients to map the improvement trajectory and predict likely hospital discharge destination. Thus this project contributes to the body of knowledge regarding the rehabilitation of intensive care patients, providing feasible strategies to enhance patient care.