Introduction
Respiratory muscle strength training (RMST) comprises exercises and techniques designed to enhance the strength and endurance of respiratory muscles, such as the diaphragm and intercostal muscles. Various disordersaffect the respiratory system and may, directly or indirectly,cause dysfunction of either inspiration, expiration, or both. RMST exercises are particularly beneficial for athletesbut are often employed as a treatment or rehabilitation tool forpatients with respiratory conditions that affect respiratory muscles, such as critical illness, chronic obstructive pulmonary disease (COPD), asthma, or respiratory muscle weakness. RMST can also be paired with other forms of pulmonary rehabilitation to remedy respiratory dysfunction and improve clinical outcomes. This article reviews the physiology, types of different respiratory muscle training, the indications and contraindications for RMST, and discusses relevant clinical significance for adult patients.
Function
Physiology of Respiratory Muscle Strengthening
Respiratory muscles are structurally similar to other skeletal muscles.However,respiratory muscles are distinct because contraction causes changes in chest wall motion during inspiration and expiration.[1]Similar to the musculature of the upper and lower extremities, respiratory muscles demonstrate plasticity during resistance and endurance trainingby remodeling their cross-sectional area, muscle fiber phenotype (fast twitch, slow twitch), and threshold for activation.[2]Introducing respiratory muscles to resistive loads for prolonged periods and varying these loads in response to tolerance is just one way to provide progressive overload.Improving respiratory muscle strength and enduranceenhances chest wall mechanics and lung volumes and reduces the clinical incidence of pulmonary diseases, such as pneumonia and atelectasis.When respiratory muscles are progressively overloaded, their physiologic response is to hypertrophy,improving contractile force.[3]
Definition of Respiratory Muscle Strength Training
RMSTisemployed in health care settings such as intensive care units and outpatient and inpatient rehabilitation units to improve the strength and endurance of inspiratory and expiratory muscles.During RMST, patients challenge inhalation or expiration to undergo a time-limited scheduled therapy protocol that includes inspiratory muscle strength(IMS) training, expiratory muscle strength(EMS) training, or a combination thereof.IMS training requires patients to use handheld devices to complete exercises intended to improve the strength and endurance of the inspiratory musculature, including, but not limited to, the diaphragm, external intercostal, and parasternal intercostal muscles.[4]EMS training requires patients to complete exercises that improve the strength and endurance of expiratory muscles, including, but not limited to, the internal intercostal and abdominal muscles.[5]
Respiratory Muscle Strength Training Devices and Protocols
EMS and IMS training utilize the following categories of devices: incentive spirometry (IS), pressure threshold devices, and resistance devices. Pressure threshold devices have a spring-loaded, 1-way calibrated valvetargeting a certain threshold of maximal pressure on muscles as the patient inhales or exhales; turning the spring increases or decreases the threshold.[6]The target pressure must be reached for air to flow through the device to facilitate exhalation or inspiration.The resistance the device applies during inspiration or expiration is independent of the volume and rate of airflow through the device.[7]Resistance devices provide constant resistance to airflow.Users can increase or decrease resistance by changing the number or size of the openings through which air can flow.[8]Incentive spirometry devices promote natural deep, slow breathing that improves lung volumes based on feedback from the device to the preset airflow during inspiration.[9]
Establishing the initial training load for either IMS or EMS training requires a pressure-measuring device such as a digital manometer, a peak flow meter, or a pressure threshold device.Depending on the goal of the RMST protocol, exercises are initiated at a percentage of the maximum inspiratory/maximum expiratory pressure.[10]Strengthening protocols typically use loads between 55% and 80% ofthis ratio with fewer repetitions.[11]Endurance protocols can use 30% to 40% ofmaximum inspiratory/maximum expiratory pressure with more repetitions.[12]
The protocol for RMST considersthe clinical indication for training, underlying diagnoses, and tolerance to therapy.Training sessions are typically conducted 2 to 3 times daily, 3 to 5 days per week, for a 5- to 6-week period; patients perform more than 10 repetitions per session under the guidance of a speech-language pathologist. Patients can also perform these exercises individually at home.[13]The training intensity can be changed by varying the number of training sessions, sets, or repetitions and the duration of sessions to promote muscle adaptation and improve respiratory muscle strength or endurance.
Indications for Respiratory Muscle Strength Training
Inspiratory muscle strengthtraining
Conditions for which IMS training is indicated include, but are not limited to:
Weaning from mechanical ventilation
Multiple sclerosis
Spinal cord injury
Congestive heart failure
Chronic obstructive pulmonary disease
Paradoxical vocal fold motion
Parkinson disease[14][15][16][17][18]
Expiratory muscle strength training
Conditions for which EMS training is indicated include, but are not limited to:
Amyotrophic lateral sclerosis
Spinal cord injury
Swallowing dysfunction and phonation in Parkinson's Disease[19][20][21]
Contraindications toRespiratory Muscle Strength Training
Although RMS training is generally well-tolerated, contraindications for the use of IMS and EMS training includeuncontrolleddisordersfor which exercise is generally contraindicated:
Acute cerebrovascular accident
Untreated hypertension
Acute delirium
Perforatedtympanic membrane
Spontaneous pneumothorax
Hemodynamic instability
Rib fractures
Osteoporosis
Recent abdominal surgery[22]
Potential Adverse EffectsofRespiratory Muscle Strength Training
There are no formally recognized adverse effects to RMST. However, muscle fatigue may occur when patients breathe at a high proportion of their functional reserve or with a prolonged inspiratory time. Fatigue may explain why adherence toRMST is often low; up to 50% of patients fail to complete the studies. Other symptoms, such as headache or dizziness, have been reported in athletes who underwent RMST.[23]
Respiratory Muscle Strength TrainingTechniques
During IMS training, patients inhale against a resistive load provided by a handheld pressure threshold or resistance device.[24]Each IMS training protocol is individualized, and variables such as resistive load, repetition, and frequency of attempts may change throughout training. IMS training is indicated when a decreased ability to perform inspiration, characterized by a lowmaximum inspiratory pressure, is noted or impairment in vocal fold movement is evidenced.IMS training primarily targets the diaphragm and external intercostal muscles.[25]
During EMS training, patients exhale against a resistive load provided by a handheld pressure threshold or resistance device (seeImage.Example of Expiratory Muscle Trainer Portable Device). EMS training is indicated for patients with weak cough, swallowing dysfunction, poor airway protection or secretion management, and poor vocal intensity to improve maximum expiratory pressure.[26]EMS training primarily targets the internal intercostal and abdominal muscles.[27]The goals of EMS training can include strengthening cough, improving clearance of secretions, and increasing vocal intensity.
Several recent developments in RMSTemploy new and portable devices, including the digital portable inspiratory muscle trainer device, POWERbreathe™ K3 manufactured by POWERbreathe International Ltd. (seeImage.Example Device For Digital Portable Inspiratory Muscle Training). This device includes the Breathe-Link Medic computer software, which provides real-time feedback andmeasures parameters such asmaximum inspiratory pressure and peak inspiratory flow. This type of device has been used in several studies to assess the role ofmuscle strengtheningin respiratory and cardiovascular outcomes.[28][29][30]
Another innovative approach utilizes distinctly sized leaks to simulate various types of muscle training, including power, strength, and endurance. This technique is available through the RT2 (DeVilbiss Healthcare, Wollaston, UK) or TRAINAIR (Project Electronics Limited, Kent, UK) devices.[31]
Clinical Significance
Clinical Efficacy of Respiratory Muscle Strength Training
RMST can alleviate shortness of breath, increase exercise tolerance, and enhance the quality of life forpatients with respiratory conditions.The clinical efficacy of RMST is evaluatedduring rehabilitation in critically ill adults or in outpatient settings by assessing changes in maximal inspiratory and expiratory pressures, lung volumes, and clinical endpoints. The role of RMST has been evaluated in several clinical conditions, including COPD, spinal cord injury, sleep apnea, asthma, various neuromuscular disorders, and exercise-induced paradoxical vocal fold motion.
An examination of the effectiveness of IMS training as a standalone intervention and in combination with pulmonary rehabilitation in patients with COPD revealed thatwhen used in conjunction with pulmonary rehabilitation, IMS training did not significantly improve dyspnea, functional exercise capacity, and quality of life compared tothe rehabilitation alone. However, IMS training will likely improve these outcomes when administered as a standalone treatment.[32]In a systematic review of randomized trials,IMS trainingsubstantially improvedmaximal andmaximum expiratory pressure compared to control groups. These findings suggested that respiratory RMS positively impacts respiratory function and may improve clinical outcomes in patients with chronic respiratory disorders.[33]Furthermore, IMS training usinga resistive loading device can increasemaximum inspiratory pressure and improve weaning outcomes in patients with severe COPD who failed to wean off mechanical ventilation. In addition, this intervention improved exercise capacity in patients who have already weaned from mechanical ventilation.[34]
The feasibility and efficacy of RMST were assessed in individuals with impaired respiratory muscle strengthsecondary to disorders such as spinal cord injury and obstructive sleep apnea. The results of a 3-month pilot feasibility study demonstrated that when combined with oropharyngeal exercises, RMST was feasible and improved maximum inspiratory pressure without significantly affectingmaximum expiratory pressure, lung function, or sleep apnea severity.[30]
A systematic research analysis on adults with neuromuscular disorders indicated thatmaximum expiratory andinspiratory pressures were improved following RMST compared to the control group without significantly affecting cough, dyspnea, physical capacity, or quality of life.[35]Another systematic review of studies on adults with mild-to-moderate asthma revealed a significant improvement in inspiratory muscle strength following an IMS training, without effect on expiratory muscle strength or lung function.[36]
IMS training could potentially treat athletes with paradoxical vocal fold motion induced by exercise.This training has demonstrated potential benefits, such as enhanced respiratory muscle strength and increased inspiratory laryngeal aperture size. However, improving the ability of inspiratory accessory muscles to generate increased flow may not be advantageous if the laryngeal patency remains the limiting factor. Nonetheless, research has indicated that IMS training can result in laryngeal abduction in healthy individuals.[37]
In patients with long-term COVID-19 symptoms, the inspiratory muscle function variables demonstrated an acceptable level of discrimination in evaluating the efficacy of respiratory muscle training programs. Among these variables,maximum inspiratory pressure demonstrated the greatest discriminative ability, with a change of 22.1% in predicted values deemed clinically significant.[38]
Enhancing Healthcare Team Outcomes
Identifying patients who can tolerate and benefit from RNST requiresa collaborative effort among healthcare professionals, including physicians, advanced practitioners, nurses, speech-language pathologists, respiratory therapists, and others.Interprofessional teammembers must possess clinical skills and expertisewhen diagnosing, evaluating, and treating patients with respiratory muscle weakness. Biosocial barriers that could interfere with program adherence and lead to relapse should be identified. Regardless of the clinical setting where RMST is practiced, consultation with atrained healthcare professionalis essential when evaluating respiratory dysfunction and establishing an RMST regimenwhich, alongside medical management and nursing care, can help prevent adverse outcomes.
Collaboration among interdisciplinary teammembers improves thesafety of patients undergoing RMST through appropriatepatient selection, refining evidence-based guidelines, and establishing IMS and EMS training and patient-specific protocols.Healthcare professionals canimproveclinical outcomes through care coordination by actively participating inpatients' RMST protocols, determining whether adjustments are needed to improve patient tolerance, and evaluating if an underlying process is inhibiting their progress.
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