Introduction
The measurement of respiratory muscle strength can be used to identify and assess respiratory muscle weakness. Vital capacity (maximum amount of air expelled from the lungs after a maximum inhalation) is decreased in individuals with significant respiratory muscle weakness, although it's a non-specific and generally insensitive measurement. A major clinical issue is respiratory muscle weakening. There are tests available to measure the strength of the respiratory muscles that range in complexity and invasiveness. The value of many tests and the relative accuracy of various tests in detecting weakness are less obvious.
Why Is Respiratory Muscle Strength Significant?
Patients with respiratory disorders affecting the lung parenchyma or airways as well as those with neuromuscular diseases are frequently found to have impaired respiratory muscles, both for inhalation and expiration. Dyspnea (difficulty in breathing) and exertion intolerance, can both result from weak respiratory muscles. However, because most dyspnea screening protocols do not include an evaluation of respiratory muscle strength, diagnoses are frequently delayed. Additionally, tests that depend on patient cooperation (called volitional tests), which produce a high percentage of false negatives, are included in the assessment of respiratory muscle strength. Therefore, it is crucial to be familiar with both non-volitional measures and even more invasive techniques, like the measurement of trans diaphragmatic pressure (the difference between esophageal and diaphragmatic pressure), for the proper investigation and potential confirmation of respiratory muscle weakness as a cause of respiratory failure.
What Are the Tests for Measuring Inspiratory Muscle Strength?
Maximal Inspiratory Pressure (MIP): This is the most common test done to measure the strength of respiratory muscles in people suspected of having respiratory muscle weakness. It is measured by calculating the pressure in the upper airway during a maximal voluntary inhalation. The pressure that is being measured is a combination of the pressure that the inspiratory muscles produce and the elastic recoil pressure of the chest wall and lungs. The advantage of this procedure includes:
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Low-cost, cost-effective equipment is used.
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Easy to be done.
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Able to get rapid results.
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Non-invasive.
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Well-established results in different populations.
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Helps to diagnose inspiratory muscle weakness early.
The disadvantages of the procedure can include:
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The procedure is dependent on the patient’s cooperation. Hence, a low value may not signify a weak respiratory muscle, but rather, a lack of cooperation from the patient.
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Low accuracy in mechanically ventilated patients.
The procedure helps in the identification of inspiratory muscle weakness, which can be caused by cardiac, neuromuscular, and pulmonary diseases. Additionally, the measurement of maximal inspiratory pressure can help with the differential diagnosis of dyspnea, obstructive lung disease of unknown origin, the evaluation of the effectiveness of cardiopulmonary rehabilitation, the prescription and supervision of respiratory muscle training, and the assessment of the success of weaning from mechanical ventilation in critically ill patients.
Sniff Nasal Inspiratory Pressure (SNIP): The search for a method for measuring the inspiratory muscle strength that would surpass the limitations of MIP, being non-invasive and avoiding the need for an esophageal balloon as in MIP, resulted in the proposal of measuring nasal inspiratory pressure during a sniff. It is a non-invasive procedure that accurately reflects esophageal pressure while measuring the diaphragm's joint activity and other inspiratory muscles. Studies using electromyography have revealed that the muscles used for breathing, particularly the inspiratory accessory muscles, selectively contract during SNIP, proving the specificity of the test. When comparing inspiratory muscle strength, SNIP is a very useful tool and has higher specificity than MIP. In recent years, SNIP has been used to diagnose and track muscle weakness in a variety of pathologies, such as neuromuscular and pulmonary diseases, where a deficit in inspiratory muscle strength is a characteristic feature of the disease's natural course. The advantages of the procedure can include:
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Uses simple and cost-effective equipment.
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Easy to perform.
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Based on intuitive maneuver. Hence the results are reproducible.
The disadvantages of the procedure include:
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The results are dependent on the patient’s cooperation.
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It cannot be done in mechanically ventilated patients.
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The procedure should be used with caution in patients with nasal obstruction.
Electrical and Magnetic Phrenic Nerve Stimulation: When patients have trouble understanding or performing the maneuvers, resulting in low values during the volitional maneuvers (MIP or SNIP), or when there is a lot of variation in the measurements, which is likely due to different levels of effort, non-volitional tests should be used to measure inspiratory muscle strength. There are two possible techniques that produce similar outcomes to achieve the maximum involuntary inspiratory contraction: electrical stimulation or magnetic (twitch) stimulation of the phrenic nerve. Although painful and has been known to cause convulsions, electrical stimulation is more specific for diaphragm stimulation than magnetic stimulation. Most patients can tolerate the minimal discomfort that magnetic phrenic nerve stimulation causes. The idea behind it is to cover the cervical region with tiny coils to generate a magnetic field there. Magnetic phrenic nerve stimulation has surpassed electrical stimulation in popularity because it offers more comfort and safety. The advantages of the procedure can include:
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Measures the strength of the inspiratory muscle accurately, irrespective of patient cooperation.
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Magnetic waves provide better stimulation, activating even the small muscle fibers.
The disadvantages can include:
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Less specific for muscles of the diaphragm.
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The procedure is costly.
What Are the Tests for Measuring Expiratory Muscle Strength?
Maximal Expiratory Pressure (MEP): It is the most commonly used method to measure expiratory muscle strength in outpatient and very ill patients. It is simple and easy to do. It uses cost-effective equipment. However, the results are dependent on the patient’s cooperation. It has low accuracy in predicting cough capacity.
Cough Gastric Pressure (CGP): Since abdominal muscles are the main muscles responsible for the expiratory flow, measuring the gastric pressure while coughing is a helpful additional test in the evaluation of expiratory muscle weakness. It is more specific than MEP. However, the procedure is invasive and requires insertion of a catheter. It is used to rule out expiratory muscle weakness. This is especially true in patients who are thought to have a weaker cough reflex as a result of an impaired expiratory pump, such as those with neuromuscular diseases and critically ill patients right before or right after extubation.
Gastric Pressure After Magnetic Stimulation of the Anterior Abdominal Muscles: After abdominal wall muscles were stimulated with neural magnetic stimulation in non-cooperative patients, the gastric pressure was measured to determine the strength of the expiratory muscles. A circular coil is placed over the dorsal spine, between the eighth and tenth thoracic vertebrae, to produce magnetic stimulation.
Conclusion
A proper assessment of respiratory muscle impairment requires the use of the right tests because it can occur not only in diseases of the respiratory system but also in many other diseases. Since they are more widely known and easier to use across centers, noninvasive volitional tests are still the most frequently used tests in clinical practice.
