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Respiratory Pressure Meter - An Overview
How are the measurements made ?
Mouth Pressures
Respiratory muscle strength is assessed by measuring maximum static expiratory and inspiratory mouth pressures. These pressures do vary with the point in lung volume that they are performed at, so PEmax is made at total lung capacity (TLC, full) and PImax at residual volume (RV, empty). The maximal pressure should be maintained for at least 1 second. Patients with under-performing respiratory muscles find this manoeuvre both hard work and difficult, so the device is provided with inspiratory and expiratory valves. These enable the patient to "seat" their teeth and lips onto the flanged mouthpiece before performing the test. The best result from 3 expiratory tests and 3 inspiratory tests should be taken and at least a minute between attempts allowed to avoid any effects from muscle fatigue
Nasal Pressures
If an airtight plug is inserted into one nostril with a pressure catheter passed through the plug so that it's distal end is within the nose opening, sniffing through the other nostril with the mouth closed generates a peak negative pressure in the nasopharynx. This Sniff Nasal Inspiratory Pressure (SNIP) is a good measure of global inspiratory muscle strength. There is no fixed testing protocol, but from the published papers, 10 sniffs, whilst seated and taking the highest value are usual. SNIF is a peak rather than sustained measurement
Respiratory Muscles
The respiratory muscles perform the vital function of sustaining ventilation. The major inspiratory muscle is the diaphragm with it's right and left domes, each innervated by the phrenic nerve. The contraction of the diaphragm lowers this dome (like a piston) and elevates and expands the lower rib cage. Other important inspiratory muscles are the scalenes and sternomastoids in the neck which elevate the upper rib cage, and the external and parasternal intercostal muscles The most important expiratory muscles are those of the abdomen wall which on contraction produce a raised intrathorasic pressure. Internal intercostals and pectoral muscles also play a role
At rest the role of the inspiratory muscles is dominant while expiration is largely passive. During expiration the expiratory muscles are increasingly recruited. The expiratory muscles also provide the effector mechanism of the cough.
Respiratory Muscles in Disease
As can be seen from the exhaustive list below there are numerous causes that can result in respiratory muscle weakness. Luckily today most such diseases are fairly rare, since however, the health implications can be very serious the MicroMPM can prove to be invaluable in detecting (or eliminating) them early on in the disease process
Causes of respiratory muscle weakness
Neurogenic
- Motor neurone disease
- Polyneuropathy (e.g. Guillian-Barré syndrome)
- Surgical trauma of phrenic nerves
- Neuralgic amyotrophy
- Poliomyelitis
- Multiple sclerosis
- Traumatic tetraplegia
- Acute porphyria
Muscular
- Muscular atrophy (particularly resulting from poor nutrition
- Muscular dystrophy
- Inflammatory myopathies
- Myotonic dystrophy
- Systemic lupus erthematosus
- Acid maltase deficiency
- Thyroid myopathy
- Biochemical abnormalities (e.g. acidosis, hypophosphatemia)
- Steroid therapy
Neuromuscular junction
- Myasthenia gravis
- Eaton-Lambert syndrome
Lung Rehabilitation and COPD
Patients with severe Chronic Obstructive Pulmonary Disease (COPD) exhibit respiratory muscle weakness due to poor nutrition and/or systemic corticosteroid treatment. These patients tend to exhibit sustained hyper-inflation thus shortening the muscle groups and the power they can generate
Training of such muscle groups in patients using inspiratory muscle training devices (semi disposable, variable inspiratory resistance units) has proved beneficial.
The perfect indicator of any improvement following such training (e.g. 20-30 minutes a day for 4 weeks ) is the result of mouth or nasal pressure measurements.