Part One : Control of Breathing
Breathing muscles require impulses from the brain to make them function. Breathing is largely an automatic process but it may also be controlled voluntarily. That means that most of the time a person breathes without having to think about it. The area of the brain responsible for this automatic breathing is the medulla. The medulla is sometimes called the "bulb", which is a part of the brain stem (the lowest part of the brain). However, when you want to hold your breath or take an extra deep breath, the super computer of the brain (the cerebral cortex) takes over.
Chemical receptors in the chest and in the medulla detect the amount of carbon dioxide, oxygen and hydrogen ions in tile blood. These sensors send input to the medulla which then triggers breathing at the right time. Impulses (like an electric current) travel from the super computer of the brain or the medulla and send messages to the anterior horn cells (motor neurons) of the respiratory muscles. The anterior horn cells of the respiratory muscles are found in the neck (cervical spine) and in the thoracic spine (rib cage area). Firing of these anterior horn cells causes a chemical message to be sent to the breathing muscles. When the muscles get this message, they contract, causing you to breathe.
Respiratory Muscle Anatomy and Function
1. The Diaphragm
The diaphragm is shaped like an upside down bowl. It is attached to the lower ribs, the bottom of the breastbone and the front and sides of the lower lumbar vertebrae. The fibers radiate inwards inserting into a central tendon. The zone of apposition (wall of the bowl) is that part of the diaphragm that is apposed to (up against) the inner aspect of the rib cage. The diaphragm muscle fibres are oriented such that when the muscle contracts and is met by counter-pressure of the abdominal contents, the diaphragm fibres pull the rib cage up and out. Thus, on inspiration (breathing in), the diaphragm moves down. The central tendon of the diaphragm meets the resistance of the abdominal contents. The continued contraction of the diaphragm then pulls the rib cage up and out. The downward movement of the diaphragm and the outward movement of the rib cage create a negative pressure within the lungs and air is drawn in.
The abdominal muscles are very important to the efficient action of the diaphragm. The dome shape of the diaphragm and the zone of apposition are maintained by the resting tension of the abdominal muscles and the firm support it provides to the abdominal contents. If the abdominals are weak, you don't get the firm counter-pressure supplied by the abdominal contents. This then lessens the force the diaphragm has to pull the rib cage up and out. In chronic bronchitis and emphysema a person often has a barrel-shaped chest due to hyperinflated lungs. The extra air in the lungs causes the dome of the diaphragm to be flattened. Because the upside down bowl shape of the diaphragm is lost (it resembles a plate instead), the diaphragm fibres pull horizontally (inward) on the ribs rather than upward and outward.
2. The Scalene Muscles
The scalene muscles are located in the sides of the neck and insert on the first and second ribs. They are active on every breath in. They lift and expand the rib cage during inspiration.
3. The Parasternal Muscles
These muscles are attached to the breastbone and run between the rib cartilages in a downward and outward direction. When they contract, the ribs are lifted and the anterior-posterior (front to back) dimension of the rib cage increases.
4. Accessory Muscles of Inspiration
These muscles help out when the person has to work harder to breathe. These include the following:
Sternocleidomastiods - These run from the mastoid process (just below and behind the ear) to insert along the inner third of the collarbone and the top of the breastbone.
External Intercostals - Run between the ribs. These muscles function when you are working harder to breathe, i.e. during exercise.
Expiratory Muscles - Breathing Out
Relaxed expiration (breathing out) is usually passive (not requiring any effort). It happens because of the elastic recoil of the lungs. When you are breathing hard because of exercise, the abdominal muscles may help to push the air out more quickly. The internal intercostals (muscles between the ribs) also work to push the air out when you are working hard.
Therefore, weakness or lack of nerve supply to the abdominals not only hinders the action of the diaphragm on inspiration, but hampers the ability to ventilate at higher than resting levels because these muscles cannot be recruited to help out on expiration (to push the air out quickly).
Post-Polio Syndrome and its Relationship to Respiratory Function
Dalakas defines post-polio syndrome as "the development of new muscle weakness and fatigue in skeletal or bulbar muscles, unrelated to any known cause, that begins 25 - 30 years after an acute attack of paralytic poliomyelitis.
Cashman and Trojan state that the "loss of motor neurons in paralytic polio induces compensatory axonal sprouting of the remaining motor neurons, which markedly increases the number of muscle fibres innervated by the motor neuron." Therefore you get extreme enlargement of the motor unit (up to 7 times the normal size).
The Weichers-Hubble hypothesis states that prominent enlargement of motor units by axonal sprouting is not indefinitely stable and that enlarged units undergo progressive loss of the terminal axon sprouts with time after polio. This notion was based on data that single fiber EMG jitter linearly increased with time after polio. In other words, muscle fibers supplied by the same nerve fibers area getting the message to contract at different times. In normal muscle, two muscle fibers that are innervated (supplied) by the same motor neuron contract at the same time when the nerve sends the signal. To illustrate, when telephone extensions ring in your house, they all ring at the same time. Jitter is when the extensions ring at all different times. Therefore when the nerve fires in a PPS survivor, the muscle fibers may not contract simultaneously. This leads to a weaker muscle contraction. Sometimes, when the nerve fires, the muscle does not receive the message at all. This is called blocking and is analogous to having a phone that doesn't ring at all.
Increased jitter may be due to poor conduction along the ends of the axonal sprouts or to inadequate release of neurotransmitter at the neuromuscular junction. The neurotransmitter is the chemical "spark" that gets the muscle going.
After acute polio, recovery of muscular strength can occur by terminal axonal sprouting of surviving motor neurons, with partial or complete reinnervation of denervated muscle fibers. These enlarged motor units are undergoing a process of continuous remodelling, with constant denervation and reinnervation after acute polio. Thus, the presence of immature terminal sprouts (which are known to have decreased conduction) at all stages following recovery from acute polio, may account, at least partially, for the observed abnormal jitter. Decades after recovery, irreversible degeneration of terminal sprouts without reinnervation may account for the new symptoms of fatigue and weakness observed in PPS.
To sum up;
1. In acute polio some anterior horn cells are lost and the muscle fibers they supply lose their innervation.
2. The surviving anterior horn cells send nerve sprouts to the orphaned muscle fibers.
3. There is a continuous process of remodelling, with some nerve fibers continuing to die off and others beginning to sprout. The baby nerve fibers are not very efficient and neither are the geriatrics. This results in weak muscle contraction because the fibers contract at all different times. This is what is referred to as "jitter".
4. Decades after recovery, a greater proportion of the sprouts may begin to irreversibly degenerate, resulting in new symptoms of fatigue and weakness.
In addition to the metabolic exhaustion of post-polio motor units, neuromuscular junction transmission defects and muscular abnormalities due to overuse occur. In PPS, the transmission defects are believed to be due to poor transmission along the sprouts. The sprouts may not transmit as well as a disease-free motor neuron. Think of how poorly a leaky hose delivers water to a sprinkler. Another cause of poor transmission may be due to limitations in the number or size of packages containing neurotransmitter at the motor nerve terminals. When the water finally gets to the sprinkler, there isn't enough volume to make the water push itself up through the sprinkler holes.
If you have had bulbar polio there are several problems that this causes for breathing. Firstly, because of damage to the respiratory center in the medulla, there may be a lack of drive to breathe, particularly when you are asleep. Secondly, there will likely be problems with the motor nerves that supply the respiratory muscles themselves. In addition, the respiratory muscles may themselves not be strong due to atrophy of some muscle fibers and overuse of others. Sometimes, obstructive apnea due to throat muscle weakness is also a factor.
Conditions that Further Hinder Respiratory Muscle Function
1. Chronic Obstructive Pulmonary Disease (Chronic Bronchitis or Emphysema)
This disease is generally caused by smoking. In chronic bronchitis, secretions in the airways decrease the airway diameter and cause increased airway resistance. To illustrate, if you dip a straw in glue and coat the inside of it, it makes the tube smaller and it is harder for liquids to move through it. In emphysema, the elastic recoil of the lung tissue is lost, hindering expiration and causing air trapping. This causes the person to have hyperinflated lungs. As was said previously hyperinflation causes the diaphragm to flatten. This reduces the zone of apposition of the diaphragm on the ribcage and hinders the mechanical advantage of the diaphragm on inspiration
Because much of the accumulation of excess fat is deposited in close proximity to the chest wall, severe obesity increases respiratory muscle load and markedly impairs pulmonary function.
3. Kyphoscoliosis (Curvature of the Spine)
Kyphoscoliosis causes increased work of breathing due to increased stiffness of the chest wall and impaired muscle action due to the altered shape of the ribcage. When you have a situation of inspiratory muscle weakness, the increased work of breathing due to changes in the lung or chest wall may result in a relative overload being imposed on the breathing muscles. This leads to fatigue and possible injury of the inspiratory muscles.
What Can Be Done?
Considerations when Training the lnspiratory Muscles
Respiratory muscles are the same as other skeletal muscles in that they can improve their function in response to training. However, because they must contract repetitively, they have no opportunity to rest and may become fatigued or injured under conditions of overload. Because of the vital fiction of these muscles, care must be taken in progression of exercise because undue fatigue could cause or exacerbate (make worse) respiratory failure.
The danger in post-polio is that because of the precarious nerve supply to the inspiratory muscles and the muscle atrophy already present, those remaining active muscle fibers may be fatigued to the extent that they become damaged and require time to repair. This would further weaken the inspiratory muscles and could potentially result in an increased need for ventilator support.
How Do We Train Respiratory Muscles?
At the Post-Polio Clinic in Edmonton, we have used a device called the P-Flex to do training of the inspiratory muscles of a few patients. The P-Flex looks like a kazoo and has a dial on it so that the diameter of the hole through which you breathe can be changed. It is similar to breathing through straws of progressively smaller diameters. In general, this technique has been known to work well for those individuals who are breathing independently (no ventilator) but become short of breath with exercise.
The general idea is to start at the easiest level on the device. The nose is plugged and the P-flex is placed in the mouth. I have directed the PPS survivor to breathe through it until they BEGIN to feel short of breath. The device is then removed from their mouth. The number of breaths taken and the resistance level recorded. If the person is comfortable taking 50 - 60 breaths at a given resistance level over several exercise sessions they may choose to increase the resistance. Exercise sessions are done a maximum of three times per week with a rest day in between. One mistake we made was that we increased the resistance level too quickly in one gentleman. He found that he had no trouble doing 30 repetitions at a particular level and we therefore increased the resistance factor at the next session. We found that he went from level one to level five over a period of several weeks. However, at level five he was able to take only three breaths. The next session, he pushed himself to do ten breaths. That extra push and the ignoring of his fatigue resulted in a relapse whereby he eventually discontinued using the P-Flex and was placed on oxygen at night. Prior to that time, however he found that he was having less difficulty breathing at night. Perhaps if we hadn't progressed the resistance level has quickly, he might have had better results. I cannot warn you enough to be extremely cautious with the use of a P-Flex. Do this only with the approval of a physician and the supervision of a physiotherapist or respiratory therapist.
Considerations for Use of the P-Flex
1. Use the P-Flex only on the advice of a physician and in consultation with a physiotherapist or a respiratory therapist.
2. Don't use it if you require ventilator support for prolonged periods (more than l 5 minutes) while you are awake.
3. Respiratory function should be measured prior to starting. Bach recommends that resisted breathing training should be done only if Vital Capacity is greater than 30% of predicted normal. Vital Capacity is defined by Sproule as "the greatest amount of air that the subject can expire following a maximal inspiration".
4. Don't use it if you have a cold or chest infection. You are already working against an increased load.
5. Don't use it on a day that you feel totally exhausted.
6. Stop exercise with the device as soon as you BEGIN to feel short of breath.
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