Characteristics of Respiratory Difficulties Caused by Neuromuscular Diseases

Respiratory complications in neuromuscular disease(NMD) are caused by hypoventilation due to the weakening of respiratory muscles and failure of removal of airway secretion, rather than problems with the lung parenchyma. Restrictive lung diseases, accompanied by a decrease in the lung capacity, including the vital capacity caused by the weakening of respiratory muscles, are found in these patients. The patients also undergo atelectasis unless their lungs are artificially expanded as their breathing is quick, shallow and irregular. A mild form of atelectasis can occur when patients do not take a deep breath for an hour. When a patient cannot take a deep breath for long periods of time or chronically stays in hypo-inflation state, permanent respiratory restriction may occur and thus, there will be a greater workload for weakened respiratory muscles to perform. An increase in the work of breathing may cause decompensating of respiratory muscles. Respiratory muscular fatigue must be avoided without any exception as the pH in the blood decreases due to the exhausted respiratory muscles and may even lead to death. To avoid this, the brain must reset its ventilation adjustability so that a patient is able to take shallow breaths.
If a patient takes shallow breaths, there will be a less load applied to respiratory muscles, but hypercapnia and hypoventilation of the lungs occur. In healthy people, if there are secretions in airway caused by a cold, etc., such secretions can be excreted from the body by coughing and complications such as pneumonia can be prevented. However, pneumonia is common among patients with NMD who have respiratory muscular weakness as these patients are less able to cough due to the weakening of muscles that control inhaling and often fail to properly excrete intra-airway secretion. It can also aggravate atelectasis. In addition to this, a decrease in compliance of the respiratory system meaning a contracture of the lungs and thorax can cause a problem as it affects the inhaling phase of a cough. Therefore, in order to lead a proper cough, inhalation of a sufficient amount of air must proceed before coughing, but removal of secretions becomes even more difficult when there is a problem in the expansion of the lungs caused by contracture of the respiratory system as induced coughing has limitations.
Complications can be minimized and the mortality rate from complications related to the diseases can be significantly decreased through a thorough understanding of the pathophysiology of patients’ respiratory system, conducting a proper patient evaluation, assisting ventilation based on the evaluation results and efficiently removing intra-airway secretion.

Maximum Inspiratory and Expiratory Pressure (MIP and MEP)

It is necessary to understand the conditions of respiratory muscles since respiratory muscular weakness is the leading cause. As a decrease in vital capacity is witnessed after respiratory muscles are weakened for a good amount of time, measuring the maximum static pressure using a spirometry would be more ideal in understanding the respiratory muscular weakness than measuring the vital capacity..
The maximum expiratory pressure is measured by the total lung capacity of the patient at the time the patient takes the deepest breath possible, then blows out all of the air through a cylindrical mouthpiece into a static pressure meter. Similarly, the maximum inspiratory pressure is measured by the residual volume of the patientat the time the patient blows out all of the air, then inhales as much air as possible.
One can understand the respiratory muscular strength of a patient indirectly by looking at the maximum static pressure measured in this manner (Figure 4).

Vital Capacity (VC)

Patients with diaphragm weakness often experience difficulty in breathing while lying down even though they do not have the same issue in a sitting position. Since the vital capacity of patients with NMD may vary depending on the patient’s posture,, it must be measured in a variety of positions. It is also necessary to determine whether to use a spinal orthosis by measuring the vital capacity since spinal movements can be restricted when patients use a spinal orthosis for the correction of scoliosis, of which is often found in patients with NMD.

Maximum Insufflation Capacity (MIC)

Weakened respiratory muscles fail to expand the lungs up to maximum capacity or compress down to minimum residual volume.
If the thoracic wall fails to make a full expansion for a long period of time, the thoracic muscle component shortens and muscle fibrosis progresses, all of which reduce the compliance of thoracic wall.
The compliance of the lung parenchyme may be reduced by microatelectasis of alveoli
Such changes in the compliance may cause serious problems in maintaining pulmonary health by reducing the capacity to cough and clear secretions.
One can understand the compliance of the respiratory system by measuring the maximum amount of air that can be inflated into the lungs and comparing such with vital capacity.
Maximum insufflation capacity can be obtained by having a patient inhale as much as possible in a sitting position and additionally inflating a maximum amount of air through a mouthpiece or oro-nasal mask using a manual resuscitator bag.

Peak Cough Flow (PCF)

Coughing is one of the most important protective mechanisms in our body. It clears out airway secretions to prevent further complications such as pneumonia.
A patients' reduced capability to cough caused by respiratory muscular weakness causes complications as intra-airway secretion is not properly excreted from the system. Coughing ability can be measured by having a patient cough hard and measuring the maximum coughing flow using a peak flow meter. The peak cough flow must be at least 160 L/m in order for secretions or unnecessary substances to be removed from the airway. Patients with a peak cough flow between 200~250 L/m may show values dropping below 160 L/m when they catch a cold or are after anesthesia. Thus, it is necessary to evaluate the cough ability of patients with NMD and take proper measures.

Measurements of oxyhemoglobin saturation (SaO2) and end-tidal CO2 (EtCO2)

Patients with NMD who have weakened respiratory muscles often go through chronic alveolar hypoventilation. In the event of acute hypercapnia caused by hypoventilation, carbon dioxide is excreted as the decreased body pH stimulates the respiratory center and it induces deep breaths. However, if patients remain in a chronic alveolar hypoventilation state, the stimulating effect is lost due to the compensatory action of the kidneys, even with hypercapnia. If the patients in this state are supplied with only oxygen to compensate for reduced oxyhemoglobin saturation levels, the patients’ breathing will weaken and cause CO2 narcosis, which aggravates hypercapnia. Therefore, when providing treatment for patients with NMD who show a desaturated level of oxyhemoglobin, hypercapnia must be taken into account.
Typically, an arterial blood gas analysis is performed when measuring pO2 and pCO2 in an effort to understand the patients’ actual ventilating condition. As it is necessary to understand the ventilating condition under various situations, consecutive measurement of the oxygen saturation using a pulse oxymeter and capnogram using a capnometer under different conditions would be ideal, although it is generally less accurate. For instance, the vital capacity of a patient with amyotrophic lateral sclerosis accompanied by the weakening of the diaphragm is measured to be much lower when the patient is lying down rather than sitting and it is the opposite for a cervical spinal cord injury patient. Thus, it is critical to analyze the ventilating conditions for different positions and for when a patient is awake and asleep, particularly for those who are at the boarder of respiratory failure since the amount of ventilation decreases while they are sleeping. Because it would in impracticable to understand all of these conditions through performing an arterial blood gas analysis and increased breathing caused by pain triggered during the test may influence the test results, it would be better to apply a non-invasive method for assessment of ventilation.

Chest Physical Therapy

Glossopharyngeal breathing, frog breathing

Glossopharyngeal breathing, a respiration method that accumulates air in the lungs using the tongue and pharyngeal muscles, can be useful for imperfect ventilation in patients with neuromuscular disease. This technique is particularly essential when there is a trouble with ventilator. A patient can accumulate 60ml to 200 ml of air at once and must practice this exercise 6 to 9 times in a single breath.

Noninvasive Ventilation

Nearly all patients with NMD eventually come to need a ventilator in order to save their lives since the function of their respiratory muscles become weaker over time. In the case where patients who are capable of retaining the function of a bulbar muscle that they can speak coherently and swallow food noninvasive ventilation method is applicable without requiring a tracheostomy. In the past, a body ventilator was commonly used as a non-invasive artificial ventilation method but nowadays a non-invasive intermittent positive pressure ventilation (IPPV) is often used.
  • 1. Body Ventilator

    A negative pressure body ventilator, which contains a chamber that wraps around the chest and epigastrium, is an apparatus that allows a patient to artificially ventilate through the addition of negative pressure. An iron lung, chest shell and coat-like poncho ventilator (Video 9) are some examples of negative-pressure body ventilators. These devices are used as breathing apparatus for spinal cord injury patients who suffer from respiratory difficulties and can be useful when changed into a non-invasive respiratory management during a tracheostomy. However, they are not used often as they are more complicated to use and less portable when compared to non-invasive intermittent positive pressure ventilation.
    Some of the body ventilators include a rocking bed (Video 8) and intermittent abdominal pressure ventilator (pneumobelt) (Figure 6). A rocking bed, a bed designed to move up and down, is a device that indirectly induces breathing by allowing abdominal organs to move about. A pneumobelt is an apparatus that induces breathing indirectly through moving the diaphragm upward by inflating air, which is either a corset or belt-type device.

  • 2. Non-invasive Intermittent Positive Pressure Ventilation (Use of a ventilator without requiring a tracheostomy)

    Non-invasive respiratory management can be still performed in patients with a low vital capacity or who are incapable of self-breathing once intra-airway secretion is properly removed. A tracheostomy was once a quite important and desired surgical procedure that saved countless numbers of patients who suffered from acute respiratory failure. However, with the development of new ventilators, various nasal masks and mouthpieces and the advancement of secretion removal technologies have resulted in an increase in the use of non-invasive ventilators that do not require a tracheostomy. Patients in a tracheostomy state undergo speaking and eating difficulties and the procedure itself can cause complications that include an increased amount of secretion and are a main source of infection. Removal of secretion through the use of an existing ventilator during a tracheostomy can cause secondary accumulation of secretions and damage a patient’s airway due to high suction pressure. Therefore, the use of a non-invasive ventilator, when compared to the use of a ventilator through performing a tracheostomy, can lower the incidence rate of pneumonia and the frequency of patients being hospitalized due to complications of the respiratory system. It also helps patients overcome psychological burdens, increases the efficiency of a caregiver’s nursing and improves the level of patients’ satisfaction in their lives. From the moment of performing a tracheostomy, every single aspect of a patients’ life becomes dependent and such patients often end up living a very limited life in bed just as ones in a vegetative state. However, when respiratory management is carried out in a non-invasive manner, not only does the patients’ life span expand, but also their quality of life is improved as the patients’ freedom in their activities are not hindered by a tracheostomy.
    A non-invasive IPPV can be done in three different ways and these include nasal, oral and oro-nasal method. Even though most of patients prefer to receive IPPV by using a nasal mask (Figure 7), 36) but when patient have severe nasal blood congestion it must be done by using a mouthpiece.
    If serious air leakage is witnessed when patients receiving IPPV by using a mouthpiece are asleep, the leakage can be minimized with a lipseal. When using a nasal mask, it would be ideal to use a variety of masks alternately to prevent pressure sores, which can be caused through the pressure of the skin.
    Portable ventilators used in the non-invasive method, just like a regular ventilator, include pressure-limited and volume limited ventilators. The most common type of pressure-limited portable ventilator used for children is the bi-level positive airway pressure (BIPAP) type ventilator. This ventilator, however, may not be capable of inflating a sufficient amount of air for lung ventilation when the air resistance increases due to atelectasis or intra-airway secretion. An air accumulation breathing exercise cannot be performed with this type of ventilator, which is required to maintain the compliance of the lungs for coughing, Therefore, volume-cycled ventilators are recommended over pressure-cycled ventilators.