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Post-Polio Health (ISSN 1066-5331)

Vol. 17, No. 2, Spring 2001

Breathing Problems of Polio Survivors (continued)

What is underventilation? The alveoli in the lungs are tiny air sacs at the end of the respiratory tract where gas exchange with the blood occurs. In underventilation (medically known as chronic or global alveolar hypoventilation), the saturation of oxygen in the blood falls due to increased carbon dioxide (CO2). Normally the alveoli should clear most of the CO2 out with each breath. Instead, the CO2 accumulates (called hypercapnia), and thus there is decreased room in the alveoli for oxygen.

Hypercapnia and decreased oxygen saturation are the hallmarks of underventilation or hypoventilation. The signs and symptoms of underventilation usually appear first during sleep.

Some people seem to suddenly experience life-threatening respiratory failure due to CO2 accumulation (hypercapnia). They may not have been aware of gradually increasing symptoms and signs, particularly since they are often not physically active nor regularly monitored with simple pulmonary function tests. Polio survivors may think that they are breathing fine until an upper respiratory infection, which makes breathing in harder for everyone, causes serious problems, partially due to an ineffective cough and the inability to eliminate secretions.

Who is the most qualified to evaluate breathing problems?

Astute family physicians will order a referral to a pulmonologist, preferably one experienced in neuromuscular disease. Pulmonologists specialize in all breathing-related disorders, however, most focus on more acute problems such as intensive care. The pulmonologist who focuses on neuromuscular diseases understands that the problem is due more to respiratory muscle weakness and the restrictive nature of the disease rather than the lungs themselves.

The Resource Directory for Ventilator-Assisted Living, available from www.ventusers.org) compiles a list of pulmonologists and respiratory health professionals who are knowledgeable about and committed to home care and long-term mechanical ventilation.

What tests will a pulmonologist order?

Pulmonary function tests can be performed in a physician's office with a simple spirometer (an instrument for measuring the capacity of the lungs) or in a fully-equipped pulmonary function laboratory.

Pulmonary function tests should include:

Vital capacity (VC) both sitting and supine (lying down), FVC, FEV1. VC measures the total volume of air one can breathe out completely after inhaling a full breath. VC is usually done forced, as fast as possible, and is known as FVC. When this fast forced expiration is performed, the volume breathed out in the first second is known as FEV1. VC is sometimes done slowly and is called SVC. These tests can be done while standing, sitting, or supine. A drop in VC over 25% in the supine position indicates significant diaphragm weakness. When VC declines to <1 L (liter), underventilation often occurs.

MIF and MEF. Maximum inspiratory force (MIF) and maximum expiratory force (MEF) are measured by breathing in and out with maximal effort, through a closed mouth tube attached to a pressure measuring device. This measurement reflects inspiratory and expiratory muscle strength.

Peak cough flow. In people who have had polio, cough is often not effective enough, due to respiratory muscle weakness. A weak cough can lead to poor secretion removal, increased respiratory infections, and pneumonia.

What is an ABG and should polio survivors who suspect breathing problems have one?

An arterial blood gas (ABG) should be ordered when VC falls or symptoms of underventilation develop. An ABG invasively measures levels of oxygen, carbon dioxide and pH in the blood and assesses pulmonary gas exchange. A noninvasive measurement of oxygen saturation in the blood is pulse oximetry, but it is not as complete or sensitive as an ABG.

What is pulse oximetry?

The blood oxygen saturation can be measured noninvasively using a pulse oximeter. It is a probe placed usually on a highly oxygenated part of the body, such as the finger. Infrared light is released and analyzed by recording its changing absorption in the arterial blood. Nocturnal oximetry is becoming more useful in screening for abnormalities that often occur first during sleep. Some oximeters have a memory module to record 8-12 hours of oxygen and pulse rate data.

What is a sleep study and when is it necessary?

Sleep studies (formally known as polysomnography) are usually performed in a sleep laboratory over one or two nights to record multiple variables simultaneously, such as sleep stages, rapid eye movement (REM), snoring, airflow at the nose and mouth, arousals, heartbeat, chest wall breathing motion and oxygen saturation, to assess sleep disorders (such as sleep apnea). These studies include EEG (brain wave), ECG (electrocardiogram), and often a video record of sleep movements.

Sleep studies are recommended for an individual exhibiting signs of nocturnal underventilation, but not daytime underventilation, or for asymptomatic individuals with a VC <1 to 1.5 L.

Sleep studies are often recommended to detect sleep apneas.

What is sleep apnea?

Defined as the lack of breathing through the nose and mouth for at least ten seconds, sleep apnea can be obstructive or central or mixed. Obstructive sleep apnea (OSA) occurs when tissues in the throat collapse and block airflow in and out of the lungs during sleep, although efforts to breathe continue. Central apnea occurs when the brain fails to send appropriate signals to the body to initiate breathing. There is neither airflow nor chest wall movement.

In sleep apnea, breathing ceases, oxygen in the blood decreases, arousal occurs, sleep ends and breathing resumes. The individual then drifts back to sleep and another apnea occurs, with this cycle continuing throughout the night, resulting in hundreds of arousals from sleep.

OSA at first occurs when individuals sleep on their backs, but eventually apneic episodes are present with any sleep position. A number of factors make snoring and apnea worse, such as obesity and nasal obstruction. Smoking causes the lining of the upper airway to swell, alcohol and sedative drugs cause the muscles in the back of the upper airway to relax, and excessive weight decreases the size of the upper airway.

When there are nighttime breathing problems in a person with neuromuscular disease, such as post-polio, they are more likely due to respiratory muscle weakness, rather than OSA. However, some individuals may have only OSA while others may have both respiratory muscle weakness and OSA.

What is the treatment for sleep apnea?

Sleep apnea is best treated with the use of a continuous positive pressure airway (CPAP) device to push the tongue out of the way and keep the airway open during sleep. If underventilation and sleep apnea occur simultaneously, a bilevel positive pressure device is recommended to help improve ventilation and also keep the airway open.

What is CPAP?

CPAP stands for Continuous Positive Airway Pressure. Air flows continuously into the airway via the nose with use of a nasal mask. CPAP keeps the airway open, but does not adequately assist respiratory muscle activity.

CPAP is primarily used to treat obstructive sleep apnea, and thus is normally used only at night during sleep. CPAP units are not ventilators. Higher pressures may make exhaling uncomfortable and difficult. Newer CPAP units, called Auto CPAPs, automatically provide varying levels of pressure based on the individual's needs during the night. Because OSA is prevalent among the general population, many companies offer CPAP units.

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