Hypoventilation in Neuromuscular Disease


Hypoventilation in Neuromuscular Disease

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Hypoventilation

Hypoventilation in Neuromuscular Disease

Oxygen Therapy is Inappropriate

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One of the most frequent problems that people with post-polio and other neuromuscular diseases experience is being inappropriately prescribed oxygen therapy as their respiratory muscle function becomes increasingly weaker.

This clinical experience, confirmed by the Mayo Clinic case series,1,2 supports the need for accurate information from physicians and respiratory care practitioners most expert in their pulmonary care.

Recognizing Hypoventilation
If progressive respiratory failure occurs in people with neuromuscular disease, an abnormal nocturnal oximetry study is often an early indication that hypoventilation is occurring with associated sleep-disordered breathing. There are significant periods of hypoxemia during sleep when lying flat.

Decreased vital capacity (VC), maximum inspiratory force (MIF) and maximum expiratory force (MEF) are expected but may initially be less severe. Decreased oxygen saturation (SaO2) combined with increasing carbon dioxide (CO2) retention, hypercapnia, are the hallmarks of hypoventilation. This is sometimes called ventilatory pump failure, due to the weakened respiratory muscles. Because COPD is more common in clinical practice, it’s easy to mistake these patients and treat them like COPD, with oxygen.

Remember the alveolar air equation?3 Calculation of the Alveolar-arterial gradient clarifies the cause of hypoxemia. If the calculated A-a gradient is normal, then the cause is hypoventilation. In contrast, if the A-a gradient is increased, then the hypoxemia is due to other causes (V/Q mismatch, diffusion defect or right-to-left shunt).

A rule of thumb, using ABG values on room air: If the sum of PaO2 and PaCO2 is between 110 and 130 mmHg, then the hypoxemia is due to hypoventilation; if the sum is less than 110 mmHg, then the cause is V/Q mismatch, diffusion defect, shunt or high altitude.

Mechanical Assistance
Patients with neuromuscular diseases who are developing progressive respiratory failure due to respiratory muscle weakness will die unless mechanical ventilation is used.

The rate of progression of the muscle weakness is often hard to predict. Some patients suddenly experience life-threatening hypercapnic respiratory failure. They may not have been aware of gradually increasing symptoms and signs, particularly because they are often not physically active and many are not being regularly monitored with even simple pulmonary function tests (of course, they should be checked regularly).

Administering oxygen does not provide mechanical assistance to the weakening respiratory muscles, but gives both the patient and the doctor the false impression that appropriate treatment is being provided. Hypoventilation is thus mistaken for another cause of hypoxemia. Indeed, administering oxygen can mask the problem.

Also there is a danger of causing respiratory depression by giving oxygen.1 Oxygen is not the treatment for hypoventilation. It will improve the SaO2, but not the hypoventilation; and it may increase the danger of dying of sudden respiratory failure.

In hypercapnic respiratory failure due to hypoventilation, the SaO2 falls due to the rise of the CO2. The alveoli in the lungs should clear out most of the CO2 with each breath. Instead, with hypoventilation, CO2 accumulates, and thus there is decreased alveolar oxygen.

When mechanical ventilation using room air is provided, it lowers the CO2 in the alveoli, corrects the SaO2 and rests the respiratory muscles. The ventilator should be adjusted to achieve a normal SaO2, on room air. If oxygen is being administered, one cannot use noninvasive oximetry to tell whether enough assisted ventilation is being provided; repeated arterial blood gas specimens (ABGs) would be needed.

Examples of situations that may require administering oxygen are as follows:

  • Pneumonia due to infection or aspiration. If this occurs in patients with respiratory muscle weakness and hypoventilation, then it is important to provide both assisted ventilation and supplemental oxygen, and use ABGs to monitor them.
  • Air travel may benefit from supplemental oxygen.
  • If the decision is made for palliative care and to forgo assisted ventilation, then the use of oxygen can be justified.

    Controversial Question
    When should assisted ventilation be started if there is evidence of progressive respiratory muscle weakness in a patient with neuromuscular disease? This is an area of active current research. There is considerable agreement that:

  • the patient needs to be symptomatic in order to be motivated enough to use the equipment;
  • noninvasive positive pressure ventilation (NPPV) should be used if this is possible;
  • NPPV should be started considerably before there is any danger of life-threatening acute respiratory failure.

    NPPV can be started initially for nighttime use, and then used more hours as needed. Nocturnal assisted ventilation provides respiratory muscle rest and also improves the sleep-disordered breathing. Both of these contribute to the ability of many patients with neuromuscular disease to breathe independently during the day after using nocturnal NPPV. A full sleep study isn’t needed.

    CPAP should not be advised; assisted ventilation is what these patients need. NPPV using a nasal mask, full face mask or mouthpiece is effective for most neuromuscular patients unless considerable bulbar impairment is present.

    Ventilator Setup
    When there is respiratory failure in neuromuscular patients (ALS, post-polio, SMA, muscular dystrophy, etc.) who have no additional pulmonary disease that impairs oxygen transfer, the NPPV ventilator setup is adjusted to:

    • be comfortable for the patient;
    • achieve SaO2 of 95 percent or higher on room;
    • assist the patient to effectively cough and clear secretions;
    • provide improved oral communication (if vocal communication is possible).

    It has been common for people using noninvasive nasal ventilation with a bilevel positive pressure unit to use inadequate settings. Frequently, they’re not monitored with clinical evaluation and oximetry.

    The EPAP is often set too high: Usually it should not be higher than 3 to 4 cmH2O. And, the IPAP is often set too low: Usually it needs to be 12 to 16 cmH2O, and adjusted to achieve an oxygen saturation of 95 percent or higher. Neuromuscular patients treated for hypoventilation, who have no additional pulmonary problems, will have a normal PaCO2 when the SaO2 is 95 percent or higher. Oximetry is usually reliable for monitoring so that ABGs are not needed when room air ventilation is used.

    These respiratory aspects of neuromuscular disease need to be disseminated more widely to alert people to the reasons why they should be cautious about using oxygen therapy when the problem is hypoventilation.

    References

    1. Gay PC, Edmonds LC. Severe hypercapnia after low-flow oxygen therapy in patients with neuromuscular disease and diaphragmatic dysfunction. Mayo Clin Proc. 1995;70(4):327-30.

    2. Hsu A, Staats B. Postpolio sequelae and sleep-related disordered breathing. Mayo Clin Proc. 1998;73:216-24.

    3. Tobin MJ (editor). Principles and Practice of Mechanical Ventilation. New York: McGraw-Hill Inc.; 1998.

    Resources

    Hill NS (editor). Long-term Mechanical Ventilation. New York: Marcel Dekker Inc.; 2001.

    Krachman S, Criner GJ. Hypoventilation syndromes. Clin Chest Med. 1998;19(1):139-55.

    Dr. Oppenheimer is a pulmonary medicine and home ventilation specialist at the Southern California Permanente Medical Group and is associate clinical professor of medicine at UCLA School of Medicine, Los Angeles. He can be reached at Eaopp@UCLA.edu. Fischer is editor of IVUN News, International Ventilator Users Network, Saint Louis, Mo. She can be reached at www.post-polio.org/ivun.html.

    This is adapted from an article originally published in the IVUN News (Spring 2000) Vol. 14, No. 1, pages 4-5.

    NPPV and Effective Cough
    It’s important to train any neuromuscular patient started on NPPV how to do effective cough and assisted cough techniques.1

    Difficulty eliminating airway secretions is a common cause for NPPV to fail. This is particularly true in cases where there is an acute respiratory infection or even the common cold.

    Manual assisted cough methods can be used by a family member or caregiver. Mechanical devices also are available, such as those that use exsufflation with negative pressure.

    NPPV with assisted cough techniques usually are successful. They result in safety, reduced hospitalization, improved function and improved quality of life.

    The Muscular Dystrophy Association (Tucson, Ariz.) has two educational videotapes reviewing many of these issues: “Breath of Life” for professional use and “Breathe Easy” for patients and families. These are available from your local MDA, or call (800) 572-1717.

    1. Bach JR. Pulmonary Rehabilitation; The Obstructive and Paralytic Conditions. Philadelphia: Hanley & Belfus Inc.; 1996.

    —E.A. Oppenheimer, MD, FCCP, and Judith R. Fischer, MSLS

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