Hypoventilation Syndromes

Introduction

Background

The respiratory system serves a dual purpose: delivering oxygen to the pulmonary capillary bed from the environment and eliminating carbon dioxide from the blood stream by removing it from the pulmonary capillary bed. Metabolic production of carbon dioxide occurs rapidly. Thus, a failure of ventilation promptly increases the partial pressure of carbon dioxide measured by arterial blood gas analysis (PaCO₂).

Alveolar hypoventilation is defined as insufficient ventilation leading to an increase in PaCO₂ (ie, hypercapnia). Alveolar hypoventilation is caused by several disorders that are collectively referred as hypoventilation syndromes. Alveolar hypoventilation also is a cause of hypoxemia. Thus, patients who hypoventilate may develop clinically significant hypoxemia. The presence of hypoxemia along with hypercapnia aggravates the clinical manifestations seen with hypoventilation syndromes.

Alveolar hypoventilation may be acute or chronic and may be caused by several mechanisms. The specific hypoventilation syndromes that are discussed in this article include central alveolar hypoventilation, obesity hypoventilation syndrome, chest wall deformities, neuromuscular disorders, and chronic obstructive pulmonary disease (COPD). Hypoventilation and oxygen desaturation deteriorate during sleep secondary to a decrement in ventilatory response to hypoxia and increased PaCO₂. In addition, diminished muscle tone develops during the rapid eye movement (REM) stage of sleep, which further exacerbates hypoventilation secondary to insufficient respiratory effort.

Hypoventilation may be caused by depression of the central respiratory drive. In patients with primary alveolar hypoventilation (the Ondine curse), the cause of hypoventilation and hypercapnia is not known. Patients with primary alveolar hypoventilation have normal alveolar-arterial oxygen gradients and are able to voluntarily hyperventilate and normalize their PaCO₂. The phrase "central alveolar hypoventilation" is used to describe patients with alveolar hypoventilation secondary to an underlying neurologic disease. Causes of central alveolar hypoventilation include drugs and central nervous system diseases such as cerebrovascular accidents, trauma, and neoplasms.

Obesity hypoventilation syndrome (OHS) is another well-known cause of hypoventilation. Abnormal central ventilatory drive and obesity contribute to the development of OHS. However, no specific body mass index is associated with the development of OHS. A recent article investigated the association of obstructive sleep apnea (OSA) to OHS in a cohort of 34 patients. In most of the cases (23 of the 26 patients) OSA was also present. The patients with both OSA and OHS had worst gas exchange abnormalities and more severe pulmonary hypertension when compared with the patients with OSA only. However, OHS is an autonomous disease. Three of the patients with OHS had no associated OSA (Kessler, 2001).

Chest wall deformities such as kyphoscoliosis, fibrothorax, and those occurring postthoracoplasty are associated with alveolar hypoventilation leading to respiratory insufficiency and respiratory failure.

Neuromuscular diseases that can cause alveolar hypoventilation include myasthenia gravis, amyotrophic lateral sclerosis, Guillain-Barré syndrome, and muscular dystrophy. Patients with neuromuscular disorders have rapid shallow breathing secondary to severe muscle weakness or abnormal motor neuron function. The central respiratory drive is maintained in patients with neuromuscular disorders. Thus, hypoventilation is secondary to respiratory muscle weakness. Patients with neuromuscular disorders have nocturnal desaturations that are most prevalent in the REM stage of sleep. The degree of nocturnal desaturation is correlated with the degree of diaphragm dysfunction. The nocturnal saturations may precede the onset of daytime hypoventilation and gas exchange abnormalities.

Hypoventilation is not uncommon in patients with severe COPD. Alveolar hypoventilation in COPD usually does not occur unless the forced expiratory volume in one second (FEV_1.0) is less than 1 L or 35% of the predicted value. However, many patients with severe airflow obstruction do not develop hypoventilation. Therefore, other factors such as abnormal control of ventilation, genetic predisposition, and respiratory muscle weakness are likely to contribute.

Pathophysiology

Control of ventilation

The respiratory control system tightly regulates ventilation. Alveolar ventilation (VA) is under the control of the central respiratory centers, which are located in the ventral aspects of the pons and medulla. The control of ventilation has both metabolic and voluntary neural components. The metabolic component is spontaneous and receives chemical and neural stimuli from the chest wall and lung parenchyma and receives chemical stimuli from the blood levels of carbon dioxide and oxygen.

Metabolism rapidly generates a large quantity of volatile acid (carbon dioxide) and nonvolatile acid in the body. The metabolism of fats and carbohydrates leads to the formation of a large amount of carbon dioxide, which combines with water to form carbonic acid (H₂ CO₃). The lungs excrete the volatile fraction via ventilation; therefore, acid accumulation does not occur. The PaCO₂ is tightly maintained in a range of 39-41 mm Hg in normal states. Ventilation is influenced and regulated by chemoreceptors for PaCO₂, PaO₂, and pH located in the brainstem and by neural impulses from lung stretch receptors and impulses from the cerebral cortex. Failure of any of these mechanisms results in a state of hypoventilation and hypercapnia.

Gas exchange abnormalities

The alveoli are perfused by venous blood flow from the pulmonary capillary bed and participate in gas exchange. This gas exchange includes delivery of oxygen to the capillary bed and elimination of carbon dioxide from the bloodstream. The continued removal of carbon dioxide from the blood is dependent on adequate ventilation. The relationship between ventilation and PaCO₂ can be expressed as follows: PaCO₂ = (k)(VCO₂)/VA. In which VCO₂ is the metabolic production of carbon dioxide (ie, venous carbon dioxide production), k is a constant, and VA is alveolar ventilation. Therefore, PaCO₂ increases as the VA decreases and is referred to as alveolar hypoventilation. Because the alveolus is a limited space, an increase in PaCO₂ leads to a decrease in oxygen, with resultant hypoxemia.

VA also can be reduced when an increase in physiologic dead-space ratio (ie, dead-space gas volume-to-tidal gas volume [VD/VT] ratio) occurs. Physiologic dead space occurs when an increase in ventilation occurs to poorly perfused alveoli. An increase in physiologic dead space results in ventilation-perfusion mismatch, which, in classic presentation, occurs in patients with COPD. The effect of physiologic dead space on alveolar hypoventilation can be expressed in the following equation: PaCO₂ = (k)(VCO₂)/VE(1 - VD/VT). In which VE (ie, expired volume) is the total expired ventilation and 1 - VD/VT measures the portion of ventilation directly involved in gas exchange. An increase in the physiologic dead space without an augmentation in ventilation leads to alveolar hypoventilation and an increased PaCO₂.

Primary and central alveolar hypoventilation

As mentioned previously, patients with primary alveolar hypoventilation can voluntarily hyperventilate and normalize their PaCO₂. These patients are unable to centrally integrate chemoreceptor signals, although the peripheral chemoreceptors appear to function normally.

Congenital central hypoventilation syndrome

Present from birth, this rare syndrome, congenital central hypoventilation syndrome (CCHS), is defined as the failure of automatic control of breathing. These patients have absent or minimal ventilatory response to hypercapnia and hypoxemia during sleep and wakefulness. Since these individuals do not develop respiratory distress when challenged with hypercapnia or hypoxia, progressive hypercapnia and hypoxemia occurs during sleep. The diagnosis is established after excluding other pulmonary, cardiac, metabolic, or neurologic cause for central hypoventilation. Patients with CCHS require lifelong ventilatory support during sleep, and some may require 24-hour ventilatory support.

Obesity hypoventilation syndrome

Patients with obesity hypoventilation syndrome have a higher incidence of restrictive ventilatory defects when compared with patients who are obese but do not hypoventilate. Studies have shown that patients with obesity hypoventilation syndrome have total lung capacities that are 20% lower and maximal voluntary ventilation that is 40% lower than patients who are obese who do not have hypoventilation.

These patients demonstrate an excessive work of breathing and an increase in carbon dioxide production. Inspiratory muscle strength and resting tidal volumes also are reported to be decreased in patients with obesity hypoventilation. Pulmonary compliance is lower in patients with obesity hypoventilation syndrome when compared with patients who are obese who do not have hypoventilation. Obesity increases the work of breathing because of reduced chest wall compliance and respiratory muscle strength. An excessive demand on the respiratory muscles leads to the perception of increased breathing effort and could unmask other associated respiratory and heart diseases.

Despite the above-mentioned physiologic abnormalities, the most important factor in the development of hypoventilation in obesity hypoventilation syndrome is likely a defect in the central respiratory control system. These patients have been shown to have a decreased responsiveness to carbon dioxide rebreathing, hypoxia, or both.

Chest wall deformities

In patients with chest wall deformities, hypoventilation develops secondary to decreased chest wall compliance with a resultant decreased tidal volume. Alveolar dead space is unchanged, but the VD/VT ratio is increased due to the reduced tidal volume. The most common chest wall abnormality to cause hypoventilation is kyphoscoliosis. It is associated with a decrease in vital capacity and expiratory reserve volume, while the residual volume is only moderately reduced. These patients usually are asymptomatic until the late stages of disease, with the most severe deformity of the spine.

Neuromuscular disorders

Patients with neuromuscular disorders have a reduced vital capacity and expiratory reserve volume secondary to respiratory muscle weakness. The residual volume is maintained. The reduction in vital capacity is greater than what is expected solely from the underlying respiratory muscle weakness, and these patients are likely to also have significant reduction in lung and chest wall compliance, which further reduces vital capacity. The reduction in lung and chest wall compliance may be secondary to atelectasis and reduced tissue elasticity. In addition, the VD/VT ratio is increased due to the reduced tidal volume, and this further contributes to hypoventilation.

During sleep, ventilation decreases because a lessening in respiratory centers function. During REM sleep, atonia worsens thus leading to more severe hypoventilation, particularly when diaphragmatic function is impaired. The effects of atonia are amplified by a low sensitivity of the respiratory centers. Nocturnal mechanical ventilation improves nocturnal hypoventilation and daytime arterial blood gases in these patients. .

Chronic obstructive lung disease

Hypoventilation in patients with COPD is secondary to multiple mechanisms. As mentioned previously, these patients usually have severe obstruction with a FEV_1.0 of less than 1 L or 35% of the predicted value. Patients with COPD who hypoventilate have a decreased chemical responsiveness to hypoxia and hypercapnia. This decreased chemical responsiveness also is observed in relatives of these patients who do not have COPD leading researchers to believe that a genetic predisposition to alveolar hypoventilation exists. These patients have a reduced tidal volume and a rapid shallow breathing pattern, which leads to an increased VD/VT ratio. Patients also may have abnormal diaphragm function secondary to muscular fatigue and muscular mechanical disadvantage from hyperinflation.

Frequency

United States

The frequency of hypoventilation syndromes varies with the underlying cause of hypoventilation. The most common of these disorders is chronic obstructive lung disease, which affects more than 14 million people in the United States. Kyphoscoliosis is the chest wall deformity most commonly associated with hypoventilation.

The prevalence of hypoventilation was studied in 54 stable hypercapnic COPD patients without concomitant sleep apnea or morbid obesity. Of these, 43% had sleep-related hypoventilation, which was more severe in rapid eye movement sleep.

Mortality/Morbidity

• The morbidity and mortality rates of patients with hypoventilation syndromes depend on the specific etiology of the hypoventilation.
• The morbidity and mortality rates of each of the above-mentioned disorders are increased secondary to the presence of respiratory failure and alveolar hypoventilation.
• Studies reported several decades ago showed significant increase in mortality in patients with obesity hypoventilation syndrome. This increased mortality is likely secondary to an increased risk of arrhythmias and cardiovascular complications. Miller reported an in-hospital mortality rate of 70% in a small cohort of hospitalized patients in 1974. This mortality rate is likely lower today due to improvements in treatment of sleep apnea and hypoventilation related to sleep apnea and improvements in the treatment of cardiovascular disease.

Sex

• Primary alveolar hypoventilation occurs more commonly in male patients than female patients.
• COPD occurs more commonly in men than in women; however, because of increased smoking in women, the incidence is increasing in females.
• Obesity hypoventilation syndrome also occurs more commonly in men because they have more upper-body obesity than women with similar body mass indices.

Age

• Most patients with hypoventilation syndromes are older. COPD and obesity increase in prevalence with age.
• Primary alveolar hypoventilation occurs more commonly in early adulthood, but it also occasionally is diagnosed in infancy.

Clinical

History

The clinical manifestations of hypoventilation syndromes usually are nonspecific, and in most cases, they are secondary to the underlying clinical diagnosis.

• Manifestations vary depending on the severity of hypoventilation, the rate of development of hypercapnia, and the degree of compensation for respiratory acidosis that may be present.
• During the early stages of hypoventilation with mild-to-moderate hypercapnia, patients usually are asymptomatic or have only minimal symptoms.
  • Patients may be anxious and complain of dyspnea with exertion.
  • As the degree of hypoventilation progresses, patients develop dyspnea at rest. Some patients may have disturbed sleep and daytime hypersomnolence.
  • As the hypoventilation progresses, more patients develop increased hypercapnia and hypoxemia. Therefore, they may have clinical manifestations of hypoxemia, such as cyanosis, and they also may have signs related to their hypercapnia.
  • As the hypoventilation progresses, the PaCO₂ increases; anxiety may progress to delirium; and patients become progressively more confused, somnolent, and obtunded. This condition occasionally is referred to as carbon dioxide narcosis.
  • Patients may develop asterixis, myoclonus, and seizures in severe hypercapnia.
  • Papilledema may be seen in some individuals secondary to increased intracranial pressure related to cerebral vasodilation.
  • Conjunctival and superficial facial blood vessel dilation also may be noted.
  • Patients with respiratory muscle weakness usually display generalized weakness secondary to their underlying neuromuscular disorder. Respiratory muscle weakness also may lead to impaired cough and recurrent lower respiratory tract infections.
  • With advanced disease, patients may develop respiratory failure and require ventilatory support.
• Patients with central alveolar hypoventilation usually have no respiratory complaints. They may have symptoms of sleep disturbances and daytime hypersomnolence.
  • In some patients, the diagnosis of central alveolar hypoventilation is made only after the development of respiratory failure.
  • Frequently, patients with obesity hypoventilation syndrome have associated obstructive sleep apnea (OSA) and complain of daytime hypersomnolence. This combination is known as pickwickian syndrome. Patients with obesity hypoventilation also may have pulmonary hypertension and chronic right heart failure (cor pulmonale), with secondary peripheral edema in advanced disease.
  • Patients with COPD and hypoventilation usually have severe disease and complain of significant dyspnea. They also may have peripheral edema secondary to pulmonary hypertension with cor pulmonale.

Physical

• In patients with alveolar hypoventilation, the findings upon physical examination usually are nonspecific and are related to the underlying illness.
• Upon thoracic examination, patients with obstructive lung disease have diffuse wheezing, hyperinflation (barrel chest), diffusely decreased breath sounds, hyperresonance upon percussion, and prolonged expiration.
  • Coarse crackles beginning with inspiration may be heard, and wheezes frequently are heard upon forced and unforced expiration.
  • Cyanosis may be noted if accompanying hypoxia is present. Clubbing may be present.
• The patient's mental status may be depressed with severe elevations of PaCO₂. Patients may have asterixis and papilledema upon examination, and conjunctival and superficial facial blood vessels may be dilated.
• Patients with central alveolar hypoventilation, COPD, and obesity hypoventilation syndrome may show evidence of pulmonary hypertension from examination findings. These findings include a narrowly split and loud pulmonary component (P2) of the second heart sound, a large a-wave component in the jugular venous pulse, a left parasternal (right ventricular) heave, and an S4 of right ventricular origin. A diastolic murmur indicative of pulmonic valve regurgitation may be auscultated.
• Patients with advanced disease develop signs of right ventricular failure (cor pulmonale) and may have elevated jugular venous pressure with a prominent V wave, lower-extremity edema, and hepatomegaly upon physical examination. A pulsatile liver develops if tricuspid regurgitation is severe. Ascites may occur but is unusual. The systolic murmur of tricuspid valve regurgitation may be present.

Causes

Hypoventilation may be secondary to several mechanisms, including central respiratory drive depression, neuromuscular disorders, chest wall abnormalities, obesity hypoventilation, and COPD.

• Chronic obstructive pulmonary disease
  • Emphysema
  • Chronic bronchitis
• Neuromuscular disorders
  • Amyotrophic lateral sclerosis
  • Muscular dystrophy
  • Diaphragm paralysis
  • Guillain-Barré syndrome
  • Myasthenia gravis
• Chest wall deformities
  • Kyphoscoliosis
  • Fibrothorax
  • Thoracoplasty
• Central respiratory drive depression
  • Drugs - Narcotics, benzodiazepines, barbiturates
  • Neurologic disorders - Encephalitis, brainstem disease, trauma
  • Primary alveolar hypoventilation
• Obesity hypoventilation syndrome

Source : http://emedicine.medscape.com/article/304381-overview