Ventilatory disorders, reflected by abnormalities in PaCO2, include alterations in CO2 production, minute ventilation, or respiratory system dead space. Many diseases can cause acute elevations in CO2 production; chronic ventilatory disorders typically relate to reduced minute ventilation or increased respiratory dead space fraction.
Chronic hypoventilation can result from parenchymal lung diseases, chest wall abnormalities (e.g., severe kyphoscoliosis), sleep-disordered breathing, neuromuscular diseases, and abnormal respiratory drive. Obesity-hypoventilation syndrome (OHS) is diagnosed by body mass index ≥30 kg/m2 and PaCO2 >45 mmHg in the absence of other causes of hypercapnia. Sleep-disordered breathing, typically obstructive sleep apnea, is seen in most OHS pts. Central hypoventilation syndrome is a rare disorder that includes a failure of the normal respiratory response to hypoxemia and/or hypercapnia.
Key symptoms of hypoventilation can include dyspnea, reduced exercise capacity, orthopnea, daytime somnolence, morning headache, and anxiety. Parenchymal lung diseases, such as chronic obstructive pulmonary disease and interstitial lung disease, often include dyspnea and cough. Sleep-disordered breathing includes daytime somnolence, snoring, and fragmented sleep. Orthopnea is common in neuromuscular disorders, although weakness of the extremities or other muscle groups often precedes respiratory system muscular weakness. Hypoventilation related to neuromuscular and chest wall disorders typically begins with nocturnal hypoventilation and progresses to daytime hypercapnia. Chronic narcotic use and hypothyroidism can lead to reduced respiratory drive.
Physical examination, chest radiographic studies (CXR and possibly chest CT), and pulmonary function tests reveal most lung parenchymal and chest wall causes of hypoventilation. Measurements of maximal inspiratory and expiratory pressures or forced vital capacity can assess and monitor respiratory muscle strength. Polysomnography to assess for sleep-disordered breathing should also be considered. When pts have hypercapnia with normal pulmonary function, normal respiratory muscle strength, and normal alveolar-arterial PO2 difference, respiratory drive abnormalities may be present, which can be revealed by polysomnography. Laboratory findings include increased PaCO2 and often reduced PaO2 as well. Compensatory increases in plasma bicarbonate levels and normal pH are seen in chronic hypoventilation. Eventually, pulmonary hypertension and cor pulmonale can develop. In central hypoventilation syndrome, hypercapnia worsens substantially during sleep.
In all forms of hypoventilation, if hypoxemia is present, supplemental oxygen should be considered. OHS is treated with weight reduction and nocturnal noninvasive positive pressure ventilation (NIPPV). Continuous positive airway pressure (CPAP) during sleep is adequate treatment for many OHS pts, but others require bilevel positive airway pressure (BiPAP).
NIPPV during sleep can provide ventilatory support and treat sleep apnea associated with neuromuscular disorders, chest wall disorders, and central hypoventilation. With progressive neuromuscular disorders, full-time mechanical ventilatory support is often required. Pts with respiratory drive disorders may benefit from phrenic nerve or diaphragm pacing.